Centre Exploration-Production

OIL AND GAS FORMATION AND ACCUMULATION

IN SEDIMENTARY BASINS:

Case study: the Wessex and Paris Basins, UK and France

Bernard Durand Alain Mascle

Claude Laffont

IFP School 2005

OIL AND GAS FORMATION AND ACCUMULATION IN SEDIMENTARY BASINS:

CASE STUDY: THE WESSEX and PARIS BASINS (UK-FRANCE)

(front cover: Blue Lias, Lyme Regis, Wessex Basin: the source rock at the origin of the oil produced at Wytch Farm oil field)

Bernard Durand, Alain Mascle and Claude Laffont (IFP School)

Summary

The Wytch Farm Oil Field is located in the Wessex Basin, which is a part of a larger Basin, the Paris-London Basin. It has been the largest onshore oil field of Europe, with a production of about 110.000 bopd in 2000.

The Paris-London basin contains Mesozoc and Tertiary sediments. The basin began to develop on a Paleozoc basement during Permo-Triassic times, and emerged under the effect of a tectonic inversion resulting from the collision of the African plate with the European plate (formation of the Alps), starting at the end of Cretaceous and still going on. In addition the western Wessex basin underwent a severe tectonic uplift during Albian times, the cause of which is still debated. In a sedimentary basin, the formation and accumulation of oil and gas is due to the linking of several causes which are mainly: The presence of intervals rich in organic matter (kerogen) in the sediment series. These

intervals (call source rocks) are rare, because their existence is possible only if several conditions were combined at the time of sedimentation: high biomass productivity in the sedimentation medium or nearby, short distances of transportation of organic debris and anoxia in bottom waters and recent sediment layers.

A thermal history allowing a "cooking" of the kerogen at temperatures and duration sufficient

to produce oil and gas. Pressure conditions and saturation of oil in the pores high enough to allow expulsion

(primary migration) out of the source-rock. Existence nearby of the source-rock of porous layers in which the expelled oil and gas can

move along (secondary migration), and of traps in which they can accumulate.

"Cooking" of kerogen is ruled by chemical kinetics, i.e. it depends on temperature, time and nature (type) of the kerogen. This is described by first order kinetic equations. Once formed in the source-rock, the primary and secondary migration of oil and gas, in porous layers are ruled by the polyphasic Darcy's law. Formation, migration and accumulation of oil and gas can be modeled according to these physical and chemical principles, and 1D, 2D and 3D models are currently used in the oil industry. It is necessary for that to have good knowledge of the evolution of the basin, subsidence and uplift of the thermal history and of porosities and permeabilities of sediment. In the absence or scarcity of such data, such models can be used to check the consistency of geological hypothesis. In the case of the Paris-London basin, oil accumulations are supplied by Jurassic kerogen rich intervals which belong to the Hettangian-Sinnemurian-Pliensbachian, to the Lower Toarcian, to the Oxfordian and to the Kimmeridgian. They basically contain a type II kerogen, i.e. derived from marine phytoplankton debris, mixed in small but variable proportions with a type III kerogen i.e. derived from land-plants debris. This kerogen is also more or less deeply altered according to the oxic-anoxic conditions prevailing at the time of deposition. In the Wessex part of the Basin, the Lower Toarcian interval is missing. All the three other intervals are outcropping along the coast. The history of the oil entrapment in the Wessex Basin is complex. First accumulations occurred in Cretaceous time before the Tertiary tectonic inversions. They were then dislocated during these inversions, the Wytch Farm structure and accumulation being fortunately preserved. A simple 1D modeling of oil and gas formation in the most probable effective area shows how critical is the evaluation of the erosion due to the inversion on the incertitude of the assessment of oil and gas formation timing and intensity. This report will start with basic reminders on the formation of sedimentary basins, reservoirs and traps.

Table of contents

1 Forewords......................................................................................................................1

1.1 Structure of the Earth and formation of sedimentary basins....................................1

1.2 Environment of deposition of sediments................................................................ 13 1.2.1 Continental environment....................................................................................................13 1.2.2 Shallow marine.....................................................................................................................14 1.2.3 Deep marine .........................................................................................................................14

1.3 Formation of traps................................................................................................... 19

1.4 Petrophysic ..............................................................................................................22

2 Introduction ................................................................................................................26

3 Reminder of the mechanisms of formation and accumulation of oil and natural gas 27

3.1 Kerogen, the raw material oil and gas originate from ............................................27

3.2 Thermal history and nature (type) of kerogen, the main factors of oil and gas formation.........................................................................................................................28

3.3 Main stages of the transformation of kerogen into oil and gas: diagenesis, catagenesis and metagenesis ..........................................................................................29

3.4 Notions of source-rock, maturation stages and maturity parameters....................30

4 Accumulation ..............................................................................................................30

4.1 Primary migration (expulsion)................................................................................ 31

4.2 Secondary migration................................................................................................33

4.3 Tertiary migration (dysmigration) ..........................................................................33

5 Numerical modeling of formation and accumulation of oil and gas .........................43

5.1 Principles for a modeling ........................................................................................43 5.1.1 Simulation of the kinetics of formation of oil and gas...................................................43 5.1.2 Simulation of burial history: the backstripping method "..............................................44 5.1.3 Simulation of thermal history ............................................................................................45 5.1.4 Simulation of migration ......................................................................................................46

6 Petroleum systems ......................................................................................................52

7 Habitat of hydrocarbons in the Paris-London Basin .................................................54

7.1 Paris Basin...............................................................................................................54

7.2 Wessex Basin...........................................................................................................55

8 Application of a simple numerical model to the reconstruction of the Wytch Farm Oil field geohistory..............................................................................................................67

9 References ...................................................................................................................73

10 Further readings (books) ............................................................................................75

List of figures Figure 1-1 Origin of the universe ...........................................................................................................5

Figure 1-2 Geological time-scale, with main events. ...........................................................................6

Figure 1-3 External envelope of the Earth ...........................................................................................7

Figure 1-4 Block diagram of the continental crust 8

Figure 1-5 Plate boundaries.....................................................................................................................9

Figure 1-6 Evolution of the lithosphere..............................................................................................11

Figure 1-7 Diagram of the oceanic lithosphere and cross-section of an ophiolite. ......................12

Figure 1-8 Sedimentary landscape ........................................................................................................16

Figure 1-9 Sedimentary processes ........................................................................................................16

Figure 1-10 Sedimentary environments.................................................................................................17

Figure 1-11 Deep marine sedimentation (Back-ground).....................................................................18

Figure 1-12 Hydrostatic sphere of constraints (a) and triaxial constraints ellipsoid (b) .................19

Figure 1-13 Dynamic interpretation of two faults ...............................................................................19

Figure 1-14 Classic examples of structural traps ..................................................................................20

Figure 1-15 Oil seep near Als, France (top) and gas seep near Grenoble, France (bottom) ........21

Figure 1-17 Wettability ........................................................................................................................24

Figure 1-18 Capillary Pressure ................................................................................................................24

Figure 1-19 Distribution of water and hydrocarbons in a deposit ....................................................24

Figure 1-20 Capillary migration ..............................................................................................................25

Figure 1-16 Sketch map showing extent and the principal structural features of the Paris-London basin during the Jurassic and Early Cretaceous (modified after a BP document, 1989). ................26

Figure 4-1 Kerogen distribution at the microscopic scale in rock samples of the Kimmeridgian of the Dorset. Kerogen, in black, is seen with an electron microscope, thanks to the "back-scattered electrons" technique. Large differences are observed in kerogen concentrations (as measured by TOC) and in kerogen distribution patterns (after S. Belin, 1992)................................34

Figure 4-2 Sketch showing the different types of influxes of organic matter debris during the formation of sediment in a sedimentary basin. ......................................................................................35

Figure 4-3 Variations of organic and mineral carbon content in a Pliensbachian organic rich interval (Belemnite Marls) of the Dorset (after Van Buchem et al. 1995). ........................................35

Figure 4-4 Burial history (a) and thermal history (b) of the Hettangian of the central part of the Paris Basin (after J. Burrus et al., 1997)...................................................................................................37

Figure 4-5 Main stages of formation of mobile products from kerogen during burial. Biomarkers are these molecules found in mobile products, the chemical structure of which demonstrates that they are derived from bio-molecules. .....................................................................38

Figure 4-6 Sketch of a classical analytical procedure used in petroleum geochemistry. Resins and Asphaltenes are fractions of the initial oils. Asphaltenes are separated by adding an excess of n-heptane, which causes their precipitation as solid particles. Resins are separated from the remaining soluble part (maltene) by liquid chromatography. ..............................................................39

