1

N

3 km

High Resolution Mega Mapping the North Sea and Chalk Group Base Horizons – Towards an Improved Petrel workflow for Multi-Resolution Grid Merges and

a Velocity Model for the Dutch Northern Offshore

July 2019

MSc Thesis - Internship Intern: Mohamed Bouchingour

Company: EBN B.V. Institute: VU Amsterdam

Student number: 2534750 Faculty: Science

Study: Earth Sciences Specialization: Geology and Geochemistry

Stream: Subsurface Resourcing Course Code: AM_1186

Credits: 27 EC EBN Guidance: MSc. L. Janssen and MSc. M. Ecclestone

EBN Supervisor: drs. G. Hoetz VU Supervisor: Prof. Dr. K.F. Kuiper

VU Second Assessor: Prof. Dr. W. van Westrenen

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Abstract The subsurface of the Southern North Sea is one of the world’s most studied regions due to its abundance of oil and gas. With many Dutch fields reaching the end of their economic life time new prospects are being explored. The Chalk Group (CK) forms an important target for these studies. This thesis contributes to exploration studies on the Chalk play with high resolution time maps of the group’s top and base horizon together with an accurate velocity model for depth conversion of this group and its overburden in the Dutch northern offshore. Moreover, the high resolution maps show a detailed overview of the structures, especially faults. Small faults require high resolution maps for proper representation. Also, well planning often benefits from identifying and avoiding faults, as they can lead to operational problems such as losses while drilling. The study is part of EBN’s High Resolution Mega Mapping Project (HIRES MEMA) which aims to compile and produce high resolution maps of key seismic horizons on 25 x 25 m grid bin sizes. Contributions to this overarching project are high resolution offshore maps of the bases of the Chalk Group (CK), Lower North Sea Group (NL) and Upper North Sea Group (NU). This was achieved with horizon interpretation on state-of-the art Pre-STM 3D seismic in combination with seismic-to-well data. Additional contributions to the project are a Petrel workflow specifically designed to combine high and low resolution grids and a velocity model. The petrel workflow implements, merges and harmonizes high resolution grids. Regional low resolution horizon maps (250 x 250 m), which are produced and published by TNO (DGM v5), are used in areas where high resolution grids are absent. The new velocity model, created in this study, can be used for regional time-depth conversion of the North Sea Supergroup (N) and CK in the Dutch northern offshore. This model was built with the new high resolution maps, careful selection of boreholes and thorough quality control of time-depth relations. The refined velocity model shows for the N and CK significantly smaller depth residuals in comparison to those that result from EBN’s current DEFAB velocity model and TNO’s VELMOD 3.1.

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Table of Contents

1. INTRODUCTION 5 1.1 REASERCH DRIVERS 5 1.2 STUDY OBJECTIVES 5 1.3 IMPORTANCE OF THIS STUDY 6 1.4 GEOLOGICAL SETTING OF THE SOUTHERN NORTH SEA 9 1.5 NOMENCLATURE OF THE DUTCH SUBSURFACE 9 1.6 STRATIGRAPHY OF THE CHALK GROUP AND NORTH SEA SUPERGROUP 10

1.6.1 THE CHALK GROUP 10 1.6.2 THE LOWER NORTH SEA GROUP 12 1.6.3 THE MIDDLE NORTH SEA GROUP 13 1.6.4 THE UPPER NORTH SEA GROUP 13

1.7 BACKGROUND INFORMATION ON THE CHALK PLAY 13 1.8 BACKGROUND INFORMATION ON PETREL WORKFLOW MODULES 15 1.9 BACKGROUND INFORMATION ON CHECKSHOT ACQUISITION IN THE OFFSHORE 15 1.10 BACKGROUND INFORMATION ON TIME-DEPTH RELATIONS 16

2. METHODS AND MATERIALS 18 2.1 PROJECT TIMETABLE AND WORKFLOW 18 2.2 DATA LOADING AND PETREL PROJECT SETTINGS 18 2.3 OVERALL OBSERVATIONS OF SEISMIC DATASETS 19 2.4 TYPICAL REFLECTION AND WIRELINE LOG RESPONSES OF THE TARGET GROUPS 21 2.5 QC OF T/Z RELATIONS 22 2.6 WORKFLOW HORIZON INTERPRETATION 24 2.7 QC OF THE INTERPRETED HORIZONS 25 2.8 APPROACH OF DESIGNING THE PETREL WORKFLOW 27 2.9 FUNCTION USED FOR THE BAGHDEF VELOCITY MODEL 27 2.10 WORKFLOW BAGHDEF VELOCITY MODELLING 29

3. RESULTS 33 3.1 HORIZON OF THE BASE UPPER NORTH SEA GROUP 33 3.2 HORIZON OF THE BASE LOWER NORTH SEA GROUP 34 3.3 HORIZON OF THE BASE CHALK GROUP 36 3.4 ARCHITECTURE AND RESULTS OF THE PETREL WORKFLOW 38 3.5 BAGHDEF VELOCITY MODEL 43

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4. DISCUSSION 48 4.1 STRENGTHS AND LIMITATIONS OF THE HORIZON INTERPRETATION 48 4.2 BENCHMARKING THE BAGHDEF VELOCITY MODEL 48 4.3 LIMITATIONS OF THE BAGHDEF VELOCITY MODEL 54 4.4 LIMITATIONS OF THE PETREL WORKFLOW 54 4.5 RECOMMENDATIONS FOR FUTURE RESEARCH 54

5. CONCLUSION 56

ACKNOWLEDGEMENTS 56

REFERENCES 57

APPENDIX 60

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Fig. 1.2 The Dutch offshore territory outlined in red, with its division in

quadrants (blocks)

A B

E F

K L

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S R

M

G D

H J

Q P

N

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N

Introduction

1.1 Research drivers Oil and gas has been a major source of energy and revenue for the Netherlands since the 1960s and will continue to be during the coming decades (Schoots et al., 2017). However, a growing number of gas and oil assets will be reaching the end of their economic life time during the coming years (Van Hoogstraten, 2016). Also, many production licenses are to expire in this period. The Dutch offshore and more broadly the Southern North Sea, has an increasing number of mature fields. For this reason there are plans for decommissioning the area’s infrastructure comprising boreholes, platforms and pipelines. To avoid pre-mature decommissioning of the infrastructure and to make maximum use of the remaining potential of the subsurface, companies are focusing on new exploration activities. The Chalk in the Dutch northern offshore forms an important target for these studies and is the focus of EBN’s ongoing Chalk Evaluation Project. The CK, while a significant play in the Danish and Norwegian offshore territory, is relatively underexplored in the Netherlands. Of the more than two hundred producing fields in the Dutch offshore, only two fields are being exploited from the CK. Both fields produce oil and are located in the northern offshore: the F2 and F17 sub blocks (Kombrink et al., 2012). This study directly contributes to exploration of the Chalk play with high resolution time maps of the CK’s base and top horizon, along with an accurate velocity model for time depth conversion of this interval.

1.2 Study objectives This study has three aims. The main aim is to produce maps of three key seismic horizons with the highest achievable resolution, from bottom to top: base Chalk Group (CK), base Lower North Sea Group (NL) and base Upper North Sea Group (NU). The NL also forms the base of the North Sea Supergroup (N) and top of the CK, if present. The maps comprise the Dutch offshore that has significant 3D seismic coverage. The Northern offshore: A, B, D, E, F, G and H quadrants (BAGHDEF area), forms the focus area (fig. 1.2). Where no 3D surveys do exist, often there is 2D seismic available. The low resolution DGM v5 grids do have typically these data included in their mapping. A parallel study is focused on mapping of the onshore Dutch territory (Beaucaire, 2019). The second aim of this study is to design a Petrel workflow for the automatization of merging and harmonizing grids of different resolution into a single grid surface. The workflow is suited for horizons mapped by TNO, because they form the input of the low resolution horizon in the Petrel workflow. These horizons are well mappable, define velocity regimes for layercake time depth conversion and form the bases of lithostratigraphic groups, following the nomenclature of Van Adrichem Boogaert & Kouwe (1997). The final aim of this study is building a more accurate regional velocity model for the N and CK in the BAGHDEF area than the currently used DEFAB velocity model (EBN) and VELMOD 3.1 (TNO). In this way it will be possibile to convert the high resolution time maps into high resolution depth maps.

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Fig. 1.3a A Grid with a resolution of 25 x 25 m bin size (top) has a 100 times more data points in comparison to a grid of 250 x 250 m bin size (bottom)

1.3 Importance of this study In 2006, for the first time, the Netherlands Organization for applied scientific research (TNO) published offshore maps of Dutch key subsurface horizons (DGM v2). These are the base horizons of lithostratigraphic groups, following the nomenclature of Van Adrichem Boogaert & Kouwe (1997). Four years later the maps were updated (DGM v3). The latest version (DGM v5) was provided by TNO to EBN in April of 2019. These maps will be published later in 2019 on the Dutch oil and gas portal www.nlog.nl. Mapping by TNO is done with horizon interpretations on public seismic datasets. However, these maps are restricted to a resolution of 250 x 250 m grid bin size, while adequate for regional geological studies, they are insufficient for many industry applications. The typical grid bin sizes of 3D seismic surveys are 25 x 25 m. This means that mapping on the highest achievable resolution yields maps that contain a factor of 100 more datapoints (fig. 1.3a). By law, EBN is a compulsory partner and stakeholder of Dutch oil, gas and geothermal assets that are explored and exploited by operators. Thus, EBN is in the possession of all subsurface (classified) data, among other high resolution seismic data sets and interpretations. However, the targets of this study (base NU, NL and CK), typically do not constitute targets for exploration studies, but are used as points of reference. Therefore, the publicly available low resolution DGM v5 horizons are often considered sufficient. The high resolution maps of the target horizons of this study that are available, are regularly of small areas of interest. This constraint makes EBN inhouse mapping necessary for the HiRes MEMA project. Fortunately this process is eased as the target horizons often yield good reflection responses with continuous loops of high amplitude that for the most part can be tracked automatically. A start has already been made where the northern offshore comprising the A, B, and partially the D, E, and F blocks (DEFAB area) have been mapped. Besides the fact that maps of higher resolution can be achieved, there are also other reasons for making own interpretations. These are that the DGM v5 regional maps are compilations of hundreds of surfaces interpreted on many different (often outdated) seismic, that lead to (unavoidable) artefacts in the horizon (see fig. 1.3b). Also, since TNO only has non-confidential data availabile for this public domain mapping project, it is forced in areas of low or no seismic coverage to extrapolate its horizons (see fig. 1.3c).

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Fig. 1.3b Artefacts in the horizons of DGM v5 in the northern offshore that are probably caused by merging of different horizons interpreted on (outdated) 2D seismic

Fig. 1.3c Artefacts in the horizons of DGM v5 in the eastern offshore, most likely caused by seismic data gaps

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Group horizons older than the target horizons of this study (e.g. Zechstein Group) are seismically significantly less defined; this is further complicated by the omnipresence of complex fault structures. Therefore, auto-tracking these older horizons will strongly be limited as result of discontinues reflections. However, many interpretations of these horizons already exist at EBN. These maps have been provided by operators and are scattered throughout projects. To manually merge and harmonize all these small grids/surfaces into one large map is time-consuming. This is where the Petrel workflow comes into play by automatically implementing, merging and harmonizing grids of different sources and resolution into one multi-resolution mega map. Also, if new interpretations are to be made of an area with new or reprocessed seismic, the improvements of the horizons can (partially) replace the outdated interpretations. This process will contribute to constantly improving and expanding high resolution maps of key seismic horizons in the entire Dutch subsurface. Lastly, with increasing interest in the Chalk play and the Dutch northern offshore, a velocity model for time-depth conversion of these targets will be greatly appreciated. Currently, EBN uses two velocity models for regional time-depth conversion in the northern offshore: EBN’s DEFAB velocity model (2015) and TNO’s VELMOD 3.1 (2017). The VELMOD 3.1 velocity model is made with velocity data from the entire Dutch (on- and offshore) territory, in contrast to the DEFAB velocity model which is focused on the Dutch northern offshore. The BAGHDEF velocity model of this study is meant to revise, expand and improve the DEFAB velocity model by minimizing depth residuals for the N and CK that yield from the conversion. Residuals pertain to the (time or depth) difference between an interpreted horizon and its corresponding well top at the location of the penetrating borehole.

