Giant Hydrocarbon Reservoirs of the World (AAPG Memoir No. 88)

Copyrig ht © 2006 By the American Association of Perroleum Cc~>logists (AAPG) and SEPM (Society fo r Sedimentary Geology) All Rights Reserved

!SllN: 978-0-89181-704-8 0-89181-704-2

AAPC a nd SEPM grant permission for a single pho tocopy of an item fro m this publicatio n fo r personal use . Authorization for add itio nal copies o f items from this publication for personal or internal use is g ran ted by AAJ>G and SEPM provided that the base fee of $3.50 per copy and $.50 per page is paid d irectly to the Co pyrig ht Cleara nce Center, 222 Roswood Drive, Danvers, Massachllsetts 01923 (phone: 978/750-8400). Fees are subject to change. Any form of electron.ic or d igital sca n.niJ>g or ot her d igital transformatio n of portions ot' t·his publication i.nto computer-readable M>d/or t·ra nsm ittable form for persoMI or corporate use re
mama Group) o f Abu Dhabi in wb icb important hydroca.rbon accumu lations occur in p latform carbonates. The Kharaib and Lower Shuaiba form ations contain th ree reservoir units separated by low-porosity and permeability dense zones. Core and well-Jog data fro m a gia nt oil field in Abu Dhabi and outcrop data fro m Wadi Rah abah in Ras Al-Khaimah were used to establish a seq uence-s tratigraphic framework and a litl1ofacies scheme. The Lower a nd Upper Kharaib Reservoir Units, as well as the upper dense zone, are part of a late transgressive sequence set of a scc.ondorder supersequence, built by two third-ord er composite sequences. The overlying Lower Shuaiba Reservoir Unit belongs to the la te transgressive sequence set and the early highstand sequence set o f this secondorder supersequence a nd comprises one third-order composite sequence. The th ree third-o rder composite sequences a re composed o f fourth-ord er parasequence sets that show p redo minantly aggradational and progradational stacking patterns, typical of greenhouse cycle.~. Reservoir lithofacies range from lowerramp to shoal crest to near backshoal open-platform deposits, whereas nonreservoir (dense) lithofacies represent an inner-ramp, restricted shallow lagoonal setting. Integration of subsurface and o utcrop data leads to more insightful and realistic geological models of the subsurface stratigraphy, and the geological model realizations based on core, outcrop, well-log, and seismic data co nstrain now-simulation models. Yose, Ruf, Strohmenger, Sch uelke, Gombos, AIHosani, AI Maskary, Bloch, Al-Mehairi, a nd Johnson integrate high-resolution three-dimensional (3-D) seismic with geologic and production data to describe the Lower Cretaceous (Aptian) reservoir in Abu Dhabi in "Volume-based Characterization of a Heterogeneous Carbonate Reservoir, Lower Cretaceous, Abu Dhabi (United Arab Emirates)." The. reservoir is positioned over a platform-to-basin transition and rec.ords a diverse range of depositional facies and stratal geometries. A second-order sequence set is divided into five depositiona l sequen ces. Sequences 1 and 2 are a transgressional phase showing the initial formation of buildup ma rgins and do minated by a lgaJ.prone f•cies. The subsequent h i.ghstand phase of Sequence 3 is mainly aggradational and records the proliferation of rudists across t he platfo rm !OJ>. A late highsta nd phase of sequences 4 and 5 is progradational showing th e progressive downstepping of the platform margin onto a low-angle slope. Three-dimen sional seismic data in the southern field area show a com plex mosaic of tidal channels, high-energy rudist shoals, and intershoal ponds. The geometry and reservoir-quality

4 1 Harris and \Veber

variations of these geologic features have a strong impact on reservo ir S\oveep and co nformance in the platform interior. ln the northern field area, seismic images of prograding slope clinoforms reveal system atic variations in architecture and reservoir qualitythat reflect multiple scales of stratigraphic cyclicity. A pat· tern gas llood has been Implemented in the clinoforms to add pressure support and improve recovery. Busi· ness applications of the reservoi r framework include (1) 3-0 seismic visua lization as a tool for optimizing well placement, identifying bypassed reservoirs and evaluating reservoir connectivity; (2) integra tion of quantitative, volume-based seismic informatio n into reservoir models; (3) m<Jximjzing recovery through full integration of a ll subsurface data; and (4) enhe, and petrographic data suggest that the dolomitization and dissolution can be attributed to deep burial and hydrothermal fluids. Sout h America