Figure 4-7 Conceptual model of the formation of oil and gas under the effect of temperature increase during burial of a source-rock. ..................................................................................................40

Figure 4-8 Profiles of average vitrinite reflectance versus depth in four areas where the kerogen is derived from land-plants (type III kerogen). Note the variety of profiles, due to very different thermal regimes. The values of vitrinite reflectance, which are typical of the main stages of oil and gas formation, are indicated......................................................................................................................40

Figure 4-9 Schematic scheme of oil and gas formation in a sedimentary basin during its geological evolution. ..................................................................................................................................41

Figure 4-10 Conceptual model of primary migration as a diphasic flow (after Ungerer et al., 1984, 1990). 42

Figure 5-1 General scheme of a five fraction standard kinetic model: up to 20 primary reactions and three secondary reactions. .................................................................................................................47

Figure 5-2 Ai and Ei distribution for five classes compositional models.......................................48

Figure 5-3 Average porosity-depth curves for the three main types of lithology, from various published sources. ......................................................................................................................................48

Figure 5-4 Example of burial history reconstruction with decompaction, on a synthetic example inspired from the Viking Graben, North Sea. .......................................................................................49

Figure 5-5 Organization and data which are necessary for a "1-D" oil and gas formation and expulsion model (after the GENEX handbook)...................................................................................51

Figure 6-1 Example of data card for a Petroleum System (Wessex Basin). ...................................53

Figure 7-1 Location of the main oil accumulations in the Paris Basin (after Burrus, 1997)........56

Figure 7-2 Comparison of potential source-rocks, reservoirs, seals and timing of trap formation in the Wessex and the Paris basins. .........................................................................................................57

Figure 7-3 West-East Geological cross-section of the Paris Basin showing locations of effective source-rocks and main reservoirs (after Espitali et al., 1988). ...........................................................58

Figure 7-4 Gas chromatograms of the C15+ saturates of the Wytch Farm and Villeperdue (Paris Basin) oil fields.................................................................................................................................59

Figure 7-5 Comparison of oil composition between Villeperdue and Wytch Farm using few classical parameters. ...................................................................................................................................60

Figure 7-6 Mass chromatograms m/z 217 (steranes) ........................................................................61

Figure 7-7 Evolution of the % of C29 Steranes and 20 S C29 steranes in relation with depth of the cores in source-rock of Paris basin, and comparison with the observed values in oils of Paris Basin and Wytch Farm pool. .....................................................................................................62

Figure 7-8 Positions of the oils of Villeperdue and Wytch Farm in a triangular diagram: C27 C28 C29 Steranes. 63

Figure 7-9 Comparison between mass chromatograms m/z 191 (triterpanes). a Villeperdue b - Wytch Farm.................................................................................................................................................64

Figure 7-10 Location of oil and gas fields, shows and seepages in the Wessex Basin (after Stoneley and Selley, 1991). ........................................................................................................................65

Figure 7-11 Interpreted structural sections through Wytch Farm field and Purbeck Disturbance. 66

Figure 8-1 Genex reconstruction of tectonic subsidence history, using the hypothesis on stratigraphic thicknesses of Colter and Havard (1981), for an area located South of Purbeck Disturbance. 69

Figure 8-2 Genex reconstruction of heat flow history, using the hypothesis of Figure 8.1. .......69

Figure 8-3 Genex reconstruction of hydrocarbon formation history, using the hypothesis of Figure 8.1. 70

Figure 8-4 Genex reconstruction of hydrocarbon expulsion history, using the hypothesis of Figure 8.1 and an expulsion threshold of 30%. .....................................................................................70

Figure 8-5 Genex reconstruction of tectonic subsidence history on a hypothesis of a Mesozoc-Tertiary pre-inversion cover thinner than in case of Figure 8.1 (3.000 m instead of 4.000 m). .....71

Figure 8-6 Genex reconstruction of heat flow history on hypothesis of Figure 8.5.....................71

Figure 8-7 Reconstruction by GENEX of the history of hydrocarbon expulsion with the hypothesis of figure 8.5 and expulsion value of 30% ...........................................................................72

List of tables Table 1: Frequencies (in ) of rock samples according to their organic carbon and mineral carbon contents, in a set of more than 10 000 samples analyzed at IFP. ..........................................36

Table 2 Parameters for the main types of lithology (default values, after Genex notice): a) main categories of sedimentary rocks; b) basement........................................................................................50

Table 3 Values of few parameters derived from steranes and triterpanes analysis Villeperdue (n 77548) and Wytch Farm (n 158981 and 158982) ..........................................................................62

Table 4 Stratigraphic data used for the reconstruction of burial area, Wytch Farm area, South of Purbeck disturbance..............................................................................................................................68

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1 Forewords A few basic definitions are proposed hereafter for non-geologist readers. 1.1 Structure of the Earth and formation of sedimentary basins Oil and gas are formed in sedimentary basins. These basins represent the surface shell of the solid globe. They are in contact at their top with the liquid envelope (seas and oceans) or with the atmosphere. The first stage in the process of oil exploration is to understand the origin of sedimentary basins, and to determine the age and types of rocks (sedimentary rocks, or more simply sediments) that they contain. This process aims to assess the potential of sedimentary basins to contain liquid or gaseous hydrocarbons, in sufficient quantities to justify the development and exploitation of an accumulation (an oil accumulation or pool). The Earth was formed 4.6 billion years ago by the gravitational collapse of interstellar matter revolving around the sun (itself formed approximately 5 billion years ago,Figure 1-1). Figure 1-2 gives the names given by geologists to the various periods of time succeeding each other since 4.6 Gy (time scale or stratigraphic time scale). The sedimentary basins are among the youngest structures of the Earths globe, since their age generally lies between 600 million years and the present. Sedimentary basins existed before that time, and certain parts of them are locally preserved at the surface of the Earth. However, the majority of these basins have been destroyed due to the effects of deformation, known as tectonic, and/or their burial to great depth. In this way, the initial sediments are transformed into rocks whose new mineralogical constituents are stable under the increased temperature and pressure (see Figure 1-3 and Figure 1-4 for the change of temperature and pressure with depth). The new types of rock so formed are known as metamorphic rocks (slates, schists, gneiss, etc.). A partial or complete fusion of these rocks may occur at very great depth, giving rise to magma. Such melts or any other type of "liquid material coming from very great depths in the Earth (several tens of km) can ascend towards the surface by convection and/or buoyancy, therefore towards conditions of lower pressure and temperature. These magmas then recrystallize either at some depth (a few km) to form intrusive rocks (granite for example), or at the surface, producing volcanic rocks (basalt, andesite, etc.). Rock-types belonging to this third category are termed magmatic or igneous. It follows on logically from the above that sedimentary basins and the sediments they contain lie on top of metamorphic or igneous rocks making up their substratum or basement. The structure of the globe is given in Figure 1-3 Apart from the surface shell, where the deepest drillings do not exceed 14 km, we have no direct knowledge of the materials making up the Earths interior. The information that we have, and thus the proposed divisions, is based essentially on parameters derived from the modeling of geophysical data, i.e. the P wave velocity and density (according to the travel times of seismic waves and measurements of the gravity field). The important point here is the distinction in the outer zone between an envelope called the asthenosphere and an envelope called the lithosphere. Within the lithosphere, we can distinguish the upper mantle at the base (but this latter includes also the asthenosphere!) and the crust at the top. In turn, we can subdivide the crust into a basement in the lower part and sedimentary basins at the top (when they exist, which is not always the case as the basement rocks may locally outcrop at the earth surface). The surface separating the crust from the upper mantle is an important discontinuity in the P wave velocity called the Mohoroviic discontinuity, or more simply the Moho. Its depth, and thus the thickness of the crust, varies considerably under the continents and the oceans. The zone of transition from one to the other corresponds to what is called a continental margin.

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Due to the convection of materials within the astenosphere, these envelopes are not fixed and immobile but are moving with respect to each other. In this way, the lithosphere is moving relative to the asthenosphere at rates of about several cm per year. However, the lithospheric envelope does not move as a single block. In fact, it is split up into more or less large segments known as lithospheric plates, or more simply plates. Each of these plates exhibits a different velocity and direction of displacement compared with the asthenosphere. Therefore, they show relative movements between themselves (always of the order of a few cm per year), which correspond to convergence, divergence or lateral displacement (known also as transcurrent faulting, see Figure 1-5), or a combination of two of these components. In the following paragraph, we describe how the formation of sedimentary basins is closely linked to these relative displacements. These lithospheric displacements are assumed to originate from the movement of convection cells within the asthenosphere, or at even greater depth. Figure 1-6 shows a simplified scenario explaining the consequence of this convective motion on the deformation of the lithosphere and the formation of sedimentary basins at the surface: 1 / The initial state of both lithosphere and asthenosphere without relative movement is given in 1 (which would be currently the case at the center of the African continent).