1.4 Geological setting of the southern North Sea 6500 boreholes have been drilled in a little over 100 years in the Dutch territory of which more than 2100 offshore. The first Dutch offshore wells were drilled in 1962 in the Q block close to the west coast of the Netherlands. Their well logs together with well data from neighboring countries provided crucial insights into the geology of the Southern North Sea Basin (SNSB) that comprises, among others, the Dutch territory. This region has undergone multiple phases of basinal subsidence and rapid uplift. Little is known about the deeply hidden Early Paleozoic sediments that in most areas lie deeper than 5 km (Cameron et al. 1992). The sediments are several kilometers thick and are likely metamorphised in many places by intruding granite plutons during the Pridoli (uppermost Late Silurian) till Emsian (uppermost early Devonion) as result of the Caledonian Orogeny. It has been suggested that the omnipresent WNW-ESE and NW-SE trending faults formed during most of the Phanerozoic are the result of the structure of the basement comprising the Lower Paleozoic sediments. During the remainder of the Devonian most of the SNSB was an upland of net erosion. Subsequently, during the Early Carboniferous (Mississippian) the basin underwent crustal extension that lead to pelagic and fluvial sedimentation of up to 4 km strata in grabens. The crustal extension caused in some places adiabetic decompression, which generated granite batholites that are believed to lie beneath Missian (Early Carboniferous) horsts (Cameron et al. 1992). At the onset of the Peian (Late Carboniferous) rifting ceased and thermal susidence commenced till the Gzhelian (uppermost Late Carboniferous). Subsequently, during the Permian the SNSB experienced folding and faulting of its Carboniferous rocks caused by the Variscan Orogeny. This was in combination with differential regional uplift that resulted in severe erosion of more than 1 km overburden in some places. This uplift lasted mainly till the Kungurian (uppermost Early Permian) and was followed by subsidence till the Late Triassic which formed

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Fig. 1.5 Key lithostratigraphic intervals for hydrocarbon plays in the Dutch subsurface (modified after Rondeel et al., 1996)

accommodation space for up to 3 km of sediments comprising, among other, the Rotliegend Group which forms the Netherland’s most important petroleum play. In some places the subsidence continued into the Early Jurassic which is believed to have reactivated the Permian-aged faults which in turn drove the halokinesis of the Zechstein salts (Glennie and Boegner 1981). During the Middle Jurassic hundreds of meters of overburden were eroded in many places as result of domal uplift (Glennie, 1986). The SNSB was further affected by overprints of diapirism during the Late Jurassic (Glennie and Underhill, 1998). At onset of the Early Cretaceous the SNSB was filled with fluvial and marine sediments of which currently some sandstones form various plays (e.g. Delft Sandstone Member). This event was followed by widespread deposition of the Chalk Group during the Late Cretaceous, where the Cleaver Bank High formed the depocenter with up to 1 km of sedimentation. During the Maastrichtian (uppermost Late Cretaceous) the N-S oriented Sub-Hercynian tectonic phase caused rapid inversion of the SNSB that re-activated faults, this overprinting is visible as upthrown fault blocks in Dutch and other NW European basins (Glennie and Boegner, 1981; Coward, 1991). Uplift gradually came to a halt during the Thanetian (uppermost Late Paleocene). Eustatic sea level rise led to transgression of the SNSB during the Eocene where the main depocenter was the extent of the Central and Viking Graben. Basinal subsidence was again a fact at the onset of the Oligocene that reactivated faults by dextral strike-slip, which in turn triggered halokinesis (Glennie and Boegner 1981). Subsidence rates reached a maximum during the Chibanian (upper Early Quaternary) that resulted in hundreds of meters thick progradational delta and glacial deposits. 1.5 Nomenclature of the Dutch subsurface In the Netherlands outcrops of older than Quaternary formations are virtually non-existent. Only late Cretaceous outcrops in the southeastern province Limburg and Triassic sediments near Winterswijk in the east of the country are locally present. Therefore, predominantly well and seismic data are used to explain the lithology and structure of the Dutch subsurface. Due to the relative small area of the Dutch territory, lateral variation of Mesozoic and Cenozoic sediments is limited. The far east and southeast of the Netherlands are commonly exception to this rule (Wong et al., 2007). TNO and other organizations and companies working with Dutch subsurface data mostly follow the nomenclature of Van Adrichem Boogaert & Kouwe (1997). This nomenclature is based on data that is integrated with well and field data from the UK, Belgium and Germany. Its stratigraphy runs down to the late Paleozoic and is divided into 19 groups that are further subdivided into subgroups, formations and members (see ‘Dutch Nomenclature.xlsx’ in the appendix). The uppermost group is the NU of

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Quaternary and Neogene age and the lowermost group is the Old Red Group (OR) of Devonian age. Fig. 1.5 shows an overview of the stratigraphy with key stratigraphic intervals that act as a source rock or reservoir. In the next paragraphs the stratigraphy and overall lithological composition of the CK, NL, Middle North Sea Group (NM) and NU are described, following the nomenclature of Van Adrichem Boogaert & Kouwe (1997).

1.6 Stratigraphy of the Chalk Group and North Sea Supergroup The CK is a Late Cretaceous sediment that is broadly found in the Northwest European subsurface and as the name implies, it consists mainly of chalk. Due to its large difference in rock properties with its over and underburden the group’s top and base contact typically form a sharp acoustic impedance contrast. The North Sea Supergroup (N) overlies the CK unconformably. It is subdivided into three groups, which are deposited unconformable on each other. The NU of Neogene and Quaternary age. The NM of late Paleogene age and the NL of early and mid-Paleogene age. The supergroup covers the entire Dutch offshore and contains sediments deposited in marine and continental environments. This succession, consisting mainly of shales and sandstones, is further subdivided into formations and sometimes members (Van Adrichem Boogaert & Kouwe, 1997). Currently, sands in the N comprise profitable reservoirs in the underexplored Northern offshore due to its abundance of shallow gas fields that mostly are produced from the NU in the A and B blocks (Wong et. al., 2007). In the next subparagraph the lithology of the groups are described.

1.6.1 The Chalk Group The CK is a sedimentary succession of Cenomanian-Danian age (lowermost Upper Cretaceous to lowermost Paleocene). The group rests conformably on the Rijnland Group (KN). The CK is omnipresent in the Dutch offshore but can be locally absent due to erosion as result of the SNSB’s inversion. This inversion was triggered by the Subhercynian tectonic phase, which in turn was driven by the Alpine orogeny. As result it has a strongly variable thickness with areas that exceed 1800 m in the K and P quadrants, while elsewhere, in the Dutch Central Graben it is relatively thin or absent. Its base reaches a maximum depth of approximately 2800 m in the central and northern offshore. The group is mainly a succession of marine, bioclastic and occasionally marly limestones. Flint concretions are omnipresent and in some locations siliciclastic rocks are found, most notably glauconite sandstones (Wong et al., 2007). The CK is for the offshore, and most of the onshore too, divided into three formations, from old to young these are: the Texel, Ommelanden and Ekofisk formations (fig. 1.6.1). The latter two formations are not subdivided into members, because they have a homogeneous lithology that make a distinguishment of members on a regional scale unfeasible (Van Adrichem Boogaert & Kouwe, 1997). The Texel Formation (CKTX) is of Cenomanian to earliest Turonian age (lowermost Upper Cretaceous) and is mostly several tens of meters thick. It consists mainly of light colored limestones and marly chalks with occasional intercalations of marls.

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S N

Fig. 1.6.1 A regional cross-section of the Chalk Group in the Dutch territory and its stratigraphic subdivision (after Wong et. al., 2007)

Towards the center of the Broad Fourteens Basin (BFB) the formation’s clay content gradually increases into deposits of marls, while in the northern offshore it consists of pure limestones. It is subdivided into three members, from old to young: The Texel Greensand, Texel Marlstone and Plenus Marl members of which the latter two are found in the Dutch offshore.

The Ommelanden Formation (CKGR) is of Cenomanian to Maastrichtian age (Upper Cretaceous) and is overall much thicker than the Texel Formation with a maximum thickness of 1500 m in the K and L quadrants. It was deposited on a shelf and upper slope of a carbonate ramp and has been found to have intraformational unconformities (Herngreen et al., 1996). The formation’s lower interval consists of white, hard and dense limestones that in the younging direction increase in clay content becoming marley. This interval is overlain by purer chalks with coarsening upwards and presence of flint nodules. The Ekofisk Formation (CKEK) is of Danian age (lowermost Paleocene) and is present in the southern and northern Dutch offshore where it is mostly several tens of meters thick. Its

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Fig. 1.6.2 Stratigraphic scheme of the Lower and Middle North Sea groups in the Dutch territory (after Van Adrichem Boogaert & Kouwe, 1997)

depositional setting is similar to that of the CKGR, but experienced redeposition as result of gravitational mass flows caused by piercing salt domes. It consists of white chalky limestones with presence of flint in both nodular and layered form. Locally, some thin laminated clays with glauconite are found at the formation’s base that hallmark the K–Pg boundary (Schmitz et al., 1992; Smit & Brinkhuis, 1996; Herngreen et al., 1998).

1.6.2 The Lower North Sea Group The Lower North Sea Group (NL) is an early Paleogene sedimentary succession of Thanetian to Bartonian age (uppermost Paleocene to upper Eocene). The group rests unconformably on its underburden which is almost exclusively the CK in the Dutch offshore (Wong et al., 2007). The NL thickens in northward direction towards the center of the SNSB with a thickness of up to 650 m in the northern part of the A quadrant. The group consist mostly of sandstones, marls and clays. It is subdivided into two formations: the Landen and Dongen Formation. The Landen Formation (NLLF) is a succession of fine-grained sands and sandy clays with alternations of clays and marls. In the Northern offshore however the formation consists exclusively of clays with its lower interval comprising marl intercalations. The formation is subdivided into five members (fig. 1.6.2).

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The Dongen Formation (NLFF) rests conformably on the Landen Formation. It comprises units of high sand content that in northward direction change into marls with some areas of silty clays. The formation is subdivided into seven members of which the Dongen Clay Member (NLFFC) makes up the bulk of the formation in the Northern offshore.

1.6.3 The Middle North Sea Group The Middle North Sea Group (NM) is a mainly Oligocene sedimentary succession of Priabonian to Aquitanian age (uppermost Eocene to lowermost Miocene) and rests unconformably on the NL or older deposits. The NM is a succession of sands, silts and clays that fine northward from the southern margin towards the center of the SNSB. The group is subdivided into three formations, from old to young: the Tongeren, Rupel and Veldhoven formations of which the latter two are found offshore with some absence in the western offshore. The Rupel Formation (NMRF) consists predominantly of grey to dark-brown clays that are typified by septaria concretions. It sand content decreases towards the center of the SNSB. The formation is subdivided into several members of which three are found in the offshore (fig. 1.6.2). The Veldhoven Formation (NMVF) comprises a high sand content unit with a coarsening upward clay unit on top of it. The formation is subdivided into three members (fig. 1.6.2).

1.6.4 The Upper North Sea Group The Upper North Sea Group (NU) comprises all post-Paleogene deposits and makes up the present day surface and seafloor. It rests unconformably on the NM or older underburden and is in many places recognizable as an angular unconformity on seismic, its base is therefore also referred to as the Mid-Miocene Unconformity (MMU). The NU is present in the entire Dutch offshore and is a succession of clays, variable grained size sands with local deposits of gravel, peat and browncoal seams. The group varies strongly in thickness with in the southernmost offshore thicknesses of less than 100 m and in the northern A and B quadrants up to 1400 m. The group has been described and subdivided by its depositional setting, either on- and offshore separately or combined (see also De Mulder et al., 2003; Rijsdijk et al., 2005 Van Adrichem Boogaert and Kouwe, 1997; Weerts et al., 2003). The group’s Neogene deposists are divided in five formations. The Breda (NUBA) and Oosterhout (NUOT) formations are marine sediments, while the Ville (NUVI), Inden (NUIN), Kieseloolite (NUIKO) and Scheemda (NUSA) formations are pre-dominantly continental sediments. Quaternary deposits of the NU are referred to as the ‘undeep subsurface’ in the Dutch nomenclature and comprises 22 formations subdivided in marine, fluvial, glacial and terrestrial sediments (De Gans, 2007).

1.7 Background information on the Chalk play As explained in paragraph 1.3 the CK is currently of particular interest to EBN for prospectivity studies. With the high resolution maps of the base NL (top CK) and base CK produced in this study, there is a detailed overview of the interval’s structures. An important area where fields produce from the CK and neighbors the Northern Dutch offshore is the UK Central Graben (UK CG). Here, the lower formations of the CK are the thickest and form the main reservoirs of the Chalk play. Isopach maps indicated NW-SE to NNW-SSE trending inversion axes of the CK basin with depocentres perpendicular to them. Upper Jurrasic claystones are considered to be the source rock (Darby et al., 1996). Fig. 1.7 is a sketch of this play in the UK CG.

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A majority of the producing fields in the UK CG are located in four-way dip closures (i.e. anticlines). A quarter of these fields have later been found to be the result of stratigraphic, diagenetic or hydrodynamic traps with dipping oil-water contacts or free-water levels (Rowe & Tomkinson, 2006). These contacts indicate trapping due to permeability constraints following post-charge tectonic tilting (Megson, 1992; Megson & Hardman, 2001; Shepherd et al., 2003) or to hydrodynamic factors (Harris & Goldsmith, 2001; Dennis et al., 2005). These type of traps are ascribed to be the result of chalk debris flows shed off from the SNSB margins and the crests and flanks of the inverted areas (Rowe & Tomkinson, 2006). They are recognizable on seismic as more disturbed reflections than the autogenic Chalk which commonly yields clear parallel reflections (Gennaro at al., 2013). These reworked Chalk deposits are also found in the Step Graben in the northwest of the Dutch offshore. A positive relation between the magnitude of seismic reflection amplitudes and porosity of the CK has been found (Anderson, 1999; Bramwell et al., 1999). The maximum negative amplitude at the top of the Tor Formation (largely coincides with the Ommelanden Formation) is the key seismic attribute of Chalk prospectivity in the Greater Ekofisk area (Bramwell et al. 1999) and UK central North Sea (Rowe & Tomkinson, 2006). However, Anderson (1999) mentions that these amplitude anomalies do not necessarily represent the presence of hydrocarbons, because no seismically detectable difference in acoustic impedance (AI) or amplitude vs offset (AVO) response has been found between brine-filled and hydrocarbon-filled chalks in the Norwegian sector. Typically, only in case of gas presence seismic responses are viable for hydrocarbon predictability, which is seldom in the case of the CK (Megson & Tygesen, 2005). Moreover, high amplitude values in the CK can also be contributed by tuning effects due to increase in shale amount (Vsh) (Bramwell et al., 1999).

Fig. 1.7 Sketch of the Chalk play in the UK Central Graben (after Rowe & Tomkinson, 2006)

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1.8 Background information on Petrel workflow modules The modelling software PetrelTM was used for, besides seismic interpretation and velocity modelling, writing a script for the automatization of multi-resolution grid merges. This was done with Petrel’s workflow module. With this module virtually any task that can be performed within Petrel can be automated by writing a script comprising lines of codes. A Petrel workflow designed with one or more processes that consists of multiple lines in order to perform a certain task. Tasks can either be operations or actions, an operation refers to a grid manipulation (e.g. surface-to-surface extraction or bulk shift) and an action refers to any non-operative task (e.g. point conversion or reading out variables). For a full description of the workflow module and its possibilities it is advised to consult Schlumberger’s Petrel manual. Since the lines of codes are pre-programmed the module is very user-friendly and does not require computer programming skills. Workflows while very handy are often underused in the industry (Eikelenboom, personal communication, 2019). They can be extremely timesaving when models (e.g. horizon maps) have to be improved, edited or updated. Maps are continuously being improved due to new (higher quality) seismic becoming available or to advances in subsurface knowledge.

