sedimentology and depo model oficina fm petrocedeno jmpg_2012 martinius etal

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Marine and Petroleum Geology 35 (2012) 354e380

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Marine and Petroleum Geology

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Sedimentology and depositional model for the Early Miocene Oficina Formationin the Petrocedeño Field (Orinoco heavy-oil belt, Venezuela)

A.W. Martinius a,b,*,1, J. Hegner a,c,2, I. Kaas a,d, C. Bejarano a,2, X. Mathieu a,c,3, R. Mjøs a,d

a Petrocedeño, Sector Venecia, CBP Edificio C e D Centro Bahia de Pozuelos PDVSA, Barcelona 6028, Venezuelab Statoil R&D, Arkitekt Ebbellsvei 10, N-7005 Trondheim, Norwayc Total Oil & Gas Venezuela, Nuevos Proyectos Faja e Geoscience, Caracas, Venezuelad Statoil ASA, Forushagen, N-4035 Stavanger, Norway

a r t i c l e i n f o

Article history:Received 28 October 2010Received in revised form27 January 2012Accepted 3 February 2012Available online 17 February 2012

Keywords:Oficina formationOrinoco heavy-oil beltPetrocedeñoInclined heterolithic stratificationFluid mudTidal deltaEarly Miocene sea-level curve

* Corresponding author. Statoil R&D, Arkitekt EbbeNorway. Tel.: þ47 97087473.

E-mail address: [email protected] (A.W. Martiniu1 Present address: Statoil Canada Ltd., 3600 e 308

0H7 Canada.2 Present address: CVP Oriente, Edificio PDVSA,

Estado Anzoategui, Venezuela.3 Present address: Total E&P, OML99 e GSR, Port-H

0264-8172/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.marpetgeo.2012.02.013

a b s t r a c t

The Early Miocene Oficina Formation in the Petrocedeño Field (Venezuela) produces extra-heavy oil withan initial gravity of 8.5� API. Reservoir zonation is successfully based on a 3rd-order sequence strati-graphic framework for which a detailed sedimentological understanding is crucial. This paper presents a,primarily core-based, detailed account of the sedimentology and sequence stratigraphy of the OficinaFormation in the Junín (formerly Zuata) Region of the Orinoco heavy-oil belt. In Petrocedeño, the OficinaFormation is typified by the long-term (approximately 7 Ma; 2nd-order sequence) change of a fluvialdominated deltaic system to a tide-dominated lower delta plain and subsequently a tide-dominated,subtidal, upper delta platform. A fluvial braidplain environment with mostly sand-dominated braidedand sinuous rivers (Sequences 1 and 2) changed into a fluvially-dominated but notably tidally-influenceddelta plain with straight and sinuous channel belts (Sequences 3e6). Subsequently, a mixed-energy(fluvial and tidal) delta front environment with numerous distributaries formed. This delta frontdrowned and changed into an estuary system with bay-head delta mouth bars (Sequence 7). Maximumtransgression was reached in Sequence 8 with the development of fully marine shale. Subsequently,a low-gradient tide-dominated lower delta plain with distributaries and meandering channel beltsestablished (Sequences 9 and 10). These were gradually transgressed to develop into proximal deltaplatform environments with a limited number of distal distributaries and large protected subtidal areas(Sequence 11). Together, the lower delta plain and upper delta platform facies form one of the fewcurrently known subsurface examples of a tide-dominated delta. The change in depositional style wascontrolled by a long-term increase of accommodation space in the developing Eastern Venezuelan Basin.The lower part of the formation was primarily controlled by compressional tectonic activity (third-orderSequences 1 through 6). The upper half, however, was primarily controlled by Early Miocene eustaticchanges (Sequences 7 through 11).

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1. Introduction

Petrocedeño (formerly Sincor; a joint venture company ofPDVSA, TOTAL and Statoil) is an oil company operating in the Junín(formerly Zuata) Region of the Orinoco heavy-oil belt in Venezuela

llsvei 10, N-7005 Trondheim,

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which is oriented parallel to the present-day Orinoco River. TheOrinoco heavy-oil belt is 460 km long and between 40 and 80 kmwide, covering a surface area of 55,314 km2. The Petrocedeñolicense is part of the Orinoco heavy-oil belt which is subdivided intofour main areas sharing a similar stratigraphic and sedimentaryhistory (Fig. 1). Petrocedeño began producing extra-heavy oil inDecember 2000. Initial gravity of 8.5� API (oil) is elevated bya delayed cooker to the 32�API of sweet synthetic crude (“ZuataSweet”; associated viscosity of 2000e7000 cP).

Petrocedeño is located at the southern margin of the oil-prolificEastern Venezuelan Basin. Oil trapping is primarily of stratigraphicnature in a thick Eocene to lower Middle Miocene sedimentarywedge onlapping from north to south onto Cretaceous rocks and/orcrystalline basement of the Precambrian Guiana Shield. Upper

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Figure 1. Location map of Venezuela with the Orinoco heavy-oil belt and the Petrocedeño Field.

A.W. Martinius et al. / Marine and Petroleum Geology 35 (2012) 354e380 355

Miocene marine mudstones provide the top seal. Oil is trapped inthe lower part of the Early to Middle Miocene Oficina Formation atshallow depths of 230e475 m with an oil column of 240 m atPetrocedeño. The reservoir is formed by unconsolidated fine- toporous coarse-grained quartz-rich sands.

From the beginning of the Petrocedeño development phase, thecombination of detailed sedimentological and stratigraphic anal-ysis, three-dimensional multi-scale reservoir architecture analysisand development of multi-scale heterogeneity models has beencrucial for oil production (Svanes et al., 2004; Labourdette et al.,2008; Devaux et al., 2009; Martinius et al., 2012). To achieve thisin the best possible way a detailed sedimentological and strati-graphic conceptual understanding is required. The conceptualunderstanding and the geomodels are necessary for evaluatingongoing cold-production schemes and for designing flexible dril-ling scenarios (Ramírez et al., 2004). They also serve to evaluaterisks and performances associated with advanced extractionschemes, such as enhanced oil recovery steam injection. In thispaper, a detailed sedimentological analysis of the Oficina Formation

SDZ-21X

SDZ-185SDZ-184

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Petrocedeño field outline

450

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VX05E

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A

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Figure 2. Location map of cored wells with well names. Each core covers only (a) part(s) oadditional cores were obtained. O ¼ observation well; PV ¼ vertical production well.

in Petrocedeño in a sequence stratigraphic context is providedincluding a significant amount of previously unreported data andinterpretations. In an accompanying paper reservoir characteriza-tion and production issues are highlighted in a 3rd -order sequencestratigraphic framework (Martinius et al., 2012).

2. Data and workflow

The large dataset used for this study includes conventionaland sidewall cores, wire-line data from vertical and slanted wells,MWD logs from horizontal wells and two- and three-dimensionalseismic data. Seismic data on the Petrocedeño field includestwo-dimensional lines (a few hundred kilometres in total) anda three-dimensional seismic survey (390 km2 full fold). Cores wereobtained from 11 existing PDVSA (formerly Maraven) wells drilledin the wider Petrocedeño area in the 1980s, and from nine partiallycored vertical observation wells drilled during the development ofPetrocedeño (Fig. 2A). These were supplemented with two addi-tional cores from the development area taken during a later phase

SDZ-86X

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0m

N

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f the stratigraphy. Inset indicates the area of the enhanced oil recovery project where

Palynozone Chronostratigraphy

Pliocene

Miocene

Jandufouria

seam

rogiform

is

Verruttric

olp

orite rotundip

oris

LateCretaceous

Tigre Fm.Canoa Fm.Carrizal Fm.

Hato Viejo Fm.Cambrian

Pilón MemberUnit III

Unit II

Unit I Lower

Upper

Middle

A 1

Ref: Key, 1977 Ref: Isea, 1987, after

Latreille et al., 1983

Ref: Petrocedeño, 1987 (after

Maraven); Gaddy, 1988

SEIS 8 = ‘M8’SEIS 1 = ‘M1’

SEIS 9 = ‘M9.3’

SEIS 12 = ‘M12’

SEIS 14 = ‘M14’

SEIS 15 = ‘Top Fluvial’

SEIS 20 = ‘Base Fluvial’

‘Top E’

Top oil column‘M9’

1

1

1

1

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2

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UpperLower

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UpperMiddle2

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Jobo Member

Yabo Member

Morichal MemberBurd

igal

ian

Aqui

tani

an

Ofic

ina

Form

atio

n

Freites Fm.

Late

Middle Ma

Early

Carribean Atlantic

Lithostratigraphy Reservoir Zonation Main markers

Figure 3. Stratigraphic and reservoir zonation scheme of the Oficina Fm in Petrocedeño as well as the main seismic and well log markers. Note that M0, M1, M9 and M20 are theonly truly reliable reflectors (caused by thick in-situ coal layers); the continuity of the other seismic reflectors carries a significant degree of uncertainty. The first 3 reflectors arevery continuous and associated with coal layers; the last is associated with the base fluvial unconformity. aAbsolute stage ages after Lourens et al. (2004).

A.W. Martinius et al. / Marine and Petroleum Geology 35 (2012) 354e380356

and 10 cored wells in the southwest corner of the field were a totalof 550 m of core was acquired as part of a hot-production pilotproject (Fig. 2B). Core recovery and quality, however, varied widely.Lithofacies analysis from core was integrated with biostratigraphic,ichnofacies and well log analyses to define a coherent facies clas-sification scheme and, subsequently, a facies association scheme.Candidate stratigraphic surfaces identified from core and/or welllogs were tied to seismic horizons and mapped for the entire fieldarea.