In "Sedimentology, Sequence Stratigraphy, and Reservoir Architecture of the Eocene Mirador !'ormation, Cupiagua Field, Llanos Footh.ills, Colombia," Ramon and Fajardo document the stratigraph.ic architecture and facies distribution in a h igh-resolution ti me-space framework to define the 3-D reservoir zonati on of the Mirador Formation (Eocene) in the Cupiagua field. The Cupiagua structure is a large, eastverging. asymmetric anticlinal fold that trends no rthnortheast in the hanging wall o f the frontal fault. The Mirador Formation accounts for approximately 55% of the recoverable oil in Ute field. Three scale.~ of strati· graphic cycles are recognized based on stacking pat· tern and general lrend of facies successions: shortterm cycles or progradational and aggradational units stack systematically into intermediate-term cycles, which, in turn, are grouped into long-term cycles. The lower half of the Mirador consists of flood-plain facies with channel, crevasse splay/ and swamp and flood-plain facies successions. Bay-head delta and bayfill facies occur in the upper half of the Mirador Formation. The Lower 1\·firador consists or two intermediatesca le cycles s howing a seaward-stepping stacking pattern overla in by a third cycle with a landwardstepping pattern, and the upper Mirador conti nues the landward-stepping pattern . This upper unit con-

sists of tluee onlapping cycles composed of a succession of aggradatio nal channel deposits, p rogradational bay-head delta and bay-fill deposits with a landward -stepping stacking pattern. The Mirador is capped by restricted marine shales of the Carbonera Formation.

ACKNOWLEDGMENTS As organizers of the core workshop and editors of the companion voltune, we acknowledge several people and our respec.tive companies for their assistance, without which the workshop and volume would not have happened. Chevron and ExxonMobil provided generous financial assistance that enabled numerous students to attend the core workshop and subsidized printing costs o f the worksl•op volume. SEPM staff helped us o ve rco me n umerous o rganizational and logistica l issues that confronted us as we organized the workshop. AAPG staff and, in particular, Beverly Molyneux provided us invaluable help in the editing and publication of the workshop volume. We thank all of the authors wh o worked bard to prepare poster and core displays, oral presentations, and the detailed manuscripts t11at describe their respective fields. Finally, we thank the many who helped us in the technical review of the manuscripts for the workshop publication: Bob Alway, Steve Bachtel, Sherry Becker, Kelley Bergman, j ohn Bova,joel Colli ns. Bob Dalrymple, Laurence Droz. Ch.ip Feazel, Sean Guidry, Jurgen Grotseh, jean Hsieh, jon Kaufman, Mike Kozar, Dale leckie, j ose Matos, jim 1\kGovney, Gary Parker, Carlos Plrmez, George Pemberton, Linda l>rJce, Gene Rankey, Rick Sarg, Toni Simo, Krishnan Srinivasan, and Niall Toomey.

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Kenter, J. A. M., P. M. Harris, J. F. Collins, L. J. Weber, G. Kuanysheva, and D. J. Fischer, 2006, Late Visean to Bashkirian platform cyclicity in the central Tengiz buildup, Precaspian Basin, Kazakhstan: Depositional evolution and reservoir development, in P. M. Harris and L. J. Weber, eds., Giant hydrocarbon reservoirs of the world: From rocks to reservoir characterization and modeling: AAPG Memoir 88/SEPM Special Publication, p. 7 – 54.

Late Visean to Bashkirian Platform Cyclicity in the Central Tengiz Buildup, Precaspian Basin, Kazakhstan: Depositional Evolution and Reservoir Development J. A. M. Kenter1

L. J. Weber

Vrije Universiteit, Amsterdam, Netherlands

ExxonMobil Development Company, Houston, Texas, U.S.A.

P. M. Harris Chevron Energy Technology Company, San Ramon, California, U.S.A.

G. Kuanysheva

J. F. Collins

D. J. Fischer

ExxonMobil Development Company, Houston, Texas, U.S.A.