1.9 Background information on checkshot acquisition in the offshore The BAGHDEF velocity model is parametrized with time-depth (T/Z) relations of wells strictly based on checkshot/VSP data. A checkshot survey is the measurement of the seismic travel time from the surface to a known depth by having a geophone in the wellbore record the P-wave signal sent by an airgun. A vertical seismic profile (VSP) is acquired in the same manner as a checkshot survey but has commonly much more geophones that are positioned at regularly spaced intervals. Checkshot surveys have a lower resolution than sonic logs, but are much more reliable as they are not subject to mud cake and filtrate effects. For this reason checkshot data is often used to perform corrections on the sonic log when generating synthetic seismograms. In the offshore airguns are hung over the side of the rig/boat to a water depth of a couple of meters. A small travel time correction back to mean sea level (MSL) for the airguns is then made as the MSL usually forms the datum (reference level). The MSL is used as datum to remove the effects of varying derrick floor/kelly bushing level from one oil platform to another (Ecclestone, personal communication, 2019). The checkshot acquisition is undertaken in measured depth (MD). This is the depth along borehole with the kelly bushing commonly being the datum. These depths are then converted back to true vertical depth (TVD) to remove any borehole deviation effects and then true vertical depth sub-sea (TVDSS) by subtracting the height of the kelly bushing with respect to the MSL. The vertical depth between the seafloor and a point in the subsurface is called the True vertical depth base mud line (TVDBML). Fig. 1.13 shows a sketch of an offshore platform running a checkshot survey.

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Fig. 1.9 Sketch of a platform (well A11-01) running a checkshot survey, nlog 2019

1.10 Background information on time-depth relations After stacking the checkshot surveys, several corrections are necessary, among other correcting for the offset and angle between the airgun and geophone. These iterations are subject to a certain degree of interpretation that can add up to travel time differences of a couple of milliseconds. For this reason some wells have multiple time-depth (T/Z) relations, often the one that can be tied best to the seismic (interpretation) is preferred. With the relation a T/Z curve can be drawn (fig. 1.14a). The average velocity (vavg) is the average seismic velocity between the datum and a certain depth or time. Excluding geological anomalies this number is excepted to increase with time honouring a compaction trend for sediments. The interval velocity (vint) is the average velocity over a certain depth or time interval and can therefore strongly differ from the average velocity. The instantaneous velocity (vins) is the velocity at a certain instant of time or point of depth and can be considered a (local) rock property.

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The BAGHDEF velocity model is concerned with the interval velocities of the N and CK. Two things of the groups have to be known: the vertical depth interval (Δz), which is the true vertical thickness (TVT), and the vertical travel time (Δt) from top to bottom. Fig 1.14b shows the different types of thicknesses that can be observed with a vertical and deviating borehole when drilling through a formation bed. In the Petrel project of this study the top contact of the N is the seafloor and the bottom contact (base) is given with well marker (B_N), this is the first well top of the CK or an older interval. The CK has B_N as top and its base is given with well marker (B_CK), this is the first well top of the Rijnland Group (KN) also known as the Cretaceous Group or an older interval.

Fig. 1.10a The different types of velocities and their interval in a T/Z curve (after Marsden, 2002)

Fig. 1.10b Different definitions of thickness for formation beds when drilling with a straight and inclined borehole

path (after quartz-reservoir.co.uk, 2019)

18

Fig. 2.2a The 3D Pre-STM seismic datasets used for the horizon interpretation in this study

N

50 km

Methods

2.1 Project timetable and workflow This project was conducted between April 1st and August 23rd 2019. The project consisted of five phases, in chronological order: data loading, quality control (QC) of T/Z relations, horizon interpretation, workflow building and velocity modelling. QC’ing the T/Z relations and horizon interpretation were interdependent and where therefore executed concurrently. The far majority of time was spent on the horizon interpretation and QC’ing it. Getting a smoothly running Petrel workflow with the desired result was difficult and involved several iterations mainly due to the time difference of horizons across seismic boundaries and differences with the DGM v5 horizons. As a result multiple workflows were produced and corrected well into the later stages of the project. QC’ing of the T/Z pairs was also a time-consuming task. The timetable of these phases is shown in table 2.1.

2.2 Data loading and Petrel project settings The entire project was conducted on the Petrel™ E&P software platform version 2018. The software was obtained by EBN as an educational license gifted by Schlumberger Limited. The seismic datasets used in this study are large 3D seismic cubes of the Dutch offshore. It consists of a large survey of the DEF quadrants (DEF Survey) and four ‘Terracubes’ purchased from FURGO B.V. (see fig. 2.2a). All seismic are Pre-STM. The Terracubes comprise a merge of smaller 3D seismic surveys acquisitioned since the 1980s, the versions reprocessed in 2011 by EBN were used. Terracube Areas 1, 2 and 3 partially overlap the DEF Survey. In these places horizons interpreted on the DEF Survey have priority over the horizons interpreted on the Terracubes, because the DEF Survey is of significant better quality. Terracube Area 1 mainly consists of seismic surveys in the A and B quadrants. Terracube Area 2 comprises the F, G, L and M quadrants. Terracube Area 3 comprises the J, K and L quadrants, while Terracube Area 4 mainly overlaps the P and Q quadrants. Table 2.2 shows the extents and settings of these seismic datasets.

Table 2.1 Timetable of the project’s phases

April May June July August

Week number 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Data loading

QC'ing T/Z pairs

Interpretation and QC of horizons

Petrel workflow building

BAGHDEF Velocity modelling

Writing and correcting report

19

Fig. 2.2b The EBN seismic convention that was used in this study

The polarity of the seismic datasets is according to the EBN convention, which is non-SEG-Y (fig. 2.2b). In this convention an increase in acoustic impedance (hard kick) gives a reflection with a negative amplitude, and a decrease in acoustic impedance (soft kick) gives a reflection with a positive amplitude. The EBN polarity convention uses a blue-gray-red color scale, where a blue loop is a soft kick while a red loop is a hard kick.

2.3 Overall observations of seismic datasets The seismic datasets used in this study are overall of good quality, but have locally poorer quality, especially near large fault systems and salt diapirs (fig. 2.3a). The reflections are of sufficient quality for interpretation of the base group horizons. Loss of bandwidth with increasing depth hampers interpretation of older horizons. The Terracubes, while carefully assembled, still show obvious signs of (seismic survey) merges in boundary areas. These artefacts are visible as (minimal and often acceptable) time shifts, change in amplitude and or change in frequency (fig. 2.3b). These time shifts cannot be the result of different uses of seismic reference datums (SRD), because in the offshore it is always the mean sea level (MSL), therefore these time shifts are most likely the result of seismic processing and zero phasing in particular (Ecclestone, personal communication, 2019). Zero phasing refers to deconvolution of wavelets to symmetrize them on time 0. This process can lead up to differences of up to 20 ms with the raw dataset. The interpreted reflections are mostly very continuous loops of high amplitudes with seismic terminations in some places. These (non-faulted) terminations indicate unconformities and can hint to depositional and erosional environments. Possible effects of faults and poor seismic imaging should be taken into consideration when assessing these reflection terminations.

DEF Survey Terracube Area 1 Terracube Area 2 Terracube Area 3 Terracube Area 4

Coordinate system ED50-UTM31 ED50-UTM31 ED50-UTM31 ED50-UTM31 ED50-UTM31

Min. X 487862.72 497227.50 482227.50 584752.50 482227.50

Max. X 649294.47 567252.50 584752.50 704752.50 612252.50

Min. Y 6019874.77 6066254.50 5881279.50 5881279.50 5748754.50

Max. Y 6106804.11 6181279.50 6066279.50 6103779.50 5881279.50

Min. latitude 54°18'13.4728"N 54°44'16.3325"N 53°04'23.3189"N 53°02'26.3566"N 51°52'37.8642"N

Max. latitude 55°06'23.7812"N 55°46'33.3783"N 54°44'32.2039"N 55°04'20.4339"N 53°04'46.4589"N

Min. longitude 2°48'35.1579"E 2°57'20.8927"E 2°43'26.2123"E 4°15'53.9584"E 2°44'4.9186"E

Max. longitude 5°20'19.9994"E 4°04'19.0861"E 4°18'58.4542"E 6°12'15.2215"E 4°40'30.9944"E

Sample Interval 4 4 4 4 4

Inline interval 25 25 25 25 25

Crossline interval 12.5 25 25 25 25

Inline length 159975 m 115000 m 184975 m 222475 m 132500 m

Crossline length 84125 m 70000 m 102500 m 119975 m 130000 m

Table 2.2 Extents and settings of the seismic datasets used in this study

20

Decrease in amplitude

Decrease in frequency

Time shifts

Fig. 2.3b At the boundary of this seismic merge artifacts are formed in the intra-Chalk, visible as time shifts and changes in amplitude and frequency of reflections

Fig. 2.3a In the study area poor seismic imaging (arrows) is frequently the case near salt diapirs (red outline) and large fault systems (black dashed line)

21

Fig. 2.4a (left), b (center) and c (right) The typical reflection responses of the base NU (a), NL (b) and CK (c) respectively and interpretation of their horizons

2.4 Typical reflection and wireline log response of the target horizons The base NU horizon was found to yield a hardkick response with a strongly variable amplitude, in most places it is a semi-continuous loop of low amplitude (fig. 2.4a). As explained in paragraph 1.6.4 the horizon usually forms an angular unconformity. Underlying strong reflections lie tilted against the relatively weak reflection of the base NU resulting in an angular relationship. This seismic phenomena causes small-scale irregularities in the reflection of the overlying horizon. This irregularity is also contributed by seismic noise due to the horizon’s relative shallowness.

The base NU’s reflectivity is further complicated in some places by seismic interference that made 3D auto-track of the horizon increasingly difficult, which could not be solved by changing the auto-track parameters. For this reason, more manual interpretation was necessary for this horizon compared to the two lower horizons. Also, a 3D auto-track workflow (see fig. 2.6c) was run more often in smaller areas. The base NL however, forms an excellent reflection response for horizon interpretation. The reflection is characterized by a hardkick of high amplitude (fig. 2.4b). The loop is very continuous and rarely is affected by seismic noise or interference. Localities of poor reflection responses of the base NL are mostly the result of tuning effects caused by penetrating salt diapirs. The base CK also yields a clear reflection response on seismic, albeit to a lesser extent than the base NL. It yields a mid to high amplitude softkick and is continuous in most places (fig. 2.4c). Of the three mapped horizons the base CK is by far the most folded and faulted horizon. The horizon is partly absent in the center Dutch offshore. The lithologies of the NU and NM are mostly a succession of shales and sands with abrupt and gradual changes from one to another. In places of (angular) unconformities these changes (and seismic responses) can be more complex. Due to the gradual change the base NU horizon does not yield a strong change in petrophysical properties, therefore it is hard to determine the exact border horizon on the basis of wireline logs other than the sonic (hardkick). Since seismic reflection is a direct product of the Fig. 2.4d The typical gamma ray and

sonic log response of the CK

22

formation’s acoustic properties, the seismic loops can be tied to well logs and subsequently the reflection loop tracking away from the wells can be conducted. In this way time maps for specific markers are being created. The CK’s lithology however is in stark contrast with that of the overlying N clastics (paragraph 1.6). With the large petrophysical difference this group is easily distinguishable on wireline logs, most notably on the gamma ray log (GR) (fig. 2.4d). In this log it yields low radioactivity levels due to its deficiency of clay minerals. Also, the CK has overall lower sonic (SON) readings than its over- and underburden as result of its high acoustic velocity. This feature is contributed by the dense, crystalline and low porous properties of carbonates. These logs together with the corresponding well tops were used for QC’ing T/Z relations and interpreting the base NL and CK horizon during seismic-to-well ties. Noteworthy is that the change in GR and SON can be more gradual in places where the CK was reworked (see paragraph 2.7).

2.5 QC of T/Z relations From seismic-to-well ties it became clear that the horizon interpretation did not always coincide with the well markers being displayed in time. In this study one-way travel time residuals of less than 5 ms are considered to fall within the range of acceptable error. Wells with residuals of more than 5 ms formed outliers and were inspected on the quality of their T/Z relationship and corrected if necessary. Frequently made errors in loading T/Z relations are the use of different datums for time and depth measurements and mistakenly using measured depth (MD) of a borehole instead of true vertical depth (TVD) or the use of uncorrected travel times. These errors can easily be overseen as they mostly result in small differences on the order of a few tens of meters or milliseconds. However, these errors can have serious impacts on models that largely depend on these parameters (e.g. velocity models). QC’ing the T/Z relations was done by analyzing reports of checkshot/VSP surveys available on nlog.nl. Summary reports of the checkshot/VSP surveys are provided by operators and are documented in variable ways and formats as they are acquired by different service companies and depend on year of acquisition (fig. 2.5).

23

Fig. 2.5 Summary report of the checkshot survey run in well E02-01. The red boxes are the T/Z pairs in true vertical depth (nlog.nl, 2019)

24

Fig. 2.6b Seismic-to-well ties formed the starting point of the horizon interpretations as seen in this example of some F and G wells

2.6 Workflow horizon interpretation Due to the large size of the area (>40,000 km2) that had to be interpreted, a workflow was set up to efficiently and partially automatically execute the task (fig. 2.6a). This workflow consisted of seven stages, of which the first two formed the framework of the horizons. The first stage was the horizon interpretation with seismic-to-well ties (fig. 2.6b). As explained in the previous paragraph this phase lead to the revision of 13 T/Z relations and therefore markers were honoured with time residuals of less than 5 milliseconds. The second stage was the horizon interpretation on additional seed lines, interpreted on in- and crosslines between and away from well correlation panels. The third stage was the 3D auto-track of the areas interpreted in the first two stages by constraining their boundaries with polygons. This way only areas with sufficient well control were tracked. In the fourth stage other areas were 3D auto-tracked. This stage was done in nine steps, where the settings of the auto-track parameters were changed with each increment as shown in fig. 2.6c. The parameters changed where the search area and correlation quality of the seed trace. The dip of a loop was only increased above 2 ms per trace in areas with steep structures (all caused by salt diapirs).

The fifth and the most time-consuming task was QC’ing the horizon interpretation. This happened in several ways. Explanation on the QC methods used is given in the next paragraph. The sixth stage of the horizon interpretation was the interpolation of the horizon, to fill in minor gaps that, with the selected settings, the 3D auto-track was unable to do. This limitation was almost exclusively the result of seismic discontinuities due to complex structures, fault zones, poor seismic imaging (chaotic seismic) and (re)processing artefacts. To prevent interpolation in locations where a horizon is absent, the function was polygon constrained to areas where the horizon is present. The interpolation function can be based on several algorithm methods of which the convergent interpolation proved to yield logical results, where the interpolated horizon did not widely cross loops and stayed within an acceptable time window. For the mathematics behind the different algorithm methods Schlumberger’s Petrel manual can be consulted.