2 / Upward-diverging convection cells in the asthenosphere produces (a) stretching of the lithosphere by pulling apart/traction and (b) an upraise of lithospheric material. The stretched lithosphere is thinned (by brittle deformation in the topmost 15 km, and by ductile deformation at greater depth). A topographic depression is formed on the surface of the lithosphere (involving subsidence), whose geometry is controlled by brittle structures called faults (we can then speak in terms of tectonic extension, leading to tectonic subsidence). This type of depression is called a rift (for example, the East African rift that extends from Ethiopia to Tanzania). A rift is a type of sedimentary basin, since the depression so created corresponds to morphology with high zones around the edges contrasting with low zones in the center. Atmospheric weathering and rivers will erode the high zones, while the action of gravity, the wind and/or water will transport the erosion products (in solid or dissolved form), which are then deposited (or precipitated) in the low zones as sediments. This process will tend to fill the basin, except if the stretching continues, i.e. if space is permanently generated for allowing sedimentation. The lifetime of a rift ranges from several million to a few tens of million years. When the convection, tectonic extension or upraise of asthenospheric material ceases, the rift development may come to an end. The basin will then be gradually filled. Cooling of the stretched lithosphere will begin as soon as the asthenospheric material ceases to rise (since it is of deep origin and therefore hot), thus nevertheless producing a depression at the surface of the lithosphere by thermal contraction (we call this thermal subsidence). This subsidence will continue for a few tens of million years to generate space for sedimentation. The North Sea is an example of this type of basin evolution, with a relatively complex history comprising at least two successive periods of rift formation (rifting); it is approximately 300 million years old.

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3 / On the other hand, if the stretching of the lithosphere continues, the continental lithosphere will break, and consequently the initial single plate will be divided into two plates, moving apart from each other. Between the two separate pieces of continental lithosphere, the ascending asthenospheric material (b) will reach the Earths surface, where it undergoes low pressure-temperature conditions that lead to the crystallization of magma. In this way, the oceanic lithosphere is created, exhibiting the physical structure illustrated in Figure 1-7. The continued spreading of the two continental plates leads to the creation of an increasingly wide ocean. The oceanic plate is oldest at the rim (i.e. at the margins), while the youngest ages - even including present-day material - are located in the middle. The type of margin thus created between continental and oceanic lithosphere is called a passive (or stable) margin. The zone of ascending asthenosphere and magma crystallization produces a sea-floor feature with strong relief known as a mid-ocean ridge. As soon as new oceanic lithosphere is created, the margins move away from the relatively hot zone situated along the axis of the ridge. The newly formed lithosphere then cools, producing a thermal subsidence that is favorable for the generation of space available for sedimentation. Continental margins are thus the sites of particularly voluminous sedimentary basins, which can cover areas of several ten of thousands of km with thicknesses sometimes exceeding 10 km. The sediments accumulated in these basins are derived from material eroded and then transported from the continents. They are called terrigenous or clastic sediments (the same term applies to sediments accumulated in continental rifts), and are mostly composed of sandstones and clays. These sediments are deposited in the marine environment, after a more or less intense reworking by wave activity or tidal currents. This is the case of the African and South American margins of the Atlantic Ocean. Two types of sediment are deposited far from the margins in open oceanic environments, where water depths are of several hundreds to several thousands of metres: the finest grained terrigenous sediments (clays), i.e. those which could be transported by marine currents far from the coast, and sediments known as pelagic, resulting from a very slow "rain" of fine shelly remains of animal and plant organisms making up the plankton. The mixture of these two types of sediment is known as hemipelagic sedimentation. Rather shallow marine waters (0 - 200 m depth) represent environments favorable for the precipitation of calcium carbonate (CaCO3) present in seawater in dissolved form. This precipitation is either purely chemical, or takes place via biological activity. The marine fauna and flora are either fixed (corals and reefs), mobile on the seabed (benthic) or floating/swimming in the water (pelagic). The sediments so formed correspond to carbonates (limestones, dolomites and marls). 4 /A period of continuous oceanic crust accretion can last for several tens of million years. For example, the Pacific Ocean was created at least 200 million years ago, and spreading is still continuing at its mid-ocean ridge. Since the volume of the Earth can be regarded as constant over several billion years, and because the ocean-floor spreading creates continuously new oceanic lithosphere, we need to assume that there is simultaneous disappearance of lithosphere elsewhere. This is indeed what occurs above asthenospheric convection cells that are converging downwards, inducing a convergence (a) of the two overlying plates. This convergence causes the plunging, or subduction (b) of one plate under another. The first plate must be of oceanic nature i.e., denser and thinner than continental plates. In this way, a new type of margin is created, known as an active continental margin or island-arc margin, according to whether the overlying plate is of continental or oceanic nature, respectively. The subducted oceanic lithosphere increasingly higher and higher pressure-temperature conditions where the mineral constituents are no longer stable. This leads to the formation of magmas that rise

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towards the surface due to their density contrast, producing volcanoes (c) which characterize this type of margin (for example, the Antilles archipelago, including Martinique and Guadeloupe, under which part of the Atlantic Ocean is currently being subducted). Basins of variable thickness develop on this type of margin, which often show complex histories (tectonic deformation, influence of volcanicity, etc.).

5 / This subduction can last as long as oceanic lithosphere material is available. But a time will come when the entire oceanic plate has been subducted, and its corresponding active margin will abut against continental lithosphere belonging to the opposite passive margin. The latter generally cannot be subducted, because it is too thick and too light. The convergence of the two plates (a) leads to a collision of the two continental lithospheres that causes the formation of a mountain chain (the Himalayas, for example, resulting from the collision of the Indian plate with the Eurasian plate). The edges of the two continental plates become imbricated (b) into each other in a complex way, involving partial or total disappearance of their initial sedimentary basins. Conversely, this collision generates strong topographic relief (cf. the Himalayas) that: gives rise to the formation of flexural basins bordering the chains. The erosion of these chains supplies the filling of these basins by clastic sediments. Melting processes in the more deeply buried pieces of lithosphere allow the generation of magmas that recrystallize at shallower depth to form intrusive rocks of granitic type (c). 6 / Eventually, the shortening within the mountain chain will no longer be able to accommodate the convergence of the two plates. The shortening will then be transferred to another place on the Earths surface, and the mountain chain will become inactive. The horizontal stresses generated by convergence will therefore cease, and vertical gravity stresses will prevail at this site, causing a collapse of the chain. This process, combined with the erosion of the last remaining relieves, will cause the disappearance of the mountain. The only remaining trace on the surface of the Earth is represented by a structurally complex substratum, composed of intrusive and metamorphic rocks where relics of the former chain and basins are more or less well preserved (this is the case of the French Massif Central and Cornwall in the U.K., evidence of a chain similar to the Alps that existed in these areas 350-300 million years ago). The term suture is used here to refer to the segment of peneplaned lithosphere preserving the trace of an ancient mountain chain. More particularly, it is possible to find local fragments of the old oceanic lithosphere, and these types of rock - which are called ophiolites - are markers of oceans that have disappeared.

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Figure 1-1 Origin of the universe

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Figure 1-2 Geological time-scale, with main events.

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The lithospheric plates

Figure 1-3 External envelope of the Earth

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Figure 1-4 Block diagram of the continental crust

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THREE TYPES OF PLATE MARGIN ARE TO BE CONSIDERED 1 DIVERGENT Divergent (also known as constructive or accretional) margins are linked to mid-ocean ridges [as well as rifted continental margins], corresponding to zones of lithospheric accretion 2- CONVERGENT Convergent (or destructive) margins are linked to subduction zones, corresponding to sites where plates are finally consumed back into the mantle 3- TRANSCURRENT Transcurrent (or conservative) margins are linked to transform faults, which provide evidence of the lateral slipping of spherical shells making up the lithosphere.

Figure 1-5 Plate boundaries

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Figure 1-6 Evolution of the lithosphere

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Figure 1-7 Diagram of the oceanic lithosphere and cross-section of an ophiolite.