Figure 4. SW to NE seismic line through a section of the Petrocedeño Field (6.5 km long) shobase and top of the fluvial section. In addition, a representation of the reservoir architectu

3. Geological setting

The Orinoco heavy-oil belt (Fig. 1) is interpreted as part of a fore-bulge that was generally unaffected by compression during theoblique collision of the Caribbean and South American plates duringthe Oligocene (Di Croce, 1995; Pindell et al., 1998; Rodríguez, 1999;Bejarano, 2006). In the Late Oligocene sedimentation in theeastewesteoriented Eastern Venezuelan Basin along the forelandbulgeof thebasin’s south sidestartedwith thickbasal sandsoffluvial-

wing basement, the early Miocene cover, the main seismic markers and the interpretedre is illustrated.

Channel fill

Mouth bar

Mudstone

Crevasse

Point bar

A1

Thick-ness

Depo-sitionalenviro-ment

FaciesGrasso

ciationResistivity

Reservoirzone

Depth Reservoir architectureNS

Fm.

B2

MFS

Coal Unconformity

C1

C2

D3

E1

E2Base Oficina

Cretaceous/Cambrian

RHOB/NPHI

-1600 ft

-1300 ft

-800 ft

500

ft

OFI

CIN

A

CIATLED

LAIVULF

300

ft

D1D2

B2B1

Figure 5. Schematic NeS stratigraphic cross section of the Oficina Fm in the Petrocedeño Field with typical well log responses and reservoir zonation indicated. Total thickness is245 m (800 ft).


deltaic origin. During the EarlyMiocene in the Junín area, a number ofriver systems delivered sediment north-westerly into the basin fromthe Guiana Shield (Fiorillo, 1984; Audemard et al., 1985; Di Croce,1995). The foreland basin was closed towards the west, but to theeast it was open and partly so to the north through an opening in theCoastal Ranges. A shallowcarbonate-dominatedmarine environmentexisted north of the Coastal Ranges and progressively deeper marineclastic depositional environments occurred to the east (Di Croce,1995; Yoris and Ostos, 1997). Consequently, the foreland basinappears to have been relatively protected from large waves andinfluenced instead mainly by tidal currents supposedly rotatingthrough the shallow, semi-enclosed Early Miocene sea.

The geodynamic evolution of the Eastern Venezuelan Basinterminated with an oblique collision phase in the Neogene andQuaternary (Eva et al., 1989; Parnaud et al., 1995; Pindell et al.,1998). During this last phase, a foreland basin developed on theLate Mesozoic passive margin of the South American craton asa result of the oblique collision between the Caribbean and SouthAmerican plates. From the Late Oligocene to earliest Miocene, thecollision migrated progressively eastwards, resulting in a triparti-tioning of the basin: a platform zone in the south that includes thePetrocedeño area, a foredeep in the central and easternparts, and anoverthrust area in the north (Macellari, 1995; Parnaud et al., 1995;Berge et al., 1998; Summa et al., 2003). The oblique collision can becharacterized as an episodic process that started 28Ma ago andwasfollowed by three main tectonic events at approximately 22 Ma, 14Ma and 7 Ma (Di Croce, 1995; Rodriquez and Oswaldo, 1996;Rodríguez,1999). The latter two events are interpreted to have beenformed due to regional isostatic rebound as a result of relaxation oftranspressive stresses during times of relatively low convergencealong the plate boundary (Summa et al., 2003).

Along a south-to-north transect, the Early Miocene OficinaFormation ranges from braided river deposits around the southernperipheral bulge to lower delta plain and delta front (shoreface)

deposits in the area around the town of Anaco (Fig. 1). Fromthis location eastwards, the formation deepens gradually intotime-equivalent open marine shales and turbidites in the foredeep,that also extends south of Trinidad and Tobago (Stainforth, 1971;Audemard et al., 1985; Erlich and Barrett, 1992; Bejarano, 2006). Theforedeep remained underfilled during the Early Miocene (Parnaudet al., 1995) and is mostly located above a large lithospheric-scalegravity anomaly reflecting the tectonic depression of the conti-nental crust formed due to the subduction of South America underthe Caribbean in north-eastern Venezuela (Summa et al., 2003).

Generally, the dominant transport direction at the Petrocedeñofield was from southwest to northeast (present-day coordinates),with themain sediment supply from the Precambrian Guiana Shield,located to the south. In the northern part of the Petrocedeño fieldadditional sediment was supplied from the Guárico fold-and-thrustbelt and from the Serranía del Interior (Rivero and Scherer, 1996;Rodriquez and Oswaldo, 1996; Pindell et al., 2009; his domain 4).

4. Lithostratigraphy and reservoir zonation

The Early Miocene Oficina Formation unconformably overliesLate Cretaceous rocks of the Tremblador Group (Tigre and Canoafms; Figs. 3 and 4). In the western part of Petrocedeño, basementrocks are unconformably overlain directly byMiocene rocks (Fig. 4).The pre-Paleogene unconformity formed paleotopographicdepressions that were filled by fluvial and fluvial-deltaic depositsof the Oficina Formation. In Petrocedeño, the productive LowerOficina Member is informally divided in lower “fluvial” and upper“deltaic” parts (Figs. 4 and 5). Approximately 65% of the oil inPetrocedeño is produced from the fluvial part of the succession andthe remaining 35% from the deltaic part reflecting reservoir quality.The fluvial and deltaic parts are further divided into three reservoirzones (F through D and C through A respectively, from the baseupwards; Figs. 3e5), and most of these zones are further divided

1200

Core

M1

M1.6

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M9M9.3

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M9.4

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M12.2

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Top E2

Base_Fluvial

M1

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RDEEP1 10000

GR0 200

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MD(ft)

MD(ft)Core RDEEP

1 10000GR

0 200 Core RDEEP1 10000

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RX055 [SSTVD] SDZ-86X [SSTVD] VD00 [SSTVD] WA00 [SSTVD]

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Figure 6. NeS oriented well section (dip-line) of the Petrocedeño Field showing reservoir succession and transition from a predominantly lower fluvial succession to the upperdeltaic succession. The main seismic markers are indicated on Figure 4. This 2D correlation line is based on 3D information obtained from all wells and the 3D seismic.


in two or three mappable reservoir units (for example, E2 and E1, orD3 and D2/D1 from base up; Fig. 3). This subdivision was estab-lished early in the field-appraisal phase and in part based onpre-existing and general stratigraphic subdivisions (Key, 1977; Isea,1987). Detailed correlationwork showed that units C2, C1 and B2 inthe deltaic part of the succession can be further subdivided intosubunits that can be mapped at field scale (Pourtoy et al., 2009,Figs. 3 and 6).

5. Biostratigraphy and paleoclimate

The current biostratigraphic zonation of the Oficina Formationin Petrocedeño is based on 76 selected core samples from 15 wells.The biostratigraphy study includes palynology and calcareousnanoplankton analysis aimed at resolving the chronostratigraphyand general paleoenvironment. Palynomorphs in analyzedsamples were well preserved but deficient in diversity andnumbers; no nanoplankton was found. The deposits can beassigned to the palynological subzones T1 and T2, corresponding

to the lowermost Miocene (Aquitanien) and Upper MiddleMiocene (Langhien). The main chronostratigraphic indicators (cf.Lorente, 1986; Mata-García, 2009; Gonzalez-Guzman, 2000, 2001)include: 1) abundant and co-occurring Verritricolporites rotundi-porus, Scabratricolportes planetensis, Retricolporites irregularis andJandufouria seamrogiformis; 2) frequent occurrence of other mor-phospecies such as Cyclusphaera euribei, Bombacacidites zuatensis,Crototricolpites annemariae and Psilatricolporites pachydermatus.

The fluvial lower half of the productive part of the succession isdominated by marsh forests and riverine swamp environments(Carbón, 2000; citing internal PDVSA reports; Gonzalez-Guzman,2000, 2001). An example is the occurrence of common Mauritiaflexuosa, a very large, palmate palm also of recent times growing toabout 25m tall, andwith petioles (the stalk of a leaf) up to 6m long. Itis currently found in north-eastern South America, around thewestern Amazon and Orinoco Rivers. In general, the palynofloraindicates a tropical or subtropical climate during deposition (Blascoet al., 1996; Woodroffe, 1990) in line with the abundance ofkaolinite, particularly in the lower fluvial half.

Figure 7. AeC: Description and illustration of the lithofacies of the Oficina Fm recognized in Petrocedeño.


Figure 7. (continued).




The deltaic upper half (zones C and B) is typified by salinephytoplankton (dinoflagellate cysts abundantly occurring inrestricted intervals); a commonly occurring Miliammina-Eggerella(M. telemaquensis) benthic foraminifera assemblage in laminatedmudstones and mudstone; and siltstone-dominated heterolithicsin which Trochammina (T. pacifica) is present, as well as Ammonia

Figure 8. Basal part of Unit E1 in well PF4-00O illustrating a poorly to moderately sorted band overlain by a kaolinitic paleosol. Note the lighter coloured silty mudstone layers, and thMuestra preservada ¼ preserved sample. For legend see B.

tepida and Ammobaculites exiguus (Keij and Nijssen, 1986; Carbón,2000; citing internal PDVSA reports; Gonzalez-Guzman, 2000,2001). This faunal composition suggests generally brackish waterconditions (5e10& salinity) in (high) marsh zones, sandy mud flatsand shallow bay environments around mean high-water level(cf. Scott et al., 1996). Extensive mangrove forests were present

raided channel (channel fill and braid bar(s)) with occasional quartz pebbles underlaine channel levee to floodplain facies at the top (1763 ft [537.4 m] to 1765.8 ft [538.2 m]).



(Gonzalez-Guzman, 2000, 2001) which contained varying degreesof phytoplankton from near absence to abundance. In addition,mangrove-swamp environments are found in the deltaic upperpart of the Lower Oficina Member, as indicated by the mangrovepalynomorphs, including Zonocostites ramonae and Psilatricolpor-ites crassus (Gonzalez-Guzman, 2000, 2001).