Fig. 2.6a The horizon interpretation workflow

comprised 7 stages

25

Fig. 2.7a Confidence map of the 3D auto-track of the base NL in the northern part of Terracube Area 2 shows low confidence levels for the northwest and southeast that formed the focus areas for QC’ing the horizon

N

Fig. 2.6c The 3D seismic auto-track workflow

comprised nine steps in which the correlation

quality was incrementally decreased

The seventh and last stage of the horizon interpretation was the creation of surfaces (grids) with the produced Petrel workflow. This stage incorporated the DGM v5 horizons in areas where a high resolution horizon is absent. Explanation of the design and final surface results of the Petrel workflow are given in paragraph 3.4 and 3.5 respectively.

2.7 QC methods of the interpreted horizons QC of seismic horizons is important when auto-tracking has been used. QC’ing was performed with several methods. Using the auto-track tool in Petrel automatically yields attribute maps of the horizon interpretation: time, confidence, amplitude, distance and seismic signature of which the first three were used in this project for QC’ing the horizons. Since large parts of the horizons were 3D auto-tracked it was necessary to inspect the areas created in stage 4 of the horizon interpretation (paragraph 2.6). These areas were tracked with limited seed lines as no wells lie nearby to tie the seismic to. The confidence map was an effective way to locate these areas of low auto-tracking confidence (fig. 2.7a). The confidence level is the correlation coefficient (r) between the attributes of the auto-tracked loop and the attributes as set in the parameters of the 3D auto-track tool. Erasing areas of low confidence levels and re-running the 3D auto-track workflow (fig. 2.6c) within their polygon-constrained boundaries greatly improved the confidence levels and fixed obvious loop jumps/skips in the time maps. This process was done iteratively and areas were subdivided if confidence levels remained low far from the picking location. Sometimes it was necessary to make additional seed lines, especially in areas where the reflections are low in amplitude and coincide with (intra-group) reflections of high amplitude causing seismic interference due to tuning effects. This phenomenon is especially the case for the base CK. The amplitude maps were used to locate seismic anomalies. The presumption is that the seismic signature of a reflection is more or less consistent and that lateral changes happen gradually, excluding the possibilities of direct hydrocarbon indicators (DHIs). Therefore strong changes in amplitude are areas of QC interest, especially when they form abrupt changes visible as sharp boundaries (fig. 2.7b). In some places these abrupt amplitude changes are artefacts caused by boundaries of seismic data merges (e.g. see fig. 2.3b).

26

Fig. 2.7b The amplitude of the base CK in the northeastern part of Terracube Area 2 shows an area of seismic anomaly in the north which lead to revision of the horizon interpretation in that area

5 km

N

Lastly, the time maps were used to locate structural anomalies in the form of abrupt time shifts, pierces and throughs (fig. 2.7c). Vertical exaggeration of the horizon in 3D view proved an effective way to locate places of jitter. These structural anomalies were corrected if they are not supported by seismic (loop jump/skip) and or cannot be ascribed to geology (e.g. fault throws). In areas of continues seismic polygon constrained auto-tracking was performed, while manual interpretation was necessary in areas of many seismic discontinuities or poor seismic imaging.

Fig. 2.7c The time map of the of the base CK in the eastern part of Terracube Area 2 shows a structural anomaly (center image) QC’ing this anomaly on a cross-section (red line) showed it to be an artefact caused by a loop jump (right image)

5 km

N

27

Fig. 2.8 The high resolution mega maps are a sum of multi-resolution maps of the entire Dutch territory with areas of a horizon interpreted on 25 x 25 m grid bins (EBN) and if not available

areas of the horizon interpreted on 250 x 250 m grid bins (TNO)

2.8 Approach of designing the Petrel workflow Designing the Petrel workflow was done simultaneously with QC’ing the horizon interpretations. EBN provided a rough roadmap of steps to perform and what tools (i.e. mathematical operations) may be useful (see ‘HiRes MEMA workflow of Guidance’ in the appendix). Additionally, TNO provided examples of the mapping workflow that are used to generate the DGM v5. Since this project only uses 3D seismic datasets, no horizons were interpreted on 2D seismic. In these areas where only 2D lines are available the DGM v5 horizons were used as input for the horizon interpretation. Additionally, the DGM v5 horizons were used to fill in gaps where interpretation of the horizons proved extremely difficult due to poor and chaotic seismic. Therefore the horizons resulting from the Petrel workflow are a compilation of TNO’s DGM v5 low resolution horizon and EBN’s inhouse high resolution horizon (fig 2.8).

To achieve the objective of merging and harmonizing these different grids many iterations of actions and operations are involved. The script of the designed Petrel workflow consists in total of 558 lines and runs a master workflow which in turn runs separate but dependent workflows. To prevent unnecessary reruns, the workflow is designed as efficient as possible, including deleting all the half-products. Specifics steps in the workflow can be de-activated by disabling the operation. This de-activation can be useful for QC’ing half-products to assess at what stage and how the final results are affected. The script is written domain independent, enabling the Petrel workflow to be used for both time and depth grids. The architecture and result of the Petrel workflow are described in the next chapter.

2.9 Function used for the BAGHDEF velocity model Many velocity models for time-depth conversion have been proposed. Sometimes simple velocity functions for general use and some advanced polynomial methods for specific case studies (Ogbamikhumi and Aderibigbe, 2019). The BAGHDEF velocity model is a layercake model (stack of layers with different velocity functions optimized per interval) and is based on the widely used so-called v0k function. Fig. 2.9 shows the concept of the function with a dipping formation bed (interval) penetrated by two wells at different depths. This function is a useful approach for sediments of which the acoustic velocity is assumed to increase linearly with depth as result of compaction and therefore describes effectively the vint as function of depth. To derive the compaction trend ‘k’ the vint of boreholes is plotted against the vertical midpoint depth of the interval (zmid), this way a line of best fit can be drawn of which its gradient is k.

28

Fig. 2.9 The ‘v0k’ function assumes that there is a positive linear relationship between the depth of a formation bed and its acoustic velocity (modified from Hoetz, 2017)

Calculating the vertical depth interval with the simple v0k function goes as follows: Δz = Δt (v0 + k*z1) / (1- Δt*k/2) (1)

- Δz is the TVT in meters - Δt is the one-way travel time interval in seconds - v0 is the normalized vint in m.s-1 (fitted for each borehole) - k is the velocity-depth gradient in s-1 - z1 is the TVD of the interval’s top in meters

For studies on large scale areas, with many (well) data points the following formula is derived, which is a more accurate velocity function and is used for this study (Japsen, 1993): Δz = ((v0/k + z1)*ek*Δt – v0/k) - (v0/k * (ek*t1 – 1)) (2)

- Δz is the TVT in meters - v0 is the normalized vint in m.s-1 - z1 is the TVD of the interval’s top in meters - Δt is the one-way travel time interval in seconds - k is the velocity-depth gradient in s-1 - t1 is the one-way travel time to the interval’s top in seconds

Solving the formula above for v0 gives: V0 = (k*(z2 – z1 ek*Δt)) / (ek*Δt) – 1 (3)

- v0 is the normalized vint in m.s-1 - k is the velocity-depth gradient in s-1 - z2 is the TVD of the interval’s base in meters

29

Table 2.10a Boreholes used for building the BAGHDEF velocity model for the N. Boreholes with strikethroughs formed outliers and were left out in the model

- z1 is the TVD of the interval’s top in meters - Δt is the one-way travel time interval in seconds

Rewriting the formula for z2 as function for v0 gives: z2 = ((v0/k + z1)*ek*Δt – v0/k) (4)

- z2 is the TVD of the interval’s base in meters - v0 is the normalized vint in m.s-1 - k is the velocity-depth gradient in s-1 - z1 is the TVD of the interval’s top in meters - Δt is the one-way travel time interval in seconds

Equation 3 was used for building the BAGHDEF velocity model by calculating the v0, basefits and k. Equation 4 was used to validate the model by calculating the depth residuals of blind well tests. In the next paragraph the workflow of building the BAGHDEF velocity model is explained.

2.10 Workflow BAGHDEF velocity modelling The BAGHDEF velocity model was built with interval velocities from boreholes that have T/Z relations based on checkshot/VSP data (see ‘BAGHDEF Wells with Checkshot Data.xlsx’ in the appendix). Tables 2.10a and b are the list of boreholes used in the BAGHDEF velocity model for the N (95) and CK (89) respectively. Fig. 2.10a shows the locations of the boreholes in the BAGHDEF area. For boreholes of which the T/Z relations in EBN’s database could not be validated, due to missing data on nlog.nl, checkshot/VSP based interval velocities from VELMOD 3.1 were used (see ‘VELMOD 3.1 Well Data.xlsx’ in the appendix). However, in VELMOD 3.1, and older models as well, the top used for the N is the datum, which is the MSL (Den Dulk, personal communication, 2019). Therefore, the water column of the sea was included into the velocity model. While its interval (ΔZsea) is relatively small (25 – 50 m), its addition to the strata has a significant impact on the interval velocities and thus affects the derived compaction trend (k) of the supergroup. This addition yields lower vint due to the relatively low acoustic velocity of seawater. Therefore this effect was corrected for in the BAGHDEF velocity model by subtracting the water column and time interval of the sea. A bathymetry map of the Dutch territory was used for determining the sea depth at the location of the wells. Δtsea was approximated with the average acoustic velocity of the North Sea by 1500 m.s-1 (Eikelenboom, personal communication, 2019). The subtraction of tsea gave new times for the horizons (tbase) as the seafloor became the new datum with z and t = 0.

A05-01 A08-01 A11-01 A12-02 A14-01 A16-01 A18-01 B10-02-S1 B13-02 B14-01 B14-02

B18-02 B18-03 D12-01 D12-03-S1 D12-04-S1 D15-03 D15-04 D18-01 E02-01 E02-02 E04-01

E06-01 E09-01 E09-02 E09-03 E10-01-S1 E11-01 E12-01 E12-02 E12-03 E12-04-S2 E13-01

E13-02 E14-01 E16-01 E16-02-S2 E16-03 E17-01 E18-02 E18-03 F01-01 F02-02-S1 F02-03

F02-04 F03-01 F03-02 F03-03 F03-04 F03-05 F03-06 F03-07 F04-01 F04-02-A F05-02

F05-03 F05-04 F05-05 F06-01 F07-02 F08-01 F08-02 F09-02 F09-03 F10-01 F11-01

F11-02 F11-03 F12-01 F12-03 F14-04 F15-01 F15-02-S1 F15-05 F15-A-01 F15-06 F15-07

F17-01 F17-02 F17-04 F17-05 F17-06 F17-07 F18-05 F18-06 F18-07 F18-08-S1 F18-09-S2

G10-01 G10-02 G11-02 G13-02 G16-01 G16-02 G16-04 G17-02 G18-01 H16-01

30

Table 2.10b Boreholes used for building the BAGHDEF velocity model for the CK. Boreholes with strikethroughs form outliers and were left out in the model

Fig. 2.10a Locations of the boreholes used for building the BAGHDEF velocity model

N

30 km

A05-01 A08-01 A11-01 A12-02 A14-01 A16-01 A18-01 B10-02-S1 B13-02 B14-01 B14-02

B18-02 B18-03 D12-01 D12-03-S1 D12-04-S1 D15-03 D15-04 D18-01 E02-01 E02-02 E04-01

E06-01 E09-01 E09-02 E09-03 E10-01-S1 E11-01 E12-01 E12-02 E12-03 E12-04-S2 E13-01

E13-02 E14-01 E16-01 E16-02-S2 E16-03 E17-01 E18-02 E18-03 F01-01 F02-02-S1 F02-03

F02-04 F03-01 F03-02 F03-03 F03-04 F03-05 F03-06 F03-07 F04-01 F04-02-A F05-02

F05-03 F05-04 F05-05 F06-01 F07-02 F08-02 F09-02 F09-03 F10-01 F11-02 F11-03

F12-01 F12-03 F14-04 F15-01 F15-02-S1 F15-05 F15-A-01 F15-06 F15-07 F17-04 F17-05

F17-06 F17-07 F18-05 F18-06 F18-07 F18-08-S1 F18-09-S2 G10-01 G10-02 G11-02 G13-02

G16-01 G16-02 G16-04 G17-02 G18-01 H16-01

31

Table 2.10c Section of the excel spreadsheet and its parameters used for the calculation of the v0, basefit and initial k for the base CK. The complete lists can be found under the tabs 020_BNS and 030_BCK in ‘BAGHDEF Velocity Functions - Multisource TZ-Relations.xlsx’

in the appendix

It was advised to use the N as one interval as it has historically been proven that the depth of the base NU is difficult to determine from drill cuttings with errors of up to 20 m (Eikelenboom, personal communication, 2019). For this same reason TNO provides two alternative layercake models: a velocity function for the NU and NM plus NL (NM+NL) separately and the North Sea groups combined to N. For the calculations of the velocities in the BAGHDEF model, the units as visible in the Petrel project were used. For time this is two-way travel time (twt) in milliseconds and vertical depth (z) in meters. Both units however are noted in positive values in the spreadsheet, as is commonly the case (see table 2.10c and ‘BAGHDEF Velocity Functions - Multisource TZ-Relations.xlsx’ in the appendix).