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1.2 Environment of deposition of sediments Different types of sediments, either clastic (mostly siliciclastic, as SiO2 , quartz, is the most common mineral) or carbonates, are being deposited within present day sedimentary basins, and are found within past sedimentary basins. Some of them may contribute to the formation of oil or gas accumulations: source rocks (sediments with more than 1% in total organic content), reservoirs and covers (sediments respectively with or without significant porosity and permeability (see definitions in the next chapter). These 3 types or rocks are important elements of petroleum systems, as defined in chapter 6. If we consider the present day morphology of the solid earth, we can consider 3 main environments were sediments are being deposited: continental, shallow marine and deep marine (Figure 1-8 and Figure 1-9; courtesy Ph. Joseph). Both siliciclastic, calcareous, or mixed siliciclastic-calcareous sediments may be found in each of these settings. The recognition of the past environment of deposition of reservoir rocks is important as this will able the geologist to characterise the reservoirs properties in term of both internal heterogeneities (for fluid flow) and lateral extent. 1.2.1 Continental environment These are the area above the sea level (Figure 1-10; courtesy Ph. Joseph). The surface of the earth, and more specially the area with the higher elevations, is exposed to weathering and erosion due to the effect of wind, rain, frost, biological activity.Pieces of rocks, either large (blocks, pebbles) or microscopic (grains) are removed from the ground (soils or solid rocks) and transported downward either by gravity alone, or by torrents and rivers, or in some case, for the finest materials, by the wind. Fallen blocks at the foot of steep slopes may form accumulations of coarse grained unsorted conglomerates and sandstones at the origin of sedimentary bodies known as alluvial fans, either dry if built by debris flow processes, or wet if built by stream flow process. Some materials can be subsequently remobilized and transported further down slope by rivers and streams. As soon as there is a decreased in the energy of the flow (due to a decrease in the amount of water as rain falls are vanishing, or to a reduction in the river slope when approaching the sea shore, or to local partition of the flow such as in meanders.), the heaviest pebbles, grains or particles will be deposited on the river beds, as fluvial conglomerates, sandstones, siltstones and, if no more current if active, clays ( as also in lakes: lacustrine sediments). Different types of rivers, and related fluvial sediments, have been recognised by geologists: braided, meandriform, anastomosed, corresponding to decreasing energies of transportation and decreasing amounts of sandy material with respect to clays. They all have in common a unidirectional direction of water flow (from upstream to downstream!). Potential reservoirs may form in these environments when clean (well sorted and without clay), thick, laterally continuous and connected sandstones are being deposited. When approaching the sea shore, some rivers may form a delta with mixed river and marine influences. Dense forests may develop there if hot and wet climatic conditions are prevailing leading to deposition of organic matter derived from these land plants. Potential source rocks such as peat, lignite and coal may formed in such conditions. Similar processes requiring a low energy environment (to allow the deposition and preservation of the fine particles of organic matter) can also occur in, or at the edge of lakes, with additional organic matter provided by algaes and bacterias. The wind may be in some places (flat open spaces and dry climate) an efficient way to transport light particles (fine sands and clays), either on the ground level or in clouds. Local fall of the wind energy may lead to deposition of well sorted and clean sands (aeolian sanstones forming aeolian dunes) as the lighter clay particles are generally transported far away. As a result aeolian sandstones may formed excellent reservoirs.

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Chemical erosion of limestones will enrich the fresh water of rivers with dissolved calcium (Ca++), that could either be transported down to the sea, or eventually combine with CO3--(from CO2 + H2O = H2CO3) to precipitate (either directly or through the formation of shells) in quiet environment such as lake to form lacustrine limestones. 1.2.2 Shallow marine Some clastic material may cross the river mouth and reach the sea. The near shore environment is usually shallow marine (less than 200 m: the continental shelf). In term of hydrodynamism, they will leave an environment of unidirectional flow (the river flow) to a new one that is either: -a bi-directional flow (the tide, twice a day), active to water depths of 50-100 meters usually, -an oscillatory movement: the waves, either the permanent fair weather waves at shallow water depth (down to a few 10th meters depending on the area), or occasionally the storm weather waves down to 100-150 meters. The hydrodynamism of both the tide and the waves may prevent the deposition of the lightest material (the clays that will remain in suspension in the water column) and well sorted sandstones may deposit (if such clastic material are actually provided by rivers), that could generate potential good reservoirs. However some thin clay laminations or lenses may be deposited during the inter-tidal periods, and clays layers may be deposited below the fair weather zone during period without storms. But both may also be removed or partly removed by the action of subsequent tides or storm-weather waves. Shallow marine environment in area remote from clastic influxes are quite suitable for the development of carbonate platforms, i.e. accumulation of limestones of either biologic origin (from pelagic-benthic micro-fauna and micro-flora, and from coral reefs), or of chemical origin (such as microscopic concretions known as oolithes). The biological productivity is enhanced by the presence of hot waters such at tropical latitudes. The classic formula of carbonate precipitation is as follow: Ca ++ (dissolved in the sea water) + CO3-- = CaCo3 With: H20 + CO2 (either atmospheric or dissolved in the water) = H2CO3 = H2++ + CO3 As carbonate are chemically less stable than siliciclastic sediments, late transformations of the initial sediment during the processes of burial (diagenesis) may significantly modify the texture and/or mineral composition of the rocks. Both processes (initial deposition and subsequent diagenesis) may form important carbonate reservoirs such as in the Middle East. 1.2.3 Deep marine The deep marine environment corresponds to the deeper edge of the continental shelf, the continental slopes and rises, down to the abyssal plains (several thousand meters). Deep marine sedimentation results generally from the mixing of 3 components (Figure 1-11): - the finest clastic material such as clays of continental origin (through the process of

weathering and erosion of land mass, transportation by rivers down to the sea, and by pass of the shelf where hydrodynamism due to the tides and the waves prevent their deposition);

- microscopic shells from the planktonic micro fauna and flora leaving near the sea surface. At great water depth (a few thousand meters depending of the area and period) solid carbonate particles are no more stable and will dissolved in the water;

- organic matter, of either continental origin (travelling with the clays), or marine origin (plancton). Excellent and widespread source rocks may form if the total amount of organic matter is in excess of 1%.

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The usual types of related rocks are either limestones (when carbonates dominate the assemblage), claystones (when clays dominate) or marls (about 50% carbonates and clays). Claystones and marls, when laminated, are also call shales. Black shales frequently refer to marine organic rich marls. Such rocks will never form reservoirs (except if later fractured). Fortunately, some siliciclastic influxes may, in a few places, and from time to time, reach the deep water realm. They are call turbidites. They occur during periods of low sea level stages (i.e. when the river mouth is close to the edge of the continental shelf, thus delivering clastic material directly to the deep marine environment), or when sandstones initially deposited on the shelf are reworked and remobilized by currents or gravity slidings down slope. Turbidites form most of the reservoirs of the producing fields in deep (500 2000m) to ultra-deep (+ 2000 m) seas or oceans.

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Figure 1-8 Sedimentary landscape

Figure 1-9 Sedimentary processes

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Figure 1-10 Sedimentary environments

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Figure 1-11 Deep marine sedimentation (back-ground)

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1.3 Formation of traps If source rocks, reservoirs and seals (covers) are fundamentals components of petroleum systems, oil and gas accumulation will only occur if some trapping mechanisms are present within the basin to prevent the free oil and gas to migrate directly to the surface (see chap. 3). This is what is indeed commonly occurring, leading to the presence of oil or gas seeps at the surface (Figure 1-15) Some oil or gas will remain at depth in the basin if some specific geometries of sedimentary layers (usually a reservoir overlain by a cover) form traps from which oil or gas can not escape. Stratigraphic traps results from some abrupt or progressive lateral variation in the sedimentary content of strata (from porous and permeable to non porous and non permeable, as at the edge of sandstones lenses for example; this type of geometry is called a pinch out). Structural traps results from tectonic deformations of the basin. They could be related to either: - extension (main stress axis 1 vertical). The resulting structures are tilted blocks or horsts

if the bounding normal faults are dipping in the same direction or not respectively (Figure 1-14

- Compresion (main stress axis 1 horizontal). The resulting structures are folds, also called anticlines, developing over reverse fault (Figure 1-14.

These deformations are induced by stresses: vertical stresses are due to the weight of overlying sediments in the basin at any given depth, whereas horizontal stresses are due to forces acting at the edge of the basin or of the plate (see the resulting 3 main stress regimes below).

Figure 1-12 Hydrostatic sphere of constraints (a) and triaxial constraints ellipsoid (b)

Figure 1-13 Dynamic interpretation of two faults

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Tectonic deformations may involved the sedimentary basin and its basement, but in some cases they will involved the sedimentary basin only, or only the upper part of it. Such types of deformation are called thick skin and thin skin respectively. This imply for the late case that a specific sedimentary layer will separate the deformed sediments above, from the undeformed sediments (or from the basement) below. Such layer is called a decollement (or detachment) and correspond to a flat fault parallel to stratification from which the fault above (cutting through the stratification, and sometimes called ramps) will develop. Traps for hydrocarbons will be different in both geometry and age above and below the decollement. Finally, during the process of tectonic deformation, the rock itself may also be damaged. A network of fractures may developed which, in some cases (if they remain open and are connected) may significantly enhance the initial (or primary) porosity of the rocks. The resulting reservoir are called fractures reservoirs, or double porosity reservoir (primary and fractures).