Figure 9. Basal part of a braided channel (fill and braid bar(s)) underlain by a kaolinitic paorganic-rich rooted zone transitionally overlain by a light brown coloured and rooted zonebase, large extra-formational rip-up clasts (<3 cm) and granule-size quartz clasts occur. Infrom fine sand to granules. The upper half is poorly to moderately sorted. Muestra preservacolour in this figure legend, the reader is referred to the web version of this article.)

6. Lithofacies and facies associations

Eight main facies associations covering the Lower Oficina Mbr atPetrocedeño have been defined based on specific stacking patternsof 15 lithofacies recognized from core. Lithofacies characteristicsare summarized and illustrated in Figure 7.

leosol (base Unit D3, well PB5-00O); note the 3 horizons in the paleosol (dark brownwhich is transitionally overlain by the bleached light grey kaolinite-rich zone). At thethe middle of the section, the sandstone is very poorly sorted with grain sizes rangingda ¼ preserved sample. For legend see Fig. 8B. (For interpretation of the references to


6.1. Fluvial-dominated braided channels

6.1.1. DescriptionIndividual and amalgamated coarse-grained sandstone

bodies predominate stratigraphic units F and E (all zones), and thelower parts of zones D3 and D2/D1. These sandstone bodies aretypified by a wide range of grain sizes, poor grain-size sorting,highly erosive nature of bed sets, and a general upward fininggrain-size distribution (Figs. 8 and 9). They have a highly erosivebase commonly overlain by an up to 10 cm thick pebble layer(lithofacies 13, conglomeratic gravel; Fig. 7). Fully preserved andwell-developed sandstone bodies typically display an upwardsuccession from lithofacies 12 (gravel-rich, angular to subangular,and clast-bearing trough cross-stratified sandstone; Fig. 7) andFigure 11 (trough cross-stratified sandstone; Fig. 7) at the baseto lithofacies 10 (planar cross-stratified sandstone; Fig. 7) tolithofacies 9 (ripple-laminated sandstone; Fig. 7) and topped bylithofacies 3. Zone F is typified by the occurrence of lithofacies 13(with intra-formational kaolinitic mudstone clasts as well as extra-formational quartz) and 12, more commonly at the base but alsodispersed throughout the sandstone bodies. The upward finingsandstone units are very poorly sorted at the base and moderatelysorted at the top. Planar and trough cross-stratified sets arecommon. Sets vary widely in preserved thickness, ranging from0.5 to 4 m. Infrequently, sets of thin mud layers with kaoliniticoverprint alternating with thin sandstone layers are observedintercalated in homogenous sand packages (lithofacies 3, butcoarse-grained and poorly sorted; Figs. 7e9). These sets are widelyspaced with highly variable thickness of homogeneous sandstonepackages in between. Thin coal layers with rootlets penetrating intothe underlying sand occur infrequently at the top of upward finingunits. Bioturbation is absent. The estimated N/G for stratigraphic

Figure 10. Distributary channel fill in Subunit C2Middle of well PF2-00E formed by dominanand medium-grained) with widely spaced double mud layers (dark grey line) and intervals wfor explanation. The gradual transition from the kaolinitic light grey coloured paleosol at thebetween Subunit C2Lower and C2Middle and the drowning of the exposed paleosurface. Theand marks the boundary between Subunit C2Middle and C2Upper. Muestra preservada ¼ prthis figure legend, the reader is referred to the web version of this article.)

zones F and E is close to 0.8, that of zone D close to 0.7. The presenceof kaolinite is decreasing upward in the succession.

6.1.2. InterpretationThe braided channel fill deposits are interpreted to have

been formed by deposition in gravel-rich but sand-dominatedfluvial rivers as channel fill deposits and mid-channel and/orbank-attached braid bars in a highly erosive depositional settingrelatively close to source area(s) (approximately 250 km). However,a full succession is rarely preserved which is inferred to have beencaused by the instability of channels in the channel belt (cf.McLaurin and Steel, 2007) or that the preserved deposits are largelythose of braid bars. Sets with mud drapes are interpreted as toesets of larger tidally-influenced fluvial dunes migrating in thebraided channel. They suggest tidal influence on fluvial flow,possibly by flow retardation during incoming flood and accelera-tion during ebb. Consequently, it is envisaged that the braidedfluvial channels were located relatively close to the paleoshore linesimilar to a braidplain. Mudstone pebbles and granules are inter-preted to have been derived from floodplain erosion as rip-up clastsduring channel incision. All mudstone present in the channel fills(granules as well as mud drapes) carries a kaolinitic overprint dueto the effects of deep penetration of soil forming processes throughthe highly permeable sands occurring shortly after channel aban-donment. The upward increase in preserved sandstone bodythickness in each stratigraphic zone which is associated witha larger upward fining grain-size range, better sorting, and betterpreservation of sedimentary structures is interpreted to reflectan upward increasing accommodation/supply (A/S) ratio due toincreasing accommodation space under the condition of a stablerate of sediment supply (and no changes in the hydrodynamicconditions of the system).

tly fine-grained moderately to poorly sorted sandstone (but ranging between very fineith significantly more frequent number of double mud layers (light grey line); see textbase of the section illustrated into a brown coloured coaly interval marks the boundarykaolinitic paleosol at the top of the section is overlain by a thin in-situ coal (not shown)eserved sample. For legend see Fig. 8B. (For interpretation of the references to colour in

Figure 11. Distributary channel floor deposits, well IZZ-52X, Unit E1, with channel levee to floodplain facies at the top (1889.5 ft [575.9 m] to 1886.5 ft [575.0 m]). A e Sedimentarylog; B e Core photograph; C e Schematic detail of the heterolithic section between 1689-1681 ft illustrating the way in which laminae thicknesses vary systematically.


6.2. Fluvial-dominated sinuous channels

6.2.1. DescriptionThe upper parts of Units D3 and D2/D1 are dominated by

relatively homogeneous and moderately sorted sandstone bodies.They are typified by thick-bedded medium-grained, moderately- topoorly-sorted sandstone at the base in which lithofacies 11 (troughcross-stratified sandstone; Fig. 7), occasionally preceded bylithofacies 12 (clast and gravel-rich trough cross-stratified sand-stone; Fig. 7), is present and an upward fining grain-size trend tolithofacies 9 (ripple-laminated sandstone; Fig. 7) in the top; thelatter part is often missing. These bodies are overlain by lithofacies1 (laminated mudstone; Fig. 7). Commonly, in the upper one third,1e2 mm thick mud- and siltstone layers of lithofacies 3 occurin thin sets that are spaced 8e20 cm apart. No fluid mud depositsare observed. The base erodes sharply into mud- or siltstone,or into preceding sandstone bodies, and occasionally largekaolinite-rich siltstone or mudstone pebbles are found overlying

the basal surface. Soil horizons and/or thin coal layers (lithofacies 1and 15; Fig. 7) commonly form the top of the succession buta brackish water bioturbated mudstone is found in many places atthe top of Unit D2/D1.

6.2.2. InterpretationThis facies succession has a number of characteristics in

common with braided channel deposits described above but isinterpreted to have been deposited in more sinuous fluvial chan-nels. The better sorting and rounding of grains, more clearlydeveloped fining-upward grain-size profile and upward decreasingset size culminating in ripple sets, and the less frequently occurringgranular lags and dispersed granules as well as the overall smallergrain-size are taken as indications for deposition in large curvedchannels on bars in the inner bend possibly developed as point barsassociated with meandering channels. Sets with mud drapes areinterpreted as toe sets of larger tidally-influenced fluvial dunessuggesting tidal influence and/or change in discharge on fluvial

Figure 12. Meandering channel thalweg fill and point bar with inclined heterolithic stratification, fluid mud deposits and mud drapes in Unit D3 (well PF4-00O). For legend seeFig. 8B.


flow (flow retardation during incoming flood and accelerationduring ebb).

6.3. Tide-dominated distributary channels

6.3.1. DescriptionThis facies association, occurring from Unit D2/1 and upward

to Unit B2, is composed of a medium- to fine-grained (occasionallyvery fine-grained), moderately to poorly sorted and troughcross-stratified to planar stratified sandstone package at the base(varying between lithofacies 12 and 11; Fig. 7, indicated by the darkgrey line alongside the core in Fig. 10). This package can be ofvariable thickness (ranging from 30 cm to 2 m) in part dependingon the total thickness of the sandstone deposit. Transitionally aboveit, a moderately to poorly sorted and planar stratified sandstonepackage interbedded with thick homogeneous or faintly-laminatedmudstone layers is found (lithofacies 4; Fig. 7, indicated by the lightgrey line alongside the core in Fig. 10). This package shows anirregular distribution of mud layers and mud layer thicknessesupwards without clear cyclicity although a tendency is observed ofmud layer clustering. A gradual transition to thickly bedded thinmud laminae-rich sandstone (lithofacies 3; Fig. 7, indicated by thewhite line alongside the core in Figs. 9 and 10) is subsequentlyobserved; lithofacies 3 deposits can be relatively thick (up to 6 ft) orabsent. A second succession of lithofacies 10 to 11, with an erosivebasal contact, overlain by lithofacies 4 is found in some instances(for example, Fig. 9).

6.3.2. InterpretationThe two lithofacies types at the base of this facies association are

interpreted as channel thalweg deposits in distributary channels.Homogeneous high-energy bed load sediment transported underrelatively strong and uniform flow conditions is transitionallyreplaced by similar sand-sized sediment interbedded with fluid

mud deposits. The fluid muds are interpreted to have been formedduring low-energy periods from flocculation of clay rich waters (cf.Ichaso and Dalrymple, 2009). Observed faint cyclicities in mudlayer distribution and thicknesses are potentially attributed to tidalcyclicity, possibly spring-neap cycles, but no quantitative observa-tions could be made to support this assumption. These deposits areoverlain by lithofacies 3 showing mud-draped toe sets with doublemud drapes (cf. Visser, 1980; Boersma and Terwindt, 1981) andripple form sets. These combined features indicate the presence ofperiodically reversing tidal flows with slackwater periods duringdeposition of the forward migrating dunes. The deposits can eitherbe part of a tidal bar (sensu Mutti et al., 1985) or a compound tidaldune (sensu Olariu et al., 2008) because master bedding surfacescan not be identified with confidence and no relation is foundbetween migration direction of the potentially present masterbedding surfaces and the cross-stratified sets.