Borehole (B_CK)

X Y Sea depth (m)

TVDBML top (m)

TVDBML base (m)

Δz = TVT (m)

z_mid TVD (m)

twt_top (ms)

twt_base (ms)

Δtwt (ms)

v_int (m/s)

v0 (m/s)

A05-01 532746.16 6170917.46 45.83 2571.69 3014.12 442.43 2792.91 2520.27 2717.96 197.69 4476.00 2342.39

A08-01 539970.30 6154385.21 34.11 2407.89 2976.91 569.02 2692.40 2430.21 2698.41 268.20 4243.25 2188.09

A11-01 535440.34 6147166.22 33.74 2338.85 2804.38 465.53 2571.62 2316.04 2537.42 221.38 4205.71 2241.71

A12-02 561890.82 6139810.91 28.63 1915.17 2347.57 432.40 2131.37 1924.80 2166.44 241.64 3578.93 1951.62

A14-01 538901.25 6117350.43 26.34 2063.82 2336.30 272.48 2200.06 2063.25 2197.49 134.24 4059.59 2378.10

A18-01 557945.32 6101629.10 46.49 2042.59 2408.11 365.52 2225.35 2044.46 2248.04 203.58 3590.92 1891.01

B10-02-S1 566898.26 6133226.58 42.30 1899.65 2296.64 396.99 2098.15 1913.52 2123.63 210.11 3778.88 2176.46

B13-02 581536.00 6124070.81 44.47 1537.59 2126.53 588.94 1832.06 1536.91 1883.23 346.32 3401.10 2005.10

B14-01 600325.22 6118772.87 48.28 1813.16 2116.88 303.72 1965.02 1763.66 1949.48 185.82 3268.97 1767.69

B14-02 585184.00 6126953.00 45.30 1951.40 2380.90 429.50 2166.15 1912.11 2147.91 235.80 3642.92 1988.93

B18-02 614671.44 6106740.69 41.51 1828.40 2016.39 187.99 1922.40 1730.91 1818.70 87.79 4282.72 2813.07

B18-03 622179.44 6097141.18 39.52 1823.22 2139.20 315.98 1981.21 1750.77 1924.73 173.96 3632.79 2119.10

D12-01 489171.00 6031405.18 29.17 881.58 1400.56 518.98 1141.07 924.94 1253.61 328.66 3158.09 2289.67

D12-03-S1 495855.37 6021100.89 37.67 745.86 1268.98 523.11 1007.42 793.35 1153.59 360.25 2904.17 2138.38

D12-04-S1 496987.37 6033581.28 30.72 958.74 1377.63 418.89 1168.19 1014.65 1284.36 269.71 3106.23 2215.67

D15-03 496211.60 6019807.65 39.92 777.80 1311.78 533.98 1044.79 823.09 1184.93 361.84 2951.47 2157.23

D15-04 494052.52 6010350.21 56.05 844.79 1494.76 649.97 1169.78 901.24 1310.34 409.10 3177.57 2289.52

D18-01 488805.99 6000450.64 47.55 846.00 1676.73 830.73 1261.37 906.81 1405.13 498.32 3334.12 2379.64

Having determined the approach as described here above building the BAGHDEF velocity model commenced and was performed in seven steps (see fig 2.10b):

1. In step one the interval velocities for the CK and N were calculated for each borehole. Boreholes used for the N and CK are shown in tables 2.9b and 2.9c respectively.

2. In step two the interval velocities calculated in step 1 were plotted against the vertical midpoint depth (zmid) of the interval at the location of the corresponding borehole. Subsequently, a line of best fit was drawn. The slope of this line is effectively the compaction trend (k).

3. In step three, the normalised interval velocity for each borehole (v0, basefit) was calculated with equation 3 (paragraph 2.9).

4. In step four outliers, in the scatter plot yielded in step 2, were left out. This exclusion was done with the naked eye. Excluding these datapoints gave a cut-off that was used to quantify and find other possible outliers in the v0’s calculated in step three. This cut-off

32

was calculated with the relative error of the v0, basefit from the v0 of the trendline (v0, global). A cut-off of 5% for the N and 30% for the CK was found. Three and four boreholes were left out for the N and CK respectively (see table 2.10a and b). Note that leaving out these outliers, does not question the reliability of the boreholes’ T/Z relations per se, but rather prevents them influencing the overall trend. This exclusion yields a k that better fits the majority of datapoints.

5. In step five the v0, basefits were re-calculated, because a new k was yielded from step four. Then these datapoints were loaded into petrel with the coordinates of the corresponding well marker and were gridded to a surface using the convergent interpolation function effectively producing the v0 map.

6. In step six wells that were not used in building the velocity model were converted from time to depth with equation 4 (paragraph 2.9). The difference between this calculated value and actual measured depth in the borehole is the depth residual. The v0’s that were used for these so-called ‘blind well tests’ were derived from the v0 map, produced in the previous step, by automatically measuring the value on the map in the corresponding location. For the N 91 boreholes were used as blind well tests, for the CK 81 boreholes were used.

7. In step seven the velocity model was tweaked on the basis of statistical analysis of the residuaIs by eliminating its bias. The bias is the mean value of the residuals and points out whether the model, on average, over- or underestimates the depths. To eliminate this bias (mean of 0 m residual) the velocity function needs to be adjusted. The velocity function comprises the variables v0 and k. The v0’s of the blind well tests are derived from the interpolated map of the v0, base fits, which in turn are derived from the compaction trend k. Thus, the k needs to be adjusted, but without changing the v0, basefits, because this interdependent process will yield a new biased model. However, this change has to be small (< 5%), otherwise the model cannot be considered representative for the study area and thus needs revision and or expansion of the boreholes and their T/Z relations used in the model.

Thus, the velocity functions for the N and CK comprise a k that is slightly different from the ‘initial’ k obtained in step 4. This method gives a velocity model with small residuals while being unbiased at the same time. The velocity model was eventually built in Petrel with the v0 map yielded from step five and the k from step seven. With these products time maps of this study can be converted to depth maps. In Petrel, depth maps can be used to automatically calculate depth residuals on the locations of blind well tests. However, to track the iterations performed, all relevant borehole data and parameters are worked out in excel spreadsheets to calculate the residuals manually (see paragraph 4.2 and ‘BAGHDEF Depth Residuals - Multisource TZ-Relations.xlsx’ in the appendix). A so-called calibrated depth map can be generated by gridding the residuals to a surface and adding/subtracting this map to/from the ‘raw’ depth map.

Fig 2.10b The BAGHDEF velocity model was built in seven steps

33

3.1a Time map (twt in ms) of the interpreted high resolution base NU horizon

3.1b Amplitude map of the interpreted high resolution base NU horizon (previous interpreted DEFAB area is irretrievable)

N

40 km

Results 3.1 Horizon of the base Upper North Sea Group The base NU was picked on a loop of negative amplitude (hardkick) that in most places forms an angular unconformity. An example of one of the well correlation panels used for the seismic-to-well ties during the horizon interpretations is shown in fig. 2.6b. Note that in this example, as in the majority of the study area the horizon is not disturbed by faults, which eased the 3D auto-track. Still, tracking the horizon remained difficult for the reasons explained in paragraph 2.4. Figures 3.1a and 3.1b show a time and amplitude map of the horizon respectively. The former shows that the horizon is flat in most areas and has very little structures. The amplitude map shows that the overall amplitude of the horizon is relatively low; the eastern and center offshore however show high amplitude values.

34

3.2a Time map (twt in ms) of the interpreted high resolution base NL horizon

3.2b Amplitude map of the interpreted high resolution base NL horizon (previous interpreted DEFAB area is irretrievable)

N

40 km

3.2 Horizon of the base Lower North Sea Group The base NL was picked on a loop of negative amplitude (hardkick) and proved to be the best traceable horizon of the three interpreted ones as explained in paragraph 2.6. This horizon has more structure than the base NU (see fig. 2.6b). Figures 3.2a and 3.2b are time and amplitude maps of the horizon respectively. The amplitude map shows that the overall amplitude of the horizon yields high values. In some places there are sharp borders of amplitude change, these are the result of seismic data merges (e.g. see fig. 2.3b). Fig. 3.2c is a time difference map between the high resolution and DGM v5 horizons. The differences are lie mostly within a time window of +/- 20 ms with negative values (shallower) having leverage. The large time differences (>40 ms) mostly pertain to the structural highs. A histogram of these time differences is shown in fig. 3.2d.

35

3.2c Time difference map (twt) between the high resolution base NL and DGM v5 in the QC’ed northern offshore. A negative value (red) means the HiRes horizon lies lower (larger negative time) than the DGM v5

3.2d Histogram of the time difference (twt) between the high resolution base NL and DGM v5 A negative value means the HiRes horizon lies lower (larger negative time) than the DGM v5

Freq

uen

cy (

%)

Time difference (ms)

N

20 km

36

3.3a Time map (twt in ms) of the interpreted high resolution base CK horizon

3.3b Amplitude map of the interpreted high resolution base CK horizon (previous interpreted DEFAB area is irretrievable)

N

40 km

3.3 Horizon of the base Chalk Group The base CK was picked on a loop of positive amplitude (softkick). Its horizon is much better traceable than the base NU, but it is more discontinues than the base NL (see paragraph 2.4). Its semi-continuous nature, besides for the reasons given in paragraph 3.1, is also contributed by fault throws and piercing salt diapirs and is therefore the most irregular horizon of the three with structures yielding steep dipping reflections (see fig. 2.6b). Time and amplitude maps of the horizon are shown in fig. 3.3a and 3.3b respectively. The amplitude of the horizons is very variable as described in paragraph 2.4. The amplitude artifacts are the result of the same causes as explained for the base NL. A map of the time differences between the high resolution and DGM v5 horizons is shown in fig. 3.3c. Fig 3.3d is a histogram of the time differences. Comparing the time difference map with the time map shows that areas with the largest differences are frequently located near structural highs. That in the study area are mostly caused by upthrown fault blocks and salt diapirs.

37

3.3c Time difference map (twt) between the high resolution base CK and DGM v5 in the QC’ed northern offshore. A negative value (red) means the HiRes horizon lies lower (larger negative time) than the DGM v5

3.3d Histogram of the time difference (twt) between the high resolution base CK and DGM v5 A negative value means the HiRes horizon lies lower (larger negative time) than the DGM v5

Freq

uen

cy (

%)

Time difference (ms)

N

20 km

38

3.4 Architecture and results of the Petrel workflow In this paragraph a description is given of the tasks performed in the Petrel workflow to achieve the objective of implementing, merging and harmonizing DGM v5 horizons in places where high resolution grids are absent. The workflow consists in total of 558 lines with a master workflow running separate workflows that are interdependent. In each separate workflow the previous and thus parent workflow is run. This reversed approach is the result of the manner in which the workflows were designed. Running the workflow chronologically would otherwise yield double or no products, as references were not set yet or were aforementioned in other processes. Furthermore, throughout the workflow multiple actions of clearing the RAM (free memory) and saving the project are incorporated after tasks that require a lot of computing power, and thus time. In summary the Petrel workflow works in the following way: first the onshore part of the DGM v5 map is cropped to preserve the map of the onshore only. Then the high resolution interpretation is converted to a surface in order to create time difference map with the DGM v5. This difference map is used for determining the bulk shift needed for the high resolution horizons in order to harmonize them with the DGM v5 horizon. The high resolution horizons are then bulk shifted. Subsequently, areas of the DGM v5 map that overlap with the high resolution interpretations are removed. This surface result, together with the high resolution interpretations, are converted to pointsets. These pointsets are appended to a single pointset which is then converted to a surface with the convergent interpolation function. This function yields logical trends for subsurface horizons, but fills all gaps (absence of the horizon) found within its given boundaries. A mask is used to preserve these gaps. However, masks in Petrel can only be made with surfaces. Therefore, a surface is produced with the moving average function, which yields illogical geological trends, but preserves the gaps. Dividing this surface by itself yields the desired mask. Lastly, the surface created with the convergent interpolation function is then multiplied with the mask that results in the final surface. A simplified overview of the workflow is shown in fig. 3.4a.

3.4a. Simplified overview of the tasks performed in the Petrel workflow

39

Below a description is given of the Petrel workflow per separate individual workflow. Master workflow:

The master workflow comprises the entire workflow from start to end. References are made to the individual workflows, which allow the workflows to run in one run. This workflow needs to be run by the user. It effectively runs all separate individual workflows by running the last workflow ‘8. Create final surfaces with mask’. In addition, it sets a reference for the folder where the final surfaces will be located in ‘Final Surfaces’ and deletes all content present in this folder to prevent that the user will mix old products with new ones.

Workflow 1: In this workflow references are set for the Dutch onshore boundary and the folder with the clipped DGM v5 horizons. To preserve the Dutch offshore maps only, DGM v5 maps are copied into a new folder and clipped with the Dutch onshore boundary using the “eliminate inside” operation.

Workflow 2: In this workflow references are set for all the cropped DGM v5 maps obtained from the previous workflow. First, string expressions are given to the horizon codes so that the references can be set for each horizon within the folder. This process is followed by setting the references with a loop of ‘else if functions’. This way, tasks on the horizons in subsequent processes will only be performed on horizons from the same stratigraphy and will prevent mixing of horizons from different stratigraphies.

Workflow 3: This workflow bulk shifts the high resolution interpretations relative to the DGM v5 horizon. Time shifts are calculated by approximating the mode of the time difference between the two horizons in a 40 ms time window. This is done by making a time difference map of the two horizons and obtaining the mean value (from the produced output sheet). Subsequently, areas in the time difference map are eliminated where values are greater than the mean plus 20 ms and smaller than the mean minus 20 ms. The mean of this cropped time difference map is used as the number for the bulk shift. However difference maps in petrel can only be made with surfaces, therefore it is necessary to convert the high resolution interpretations to surfaces first. This is done with the moving average algorithm method with an inverse distance squared point weighting and a max search radius of 50 m. This function preserves gaps that are present in the interpretation. The algorithm settings were determined by trial and error where the surfaces with different settings were compared to each other. A max search radius of 50 m gives the best replication of a horizon with a 25 x 25 m resolution.

Workflow 4: This workflow converts the high resolution interpretations to separate point files. As in workflow 2, string expressions are made first, this is done for the high resolution interpretations. Followed by a process of ‘if’ statements with a new folder created for a horizon if a high resolution interpretation of it exists. This interpretation is then converted to a pointset and moved to a folder where all pointsets of the horizons of the same stratigraphy are put together.

Workflow 5: In this workflow the high resolution point sets obtained from the previous workflow are appended to a single point set. Subsequently, surfaces are made to remove areas in the DGM v5 horizon that overlap with high resolution horizons. Surfaces are made using the

40

moving average function with the same settings as in workflow 3. These surfaces are then cropped at the edges by two cells, to avoid artefacts as result of poor seismic imaging at the survey edges. This operation is followed by creating a polygon around the edge of the implemented high resolution horizon for visualization purposes.