Figure 1-14 Classic examples of structural traps

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Figure 1-15 Oil seep near Als, France (top) and gas seep near Grenoble, France (bottom)

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1.4 Petrophysic In the following chapter, we give some basic definitions to make the rest of the course more comprehensible to the reader. Sediment: Mineral (grains and cements) and porous network (intra- or extra-granular porosity) filled by a fluid (generally water, sometimes hydrocarbons). The fluid is either mobile, or attached (at a molecular scale) to the matrix. In oil fields, both oil and water are present. Porosity: Fluid are contained in a porous system that can be primary and/or secondary (joints, fractures, etc.).

= Vpores / Vtotal (expressed in %) We distinguish an effective porosity, , when the pores are connected between themselves and with the external medium, and a residual porosity, R, when the pores are not interconnected. Their sum is the total porosity t. Permeability: The intrinsic (or absolute) permeability is the aptitude of a rock to let a fluid circulate through its pores. In a steady-state regime and for laminar flow, Darcy's (experimental) law expresses it:

Q = A K / dP/dx Where: Q: flow rate in m3/s A: cross-sectional area of the sample in m K: coefficient of permeability (or permeability) in m or Darcy (with 1 mD: 10 -15 m) : viscosity in Pascal.seconds (or centipoise, 1000 cP = 1 Pa.s) dP: difference in pressure between upstream and downstream sides of the sample, in pascals dx: length of the sample, in m More complex formulae exist for gas, high-velocity flows and circular radial flows (in wells). The absolute permeability is not isotropic in sediments because of sedimentary and structural anisotropies. In particular, we define horizontal (kh) and vertical (kv) permeabilities (the ratio between them may vary from 1 to 10). If there are two fluids in the sediment (for example, water and oil) we define a permeability that is said to be effective for each fluid:

Q1 = A k1/1 dP1/dx and Q2 = A k2/2 dP2/dx

Relationship between porosity and permeability: the following approximate relation has been proposed from experiments in a homogeneous medium: k = r / 8

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Saturation: A given pore space volume, Vp, contains a volume of water: Vw, oil: Vo and gas: Vg. The degrees of saturation in water, oil and gas are given as follows:

Sw = Vw / Vp So = Vo / Vp Sg = Vo / Vp Wettability: This parameter defines the geometry of the contact between a liquid and the surface of the mineral matrix. It corresponds to the molecular attraction or repulsion between the fluid and solid involved. On Figure 1-2, the liquid at the top is said to be wetting (it is often water, except in some limestones), while the liquid at the bottom is said to be non-wetting (it is often oil or gas). Capillary pressure: This corresponds to the difference in pressure Pc existing between two infinitely close points A and B located on either side of the interface between two fluids: PC = Pa - Pb, where Pa represents the pressure in the non-wetting liquid (Fig. 1-3). The capillary pressure is also expressed by the formula:

Pc = 2 cos /r (1) With : inter-facial tension (force per unit length necessary to maintain contact between the two sides of an incision/indentation on the interface).

: contact angle of the wetting liquid r: radius

From Figure 1-3, we may conclude that: Pa = Pa' h air g

and Pb = Pb' h water g Therefore: Pa - Pb = Pa' - Pb' + h. (water - air) That is, Pctop = Pcbottom + h g (water - air) In a deposit where the wetting phase is water and the non-wetting phase is a hydrocarbon product, the capillary pressure at the base is close to 0 (water saturation close to 100%: Fig. 1-4). The preceding equation is thus simplified to:

Pctop = h g (water - HC) (2) Capillary migration: Figure 1-5 shows an oil drop migrating within a porous medium saturated with water. While migrating towards the surface (by buoyancy), the oil drop meets a succession of constrictions and widening/bulges. The smaller the constriction, the smaller the radius r, so, according to (1), the higher will be the capillary pressure preventing the oil drop to migrate. To allow the drop to keep on migrating upward, the Archimede pressure (B = h . g . (HC - H2O)) must increase. The drop will pass the constriction when the Archimede pressure will become higher than the capillary pressure. According to the previous formula, this can be achieved with an increase of h, i.e. when additional oil at the base of the drop will increase its height.

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Figure 1-16 Wettability

Figure 1-17 Capillary Pressure

Figure 1-18 Distribution of water and hydrocarbons in a deposit

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Figure 1-19 Capillary migration

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2 Oil and Gas formation and occurrences: introduction The aim of this course is to give a general overview of the formation and accumulation of oil and natural gas, including modelling of the phenomena involved. This is supported by data from the Wessex and Paris Basins, and from the Wytch Farm oil field. The Wytch Farm oil field is located in the Wessex basin, which forms part of the Paris-London basin. This latter started to form in the Permo-Triassic (- 280 / -250 My) and developed during the Jurassic and early Cretaceous (figure 1.1). Compression began at the end of the Cretaceous, along with a tectonic inversion caused by the collision of the Iberian and European plates farther to the south. During the Tertiary, this inversion was amplified by the collision of the African and the European plates in the south-east leading to the formation of the Alps. The Paris-London basin was then differentiated into several parts, with the Wessex basin finally emerging in its current form. The western Wessex basin underwent a severe tectonic uplift during the Aptian (Middle Cretaceous), the cause of which is still the subject of debate (opening of the Bay of Biscaye oceanic basin?). This course starts with a presentation of the mechanisms of the formation and accumulation of hydrocarbons in sedimentary basins, followed by a simple presentation of the principles currently used for modeling these mechanisms. This course is then focused on the environment of hydrocarbons in the Wessex basin. Finally, we present a simple but numerical model for the formation of the Wytch Farm oil field.

Figure 2-1 Sketch map showing extent and the principal structural features of the Paris-

London basin during the Jurassic and Early Cretaceous (modified after a BP document, 1989).

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3 Mechanisms of formation and accumulation of oil and natural gas 3.1 Kerogen, the raw material oil and gas originate from The raw material of oil and thermal(1) gas formation is kerogen, i.e. the organic matter that is solid and insoluble in organic solvents found in sedimentary rocks (B. Durand et al. 1980). The usual solvent is chloride of methylene (CH2Cl2). Kerogen is generally present in these rocks in small quantities and in a dispersed state. Figure 2.1 shows a visualization of kerogen (in black) distributions in samples of the Kimmeridge clays of the Wessex Basin containing various abundance of kerogen (as checked by Total Organic Carbon measurements TOC, see later on and bottom note (2)). Kerogen forms during deposition of the sediment from remains more or less deeply altered of dead organisms having lived in the sedimentation medium or having been transported in this medium by fluvial or marine streams. Unicellular algae, bacteria, and terrestrial higher plants, because they make up most of the living biomass are the organisms whose rests and alteration products are the main contributors to kerogen formation. The rests of algae and bacteria are called the autochthonous contribution because they lived in the sedimentation medium. The rests of higher plants, directly or via the terrestrial soils, are called allochthonous. Ancient strata may also contribute via erosion and this material is called reworked (Figure 2.2). The measurement of the quantity of organic matter contained in a sedimentary rock is done through the measurement of its Total Organic Carbon (T.O.C.). T.O.C. includes, in addition to organic carbon contained in the kerogen, the organic carbon of fluid and mobile phases (oil, gas...) still present in the rock at the moment of the analysis(2) Sedimentary rocks containing at the moment of their deposition more than 1% weight of TOC (which is one of the conditions to give them an interest for petroleum, and a possibility to be source-rock) are rare. Table 1, which is a table of frequencies, in thousands, of organic carbon and mineral carbon values in a set of more than 10 000 samples analyzed at IFP, shows that 60% of the samples have less than 1% weight of TOC. They are mostly clays and marls. Indeed it is necessary that an assembly of favorable conditions be realized in the sedimentary medium and particularly:

The existence of an abundant living biomass in the medium or close to it; A transportation of debris from the living medium on to the sediment on short distances

so as to minimize alteration and dispersion; (1) It also exists a "bacterial" gas, made solely of methane (CH4), which is produced by methanogenic bacteria

from organic substrates dissolved in water in very specific environments: marsh, fermented agricultural wastes. This gas is responsible for will -O'- the wisps in cemeteries and for marsh gas. It may also form in large quantities in recent sediments deprived of oxygen, sulfates and nitrates and in this case is responsible for large accumulations (20% of known reserves of gas according to Rice and Claypool, 1981).