6.4. Inclined heterolithic meandering channels

6.4.1. DescriptionHeterolithic sandstone bodies typified by a 3-partite lithofacies

succession occur commonly in Zone B (and appear already occa-sionally in Unit D3), but are different from the distributary channelsdescribed above. They are composed of i) gravelly and poorly sortedcoarse-grained sandstone (lithofacies 11 to 12; Fig. 7) in which thegravel and granules are composed of kaolinitic mudstone andsiltstone; this part is of variable thickness ranging from 10 cm to50 cm; ii) a poorly to moderately sorted trough cross-stratifiedsandstone (lithofacies 10 to 11; Fig. 7), and iii) a heterolithicsandstone formed by regularly alternating relatively thicker sand-stone packages and thinner mudstone-dominated packages (clas-sified as a-typical lithofacies 4; Figs. 7, and 11e13). Grain-sizecontrasts between the mudstone-dominated packages and the

Figure 13. A: Meandering channel fill and point bar inclined heterolithic stratification with double mud drapes in Subunit B2-2 (well PD2-00O); B: Double mud drapes in a pointbar deposit in well SDZ-187 at 1760 ft (536.4 m; base Unit D2/D1). Circle: ripple form sets draped with mud. C: Erosive base of a thin (0.6 m thick) channel fill with double muddrapes encased in a mud-rich succession of an interdistributary bay in Subunit B2Upper (well PE4-00O at 1315 ft [400.8 m]). For legend see Fig. 8B.


sandstone packages are commonly large. The sandstone bodytypically has a bell-shaped gamma-ray log signature.

The mudstone-dominated packages are typically composedof relatively thin (1e3 mm thick) mudstone laminae alternatingwith approximately equally thick sandstone laminae. Fromthe base upward, the sandstone laminae become thinner, lessfrequent and virtually disappear towards the middle part ofa mudstone-dominated package. From the middle part towardsthe upper part of the package the frequency and thickness ofindividual sandstone laminae increases again (Fig. 12C). Rippleform sets are common as well as double mud drapes (e.g. Fig. 13B).The shape of both sandstone and mudstone laminae are irregularand their thicknesses laterally is variable. In addition, somethicker (3e20 mm thick) homogeneous non-laminated mudstonelayers occur within the mudstone-dominated package. Frequently,cracks occur in the mudstone layers that are filled with sand fromthe overlying sandstone layer.

6.4.2. InterpretationThe facies association is interpreted as laterally accreted subtidal

bars and point bar deposits formed by meandering channels ina tide-influenced deltaic setting. Seismic attribute maps generatedfor Zone B showed curved features and patterns suggestingthe abundant presence of scroll bars that were part of largecomposite point bars. However, it was found impossible tounequivocally delimit individual occurrences of the facies associa-tion. The alternating succession of sandstone dominated packages

and mudstone-dominated packages is interpreted as inclinedheterolithic stratification. Within a mudstone-dominated package,the transition from sandstone dominance to mudstone dominancereflects decreasing fluvial bed load transport and increasingdeposition from suspension. The relatively large amounts ofmudstone suggest deposition close to or in the turbidity maximumzone. Thicker homogeneous mudstone layers are interpreted asfluid mud deposits (cf. Ichaso and Dalrymple, 2009). The influenceof tidal currents is reflected by the presence of mud drapesdeposited from suspension particularly during low fluvial currentenergy (high to falling stage).

Sand-filled cracks have been interpreted as either desiccationcracks if upward curling of mud polygons could be proven (cf. VanStraaten, 1959), or as sub-aqueous shrinkage (“synaeresis”) cracksgenerated by salinity changes if no decisive evidence for desiccationon an intertidal flat depositional environment could be found(e.g. Plummer and Gostin, 1981). The interpretation as synaeresiscracks is favoured because of the association of lithofacies inwhich the cracks occur, their physical shape, and the fact that noevaporitic minerals or residues are found.

6.5. Mouth bars

6.5.1. DescriptionThis facies association (Figs. 14e16) is limited to Zone C,

particularly Unit C2. It is often underlain by either dark,greyish-brown coloured mudstone (classified as lithofacies 15;

Figure 14. Succession formed by mouth bar deposits and interdistributary fines in well IZZ-52X (Unit C2) e see text for explanation. 1: bioturbated thinly bedded mudstone-dominated heterolith (lithofacies 7); 2: trough cross-stratified coarse-grained sandstone (lithofacies 11); 3: thickly bedded thin mud laminae-rich sandstone (lithofacies 3). Thedeposits are typified by an upward coarsening grain-size trend, increasingly better sorting and associated upward increase in bed thicknesses with fewer shale layers and doublemud drapes; 4: Homogeneous fine- to medium-grained planar cross-stratified sandstone (lithofacies 10) and ripple-laminated top (lithofacies 9, formed in the proximal part of themouth bar; 5: thinly laminated mudstone (lithofacies 1); 6: bioturbated thinly bedded siltstone-dominated heterolith (lithofacies 6) interpreted to have formed by deposition ona sand-dominated tidal flat and underlain by a thin in-situ coal and carbonaceous mudstone (at 1620 ft [493.8 m]). For legend see Fig. 8B.


Fig. 7) with indistinct lamination and often containing abundantshallow marine faunal indicators, including Planolites andTeichichnus ichnofabrics (Fig. 17), or a heterolithic intervalcomposed of a regular alternation of mud- (or silt) stone(0.5e2 cm thick) and very fine-grained sandstone layers that aretypically 2e3 cm thick (lithofacies 5; Fig. 7). In some cases, themouth bar is underlain by trough cross-stratified coarse-grainedsandstone (lithofacies 11; Fig. 7). This facies association is typi-fied by a succession of thickly bedded thin mud laminae-richsandstone (lithofacies 8; Fig. 7) and homogeneous fine- tomedium-grained planar cross-stratified sandstone (lithofacies 10;Fig. 7) with ripple-laminated top (lithofacies 9; Fig. 7). In general,these deposits are typified by an upward coarsening grain-sizetrend, increasingly better sorting and associated upwardincrease in bed thicknesses with fewer shale layers and doublemud drapes; bioturbation is common and includes Ophiomorphaichnofabrics (Fig. 15). In a fully preserved succession, sandstone isabruptly overlain by a (in some occasions rooted) mudstoneinterval that contains mm thick and very fine-grained sandstonelayers at the base.

6.5.2. InterpretationThis facies association is interpreted as mouth bars that

have prograded into shallow marine brackish water andtidally-influenced environments at the mouth of distributarychannels (see Labourdette et al., 2008; for a more detailed

geomodelling study on the mouth bars). The characteristicsuccession of thickly bedded thin mud laminae-rich sandstonegradually changing into homogeneous fine- to medium-grainedplanar cross-stratified sandstone is interpreted to have beenformed by aggradation and progradation of migrating dunes withmud-draped toe sets. The mud-draped toe sets show double muddrapes and ripple form sets indicating the presence of tidal reversalflows and slackwater periods during deposition (Figs. 15 and 16).Underlying trough cross-stratified coarse-grained sandstone isinterpreted to have been formed in a distributary channel feedingan older and more distant mouth bar. Laterally and vertically,mouth bars are intercalated with either bioturbated laminatedmudstone or thinly bedded mudstone-dominated heterolithicdeposits formed in interdistributary bays dominated by minortraction currents and deposition from suspension.

6.6. Crevasse splays

6.6.1. DescriptionThese deposits occur throughout the Lower Oficina section.

They are seldomly observed in Zones F through D, but occurcommonly in Zones C and B. Crevasse splay sandstones range inthickness from 30 cm to 2 m. They are generally fine to very fine-grained (lithofacies 3) and may show a gradual or rapid upwardfining grain-size trend to lithofacies 2, or no trend. Sandstonebodies are ripple-laminated and they are often under- and/or

Figure 15. Mouth bar deposits; see text for explanation. A: well RX-05S, Unit C2. B: well IZZ-52X, base Unit C2 just above ‘Top Fluvial’. 1: thinly bedded mudstone-dominatedheterolith (lithofacies 7); 3: thickly bedded thin mud laminae-rich sandstone (lithofacies 3); 4: Homogeneous fine- to medium-grained planar cross-stratified sandstone (lith-ofacies 10); 5: thinly laminated mudstone (lithofacies 1); 6: bioturbated thinly bedded siltstone-dominated heterolith (lithofacies 6, interpreted to have formed by deposition ona sand-dominated tidal flat.


overlain by an in-situ coal layer (lithofacies 15) that may haveassociated rootlets preserved in the sandstone beneath it. In ZonesC and B, mud drapes (infrequently double mud drapes) are occa-sionally preserved.

6.6.2. InterpretationThe facies association is interpreted as crevasse splays depos-

ited in floodplain areas during high discharge periods by breach-ing through channel levees or overbank flow. They may occur asmulti-storey events and thus could show a tendency for coars-ening upward (typically 1e2 m in bed thickness). Especially in thenon-channelized areas of Zone B they are well preserved andoccur frequently suggesting a relatively high preservation poten-tial. The examples from Zone C and B are typically fine-grainedand probably represent crevasse splays in the more distal deltaicsettings. In the fluvial zones F, E, and D similar overbank flows(levees) or crevasse splays are likely to occur, however, it seemsthat here the preservation potential is low and they have not beencored.