Workflow 6: In this workflow the surfaces yielded from the previous workflow are expanded at the boundaries with three cells, for subsequent interpolation purposes. Then in areas where high resolution surfaces overlap with DGM v5, the latter surface is removed using the ‘where A, not B’ operation. In this way, the DGM v5 surface is only preserved where no high resolution surface is present. In the last operation, this newly obtained DGM v5 surface is converted into points and moved automatically into a folder.

Workflow 7: This workflow appends the single high resolution point set, obtained from workflow 5, with the low resolution point set (minus the overlap with HiRes) produced in workflow 6. For this task the ‘append points’ operation is used. References are set for both point sets in order to prevent mixing of horizons of different stratigraphies.

Workflow 8: In this workflow the final surfaces are generated. First two surfaces are produced: one with the moving average algorithm and one with the convergent interpolation function. The latter is done by gridding on a 25 x 25 m bin size to preserve high resolution, where actually present. However, gridding with the convergent interpolation function happens everywhere within the boundaries. To preserve gaps of the interpretation, absence of the group due to unconformities/non-deposition, a so-called mask is made first. This is achieved by dividing the surface produced with the moving average function with itself resulting in an omnipresent grid. Where data is present in the mask the value 1 is appointed and where it is absent it is 0. Multiplying this mask with the surface produced by the convergent interpolation function yields the final surfaces.

Fig. 3.4b - e are some of the results of the final surfaces, produced by the Petrel workflow, and comparisons with the DGM v5 surfaces. Fig. 3.4b shows a boundary between DGM v5 horizon and the high resolution horizon in the final base NL map. On this scale (250 km2) clear structures of folds and faults can be observed in the high resolution horizon, while in the DGM v5 virtually no structures are visible. In the latter, structural highs are visible as vague shades of green colours and no faults are observed. Moreover, three faults running from the high resolution horizon into the DGM v5 stop being visible (pointed out with red arrows). This means that for these faults the resolution of the DGM v5 surface is insufficient for proper representation. It has been suggested that the throw and dip of a fault are the key factors for fault detectability (Beaucaire, 2019). Statistical analysis of small faults systems have showed (at least in the case of the base NL) that the minimum fault throw for detection on maps of 250 x 250 m grid bin sizes (i.e. DGM v5 horizons) is 10 ms (~10 m). However, testing this hypothesis is beyond the scope of this study. Fig. 3.4c shows a boundary area between the DGM v5 and high resolution surface in the final base NL surface in the eastern offshore. Small scale structures visible in the high resolution surface cease to exist going into the DGM v5 surface. It’s plausible that some of these structures in this high resolution surface are contributed by seismic noise. Fig. 3.4d shows before (DGM v5) and after high resolution interpretation of the base NL in the centre part of the northern offshore. Faults, pointed out with the red arrows and triangle, that are visible in the high resolution surface are not visible in the DGM v5. Fig. 3.4e shows a boundary area between the DGM v5 and high resolution horizon of the final base Cretaceous

41

surface (not interpreted in this study) in the eastern offshore. Even though, this view covers an area of approximately 1500 km2 the difference in resolution between the two surfaces is still significant.

3.4b A boundary between DGM v5 horizon (left) and the HiRes horizon (right) in the final base NL map in the in the northern part of Terracube Area 2

3.4c The final base NL surface in 3D view clearly shows the difference in resolution between the implemented DGM v5 (bottom) and the HiRes horizon (top) in the eastern border of Terracube Area 3

2 km N

42

3.4d A comparison between the base NL horizon of DGM v5 (top) and the HiRes horizon (bottom) in the southern part of the DEF Survey

3.4e The final surface of the base Cretaceous in the eastern extent of the offshore shows the difference between the HiRes (interpreted by Ward Teertstra) in the center and DGM v5 in the left and bottom

5 km N

3 km N

43

Table 3.5 Boreholes with time residuals of greater than 5 ms lead to the revision of 10 T/Z relations that effectively decreased the residuals to less than 4 ms. A positive residual means that the interpreted

horizon lies higher (smaller negative time) than the corresponding well top

3.5 BAGHDEF velocity model Prior to time-depth conversion of horizons it is important that any time residuals are eliminated so that depth residuals that may result from time-depth conversion are purely caused by the velocity model's inaccuracy. Seismic-to-well ties during the phase of horizon interpretation showed minimal time residuals. With the well markers of the base N and base CK mostly plotted slightly off from the maxim amplitude of the reflection with a one-way travel time difference of <2 ms and sometimes more significant on or near zero crossings (2 - 5 ms). For 13 boreholes (10 wells) the difference was greater than 5 ms where the marker sometimes plotted on other loops (see table 3.5). In all 13 cases inspection of their T/Z relation lead to correction of the relations that decreased the time residuals to less than 4 ms (see table 2.5).

Thus small residuals of <5 ms where still present, to prevent these differences affecting the BAGHDEF velocity model time of the well tops were used in calculating the interval and instantaneous velocities. Velocity functions for the N and CK were calculated in the manner as explained in paragraphs 2.8 and 2.9. These are v0+kz functions where v0 is the normalised interval velocity parametrized at the depth of the corresponding well top and k is the slope of the line of best fit. The global velocity function for the N and CK are respectively: 1797.4 + 0.257 * z (5)

- 1797.4 is the v0, global in m.s-1 - 0.2566 is the k in s-1 - z is the TVD in meters

Correlation coefficient (r2) of this global velocity function is 0.53

2214.9 + 0.744 * z (6)

- 2214.9 is the v0, global in m.s-1 - 0.744 is the k in s-1 - z is the TVD in meters

Correlation coefficient (r2) of this global velocity function is 0.49

Borehole OWT residual B_CK (ms)

OWT residual B_CK (ms) Corrected

OWT residual B_N (ms)

OWT residual B_N (ms) Corrected

A05-01 44.09 -2.74 50.97 -3.81

A11-01 15.60 -2.67 5.38 3.96

B14-02 -11.14 1.09 -12.31 0.97

D18-01 10.82 3.01 13.96 2.71

E09-01 8.55 2.01 9.04 1.50

F05-04 -17.53 -0.42 -15.42 2.56

F17-01 (+S1 & S2) 11.45 2.19 15.07 1.36

F17-02 (+S1) 16.16 1.50 18.15 2.20

F18-06 5.6 -1.10 9.97 3.32

G16-02 21.00 -1.28 23.52 -2.93

44

Fig. 3.5a and b are the scatter plots of the interval velocities against their zmid with labels of the corresponding borehole names. The trendline is the line of best fit (unbiased model) that yielded the global velocity functions for the N and CK respectively. Fig. 3.5c and d are the v0 maps of the N and CK respectively.

45

Interval velocity (m.s-1)

zm

id TVD

BM

L (m)

Fig. 3

.5a

Scatter p

lot o

f the in

terval velo

cities in th

e N a

ga

inst th

eir zm

id with

the co

rrespo

nd

ing

bo

reho

les lab

eled.

The d

otted

line is th

e line o

f best fit

vin

t = 17

97

.4 +

0.2

57

z r

2 = 0

.53

46

Interval velocity (m.s-1)

zm

id TVD

BM

L (m)

Fig. 3

.5b

Scatter p

lot o

f the in

terval velo

cities in th

e CK

ag

ain

st their z

mid w

ith th

e corresp

on

din

g b

oreh

oles la

beled

. Th

e do

tted lin

e is the lin

e of b

est fit

vin

t = 22

14

.9 +

0.7

44

z r

2 = 0.4

9

47

V0, basefit

3.5c V0 map of the N with locations of the wells used in the model (v0, basefit)

3.5d V0 map of the CK with locations of the wells used in the model (v0, basefit)

V0, basefit

N

25 km

V0, basefit

N

25 km

48

Discussion 4.1 Strengths and limitations of the horizon interpretation The reliability and quality of the horizon interpretation is the result of the use of state-of-the-art Pre-STM 3D seismic datasets and that the interpreted horizons, especially the base NL and base CK, commonly yield good reflection responses with loops of high amplitude and continuity. QC of seismic attribute maps aided in locating artefacts caused by the 3D auto-tracking. However, the horizon interpretation was not without limitations. The time maps yielded from this study are result of many iterations that are partially subject to personal interpretation. Picking of the horizon was done with snapping (tracking the maximum amplitude within the loop) this created jitter, as result of seismic noise. These artefacts can be removed with smoothening of the horizon, but resolution will be lost in this process. Another, limitation of snapping is that the possible effects of seismic interference are excluded, where the actual horizon may not form the maximum amplitude of the loop. However this phenomenon pertains to small local and minimal time differences of a couple of milliseconds. The same could be said about the choice to exclude the possibility of phase reversals. Also, in places where the tracked loop separates into two loops (doublet) it is difficult to determine which one belongs to the target reflection without a well(marker) to tie to. The most uncertain areas in the horizons are the places created by the operation of horizon interpolation, which were necessary to fill in minor gaps caused by areas of poor or complex seismic imaging (see fig. 2.3a). Thus, the biggest horizon uncertainties are located near large fault structures and salt diapers. However, in spite of these limitations, the seismic interpretation cannot have impacted the BAGHDEF velocity model, because the velocity model was parametrized on the exact time of the well markers. Nevertheless, QC of T/Z relations confirmed/corrected time residuals are/to less than 5 ms (see paragraphs 2.5 and 3.5).

4.2 Benchmarking the BAGHDEF velocity model Comparing the velocity data of the BAGHDEF velocity model with that of VELMOD 3.1 showed that the former has commonly (much) higher interval velocities. Differences between the v0

maps of the two models show large differences, especially for the base CK (fig. 4.2a and b). These significant velocity differences can be ascribed to the lower k value used in VELMOD 3.1, which is probably the result of the model having been parametrized with well data from the entire Dutch territory both on- and offshore. Other possible contributors to this significant differences are that in VELMOD 3.1 multiple T/Z sources were used (e.g. sonic data and pseudo-seismic picks), v0 maps were smoothed and sea velocities were included. Currently, for regional study purposes, EBN uses mainly two models for regional time depth conversion of the Dutch northern offshore. These are the DEFAB velocity model and VELMOD 3.1. The model that yields the lowest residuals for the set of boreholes used in the case study is then preferred. Therefore, it is important to benchmark the different models by comparing and quantifying the residuals that result from the three different models. Residuals of this study are from blind well tests that were not used in building any of the models.

49

Fig. 4.2a A difference map (m/s) between the v0 of the BAGHDEF velocity model and VELMOD 3.1 of the base N. A positive difference (red) means the BAGHDEF velocity

model has a greater v0 than VELMOD 3.1

Fig. 4.2b Difference map (m/s) between the v0 of the BAGHDEF velocity model and VELMOD 3.1 of the base CK. A positive difference (red) means the BAGHDEF velocity

model has a greater v0 than VELMOD 3.1

N

25 km

N

25 km

50

Table 4.2c The mean, standard deviation and average RRMSE of residuals per subarea show that the BAGHDEF velocity model is significantly more accurate than the DEFAB velocity model and VELMOD 3.1

A positive residual means the model overestimates the depth

Table 4.2a and b in the appendix show the residuals yielded from the blind well tests for all three velocity models in the BAGHDEF area for the N (91 boreholes) and CK (81 boreholes) respectively. Note that the DEFAB velocity model covers the BAGHDEF quadrants only partly. Table 4.2c shows the average residuals of all three models in the BAGHDEF, DEFAB area and subareas of the BAGHDEF: northern, center, southwestern and southeastern BAGHDEF. The DEFAB area comprises the BAGHDEF area minus the southeastern BAGHDEF. The BAGHDEF velocity model is more accurate than the DEFAB velocity model in its own suited DEFAB area. Also, the BAGHDEF velocity model is the most accurate model for the N in all areas. For the CK the BAGHDEF velocity model is also the most accurate with exception of the southwestern BAGHDEF subarea, where the DEFAB velocity model shows smaller residuals on average. Furthermore, from the table it becomes evidently clear that the VELMOD 3.1 is a very poor model for depth conversion of the northern offshore with large standard deviations and average relative residual. A relative residual is the relative root mean square error (RRMSE) from the actual depth of the marker. Commonly, <2% is considered an acceptable RRMSE for a residual. In all instances the VELMOD 3.1 falls far behind the BAGHDEF and DEFAB velocity model. Fig. 4.2c and d show the statistics from table 4.2c on map view for the N and CK respectively. Fig. 4.2e – h are gridded and kringed residual maps of the BAGHDEF velocity model that can be used to get calibrated depth maps of the horizons (paragraph 2.9). These maps need to be subtracted from the ‘raw’ depth maps, because in this study a positive polarity was chosen for the depth domain.