(2) T.O.C. analysis is made after crushing of the rock by oxydation in an oven at a temperature around 1000C. Oxydation of the organic carbon produces CO2, which is measured thanks to a specific detector. However sedimentary rocks often also contains mineral carbon in form of calcite [CO3Ca], dolomie [CO3(Ca, Mg)], siderite [CO3Fe] and it is necessary, after crushing and before oxydation, to proceed to a "decarbonatation" by means of strong acids [HCl...]. A recent apparatus, ROCK EVAL 6, is able to measure organic carbon and mineral carbon on the same crushed sample, without decarbonatation.

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The absence of dissolved oxygen, i.e. anoxia, in water, so as to avoid the consumption of organic debris by aerobic bacteria and the return of organic carbon to the atmosphere in form of CO2.

Organic rich sediment intervals are rarely homogeneous. Due to climatic and sea level variations, abundance and nature of kerogen may vary at a small scale within the sediment, as demonstrated for instance by TOC variations in the Pliensbachian of the Wessex basin (Figure 2.3). These variations also result of course in strong variations in the petroleum potential of the Koreans, which must be evaluated to decide of the petroleum interest of the sediment. 3.2 Thermal history and nature (type) of kerogen, the main factors of oil and gas

formation After its deposition, a sedimentary rock may be buried and recovered by other sediment strata. This phenomenon is called subsidence. It may afterwards be uplifted. Therefore a sedimentary rock has a burial history (Figure 2.4) which may be complex. Burial is always accompanied by a temperature increase, and uplift by a temperature decrease. This is due to the existence in the sediment pile of a temperature gradient, the average of which is 30C/km and the variation between 15 and 50C/km approximately. This gradient is created by the dissipation of the terrestrial heat flow(3) through the sediment pile. Therefore a sedimentary rock has a thermal history (Figure 2.4) which depends on its burial history and on the history of thermal flux in the sedimentary basin where it is located (sedimentary basins are these places of the Earth Crust where sediments accumulate).

In kerogen-containing sedimentary rocks temperature increase results in a gentle cooking of the kerogen (and not a combustion because sediments do not contain oxygen). This is accompanied by the formation of liquid and gaseous products(4) and more particularly oil and "thermic" gas. The yield of this transformation, i.e. the transformation ratio in % weight of kerogen into oil and gas depends on the nature of the initial kerogen, which is called its type(5). It

(3) The sources of heat in the Earth are:

- For one half approximately the remaining part of the Heat created during Earth's formation, 4,6 billions years ago.

- For another half the heat produced by the desintegration of radionuclides, the concentration of which is the greater in the lithosphere, including sediments.

Therefore a part of the Heat flow comes from the asthenosphere, another part from the lithosphere beneath the sediments and another part from the sediments themselves. Of course the asthenospheric flow is the greater above the ascending part of the convection cells which create the lithosphere plates movements.

As a whole, the heat escaping Earth is 42 TW, i.e. the equivalent of the heat of combustion of 1.000 t of petroleum per second.

(4) These products are gaseous in standard surface conditions. In the subsurface they are most often dissolved in liquids (water, oil) or in a supercritical state with a density closer to that of a liquid than to that of a gas.

(5) Petroleum geochemists (Durand and Espitali 1973, Tissot et al 1974, Tissot and Welte 1984) classify kerogens in three main types according to the initial living organisms and associated sedimentation media: Type I is mostly formed from membranes of bacteria having fed on rests of unicellular algae living in

large evaporitic lakes and lagunas. The reference formation is that of the Green River Shales in the U.S. This type is not very common. It is for instance found in the organic-rich rocks of the Neocomian of the West African Margin, or in lacustrine rocks of intracontinental Chinese Basins. This type can produce up to 75 to 80% of its weight as hydrocarbons during burial.

Type II is very common and is mostly formed from rests of marine phyto-plancton. Corresponding rocks are clays or marls deposited in sags or epicontinental seas. Type II may contain high proportions

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also depends on temperature and time, i.e. on the thermal history of the kerogen-containing rock.

Speeds of burial and of temperature increases are very small. In the example of figure 2.4, they are respectively of 20 m and 1C per million of years at the maximum; this is a common situation.

Depths which are reached at maximum burial depths are rarely over 6-7 km and the corresponding temperature rarely exceeds 200C. Notable exceptions exist, such as in the South Caspian Basin, where maximum depths are in the order of 25 km and the corresponding temperatures are likely to exceed 400C (however nobody is able to drill and see at this depth). 3.3 Main stages of the transformation of kerogen into oil and gas: diagenesis,

catagenesis and metagenesis Three successive stages of kerogen transformation into oil and gas, also called maturation, are observed during burial (Durand and Espitali 1973, Tissot et al. 1974) (Figure 2.5). In a first stage, called diagenesis, kerogen forms mobile products, which are highly oxygenated: water, carbon dioxide, resins(6) and asphaltenes also called NSOs and no hydrocarbons. This stage is also called the immature stage. In a second stage, called catagenesis, the kerogen, which is now mature, produces hydrocarbons, first oil, and then gas. Gas is mostly produced by a thermal cracking of the oil previously formed, and a smaller part directly from the remaining kerogen. Therefore oil progressively disappears and we can observe successively an oil zone and a wet gas zone (Figure 2.5). The cracking of oil is called secondary cracking, by opposition to the primary cracking by which CO2, H2O, resins, asphaltenes, oil and gas are directly produced from the kerogen. Secondary cracking is accompanied by the formation of pyrobitumen, i.e. a carbonaceous residue which is insoluble in organic solvent as kerogen is, and is the equivalent of the coke produced in refineries by the cracking of crude oil to produce lighter fractions. For this reason, pyrobitumen is sometimes also called coke by geochemists. In a third stage called metagenesis, only methane, i.e. dry gas, is produced by the remaining kerogen.

of sulfur and it is then called type II-S. This high sulfur content results from an attack of kerogen by hydrogen sulfide (H2S) produced in the sedimentation medium by sulfate reducing bacteria. Hettangian-Sinemurian and Lower Toarcian of the Paris-London Basin, Kimmeridge Clays of the Viking Graben, Oxfordian of Saudi Arabia are Type II rich rocks. Monterey Shales of California contain a Type II-S kerogen. Type II may produce up to 50-60% of its weight as oil and gas.

Type III is mostly formed from higher plants remains. Corresponding sedimentary media are clayey sediments in deltas of streams located in areas having a dense vegetation (equatorial areas for instance). Typical examples are clays of the Niger delta and of the Mahakam delta. This type, very frequent, may produce up to 25-30% of its weight as oil and gas. It may form rocks formed almost entirely of kerogen, which are the so called humic coals.

Sometimes geochemists also define a type IV, which is made of reworked or oxydized particles of organic matter, with a very small hydrocarbon potential.

(6) Resins and Asphaltenes are high molecular weight compounds containing, in addition to carbon and hydrogen, heteroatomic elements, mainly N, S and O. For this reason, they are often called NSOs. Asphaltenes can be distinguished from resins because they are insoluble in light saturated hydrocarbons (n-heptane is taken as a standard) while resins are soluble (see Figure 2.6 for analytical procedures).

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In a very simplified manner, the conceptual model of figure 2.7 can be used to schematize these transformations. The role of kerogen type in these transformations is apparent mostly at the level of primary cracking. According to its type, the kerogen will produce by primary cracking different proportions of the main classes of products: CO2, H2O, NSOs, oil and gas, and the composition of oil will also vary. The speed of formation of these products will also vary, even if the thermal history is the same. 3.4 Notions of source-rock, maturation stages and maturity parameters A kerogen containing sedimentary rock which reached a mature stage (catagenesis, metagenesis), i.e. which has been in the right temperature and time conditions to produce oil and gas is called a source-rock. The term is often used, in an improper way, for all sedimentary rocks containing a significant proportion of kerogen (> 1%), even if they are immature. Such rocks are only potential source-rocks. Their petroleum potential can be realized only if they are buried in proper conditions. When a kerogen containing rock is at the present time at the stage of oil and gas formation it is called a mature source-rock, at the oil stage (or in the oil kitchen), wet-gas stage (in the wet-gas kitchen), or dry-gas stage (in the dry-gas kitchen) according to the situation. When the rock is in the dry-gas kitchen, it is often called overmature. It is easily understandable that a source-rock will pass successively through different stages of maturation during geological times, according to its thermal history, the reconstruction of which is therefore of paramount importance in oil and gas exploration. When maturation proceeds, a number of physical and chemical parameters will be modified in both the remaining kerogen and in the products of its degradation. Some of those can be used to calibrate the maturation stages. They are called maturity parameters, the most popular of them being vitrinite reflectance(7). 4 Accumulation Oil and/or gas are expelled from the source-rock after their formation. This is called primary migration. Then they continue on their way to a trap where they are stopped and therefore accumulate. This is called secondary migration. They eventually may leak from the trap to form other pools or seeps. This is called tertiary migration or dysmigration (Figure 2.9).