6.7. Interdistributary fines

6.7.1. DescriptionZones E and D contain finely-laminated mudstones and muddy

siltstones that are kaolinite-rich with paedogenic imprints and

have a characteristic white colour. When occurring, an in-situ thincoal layer is often found overlying the mudstone. These coals arethin and immature, and can only occasionally be correlated fromonewell to another. Occasionally, thin siltstone layers are preservedwithin the mudstone. Not uncommonly, the in-situ coals are foundoverlying paleosol horizons.

The interdistributary fines that occur in stratigraphic Zones Cand B are generally more complex. They are formed by a siltstone tovery fine-grained sandstone layer at the base (lithofacies 2; Fig. 7)which often has a sharp base and top. This layer is overlain bya heterolithic package formed by thin laminae (0.5e1 cm thick) offinely-laminated mudstone (lithofacies 1; Fig. 7) and very fine- tofine-grained sandstone layers of 1e5 mm thick (lithofacies 9;Fig. 7). Each of the sandstone layers is formed by ripple struc-tures. At the top, a gradual transition into mudstone with kaoliniticoverprint and often an in-situ coal layer on top (lithofacies 2 or 15;Fig. 7) occurs with rootlets penetrating into the underlyingmudstone. Thin (smaller than 0.5 m), immature coals and thicker(0.5e1.5 m), more mature coals occur; these coals can normallybe correlated from one well to another (over distances morethan 1 km). Occasionally, coals are correlatable over the entirePetrocedeño area (that is, more than 30 km) such as the coals ontop of Zone B2 and A1. In fact, the M0 seismic marker (Figs. 3 and 4)represents a coal layer that can be followed semi-regionallyacross most of the Junín area.

Figure 16. Small mouth bar deposit in well PF4-00O at 1540e1542 ft (469.4e470.0 m; Unit C2). 1: thinly bedded mudstone-dominated heterolith (lithofacies 7), interpreted to haveformed in an interdistributary bay, underlying a small prograding mouth bar succession formed by a thickly bedded thin mud laminae-rich sandstone (lithofacies 3) of the distalmouth bar (3) gradually coarsening upward into homogeneous planar cross-stratified (lithofacies 10) and ripple-laminated sandstone (lithofacies 9) of the proximal mouth bar (4).For legend see Fig. 8B.


6.7.2. InterpretationIn zones E and D, the mudstones and muddy siltstones are

interpreted as floodplain deposits of a dominantly braided tosinuous river system. Thick kaolinite paleosols developed possiblyunder humid climatic conditions (leaching of cations requires acidsoil conditions and extensive percolation of groundwater). Frac-tured zones in some mudstone intervals that are rich in kaoliniteare interpreted as subaerial exposure surfaces.

In Zones C and B, the heterolithic mudstones of lithofacies7 (Fig. 7) are interpreted to have been formed in shallow-waterlow-energy interdistributary bays dominated by deposition fromsuspension. Occurrences of lithofacies 4 (Fig. 7) are attributed tointertidal flat to upper marsh and interdistributary floodplaindeposition. The in-situ coals as well as mudstone layers withan abundant mangrove pollen suite are interpreted to havebeen formed beneath mangrove forests. In large deltas, thatare river-dominated, mangroves are geographically distributedin response to geomorphological characteristics such as alongdistributary channels, on natural river levees and on point bars(Thom, 1967). The mudstones formed during periods of highfine-grained sediment supply whereas the in-situ coals developeddue to poorer drainage during periods of sediment starvationand moderate rates of accommodation space generation which

provided relatively stable conditions and did not exceeding organicproductivity rates in mires (cf. Woodroffe, 1990; Bohacs andSuter, 1997).

6.8. Transitional lower delta plain to delta platform deposits

6.8.1. DescriptionThis facies association is only observed in the uppermost

part of the productive succession of the Oficina Fm (Unit A2).It is composed of four parts. From the base up, the lowermostone-fourth is formed by clean, upward fining, fine-grained,moderately sorted sandstone of which the lowermost metre issignificantly coarser, contains coal clasts, is and composed of verypoorly sorted medium- to coarse-grained sandstone (lithofacies 10to 11; Fig. 7). The basal contact often erodes into an underlyingin-situ coal (Fig. 18a-C and 18b-C). A gradual transition across thesecond one-fourth part (lithofacies 8; Fig. 7) occurs in whichprogressively more faintly-laminated and thick (up to 0.3 ft)mudstone layers are interbedded. Many of these mudstone layershave irregularly eroded tops (for example, at 1251 ft, Fig. 18b-C)and are composite features. Other mudstone layers are broken-upand preserved as large fragments encased in sandstone. A faintcyclicity is observed in 1) the distribution of mudstone layers

Figure 17. Examples of ichnofabrics and trace fossils in Petrocedeño. 1: Monospecific Teichichnus rectus brackwater ichnofabric (opportunistic colonizer) in a thinly bedded mixedsandstone-mudstone heterolith (lithofacies 5) interpreted to have formed by deposition under stressed conditions in a brackwater environment on a tidal flat intersected by smalltide-dominated channels, well VD00 at 1891 ft (576.4 m; lower part Unit D2/D1); 2: Backfilled large specimen of Beaconites antarcticus ichnofabric indicative for low-energyfreshwater environments periodically exposed to air. Colonized upper interval of an abandoned tide-dominated channel deposit with double mud drapes in well VD00 at1879 ft (572.7 m; upper part Unit D2/D1); 3: Beaconites ichnofabric in well IZZ-52X at 1701 ft (518.5 m; top of Unit D2/D1); 4: Teichichnus brackwater ichnofabric in pervasivelybioturbated thinly bedded mudstone-dominated heterolith of lithofacies 7 (Fig. 7) formed in a tidally-dominated interdistributary bay to muddy tidal flat, well PB5-00O at 1452 ft(442.6 m; Subunit C2Lower); 5: Monospecific Palaeophycus tubularis ichnofabric indicative for opportunistic colonization of brackish water as well as freshwater environments. It isa common ichnofabric in many wells. Abandoned tide-influenced channel deposit in well VD00 at 1850 ft (563.9 m; Subunit C2Lower); 6: Gyrolites (salt water indicator) in marinemudstones of lithofacies 1 (Fig. 7) in well PD2-00 at 1270 ft (387.1 m; lowermost part of Unit B2); 7: Planolites montanus ichnofabric, typical for mud fallout deposition under low-energy conditions in freshwater as well as brackish water conditions. It occurs abundantly in Units D and C in many wells (example from well FX05D at 1900 ft [579.1 m]).


typified by sections only containing very thin sandstone layers(for example, 1216 to 1213.8 ft, Fig. 18a-C), and 2) in sections con-taining thicker sandstone layers. The third one-fourth part of thesuccession is formed by thick composite mudstone layers of up to30 cm thick alternating with fine-grained sandstone layers ofvariable thickness (up to 30 cm thick; lithofacies 8; Fig. 7). Theuppermost one-fourth part is formed by strongly bioturbated andhomogenized very fine-grained sandstone (lithofacies 14; Fig. 7).

6.8.2. InterpretationThis facies association is interpreted to have been formed by

a relatively shallow lower delta plain channel eroding into coastalinter- and supratidal flats (marshes) which were probably vege-tated by mangroves. The channel initially filled with relativelyhomogenous sand but subsequently became dominated by depo-sition of alternating thick muds and fine sand. The muds areinterpreted to have been formed by deposition from fluidized mudflows under relatively distal quiet water conditions in and in frontof wide distributary channels. Even though the seismic quality in

general can not provide details on channel morphology it ispossible in some levels to estimate the morphology of the channelssuggesting a width on the order of 3e5 km.

Based on 1) the large amounts and dominance of tidallygenerated composite fluid mud deposits, 2) the proximity andgenetic link to delta distributary channels, 3) the possible presenceof tidal periodicities in mudstone layer distribution patterns, and 4)the absence of mouth bar deposits, it is concluded that this faciesassociation was most likely formed in a tide-dominated delta.Figure 18 (a and b) illustrates the differences between the fill ofa tide-dominated lower delta plain to delta platform channel witha braided channel fill and an abandoned channel fill. Note, forexample, that thicker mud layers are commonly a compositeformed by 2 mudstone layers separated by a very thin (1 mm thick)sandstone layer (for example, at 1500.9 ft [457.5 m] and 1496.7 ft[456.3 m] in Fig. 18a-B).

Similar facies association has been described from the deltafront of the Fly River (Dalrymple et al., 2003). However, due to thepresence of the genetically related channel underneath the


composite fluid mud dominated succession, a proximal subtidaldelta platform depositional environment, analogues to theMahakam Delta (cf. Roberts and Sydow, 2003; Crumeyrolle et al.,2007), located somewhat closer to the mouth of the delta distrib-utaries is preferred.

7. Depositional sequence model

7.1. Rationale

The conceptual depositional model for the Oficina Fm inPetrocedeño is summarised in a hierarchical 3rd-order sequencestratigraphic framework and has been described elsewhereincluding the analytical method applied (Martinius et al., 2012).Only a summary is presented here and focus is extended to 4th-order sequences and the relation with the Early Miocene eustaticcurve.