BAGHDEF

area Northern BAGHDEF

Center BAGHDEF

Southwestern BAGHDEF

Southeastern BAGHDEF

DEFAB area

BAGHDEF VM (B_N) Mean 0 m 16 m -5 m -3 m -1 m 1 m

Standard d. 21 m 32 m 14 m 17 m 20 m 22 m

Avg. RRMSE 1.1% 1.4% 0.8% 0.9% 1.2% 1.0 %

DEFAB VM (B_N) Mean -37 m -52 m -34 m -40 m

Standard d. 51 m 11 m 26 m 32 m

Avg. RRMSE 2.7% 3.4% 2.6% 2.9%

VELMOD 3.1 (B_N) Mean -95 m -76 m -84 m -119 m -91 m -89 m

Standard d. 67 m 127 m 39 m 75 m 39 m 80 m

Avg. RRMSE 6.8% 6.9% 5.5% 3.5% 6.3% 6.9%

BAGHDEF VM (B_CK) Mean 0 m 7 m -8 m 0 m -2 m -1 m

Standard d. 40 m 54 m 34 m 29 m 46 m 38 m

Avg. RRMSE 1.3% 1.3% 1.2% 1.2% 1.4% 1.3%

DEFAB VM (B_CK) Mean -45 m -12 m -3 m -15 m

Standard d. 63 m 40 m 28 m 44 m

Avg. RRMSE 2.4% 1.9% 1.0% 1.6%

VELMOD 3.1 (B_CK) Mean -142 m -128 m -104 m -148 m -162 m -132 m

Standard d. 79 m 79 m 85 m 47 m 95 m 69 m

Avg. RRMSE 6.8% 5.8% 5.1% 8.1% 6.9% 6.8%

51

25 km

Fig. 4.2c The mean + standard deviation and average RRMSE of the base N that result from three regional velocity models in six subareas of the Dutch northern offshore A positive residual means the model overestimates the depth

Fig. 4.2d The mean + standard deviation and average RRMSE of the base CK that result from three regional velocity models in six subareas of the Dutch northern offshore. A positive residual means the model overestimates the depth

52

N

25 km

Fig. 4.2e A gridded residual map (m) of the base N belonging to the BAGHDEF velocity model A positive residual (red) means the model overestimates the depth

Fig. 4.2f A kriged residual map of the base N belonging to the BAGHDEF velocity model A positive residual (red) means the model overestimates the depth

Blind well tests

N

25 km

53

Fig. 4.2h A kriged residual map (m) of the base CK belonging to the BAGHDEF velocity model. A positive residual (red) means the model overestimates the depth

Fig. 4.2g A gridded residual map (m) of the base CK belonging to the BAGHDEF velocity model. A positive residual (red) means the model overestimates the depth

Blind well tests

N

25 km

54

4.3 Limitations of the BAGHDEF velocity model The BAGHDEF velocity model approximated the lateral geological variation of the area with a variable v0 map and constant k’s are used. Tweaking the model according to this variation would most likely yield a variable k (map). Another limitation of the velocity model is that there are still large RRMSEs of up to 4.78%. While such values are common for a regional velocity model, they can and should be improved if possible. The model demonstrated relatively poor results for the northern part of the BAGHDEF area with overall the largest residuals and standard deviation. A significant uncertainty of the BAGHDEF velocity model is that blind well tests were performed on boreholes of which the source of T/Z relations cannot be retrieved and thus not be QC’ed. This uncertainty leaves the possibility that some boreholes have a significantly wrong T/Z relation which could either be to the advantage or disadvantage of the velocity models analyzed in this study. This possible effect may be minimized by the large number of blind well tests as the incorrect depth to time converted boreholes will fall out of resolution in the average residuals.

4.4 Limitations of the Petrel Workflow The Petrel workflow designed in this study is fully tested and some suggestions for further improvements can be made:

- The most notable shortcoming is the process of harmonizing the surface merges achieved by the actions and operations of points expansion (workflow 5), points elimination (workflow 6) and interpolation (workflow 8). This process causes artefacts in the boundary areas, but this is greatly limited by the preceding bulk shift module of the workflow. Also, it pertains to small and local artefacts. It is noteworthy that this method turned out to be the same approach used in TNO’s Petrel workflow (personal communication, Den Dulk, 2019).

- Another limitation of the Petrel workflow is that no prioritization of surfaces in places of overlap is incorporated. It still remains the question if such a prioritization can be quantified as it mainly is dependent of the quality of seismic (for interpretation purposes) on which the horizon is interpreted. Also, such priorities are subject to personal preferences. Nevertheless, the Petrel workflow works by default on a top-to-bottom basis where the surfaces in the folder can be moved to higher or lower positions in the working tree. This method is an easy way for the user to build the needed hierarchy and is (in almost all cases) constrained to just a couple of movements.

- Lastly, the workflow still has for some processes repetitions of lines for horizons of different stratigraphies (e.g. in workflow 3) that can be run with one process instead. This option cannot be solved with string expressions, because results from preceding tasks are aforementioned prior to the operation. This limitation however does not affect the desired results and only concerns the consensus that a Petrel workflow, or every automatized program for that matter, should be as short as possible.

4.5 Recommendations for future research For future research it is first and foremost recommended to perform seismic attribute analyses on the high resolution maps. The key is looking for direct hydrocarbon indicators (DHI) and seismic anomalies on maximal negative amplitude maps in the CK interval (see paragraph 1.7).

55

It is advised to QC the horizons interpreted in this study, especially outside the focus area. This little inspected area comprises Terracube Area 4 and the southern parts of Terracubes Area 2 and 3. Also expansion of the horizon interpretation on 2D seismic is advised. Furthermore, the Petrel workflow can be improved with a statistical method that is more accurate for determining the bulk shift needed between horizons. Also a quantitative way to harmonize surfaces in places of merging areas may be more precise than the currently used method of smoothing. Another focus of study could be the improvement of the BAGHDEF velocity model by expansion of the v0, basefits with wells of which the T/Z relation may be improved with pseudo-seismic picks. In addition, no study was performed on whether certain trends in the v0 map can be ascribed to geological phenomena. Effects such as geological setting, thickening, intra-group sedimentological change, pore pressure and other causes of diagenesis can then be correlated with the v0 map. If such a relationship is found it can strengthen the case for using the velocity model. Also, expansion of the layercake model by adding velocity functions for older intervals of the underburden should be envisaged. Lastly, the recommended parameters and settings for ant-tracking performed on the West Netherlands Basin advised by Beaucaire (2019) could be validated by performing fault extraction on the Broad Fourteens Basin with these settings. Hypothetically, this also should yield desired results as the basin was subject to the same tectonic forces as the West Netherlands Basin.

56

Conclusion Conclusions drawn from this study are that:

- High resolution horizon maps (25 x 25 m grid bin) greatly emphasize structures and faults in particular, in comparison to maps of lower resolution. These differences are especially relevant when working on field scale or smaller scales (e.g. well planning).

- High resolution maps of horizons in combination with suitable display setting (as present in Petrel) are very powerful in analysing fault systems. It allows identifying detailed faults which intersect the well imaged reflectors as the base NL and base CK.

- The high resolution maps yielded from this study coincide in most places with the DGM v5 horizons. Time differences are minimal (<20 ms) in places of little structures and are significantly larger (>50 ms) in structurally complex areas or areas where TNO did not have access to 3D seismic data.

- The BAGHDEF velocity model is suitable for depth conversion of the high resolution time maps of the base N and base CK. Based on residuals from blind well tests the BAGHDEF velocity model (avg. RRMSE: 1.2%) is a better model compared to EBN’s DEFAB velocity model (avg. RRMSE: 2.3%) and significantly better than the TNO’s VELMOD 3.1 (avg. RRMSE: 6.8%).

- All three above mentioned velocity models underperform in the northernmost part of the Dutch offshore with relatively large residuals for boreholes in the A and B blocks. The BAGHDEF velocity model overestimates the depth, meaning that on average the vint

is overestimated. To correct for this overestimation either an expansion of the v0 map is needed by adding wells with pseudo-seismic picked T/Z relations (the blind well tests used in this study) or a gentler compaction trend is needed for the subarea. The latter scenario calls for a variable k (map) in the model.

- The Petrel workflow designed in this study is a useful tool to automatically merge and harmonize grids of different resolutions into a single multi-resolution grid (on a 25 x 25 m grid bin). While the workflow is suited to the DGM v5 horizons it can be applied to any type of grid merging if reference names are adjusted accordingly in the script.

Acknowledgements

First and foremost I would like to thank MSc. M. Ecclestone and MSc. L. Janssen and for sharing their vast knowledge of seismic interpretation and their professional advice during the project. I felt very motivated by their active and enthusiastic contribution. I am also grateful to MSc. W. Eikelenboom for sharing his experience in mapping and velocity modelling. I also would like to thank drs. Guido Hoetz for designing and coordinating this project and other people of the Exploration and E&P themes for their input. My gratitude is also expressed to MSc. Den Dulk who provided the TNO DGM v5 maps and was very keen to help on explaining the architecture of the TNO Petrel workflow. It was a pleasure working with Y. Beaucaire who has been a cooperative and pleasant colleague. The presence of fellow interns W. Teertstra, P.L. Reinhard and V. Wani was also greatly appreciated. Lastly I would like to thank Dr. J. Verbeek, Prof. Dr. K.F. Kuiper and Prof. Dr. W. van Westrenen for their kindness in organising this project. It was a great experience working with scientists and engineers that have decades’ worth of experience in the petroleum industry. I mostly will remember them for treating me as one of their own colleagues.

57

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Appendix BAGHDEF Wells with Checkshot Data.xlsx: All wells in the Dutch offshore A, B, D, E, F, G, H blocks listed on their checkshot/VSP data availability. VELMOD 3.1 Well Data.xlsx: All well data used by TNO for building their latest velocity model VELMOD 3.1. BAGHDEF Velocity Functions - Multisource TZ-Relations.xlsx: All relevant well data used in building the BAGHDEF velocity model to determine V0, basefits and the k of intervals. BAGHDEF Depth Residuals - Multisource TZ-Relations.xlsx: Depth residuals calculated for wells that were not used in building the BAGHDEF velocity model (blind well tests).

Borehole (B_N)

TVDSS measured in borehole (m)

TVDSS BAGHDEF (m)

TVDSS DEFAB (m)

TVDSS VM 3.1 (m)

Residual BAGHDEF (m)

Residual DEFAB (m)

Residual VM 3.1 (m)

Relative residual BAGHDEF (%)

Relative residual DEFAB (%)

Relative residual VM 3.1 (%)

A08-01 2442.00 2462.35 2352.49 2551.49 54.46 -55.40 109.49 2.26 2.30 4.55

A14-02 2113.22 2047.22 1975.17 2154.67 -37.10 -109.15 41.45 1.78 5.24 1.99

A15-01 2155.05 2130.85 2064.99 1921.24 14.06 -51.80 -233.81 0.66 2.45 11.05

A15-02 2155.78 2161.67 2044.68 2216.86 35.02 -81.97 61.08 1.65 3.85 2.87

A17-01 1883.29 1854.46 1797.70 1628.65 13.41 -43.35 -254.64 0.73 2.35 13.83

A18-02-S1 1813.43 1763.43 1785.88 1681.42 -5.72 16.73 -132.01 0.32 0.95 7.46

B13-01 1770.85 1793.00 1724.75 1605.79 65.12 -3.13 -165.06 3.77 0.18 9.55

B14-03 1965.14 1931.13 1869.44 1847.39 13.13 -48.56 -117.75 0.68 2.53 6.14

B17-02 1789.84 1784.94 1695.96 1649.87 42.56 -46.42 -139.97 2.44 2.66 8.03

B17-04 1325.33 1278.20 1244.71 1195.32 -2.44 -35.93 -130.01 0.19 2.81 10.15

B18-06 1971.99 1955.22 1858.47 1917.78 25.63 -71.12 -54.21 1.33 3.69 2.81

D12-05 1125.25 1095.96 1095.99 1008.17 0.58 0.61 -117.08 0.05 0.06 10.69

D12-A-03 568.56 548.48 542.16 552.26 12.04 5.72 -16.30 2.25 1.07 3.04

D15-01 481.60 455.73 449.55 461.10 11.98 5.80 -20.50 2.70 1.31 4.62

D15-02 931.84 880.46 867.53 872.98 -0.50 -13.43 -58.86 0.06 1.52 6.68

D15-05-S2 1016.98 962.72 947.16 951.99 -2.32 -17.88 -64.99 0.24 1.85 6.73

E04-01 976.00 947.97 928.23 924.42 11.13 -8.61 -51.58 1.19 0.92 5.51

E10-02 1131.98 1101.12 1080.43 1022.67 8.25 -12.44 -109.31 0.75 1.14 10.00

E10-03-S2 1473.46 1419.05 1382.89 1391.17 -6.64 -42.80 -82.29 0.47 3.00 5.77

E16-04 1063.76 1016.19 998.14 1007.71 -11.46 -29.51 -56.05 1.12 2.87 5.45

E16-05 1315.18 1264.79 1232.85 1246.13 -9.46 -41.40 -69.05 0.74 3.25 5.42

E17-02 1082.75 1042.61 1016.07 1030.74 -2.93 -29.47 -52.01 0.28 2.82 4.97

E17-A-03 989.59 949.76 921.49 935.41 -1.18 -29.45 -54.18 0.12 3.10 5.70

E18-01 1438.81 1386.94 1338.98 1265.74 -4.55 -52.51 -173.07 0.33 3.77 12.44

E18-04 1788.41 1737.64 1687.60 1601.54 -2.54 -52.58 -186.87 0.15 3.02 10.74

E18-05 1810.66 1758.21 1706.95 1625.16 -5.14 -56.40 -185.50 0.29 3.20 10.52

E18-06 1628.57 1570.54 1517.61 1434.83 -9.94 -62.87 -193.74 0.63 3.98 12.26

F02-01 1659.09 1614.55 1557.53 1546.56 0.12 -56.90 -112.53 0.01 3.52 6.97

F02-05 1364.81 1294.14 1254.83 1305.32 -26.23 -65.54 -59.49 1.99 4.96 4.51

F02-06 1441.00 1398.06 1336.17 1408.09 1.43 -60.46 -32.91 0.10 4.33 2.36

F02-07 1900.40 1845.16 1808.54 1834.37 -11.09 -47.71 -66.03 0.60 2.57 3.56

F03-08 1799.16 1751.38 1715.13 1733.47 -8.35 -44.60 -65.69 0.47 2.53 3.73

F03-FA-01 1835.80 1776.39 1740.31 1760.78 -20.06 -56.14 -75.02 1.12 3.12 4.18

F03-FB-108-S1 1631.60 1576.69 1526.36 1533.11 -10.44 -60.77 -98.49 0.66 3.83 6.21

F04-03 1782.87 1749.20 1696.65 1693.07 14.84 -37.71 -89.80 0.86 2.17 5.18

F05-01 1820.64 1774.03 1715.73 1660.43 1.70 -56.60 -160.21 0.10 3.19 9.04

F06-02 1396.87 1343.87 1313.47 1345.76 -9.34 -39.74 -51.11 0.69 2.94 3.78

F06-03 1508.55 1448.94 1392.12 1419.58 -13.52 -70.34 -88.97 0.92 4.81 6.08

61

Table 4.2a Residuals of the base N in the BAGHDEF area. A positive residual means that the depth is overestimated