(7) Vitrinite is a constituant of type III kerogens. It is a product of the gelification, in peats and recent

sediments containing type III kerogen, of the lignocellulosic part (wood) of higer plants. When examined under a microscope, it appears generally as an amorphous grey material, sometimes as wood cells remain. Its reflectance (% reflected lilght/incident light) increases with maturation and can be taken as a mark of maturation. Typical values are between 0.2 and 2.0% (under green (546 nm) normal incident light, using a oil immersion objective, the refraction index of the oil being 1.517). Values between 0.2 and 0.6% indicate diagenesis, 0.6 to 1 the oil zone, 1 to 2.0 the wet gas zone and over 2.0 the metagenesis (Figure 2.8). Vitrinite reflectance can be safely used only on type III kerogens.

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4.1 Primary migration (expulsion) Oil and gas are expelled from source-rock by a pressure driven mechanism: pore fluids pressure is higher in source-rocks than in adjacent permeable drains and it exists a pressure gradient, which is responsible for the movement of fluids. However the mechanism is not simple, because oil and gas are mostly insoluble in pore water (except methane, ethane and benzene) and therefore constitute separate phases in the porous network of the source-rock. The sketch of figure 2.10 will help to understand how it works: The source-rock, which is clay or marl with a very low permeability (10-8 to 10-10 Darcy),

bears the load S of the sediment pile on top. S is accommodated partly by , the effective stress on the mineral matrix and for the rest by the pore fluid pressure p, according to the effective stress law:

S = + p

p is higher than in the surrounding permeable drains, because in this very low permeability medium, the fluids do not escape easily. When the rock is still immature (first stage of the sketch of Figure 2.10), no hydrocarbons have been formed and only water is expelled under the effect of the pore pressure gradient. The volume of the rock decreases slowly, and its thickness as well: this is compaction. The Darcy's law describes the speed of fluid expulsion:

( )gPKV ww

= With V speed of expulsion (m/s) K permeability (m2) w dynamic viscosity of water (Pa.s) w volumic mass of water (kg/m3) g acceleration of gravity (9,81 m/s2) P pore water pressure (N/m2)

When oil begins to form from kerogen (stage 2 of figure 2.10), it forms at first in small

quantities at the contact of kerogen particles (the volume of which decreases correlatively). Its viscosity is high.

Between the oil phase and the water phase it exists capillarity forces which result in a pressure difference between oil and water opposed to the movement of oil, called capillary pressure Pc. Pc is calculated according to the Laplace's law

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r2Pc

= being the interfacial tension between the two phases (in N/m) and r the radius of pore throat (in m). r being very small in source-rocks (a few m) oil cannot move and water continues to be expelled.

Water being expelled and oil and gas going on to be formed, continuous network of oil

and/or gas are created along kerogen layers. In these networks there are no more capillary forces and oil and/or gas begin to be expelled (stage 3 of figure 2.10).

Therefore oil and gas can be expelled only when a certain threshold of saturation of the porous network of the source-rock by oil and/or gas (i.e. a certain proportion of the porosity is occupied by hydrocarbons, the rest being occupied by water) is reached and this can be well beyond the threshold of oil and/or gas formation.

The overall mechanism will therefore be described (Ungerer et al. 1993) by the so-called multiphase Darcy's law, in the same way as reservoir engineers do to describe the flow of oil and/or gas during production.

( )gPKkrV ooo

oo =

r

( )gPKkrV www

ww =

r

r2PPP woc

== With Vo, Vw speeds of expulsion of oil, of water (m/1) K intrinsic permeability (m2) kro, krw relative permeabilities to oil, to water (ratios, no dimension) o, w dynamic viscosities of oil, of water (Pa.s) o, w volumic masses of oil, of water (kg/m3) g acceleration of gravity (9,81 m/s2) Po, Pw pore pressure of oil, of water (N/m2) Pc capillary pressure (N/m2) This law is pragmatic and parameters like kro and krw and Pc have to be adjusted to observed facts (or experimentally in reservoir engineering). An additional mechanism may be a microcracking of the rock if the pore pressure exceeds the mechanical resistance of the rock.

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Expulsion of water and hydrocarbons results in a pore volume decrease as a function of depth as on figure 2.10 and therefore in a total volume and height reductions. 4.2 Secondary migration Once expelled from the source-rock, hydrocarbons, which constitutes phases separated from the water phase because of their insolubility move in drains under the effect of buoyancy which exerts a pressure proportional to the height of the hydrocarbon phase and to the difference of volumic mass between the phases

B = (o - w) gh h being the height of the hydrocarbon phase. The opposing pressure is the capillary pressure as explained above, which is all the stronger, as the mineral grain size of the drain is the smaller. Pools are formed when hydrocarbons are stopped by a permeability barrier (cap rock, faults which put an impermeable layer in front of the drain, strong decrease in grain size of the drainage layer...). 4.3 Tertiary migration (dysmigration) Once constituted, the pool may leak and the contained hydrocarbons may supply another pool or seepage. Different mechanisms may be responsible for this: Once the trap is filled up to the closure, the excess may pass from under the closure on

to another structure (Gussow effect). The pressure created by buoyancy at the top of the structure may exceed the capillary

pressure (entrance pressure) in the cap rock at places where grain size is the coarser, or create or reactivate cracks.

Therefore a pool is a provisional storage which exists as long as hydrocarbon supplies exceed hydrocarbon leaks.

- 34 -

Figure 4-1 Kerogen distribution at the microscopic scale in rock samples of the

Kimmeridgian of the Dorset. Kerogen, in black, is seen with an electron microscope, thanks to the "back-scattered electrons" technique. Large differences are observed in kerogen concentrations (as measured by TOC) and in kerogen distribution patterns (after S. Belin, 1992).

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Figure 4-2 Sketch showing the different types of influxes of organic matter debris during

the formation of sediment in a sedimentary basin.

Figure 4-3 Variations of organic and mineral carbon content in a Pliensbachian organic rich interval (Belemnite Marls) of the Dorset (after Van Buchem et al. 1995).

- 36 -

ORGANIC CARBON (weight %)

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Total

1 82 52 38 21 15 7 10 8 4 4 241

2 44 29 18 17 12 9 5 6 2 2 144

3 29 19 10 8 5 3 1 1 1 2 79

4 24 22 13 6 3 3 1 1 1 2 76

5 24 26 10 5 2 2 2 2 1 1 75

6 24 16 7 4 3 1 2 1 1 1 60

7 19 10 5 3 2 2 2 1 1 1 46

8 25 14 5 3 4 2 0 1 0 1 55

9 23 8 3 2 1 1 1 0 0 0 39

10 22 6 3 2 1 0 0 0 0 0 34

11 29 7 2 0 0 0 0 0 0 0 38

13 39 6 0 0 0 0 0 0 0 0 45

Total 384 215 114 71 48 30 24 21 11 14 932

MINERAL CARBON (weight %) Table 1: Frequencies (in ) of rock samples according to their organic carbon and mineral

carbon contents, in a set of more than 10 000 samples analyzed at IFP.

- 37 -

Figure 4-4 Burial history (a) and thermal history (b) of the Hettangian of the central part of the Paris Basin (after J. Burrus et al., 1997).

- 38 -

Figure 4-5 Main stages of formation of mobile products from kerogen during burial.

Biomarkers are these molecules found in mobile products, the chemical structure of which demonstrates that they are derived from bio-molecules.

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Figure 4-6 Sketch of a classical analytical procedure used in petroleum geochemistry.

Resins and Asphaltenes are fractions of the initial oils. Asphaltenes are separated by adding an excess of n-heptane, which causes their precipitation as solid particles. Resins are separated from the remaining soluble part (maltene) by liquid chromatography.

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Figure 4-7 Conceptual model of the formation of oil and gas under the effect of

temperature increase during burial of a source-rock.

Figure 4-8 Profiles of average vitrinite reflectance versus depth in four areas where the

kerogen is derived from land-plants (type III kerogen). Note the variety of profiles, due to very different thermal regimes. The values of vitrinite reflectance, which are typical of the main stages of oil and gas formation, are indicated

- 41 -

Figure 4-9 Schematic scheme of oil and gas formation in a sedimentary basin during its

geological evolution.

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Figure 4-10 Conceptual model of primary migration as a diphasic flow (after Ungerer et al.,

1984, 1990).