Figure 18. a.) Two core examples of channel fill facies associations departing from the classsedimentary descriptions are given in Figure 18b. A: Typical stack of braided channel fills ina channel base erosion surface at 1632 ft (497.4 m) is recognized from well logs but not corethe boundary between Unit E1 and Unit D3 and classified as 3rd order SB. B: Abandonedchannel thalweg deposit and thick tidally-dominated rhythmic channel abandonment fill tylayers of siltstone and very fine-grained sandstone. A coal forms the top of the succession. C:(well PE4-00O). At the base, a thin coarse-grained thalweg lag is present fining-upward to fi

grained sandstone in which numerous thick (up to 5 cm) and massive composite fluid mudbeen deposited in the turbidity maximum zone which was located on the delta front indicati2003). The top of the succession is bioturbated and homogenized and possibly depositeddescriptions of the core examples shown in Figure 18a. A: Coarse-grained braided channel fiA2 (well PE4-00O); C: Abandoned channel fill succession in Unit D2/D1 (well PC2-00O). Fo

The 3rd-order sequence stratigraphic framework is based on 1)the recognition of key stratigraphic surfaces that are at leastcorrelatable at the scale of the Petrocedeño area, some of whichhave been mapped on 3-D seismic; 2) the interpretation of faciesassociations, their stratigraphic distribution and stacking patterns;and 3) biostratigraphic age determinations and correlations; and 4)the generally accepted average duration of 3rd-order sequences of0.5e3 million years (Kerans and Tincker, 1997; Vail et al., 1991).

Base level (sensu Cross, 1988) in fluvial and delta plain strati-graphic successions is commonly defined as the ratio between therate of accommodation space creation (sensu Jervey, 1988;Posamentier et al., 1988) and sediment supply through time (A/Sratio). These two factors have been recognized as the dominantcontrol on coastal deposition (e.g. Curray, 1964; Jervey, 1988;Thorne and Swift, 1991; Schlager, 1993; Shanley andMcCabe,1994).The A/Smethodology (cf. Swift and Thorne,1991; Thorne and Swift,1991), applying a primary transgressive-regressive systems tract

ified examples shown in Figures 8e16, compared with a delta front facies association;Unit E1 crossing over into the lower part of Unit D3 (well PC2-00O). The presence ofd. Detailed well correlation and seismic analysis concluded that this erosion surface ischannel fill succession (oxbow lake?) in Unit D2/D1 (well PC2-00O) formed by a thinpified by deposition of mud from suspension alternating with thin (up to 1 cm thick)Core expression of delta front deposits in the lower part of the undifferentiated Unit A2ne-grained sandstone. The succession is typified by well sorted, silt-bearing upper fine-deposits occur often stacked on top of each other. The succession is interpreted to haveng that the succession of Unit A2 formed in a tide-dominated delta (cf. Dalrymple et al.,on the proximal prodelta. Muestra preservada ¼ preserved sample. b.)Sedimentaryll in Unit E1 and lower part Unit D3 (well PC2-00O); B: Distributary channel fill in Unitr legend see Fig. 8B.


terminology, is followed mainly for two reasons. Firstly, the fluviallower part of the Oficina Fm (where the marine transgressivesurfaces and the laterally extensive coals do not occur) is fullycontinental and can not be correlated to contemporaneous marineor marine-influenced environments at the Petrocedeño field scale.Secondly, the tectonic control on deposition is envisaged to beimportant (Petrocedeño internal data; Parnaud et al., 1995;Rodriquez and Oswaldo, 1996). Consequently, correlation of wellswas done mainly based on trends and/or stacking patterns and inaddition, where possible, using extensive coal layers.

The subaerial unconformity (SU), or maximum regressionsurface, is chosen as the sequence boundary for sequences 1 to 5(corresponding to reservoir Units F through to D1/D2), wheremaximum flooding surfaces are difficult to identify in the datasetavailable (or not at all, as also recognised by Di Croce, 1995),following the Exxon school of sequence stratigraphy (VanWagoneret al., 1988). In the deltaic part of the succession, each of thesequences represents one regressiveetransgressive cycle. Themaximum flooding surface (MFS) is chosen as the sequenceboundary for sequences 6 to 10 (corresponding to Units C2 throughto A2), following the genetic stratigraphy approach of Galloway(1989), because SU or correlative key stratigraphic surfaces cannot be identified in well logs and core and/or mapped on seismic.

The 3rd-order sequences are superimposed on one 2nd-ordertransgressive-regressive sequence (approximately correspondingto seismic Unit 5 of Di Croce, 1995; bounded by an unconformity at

Figure 18. (co

its base and top, and tectonic event II of Rodríguez, 1999) with thebase at the base of Sequence 1, a base level turn-around at the topof Sequence 7 (MFS ‘M12’), and at the very top of Sequence 10(MFS ‘M8’). In areas in the vicinity of the Petrocedeño Field, the rateof tectonic subsidence during event II is estimated to be variablebut high ranging from 88 to 119 m/Ma or even more dependingon location in the foreland basin and stratigraphic age. However,estimated sediment supply rates easily kept up with the rate oftectonic subsidence (Rodríguez, 1999).

7.2. Depositional sequence model at 3rd order level

Sequence 1 (Zone F) is characterized by deposition on a paleo-relief which is defined partly by large-scale structural elementsand partly by more subtle structural features formed during theexhumation phase at the end of the Cretaceous and the start ofthe Early Miocene. A number of alluvial valleys formed that werefilled by gravel- and coarse sand-dominated wide and relativelystraight braided rivers typified by erosive bases and a braidedthalweg pattern. The available accommodation space was minimal.Mature kaolinitic paleosols developed by chemical weathering ofaluminum silicate minerals like feldspar. The associated processesare relatively rapid but preservation of kaolinitic paleosols wasenhanced if the area remained unaffected by alluvial erosiveprocesses (such as on paleohighs).

ntinued).


During Sequences 2 and 3 (Units E2 and E1) fluvial environmentscontinued to dominate the Petrocedeño area. Frequent channelswitching created broad channel belts occupied by coarse-grainedbraided and low-sinuosity streams (Martinius et al., in press). Thedepositional environment is interpreted as a braidplain.

Each of the subsequent Sequences 4, 5 and 6 (Units D3 and D2/D1) is characterized by deposition on a braidplain in wide sand-dominated and relatively straight rivers typified by erosive basesand a braided thalweg pattern around irregularly offsetting repet-itive bars. More sinuous rivers developed at the end of eachsequence (Fig. 5). Channel belts are present across the entire Pet-rocedeño area (Martinius et al., in press, Fig. 19A). During Sequence6, A/S increase continued sufficiently to allow brackish waters toenter the Petrocedeño area and form a mappable MFS. From thislevel upward in the stratigraphy no mappable unconformities arerecognized and therefore this MFS ‘Top Fluvial’ (Fig. 4) is chosen asthe top of Sequence 6 (and base of Sequence 7; Fig. 3).

Sequence 7 (Unit C2) is dominated by lower delta plain depositsformed in a tidal and brackishwater influenced distributary channeland mouth bar system; marsh forests fringed the distributarychannels and coastline. ‘M14’ at the top of the sequence representsthe first major transgressive event in the area. Long distance wellcorrelations showsimilardepositional environments at least 150kmnorthward. This implies a low-gradient lower delta plain anda shoreline at a large distance from Petrocedeño.

At the base of Sequence 8 (Unit C1), mudstones are founddominated by a brackish water fauna with marine influence.Ensuing delta progradation introduced freshwater in near-coastalzones of the lower delta plain. Sequence 8 can be subdivided intotwo 4th order sequences (8-1 and 8-2) corresponding to reservoirSubunits C1Lower and C1Upper.

The mudstone at the base of Sequence 9 (Unit B2) is interpretedas marine shale formed during maximum inundation of marinewaters over the area during the Early Miocene (cf. Demchuket al., 2003; for Distrito Cabrutica, the former Petrozuata Field).

15 / 4.5

30 / 9.1

45 / 13.5

60 / 18.3

75 / 22.5

90 / 27.4

105 / 31.5

120 / 36.6

Amalgamated channelthickness (ft / m) A

C

0

10 / 3.0

20 / 6.1

30 / 9.1

40 / 12.2

50 / 15.2

60 / 18.3

Amalgamated channelthickness (ft / m)

Figure 19. Net sand maps of successive sequences. A e Sequence 5 (Unit D1/D2); B e Sequcorner of the Petrocedeño Field of sequence 10 (Unit A2; indicated with a box in C).

The regional base level rise associated with this flooding probablychanged the bathymetry and geometry of the basin causing a rela-tive increase of tidal resonance. Depositional environments aretypified by large meandering tide-influenced to edominatedchannels and tide-dominated distributary channels with adjacentintertidal and supratidal floodplains (Fig. 19B) covered by brackishwater mangrove swamps and forest marshes. Sequences 7 and 9are discussed in more detail in the next section.

Sequence 10 (Unit B1) is typified by a few tide-influencedto edominated meandering channel belts with point bar depositssimilar to those of Sequence 9 which developed along a narrow N-Strend located only in the eastern part of Petrocedeño (Martiniuset al., in press).

Sequence 11 (Unit A2) is dominated by deposits formed in thedistal part of wide distributary channels in shallow-water depths(probably less than 5 m) on the upper part of the subtidal deltaplatform analogues to the modern Mahakam delta (cf. Roberts andSydow, 2003). Distributaries followed a clear SW-NE trend butwere dominantly located in the western part of Petrocedeño(Martinius et al., in press, Fig. 19C and D). Within the distributaries,sandwasdepositedas tidal bars and the thalwegof thedistributariesappears to be filled by a combination of sand and shale deposited asfluidmuds. Interdistributary areaswerefilled bycrevasse splays andunconstrained bodies of siltstone andmudstone; both contain thickfluid mud deposits (lithofacies 8). Strongly bioturbated sands (lith-ofacies 14, Fig. 7) of the more distal parts of the subtidal delta plat-form overly the facies association. The absence of mud in thislithofacies is explained by the influence of relatively strong tidalcurrents and/or waves that were capable of keeping fine-grainedsediment in suspension; however, no direct evidence is found ofpreserved wave-influenced or wave-dominated sedimentarystructures. It is likely that the sandstonesweredeposited as thebasalpart of barrier islands (possibly initially as sandflats or bars).Most ofthe upper part of these barrier islands was subsequently erodedduring the transgression recorded just above the top of Unit A2

15 / 4.5

30 / 9.1

45 / 13.5

60 / 18.3

75 / 22.5

90 / 27.4

Amalgamated channelthickness (ft / m)

B

D

Meanderingchannel beltwith point bars

Crevasses

Floodplainmudstones

ence 8 (Unit B2); C e Sequence 10 (Unit A2); D: Deterministic map of the southwest


preserving only the basal part. The presence of barrier islands andassociated tidal inlets in the upper part of the Lower Oficina Fm inJunín was already suggested by Audemard (1983, cited in Carbón,2000, Fig. 20).