F06-04 1488.69 1470.95 1406.31 1459.48 29.61 -35.03 -29.21 2.05 2.43 2.03

F07-01 1465.40 1411.88 1377.02 1359.38 -7.02 -41.88 -106.02 0.49 2.95 7.47

F09-01 1248.10 1182.11 1143.69 1092.17 -18.31 -56.73 -155.93 1.53 4.73 12.99

F10-02 1588.24 1491.75 1479.20 1438.80 -46.58 -59.13 -149.44 3.03 3.84 9.71

F10-03 1595.39 1512.84 1493.31 1481.50 -32.81 -52.34 -113.89 2.12 3.39 7.37

F12-02-S1 1585.50 1553.92

1532.97 14.40

-52.53 0.94

3.41

F12-03 1645.49 1611.75 1586.90 13.51 -58.59 0.85 3.67

F12-04-S1 1631.23 1534.33 1546.64 -49.47 -84.59 3.12 5.34

F12-05 1439.00 1370.96 1355.32 -20.79 -83.68 1.49 6.01

F13-01 1272.33 1267.98 1235.20 1256.88 46.26 13.48 -15.45 3.79 1.10 1.26

F14-01 1639.90 1606.44

1561.04 14.75

-78.86 0.93

4.95

F14-02 1423.00 1396.86 1358.24 21.62 -64.76 1.57 4.71

F14-03 1523.90 1476.88 1424.19 0.18 -99.71 0.01 6.75

F14-05 1594.02 1544.18 1499.61 -1.56 -94.41 0.10 6.11

F14-06 1618.08 1585.16 1538.75 15.11 -79.33 0.96 5.05

F15-03 1611.00 1585.41 1506.97 21.25 -104.03 1.36 6.65

F15-08 1647.01 1569.21 1510.08 -32.31 -136.93 2.02 8.55

F15-A-05-S3 1284.60 1205.34 1111.12 -33.28 -173.48 2.69 14.01

F16-01 1972.90 1937.55 1679.99 13.09 -292.91 0.68 15.22

F16-02 1339.20 1286.30 1207.79 -7.13 -131.41 0.55 10.16

F16-03 1813.88 1761.78 1705.48 1604.17 -3.17 -59.47 -209.71 0.18 3.37 11.88

F16-04 1968.84 1921.61 1852.54 1765.80 0.16 -68.91 -203.04 0.01 3.59 10.57

F16-05 1681.39 1628.08 1567.62 1499.08 -3.76 -64.22 -182.31 0.23 3.94 11.17

F17-08 1583.29 1536.29

1504.74 0.09

-78.55 0.01

5.11

F17-09 1678.95 1621.18 1591.38 -12.84 -87.57 0.79 5.36

F17-10 1339.65 1280.59 1267.66 -15.59 -71.99 1.20 5.55

F18-01 1561.48 1485.15 1437.02 -32.78 -124.46 2.16 8.20

F18-02 1507.50 1472.17 1380.57 7.76 -126.93 0.53 8.67

F18-03 1479.46 1443.97 1421.78 6.04 -57.68 0.42 4.01

F18-10 1449.58 1400.78 1340.22 -5.59 -109.36 0.40 7.78

F18-11 1530.50 1474.34 1457.33 -15.13 -73.17 1.02 4.91

G07-02 1636.48 1600.11 1537.14 1565.56 7.48 -55.49 -70.92 0.47 3.48 4.45

G10-01 1430.99 1385.51

1344.20 -1.95 -86.79 0.14

6.26

G10-03 1577.77 1565.73 1527.74 31.95 -50.03 2.08 3.26

G13-01 1462.28 1441.62 1401.07 22.20 -61.21 1.56 4.31

G13-03 1482.39 1459.36 1424.04 21.68 -58.35 1.51 4.06

G14-01 1325.91 1270.68 1249.07 -15.57 -76.84 1.21 5.97

G14-02 1437.28 1391.37 1348.29 -6.30 -88.99 0.45 6.37

G14-04 1479.86 1446.62 1386.34 6.57 -93.52 0.46 6.49

G14-06 1374.30 1291.72 1297.87 -41.98 -76.43 3.15 5.73

G14-A-01 1382.61 1322.26 1307.62 -19.97 -74.99 1.49 5.59

G16-03 1387.07 1356.12 1331.95 10.10 -55.12 0.75 4.09

G16-05 1481.24 1423.92 1423.38 -18.09 -57.86 1.25 4.01

G16-06 1395.09 1373.53 1333.59 18.96 -61.50 1.40 4.54

G16-08 1576.66 1532.30 1521.71 -1.85 -54.95 0.12 3.58

G16-11 1480.47 1458.44 1372.86 18.23 -107.61 1.27 7.47

G16-B-02 1511.28 1494.70 1425.47 23.96 -85.81 1.63 5.83

G17-01 1376.03 1352.88 1263.39 15.27 -112.64 1.14 8.42

G17-03 1375.59 1349.19 1214.38 12.96 -161.21 0.97 12.06

G17-06 1274.35 1225.37 1216.26 -11.65 -58.09 0.94 4.70

G17-A-02 1527.50 1463.16 1282.07 -25.73 -245.43 1.73 16.48

G17-S-01 1462.26 1441.87 1315.76 19.16 -146.50 1.35 10.30

62

Borehole (B_CK)

TVDSS measured in borehole (m)

TVDSS BAGHDEF (m)

TVDSS DEFAB (m)

TVDSS VM 3.1 (m)

Residual BAGHDEF (m)

Residual DEFAB (m)

Residual VM 3.1 (m)

Relative residual BAGHDEF (%)

Relative residual DEFAB (%)

Relative residual VM 3.1 (%)

A12-01 2667.56 2518.55 2429.06 2402.10 -103.18 -192.67 -265.46 3.94 7.35 10.13

A14-02 2379.66 2362.45 2359.69 2315.43 16.90 14.14 -64.23 0.72 0.60 2.74

A15-01 2450.71 2411.68 2365.90 2341.94 -0.77 -46.55 -108.77 0.03 1.93 4.51

A16-01 2000.30 1969.36 1857.48 1841.59 -2.85 -114.73 -158.71 0.14 5.82 8.05

A17-01 2057.18 1994.72 1992.65 1966.75 -20.22 -22.29 -90.43 1.00 1.11 4.49

A18-02-S1 1933.88 1933.24 1922.98 1915.58 43.64 33.38 -18.30 2.31 1.77 0.97

B13-01 2270.82 2246.70 2148.16 2118.57 18.85 -79.69 -152.25 0.85 3.58 6.83

B14-03 2294.09 2250.91 2229.50 2192.51 3.96 -17.45 -101.58 0.18 0.78 4.52

B17-02 2664.68 2742.28 2551.84 2405.53 125.06 -65.38 -259.15 4.78 2.50 9.90

B17-04 1501.33 1458.70 1389.75 1390.64 2.06 -66.89 -110.69 0.14 4.59 7.60

D12-05 1675.84 1637.34 1639.09 1568.84 -8.63 -6.88 -107.00 0.52 0.42 6.50

D12-A-03 1161.18 1105.55 1123.85 1027.68 -23.51 -5.21 -133.50 2.08 0.46 11.82

D15-01 1056.43 1004.11 1019.77 917.19 -14.47 1.19 -139.24 1.42 0.12 13.67

D15-02 1512.81 1449.58 1446.56 1374.50 -12.35 -15.37 -138.31 0.85 1.05 9.46

D15-04 1550.74 1483.40 1478.95 1412.67 -11.29 -15.74 -138.07 0.76 1.05 9.24

D15-05-S2 1551.08 1487.44 1480.25 1418.14 -11.70 -18.89 -132.94 0.78 1.26 8.87

E10-02 1603.26 1622.69 1551.90 1512.63 57.28 -13.51 -90.63 3.66 0.86 5.79

E10-03-S2 2028.32 2062.16 1954.95 1927.19 81.61 -25.60 -101.13 4.12 1.29 5.11

E16-04 1980.82 1914.72 1899.25 1790.50 -29.99 -45.46 -190.32 1.54 2.34 9.79

E16-05 2049.91 2018.94 2005.06 1915.60 9.96 -3.92 -134.31 0.50 0.20 6.69

E17-02 1960.87 1924.66 1906.38 1798.97 1.00 -17.28 -161.90 0.05 0.90 8.42

E17-A-03 1598.47 1584.61 1562.29 1495.02 24.79 2.47 -103.45 1.59 0.16 6.63

E18-01 2084.49 2059.14 2054.63 1923.92 21.97 17.46 -160.57 1.08 0.86 7.88

E18-04 2483.59 2418.92 2421.97 2274.12 -16.44 -13.39 -209.47 0.68 0.55 8.60

E18-05 2504.68 2456.30 2457.60 2309.53 -1.07 0.23 -195.15 0.04 0.01 7.94

E18-06 2298.71 2260.06 2260.68 2113.88 9.44 10.06 -184.83 0.42 0.45 8.21

F02-01 1836.10 1787.56 1794.70 1762.88 -3.88 3.26 -73.22 0.22 0.18 4.09

F02-05 1515.31 1457.71 1425.71 1442.84 -13.16 -45.16 -72.47 0.89 3.07 4.93

F02-06 1599.00 1565.77 1530.33 1553.15 11.14 -24.30 -45.85 0.72 1.56 2.95

F02-07 2305.40 2253.73 2210.27 2213.25 -7.52 -50.98 -92.15 0.33 2.25 4.08

F03-08 2063.11 2020.58 2019.58 1988.61 -3.10 -4.10 -74.50 0.15 0.20 3.68

F03-FA-01 2184.80 2160.97 2164.63 2123.25 15.52 19.18 -61.55 0.72 0.89 2.87

F04-03 2010.58 1949.73 1915.53 1902.08 -12.34 -46.54 -108.50 0.63 2.37 5.53

F05-01 2215.66 2154.30 2160.82 2064.20 -13.05 -6.53 -151.46 0.60 0.30 6.99

F05-04 1335.92 1330.75 1334.79 1307.18 42.42 46.46 -28.74 3.29 3.61 2.23

F05-05 1696.60 1584.33 1586.00 1553.43 -62.73 -61.06 -143.17 3.81 3.71 8.69

F07-01 1714.40 1688.02 1734.14 1649.81 20.12 66.24 -64.59 1.21 3.97 3.87

F08-02 1582.50 1545.92 1543.44 1539.58 11.91 9.43 -42.92 0.78 0.61 2.80

F09-01 1496.10 1443.67 1444.40 1425.37 -4.75 -4.02 -70.73 0.33 0.28 4.88

F10-02 2152.16 2072.80 2174.36 2000.19 -29.45 72.11 -151.97 1.40 3.43 7.23

F10-03 1829.43 1825.07 1851.69 1782.68 45.38 72.00 -46.75 2.55 4.05 2.63

F11-03 2166.57 2047.53

2044.82 -68.42

-121.75 3.23

5.75

F12-02-S1 2427.00 2462.78 2346.55 81.76 -80.45 3.43 3.38

F12-05 1577.00 1536.62 1515.75 6.87 -61.25 0.45 4.00

F13-01 1556.06 1483.21 1486.98 1429.55 -22.24 -18.47 -126.51 1.48 1.23 8.40

F14-02 1469.00 1376.87

1376.74 -44.37

-92.26 3.12

6.49

F14-05 1607.61 1561.68 1555.73 2.35 -51.88 0.15 3.33

F15-03 1763.00 1695.06 1681.12 -21.10 -81.88 1.23 4.77

F15-08 2832.76 2764.38 2593.93 -22.89 -238.83 0.82 8.57

F16-02 1555.02 1505.18 1454.12 -4.07 -100.90 0.27 6.69

F16-03 2524.73 2450.57 2456.09 2289.33 -25.23 -19.71 -235.40 1.02 0.80 9.51

F16-04 2595.59 2515.82 2523.01 2375.93 -32.38 -25.19 -219.66 1.27 0.99 8.62

F16-05 2284.12 2222.98 2227.89 2077.90 -11.59 -6.68 -206.22 0.52 0.30 9.23

F17-09 1724.48 1686.67

1674.18 7.12

-50.30 0.42

2.99

F18-01 1841.47 1786.91 1741.54 -11.01 -99.93 0.61 5.56

F18-02 1622.50 1590.04 1561.03 10.63 -61.47 0.67 3.89

F18-03 2141.46 2090.07 1981.41 -9.86 -160.05 0.47 7.62

F18-07 1714.00 1635.04 1600.74 -35.42 -113.26 2.12 6.78

63

Table 4.2b Residuals of the base CK in the BAGHDEF area. A positive residual means that the depth is overestimated

F18-11 2199.77 2208.40 2056.16 49.66 -143.61 2.30 6.65

G07-02 3119.64 2980.93 3022.65 2759.11 -94.86 -53.14 -360.53 3.08 1.73 11.72

G10-03 2904.99 2726.65

2378.46 -134.35

-526.53 4.70

18.40

G13-01 2161.71 2120.76 2029.37 1.91 -132.34 0.09 6.25

G13-03 2468.71 2386.52 2227.80 -37.48 -240.91 1.55 9.94

G14-01 2302.95 2292.62 2199.15 29.33 -103.80 1.30 4.59

G14-02 2617.87 2595.09 2484.62 16.83 -133.25 0.65 5.17

G14-04 2653.19 2601.99 2472.37 -11.39 -180.82 0.44 6.92

G14-06 2466.80 2481.75 2353.56 55.55 -113.24 2.29 4.67

G14-A-01 2420.95 2410.52 2306.94 29.95 -114.01 1.26 4.79

G16-03 2130.47 2097.87 1986.60 8.45 -143.87 0.40 6.89

G16-05 2045.21 1997.30 1919.19 -8.68 -126.02 0.43 6.28

G16-06 2303.28 2246.05 2115.25 -16.71 -188.03 0.74 8.31

G16-08-S1 2739.82 2720.84 2543.38 23.53 -196.44 0.87 7.28

G16-11 2307.24 2257.66 2136.34 -9.32 -170.90 0.41 7.54

G16-A-02 2122.40 2106.49 2399.23 25.12 276.83 1.21 13.30

G16-B-02 2625.33 2589.16 2408.96 4.37 -216.37 0.17 8.37

G16-B-04-S1 2542.04 2601.76 2414.45 100.26 -127.59 4.01 5.10

G17-01 2519.94 2431.59 2257.15 -49.93 -262.79 2.01 10.59

G17-03 2123.64 2134.33 2014.75 50.05 -108.89 2.40 5.22

G17-06-S1 2396.26 2363.41 2213.02 4.48 -183.24 0.19 7.77

G17-A-02 2569.50 2548.37 2368.28 17.48 -201.22 0.69 7.95

G17-S-01 2669.35 2607.91 2410.34 -21.89 -259.01 0.83 9.85