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5 Numerical modeling of formation and accumulation of oil and gas 5.1 Principles for a modeling 5.1.1 Simulation of the kinetics of formation of oil and gas One refers to the conceptual model of figure 2.7. This model simplifies considerably the reality, where billions of reactions occur, but it is impossible to describe the reality in the detail. Following the work of the petroleum geochemistry group of IFP (B. Tissot and colleagues), most models now in use in the industry use to describe the primary and the secondary cracking a set of first order parallel kinetic equations. Parallel means that they do not interfere with each other. First order means that the speed of formation of a class of product is proportional to the concentration of the parent class of product remaining in the kerogen, i.e. for reaction i

iii Ck

dtdC =

The basis of this choice is by comparison with the disintegration of parent radionuclides into daughter elements, where speed of disintegration is proportional to the number of remaining parent atoms, the proportionality constant being a characteristic of the parent atom:

ndtdn =

Here these are chemical bonds such as C-C, C-H, C=O, O-H which are considered, ki being a characteristic of the class n i of these various chemical bonds. However, contrarily to of radionuclides, ki is in case of chemical bonds strongly dependent on temperature, and is described by the Arrhenius's formalism:

RTE

iii

eAk = With Ai frequency factor (s-1) Ei activation energy (joules/mole) R constant of perfect gases ( 2) T = To + gz

(K) (T (K) = T (C) + 273,15)

With T temperature (K), To soil temperature (K), g geothermal gradient (K/m) and z depth (m). Ai and Ei are characteristic of reaction (class of bonds) i. Primary cracking is described by 10 to 20 reactions relating to the various types of bonds in the kerogen (Figure 3.1). The distribution of the initial concentrations of these bonds (Cio) in a kerogen characterizes the nature (the type) of the kerogen. In other words, these distributions are different according to the different types (Figure 3.2).

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Secondary cracking is described by 1 to 10 reactions according to the different models (Figure 3.1). These reactions relate to the cracking of chemical bonds in oil, where the variety of these bonds is lesser than in a kerogen. 5.1.2 Simulation of burial history: the backstripping method " The burial or uplift of a sedimentary rock in a basin is due to the addition of several causes: Convection in the Asthenosphere which results in extension or compression of the

Lithosphere. As a whole, extension generally results in a subsidence, while compression generally results in uplift.

Another cause for uplift is the warming up of the lithosphere by an ascending part of a convection cell in the Asthenosphere. This creates a doming by dilatation of the lithosphere.

The piling up of sediments imposes an additional load to the substratum and therefore an

additional subsidence so as to reach isostatic equilibrium with the Asthenosphere. Uplift results in an erosion and therefore an unloading of the substratum and therefore an additional uplift.

The load on a sediment layer caused by a piling up of successive layers results in a

compaction and therefore a height reduction and an additional subsidence. The total subsidence is the sum of the subsidence of the substratum (sometimes referred to as the "tectonic subsidence") and of this additional subsidence created by compaction.

All these phenomena are taken into account to reconstruct the burial history of a sediment layer and for that the so-called backstripping method is used: to begin with, the sediment pile is divided, with help of available data (well logging,

sedimentological analysis, stratigraphy) in discrete layers to which are attributed a lithology (chosen on a "catalog"): clay, silt, marl, sandstone ...) and ages of boundaries;

to each lithology corresponds (pragmatically) a curve (z) describing the variation of its

porosity and therefore of its height, with depth (Figure 2.13); if 1 is the number of the most recently deposited layer, 2 the number of the layer deposited

just before, and so on..., layer 1 is taken away and its initial height is restored thanks to the corresponding curve (z) and the repercussion of the unloading (decompaction + isostasy) on all the other layers is calculated. Then layer 2 is taken away, then layer 3, etc. This is done thanks to a computer program. In such a work, it is necessary to have an evaluation of the water thickness at the age of deposition of each layer, because the water slice has a weight, which acts also as a load. Thicknesses of eroded layers have also to be evaluated, for the same reason, and this is often rather uncertain.

Finally it becomes possible to restore the burial history of each layer as on Figure 2.14.

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5.1.3 Simulation of thermal history

The temperature increase with depth zT

, i.e. the geothermal gradient, is due to the dissipation

of terrestrial heat flow through the sediment pile. This pile may also contain heat sources, such as radionuclides, or can be warmed or cooled by convection, i.e. transportation of heat by movements of matter: uplifts of salt diapirs, volcanic intrusions, warm waters circulation (hydrothermalism) or of cold waters (meteoric water).

In the absence of convection, heat is dissipated by conduction, and the geothermal gradient is linked to the heat flow by the so-called Fourier's law:

zTF

= F (W/m2) being the heat flow and (W/m/K) a constant characteristic of a sediment layer, which is called its thermal conductivity. In sedimentary basins, F ranges approximately from 50 to 100 mW/m2 (i.e. the power of an electric lamp for a surface of 1000 m2); F is often expressed in Heat Flow Unit (HFU). 1 HFU = 41,8 mW/m2 and therefore F varies from 1 to 2 HFU approximately. In case of a heat transfer by convection, which is added to conduction, the heat transfer is described by a more general equation(8). In simple models, convection is not taken into account but immobile heat sources (normally due to radioactivity) are considered: to each lithology is attributed a thermal conductivity (table 2) a heat capacity and a heat production related to the radionuclides content. Because thermal conductivity depends on the water content, i.e. on porosity, thermal conductivity is recalculated(9)

(8) For those who want to know more, the general equation to describe heat transfer in sediments (conduction

+ convection) is:

( C) Tt = C

r V s + w Cwr u ( ) Tt x

z +

Tz

+ A (Ungerer et al 1990)

with: u: water filtration velocity with respect to solid matrix (m/s) Vs: burial velocity (m/s) , w: volumic mass of sediment, of water (kg/m3) : thermal conductivity of bulk sediments (W/m/K) T: temperature (K) t: time (s) z: depth (m) A: heat generation in the sediment (W/m3) C, Cw: heat capacity (J/kg/K) of the rock matrix, of water. (9) A formula which can be used for the dependency of thermal conductivity on water content is:

= sws

with w, s thermal conductivity of water and solid matrix and porosity, and for the temperature dependency:

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at each depth according to the corresponding porosity. Thermal conductivity also depends on temperature(9). Heat capacity depends on the water content, i.e. porosity(10). Then it is possible to calculate the temperature of each sediment layer as a function of its depth and, by using the burial history, to construct a temperature-time trajectory for these layers and more particularly for kerogen-rich layers, as on Figure 2.4. For this, it is necessary to have data such as present day heat flows and temperatures, history of heat flow, soil temperature..., or default data are used (see Figure 3.5). Finally it is possible to simulate oil and gas formation thanks to a kinetic model, or default data are used. Table 2 shows the list of data, which are necessary. Very often many of these data are not available and hypotheses must be made. Therefore it is of paramount importance to control the results by their consequences. Paleothermic indicators and among them, particularly maturity parameters are often used for that: reflectance of vitrinite, fission tracks lengths in apatites, composition of fluid inclusions found in some minerals are among the most popular for such a work. 5.1.4 Simulation of migration As already presented, the physical laws which are used to describe the different steps of migration are those used by reservoir engineers to describe fluid flow during production. Using these equations can make simulation of migration, but it is a very difficult exercise because it is necessary to know the porosities and permeabilities of the sedimentary layers in any point of the concerned part of the basin and throughout the whole geological history. This may seem impossible to do. However 3D basin simulators are now able to treat this problem (Schneider ET al., 1997). In simple models, it is assumed that hydrocarbons are expelled as soon as a saturation threshold is reached. The method is the following: A variation of porosity as a function of depth is attributed to the source-rock as for other

sedimentary layers. The formation and the composition of the hydrocarbons formed in the source-rock are

calculated by a kinetic model, which gives them as a function of thermal history. The corresponding volume of hydrocarbons is calculated by estimation of their density at the considered depths and pressures.

= o1

1 + (T To

being a temperature dependency coefficient, To the soil temperature and o the value of at To. (10) Heat capacity c dependency on water content is calculated according to the following formula:

c = w cw + s cs (1 ) , w,s: volumic mass of the bulk rock, of water, of the solid matrix (kg/m3) c, cw, cs: specific heat of the bulk rock (Joule/K) : porosity (%)

- 47 -

As soon as a given saturation threshold is reached the excess volume of fluids (water + hydrocarbons) with respect to the available porosity at this depth is expelled. The composition of expelled hydrocarbons is assumed to be the same outside and inside the rock.

The remaining hydrocarbons are submitted to further thermal history and subsequent

cracking. Such a model is said to be 1D. It permits to describe the history of expulsion and to produce various diagrams (graphs of formation and expulsion of hydrocarbons as a function of time or depth, maps or cross-sections realized at different geological times using the results of simulations made at different locations on the basin (fictive wells, etc.).