7.3. Sequences at 4th order

7.3.1. Sequences 7-1 to 7-33rd order Sequence 7, corresponding to Unit C2, can be sub-

divided into three 4th order sequences (7-1 to 7-3, correspondingto reservoir Subunits C2Lower, C2Middle and C2Upper.

Caracas

Cumaná

El Tigre

68°W 67°W 66°W 65°W64°W

66°W68°W 64°

MARGARITA

TOB

ARAYA

Depositional environmentNo deposition or later eroded

Fluvial

Delta plain / coastal plain

Inner shelf

El Baúl Arch Chaguaramas Fm.

lower Oficina

A

B

Tres Puntas Fm.

Outer shelf / Intra-shelf slope

Deep sea

Tide dominated deTidal flats

Tidal barsTide dirand int

Tidal

inlets

NOT TO SCALE

NINUJÁCAYOB

Estuarine channels

Figure 20. A: Palaeogeographic sketch map of the Eastern Venezuelan Basin representing tcomprises retro-deformed latitude and longitude coordinates. The islands of Margarita, Tobapoints (cf. Sztanó and De Boer, 1995) assuming sufficient basin width (w150 km?), length (woil belt, including the Junín region (indicated with a box in A), modified after Audemardbarriers, lagoons, estuaries and deltaic systems during the Early Miocene. It is assumed hereshore tidal currents and deposited as shore-parallel bars. These bars were subsequently tra

The boundary between Subunit C2Lower and C2Middle (Fig. 3)as well as the boundary between Subunit C2Middle and C2Upper isdefined as a 4th order MFS which can in both cases be correlatedacross the Petrocedeño area using N/G differences expressed onwell logs. In core, both these 4th order MFS are expressed as anin-situ coal locally with in-situ coaly mud- to siltstone underneath(Fig. 21). In all 3 cases, the 4th order MFS overlies a kaoliniticfloodplain paleosol and contains brackwater floras. No regressive totransgressive turn-around surfaces, such as an interfluvial SU, areidentified above the MFS and consequently the base of the channelsand overlying the MFS in all 3 reservoir subunits is taken as the

San Fernando

Maturín

Ciudad

Bolívar

63°W62°W 61°W

60°W

W 62°W 60°W

8°N

10°N

12°N

TRINIDAD

AGO

Fm.

Bupper Carapita Fm.

upper Cipero Fm.

Brasso Fm.

ltas

ectionensity

Barrier islands

Costal plain andolder deposits

Delta plain

OBOBARACOHCUCAYA

Distributary mouth bars

Wave - tide dominated delta

N

he late Early Miocene on palinspastic base (modified after Pindell et al., 1998). The gridgo and Araya are in restored position. The basin probably had one or more amphidromic500 km?) and depth (>100 m?). B: Palaeogeographic sketch map of the Orinoco heavy-et al. (1985) as reproduced in Carbón (2000), showing the interpreted distribution ofthat sand delivered to the inner parts of the basin was re-distributed by rotating long-nsformed into a barrier islands e inlet system.

Figure 21. Expression of base and top of Sequence 5. A: Cartoon-like sketch to illus-trate the channel belt erosion surface in well WA00 at base Unit D2/D1 which corre-lates with a soil horizon in floodplain deposits in well VD00. B: Core expression ofinterfluvial subaerial unconformity at the base of Unit D2/D1, defined as 3rd order SB,in well VD00 at 1918.6 ft. C: Expression of the top of Unit D2/D1, defined as 3rd orderMFS (‘Top Fluvial’) in well PB5-00O at 1464 ft (446.2 m) showing a dark brownorganic-rich rooted zone transitionally overlying a bleached light grey kaolinite-richzone. (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)


regressive to transgressive turn-around. In well PB5-00O as well asin well PF2-00E a muddy siltstone 3 m above the 4th order MFSpicked as top C2Middle (based on well correlations) haswell-developed cone-in-cone calcite structures which are ascribedto the diagenetic alternation of shell fragments (coquina typehardground). A similar feature is observed in well SDZ-4X at theposition of ‘M12’ (top Sequence 8, corresponding to Unit C1).

Subunit C2L is channel dominated, Subunit C2M is formed bya mixture of channels and mouth bars and Subunit C2U is mouthbar dominated. In addition, Subunits C2M and C2U do not havecoals in certain parts of the field. Small specimens of Ophiomorphanodosa are observed in Subunit C2Lower (for example, well FX05D)which is an indication for marine influence under stressed salinityand moderate energy conditions. Furthermore, Planolites andPalaeophycus ichnofabrics, indicative for mixing of freshwater andbrackish water, are common throughout the C2 subunits. A singlesandstone body in Subunit C2Middle in well FX05D, recordingchannel abandonment, contains the brackish water Teichichnusichnofabrics in the middle and the freshwater to terrestrial Bea-conites ichnofabric, dominated by small forms, at the top (Fig. 17-2).

This indicates temporal fluctuations between freshwater andbrackish water even at the scale of individual channels. Most topsurfaces of themouth bars are not bioturbated apart from two caseswhich contain well-developed ichnofabrics. These two cases cannot be correlated but point to marine water influence and mayindicate high-frequency base level changes and marine flooding.

7.3.2. Sequences 9-1 to 9-43rd order Sequence 9, corresponding to Unit B2 (Fig.19B), can be

divided into four 4th order sequences (9-1 to 9-4), each separatedbyan MFS at 4th order level, and corresponding to reservoir SubunitsB2.4 to B2.1 from base to top. The regressive part of the first 4th-order sequencecorresponds toSubunitB2-4 (boundedat thebaseby‘M12’; Figs. 7 and 22). At the top, this regressive part of the first 4thorder sequence is bounded by a SU, in places expressed as aninterfluvial exposure surface (cf. Aitken and Flint, 1996; NE part ofPetrocedeño; see also example of Fig. 21) at 2.4 m above ‘M12’ ina 2.1m thickmudstone interval. It is typified bya distinct soil profilewith large root traces that deeply penetrate into the underlyingmudstone. This surface correlates into the erosive surface under-lying tidally-influenced point bar deposits, with a SE-NW orienta-tion, locally developed in the SW of the field (Fig. 22).

The overlying transgressive part of this 4th order sequencecorresponds to Subunit B2-3 and is bounded at the top by a 4thorder MFS (defined as 4th order SB), lined by large burrows, andoften expressed in core as a siderite-cemented mudstone (forexample, IZZ-52X) similar to ‘M12’. This MFS is most clearlydeveloped in the NE which suggests flooding from the NE. Towardsthe SW, it correlates into the shale unit which directly overlies theinterfluves in the regressive part of Sequence 9-1 developed in theNE. Differences in N/G between the regressive and transgressivepart of this 4th order sequence aremapped based on correlatedwelllogs. Palaeophycus ichnofabrics, indicative for mixing of freshwaterand brackish water, are common throughout B2.4 and B2.3.

Sequence stratigraphic models developed for coals in paralicsettings using outcrop (Flint et al., 1995; Bohacs and Suter, 1997)and/or subsurface data suggest that laterally extensive coal seamsdevelop in response to intermediate rates of subsidence and risinggroundwater tables (base level), the latter being strongly controlledby sea-level and the precipitation/evaporation ratio. In particular,this is the case during the middle lowstand and middle highstandsystems tracts when the rate of change of base level is low tomoderate (Bohacs and Suter, 1997). Consequently, the extensivecoal layer on top of Subunit B2 (and the coal layer forming the M0seismic marker) is interpreted to i) have formed during regionaltransgression and ii) be time correlative to a significant floodingsurface at the coeval coastline.

Consequently, the regressive and transgressive parts of thesecond 4th order sequence correspond to reservoir Subunits B2-2and B2-1. No SU is identified in core or well logs but the twosubunits are mapped based on differences in N/G using correlatedwell logs. The top is formed by 3rd order MFS ‘M9.3’.

8. Discussion e Relation with the Early Miocene eustaticcurve

Comparison of a number of studies of Neogene depositionalsystems located at relatively large distances from each other haveshown that in those cases where detailed chronostratigraphic datapermit accurate and precise global correlation, a stratigraphicsignature of eustatic change with a 106-year periodicity, probablydriven by Antarctic ice fluctuations, can be demonstrated for theNeogene record (Miall, 2009). This is in part based on oxygenisotope fluctuations in the Early Cenozoic that suggest periodicdevelopment of small to moderate-sized ice caps on Antarctica

Figure 22. An interfluvial exposure surface within the lower part of Sequence 9 corresponding to Subunit B2-4 (A; well E20-1 at 2032 ft [619.4 m]) which correlates (based on welllogs) into an erosional surface in B (well SDZ-86X at 1769.7 ft [539.4 m]) underlying a tide-influenced distributary channel fill. The distance between the 2 wells is 10.5 km; thissurface is identified as a (candidate) 4th-order SB. Core depth location i: Normal saline marine laminated mudstone (lithofacies 1) with Hanzawaia, Siphonina, Ammonia, Florilus,Thecamoeba, Gastropoda, echinoida, fish remains and Gyrolites. Core depth location ii: Non-bioturbated, rhythmic fluid mud deposits, part of lithofacies 4, interpreted to haveformed at a distributary channel bottom with large grain-size contrasts and synaeresis cracks.


through the so-called greenhouse period of the Early Cenozoic (e.g.Miller et al., 1999, 2005, 2009). It is therefore not unlikely that,despite ongoing foreland basin development, the sequencesrecognized in the Oficina Fm can in general be correlated to the

Figure 23. Schematic diagram illustrating the assumed relation of the 3rd order sequences2009). See text for explanation. aAbsolute stage ages after Lourens et al. (2004).

global eustatic curve for the Early Miocene (Miller et al., 2009,Fig. 23).