TengizChevroil, Atyrau, Kazakhstan

TengizChevroil, Atyrau, Kazakhstan

ABSTRACT

T

he Tengiz buildup, an intensely cored and studied isolated carbonate platform in the Precaspian Basin, contains a succession of shallow-water deposits ranging from Famennian to Bashkirian in age. From a reservoir perspective, Tengiz can be subdivided into platform (central and outer) and rimslope (flank) regions. The upper Visean, Serpukhovian, and Bashkirian form the main hydrocarbon-bearing interval in the platform. Depositional cycles (highfrequency sequences) in this interval are several to tens of meters thick for the Visean and Serpukhovian, and decimeter to meter scale for the Bashkirian. Cycles are made up of a succession of lithofacies overlying a sharp base that locally shows erosion, calcretes, meteoric diagenesis, and other evidence for subaerial exposure. At the base of the succession, tight peloidal mudstone and ash beds are associated with sequence boundaries and are thought to reflect lowenergy conditions developed in deeper platform areas at lowstand and during 1

Present address: Chevron Energy Technology Company, San Ramon, California, U.S.A.

Copyright n2006 by The American Association of Petroleum Geologists. DOI:10.1306/1215873M88374

7

8 / Kenter et al.

initial flooding. Above this, beds with in-situ articulated brachiopods signal initial open-marine but still low-energy conditions. Succeeding crinoid-dominated intervals represent maximum marine flooding and overlying skeletal-peloidal grainstones highstand shoaling phases. Visean and Serpukhovian cycles are generally easy to correlate from well to well over several kilometers distance. Volcanic ash beds are identified by gammaray spikes, and flooding intervals show as low-porosity zones. In contrast, Bashkirian cycles are thinner and incomplete, dominated by thin, peloidal mudstone intervals alternating with high-energy coated-grain and ooid grainstone, and are more difficult to correlate. High-frequency icehouse sea level fluctuations exposed the platform during each fall of sea level, and rapid flooding resulted in incomplete cycles and complex lateral facies changes that may explain relatively poor lateral continuity of log character. The distribution of reservoir rock types in the central platform is determined by burial diagenetic modification of an earlier reservoir system that includes meteoric alteration and porosity enhancement below major sequence boundaries and reduced dissolution along higher order sequence boundaries associated with the presence of volcanic ash. The lateral continuity of tight layers at sequence boundaries probably greatly affected later fluid flow as well as the ultimate distribution of cements, dissolution, and bitumen in the central platform reservoir. The burial diagenetic overprint included two major phases of reservoir modification. First, a corrosion and cementation phase significantly enhanced existing matrix porosity in the interior central platform while reducing porosity in the exterior central and outer platform by pore-filling equant calcite cement. This was followed by bitumen emplacement and associated corrosion. These processes not only exerted an overall porosity-reducing effect prior to and associated with bitumen invasion toward the exterior central platform, but also dampened or flattened the initial cyclic porosity variations and obscured relationships between pore types and permeability. The bitumen overprint is nearly absent in the innermost platform wells; bitumen concentrations are highest near the bases of the cycles, which may imply that the first fill of hydrocarbons migrated through the flanks laterally into the platform cycles.

INTRODUCTION Tengiz Field History The Tengiz field, located in western Kazakhstan, near the northeastern shore of the Caspian Sea (Figure 1), produces oil from an isolated carbonate platform (aerial extent of >110 km2 [42 mi2]) of Devonian and Carboniferous age. Tengiz was discovered in 1979 by the Ministry of Oil Industry of the Soviet Union. The discovery well Tengiz 1 (T-1) reached a total depth of 4095 m (13,435 ft). Development drilling of the Tengiz field commenced in 1983, and onsite construction of plant-processing facilities began in 1987. Field production officially began in April 1991. Since 1993, TengizChevroil, an in-country joint venture company run by Chevron, has operated Tengiz and the adjacent Korolev field.

Tengiz field produces a light, intermediate-sulfur, stabilized tank oil of approximately 478 API. As of midyear 2005, more than 115 wells have been drilled on Tengiz. The highest rate wells are located in the platform margin and slope in fractured carbonates with low (