Only one of the main Neogene tectonic events (at approximately22 Ma; Rodriquez and Oswaldo, 1996; Rodríguez, 1999) falls within

recognized in Petrocedeño with the eustatic curve for the Early Miocene (Miller et al.,

Net

/ G

ross

Gen

etic

faci

es

Coals, crevassesand floodplainmudstones

Key togenetic facies

Braided channeldeposits

Coals, floodplainmudstones, crevasses,clay plugs, andpointbars

Bay and delta-plainmudstones

Bay mudstones,mouthbars, distributarychannel deposits, andincised-valley fill

Coals, floodplainmudstones, crevassesand pointbars

Braided channelsandstones withintercalatedmudstones andoccasional coals

Very

low

Very

low

Low

Inte

rmed

iate

Very

hig

h

Bad

Seis

mic

resp

ons

Cor

rela

tion

unce

rtain

ty

doog yreVetaide

mretnI

Very

low

Inte

rmed

iate

woLhgih yrev ot hgi

H

Point bar deposits

Sandy mouthbar(Bedded Sandstone)

Distal mouthbar(Bay mudstone)

Heterolithic sandstone

Crevasse & levee

Interdistributary fines(floodplain mudstones)

Coal

Figure 24. Summary log of one well (“type” well) through the reservoir section of the Oficina Formation in the Petrocedeño Field. The sandstones in reservoir zone A and B appearsporadically, and are replaced by mudstone in other wells.



the depositional time frame of the Lower Oficina Fm and has beenshown to have had an influence on the generation of accommo-dation space (Rodríguez, 1999) in addition to eustatic changes. Thisevent occurred at the base of the Oficina Fm and is assumed to beassociated with the eastward movement of the Serranía del Interiorthrust front causing repeated forebulge uplift, passive margindeepening and subsequent forebulge movement away from theJunín area (cf. Di Croce, 1995; Rodríguez, 1999; Suter and Fielder,2003).

These tectonic pulses are interpreted to have been responsiblefor the generation of multiple unconformities which have beenused to define and map 3rd order Sequences 1 to 6. The EarlyMiocene eustatic curve for this period indicates fall (Fig. 23) whichdoes not match with the developments in the depositional envi-ronment of the Oficina Fm as observed in Petrocedeño. It isconcluded that the structural development in the basin overprintedthe eustatic fall enforcing an increase in accommodation spaceinstead.

Recorded MFS’s used to define 3rd order sequences 7 to 11 cantentatively be linked to the Early Miocene eustatic curve (Fig. 23; cf.Miller et al., 2009). Tectonic subsidence remained high during thisperiod but sediment supply was also high and kept up, at timeoutpaced, the rate of accommodation space generation (Rodríguez,1999; sedimentary facies data presented herein associated with animproved water depth estimation). Sequences 7 and 8 representdelta front deposits and the most marine environments e theeustatic curve shows a significant and rapid rise approximatelyduring this time. Sequence 9 is dominated by meanderingtide-influenced and edominated channel belts on the delta plainindicating a relative base level fall. The eustatic curve showsa significant fall approximately at this time. Sequence 10 is themostmarine interval whichmatches with the eustatic curve for this timeperiod. Deposits of Sequence 11 indicate less marine but signifi-cantly more distal deposits as compared to Sequence 9 suggestinga relative base level rise; the eustatic curve shows a significant riseapproximately at this time. It is concluded that eustatic changeswere able to affect deposition due to an overall A/S ratio of 1 orless than 1.

No attempt is made to correlate the 4th order sequences withinSequence 7, 8 and 9 of the Lower Member to the eustatic curve forthe Early Miocene. Furthermore, the Middle Member of the OficinaFm was deposited during the maximum transgression of the EarlyMiocene, and the Upper Member represents a progradationalwedge covered by transgressive deposits (Audemard et al., 1985;see also Rodríguez, 1999).

8.1. Reservoir properties

The Oficina Fm has awide porosity range; however, the net sandvalues (for example channel facies with fine to coarse sands)typically range from 30% to 38% with a mode around 32%e34%. Themeasured core permeability varies from a few Darcies to more than12 to 14 Darcy with a few outliers of around 20 Darcy; thearithmetic average is close to 10 Darcy. In general, however,the quality of the laboratory data is strongly influenced by coringconditions, core-barrel preservation and laboratory cleaningprocedures prior to experiments. Pressure build-up analysis(derived from drill stem tests) indicates a somewhat higher averagepermeability than measured from core. It shows a clear anisotropywith maximum permeability values along the depositional axis ofsandstone channels, similar to the experience from the DistritoCabrutica Field (ex- Petrozuata; Briceño et al., 2002). Furthermore,trends in petrophysical log responses (Vshale, Phie and Resistivity;Fig. 24) are analyzed and employed for facies recognition in non-cored wells including vertical observation wells as well as

horizontal drains. A Kv/Kh estimation from the pressure build-upanalysis is further applied to characterize the different sedimen-tary facies in simulations.

8.2. Production characteristics

Production in Petrocedeño is based on horizontal wells drilledfrom a central cluster in a radial pattern. This configuration wasselected for providing the optimum production from drains(maximize net pay meters) and taking into account various surfaceconstrains in the field e.g. topography, small-scale rivers andvegetation (called morichal). Furthermore, the radial patternsprovide flexibility to drill wells in accordance to reservoir sandbodies and their orientation.

The lower fluvial dominated part of the Oficina formation isoften referred to as the “Arena Basales” (Sequences 1 and 2) andexhibits by far the best production characteristics. Typical initialflow-rates are found in the range from 1000 to 3000 bpd sustain-able for 2e5 years and after 10 years may still have sufficient viableproduction. However, production in this lower part is affected bythe presence of a flush-zone with low-saline water and precau-tions have to be taken in order to avoid excessive water production(Foulon et al., 2009).

The upper deltaic dominated part (Sequences 3e6) typically hasinitial flow-rates in the order of 500e1000 bpd. These rates varydepending on i) palaeogeographic position (proximal or distal);ii) morphology of the channel sandstone body, and iii) drainorientation versus prevailing paleotransport direction. Productiondecline is less than for the lower fluvial dominated part andproduction can be sustained for several years.

However, the uppermost part of the deltaic dominated part(Sequence 7), which comprises mouth bars, tidal bars and tide-influenced fluvial bars, has a poor production history. Drains drilledin these facies types have typically flow-rates below 500 bpd witha fairly rapid decline suggesting poor volume connectivity mostlikely caused by the heterolithic nature of the facies (Fig. 15). Build-up flow tests also support this interpretation as they typically havea rather low average Kv/Kh ratio of 0.2.

9. Conclusions

Deposits of the Oficina Formation in the Petrocedeño Fieldpresent awell-developed example of the long-term (approximately7 Ma; 2nd-order sequence) change of a deltaic depositional systemas a result of long-term increasing accommodation space ina foreland basin. A number of 3rd- and 4th-order transgressionsand regressions are superposed on the 2nd-order trend.

Fluvial braidplain environments with mostly sand-dominatedbraided and sinuous rivers, often with lateral amalgamation, inthe lowermost stratigraphic part (Sequence 1 and 2) developed intoa fluvially-dominated but notably tidally-influenced delta plainwith straight and sinuous channel belts (Sequences 3e6).Subsequently, a mixed-energy (fluvial and tidal) delta front envi-ronment with numerous distributaries of various dimensionsformed. This delta front drowned and changed into a multipleestuary system with bay-head delta mouth bars containing fluidmuds (Sequence 7). Maximum transgression was reached inSequence 8 with the development of fully marine shale. Subse-quently, a major change of the depositional setting is registeredwith the development of a low-gradient tide-dominated lowerdelta plain in Sequences 9 and 10. Large tide-dominated distribu-taries and meandering channel belts with inclined heterolithicstratification dominated. These were gradually transgressed todevelop into proximal delta platform environments with a limitednumber of distal distributaries and large protected subtidal areas


dominated by tidally controlled fluid mud deposition (Sequence11). The distal delta platform possibly had barrier islands whichprotected the proximal delta platform zone from the wave fetchallowing it to become tide-dominated and enabling fluid muddeposition. Only the basal part of these barriers is preserved due toerosion during the transgression recorded at the top of Sequence 11.Tectonic pulses related to a main Neogene tectonic event at 22 Maare interpreted to have controlled the development of third-ordersequences 1 through 6. Sequences 7 through 11 can tentatively belinked to the dominant control of the EarlyMiocene sea-level curve.Consequently, the reservoir zonation is successfully based on the3rd-order sequence stratigraphic framework which integratessedimentological analysis, static reservoir description and dynamicproduction data.

Acknowledgements

The authors would like to thank Petrocedeño and the share-holders of Petrocedeño (PDVSA, Total and Statoil) for permission topublish this paper. Particular gratitude is expressed to Jhonny Casas(former Petrocedeño sedimentologist, now with Gazprom,Venezuela) who contributed significantly to the increased sedi-mentological and stratigraphic understanding of the Oficina Fm inPetrocedeño. Palynoflora analysis was carried out by E. Gonzalez-Guzman (Consultores Geostrat C.A, Venezuela). The ichnofabricsand trace fossils were analyzed in a pilot study by Luis Buatois(University of Saskatchewan, Canada). Lars Reistad (Statoil) skill-fully and promptly computer drafted the figures. The comments bythe two reviewers, particularly J. Suter, are gratefully acknowledgedand improved the manuscript.

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