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AAPG Bulletin, v. 85, no. 10 (October 2001), pp. 1713–1730 1713

Regional synthesis of theproductive Neocomian complexof West Siberia: Sequencestratigraphic frameworkO. V. Pinous, M. A. Levchuk, and D. L. Sahagian

ABSTRACT

We have developed a regional sequence stratigraphic frameworkfor the productive Neocomian complex of central West Siberia.Sixteen depositional sequences have been identified on the basis ofanalysis of regional seismic lines and well logs. A dip-oriented well-log transect that was constructed across the Nizhnevartovsk andSurgut arches reveals detailed features of the stratigraphic architec-ture and depositional history of the Neocomian section in two-dimensional view. Integration of ammonite biostratigraphic data ledto development of a reliable chronostratigraphic framework for oursequence interpretation.

The Neocomian marine complex ranges in thickness from 350to 700 m and consists of the clinoform (lower part) and topsetpackages. Deposition of the clinoform package occurred during aperiod of at least 9 m.y. when clinoforms prograded more than 550km from east to west. Progradation occurred through lateral shelfoutbuilding mostly during lowstand periods when sandstone unitsaccumulated in shelf-edge deltas, shoreline-shelf systems at theshelf-break zone, and submarine fans in basinal parts (Achimov For-mation). Periods of progradation were commonly interrupted byregional transgressions with significant retreat of depocenters land-ward over the shelf (up to 200 km). The transgressive systems tractson the shelf are laterally extensive shale horizons that representuseful markers for correlation. The overlying highstand deposits areinterpreted as thin but broad sand-prone packages with generallyprogradational appearance. The lowstand systems tracts on the shelfare locally present as fluvial sandstones that are incised into under-lying highstand deposits.

INTRODUCTION

The West Siberian basin is one of the largest sedimentary basins inthe world with an area of 2 � 106 km2. As the largest hydrocarbon

Copyright �2001. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received June 24, 1999; revised manuscript received September 11, 2000; final acceptanceNovember 9, 2000.

AUTHORS

O. V. Pinous � Climate Change ResearchCenter, Institute for the Study of Earth,Oceans and Space, University of NewHampshire, Durham, New Hampshire, 03824;[email protected]

Oleg Pinous is a research scientist at theInstitute of Earth, Oceans and Space. Hereceived his M.S. degree in geology fromNovosibirsk State University in 1993 and aPh.D. in geology from the University of NewHampshire in 1997. For the last five years hehas worked in several research projects onstratigraphic analyses of several basins inRussia and Kazakhstan. His research interestsinclude petroleum geology of the CIS basins,basin modeling, sequence stratigraphy, andsea level change.

M. A. Levchuk � Institute of Oil and GasGeology, Russian Academy of Sciences,Novosibirsk 630090, Russia;[email protected]

Mikhail Levchuk is a senior research scientistat the Institute of Oil and Gas Geology(Novosibirsk, Russia). He obtained his M.S.degree in sedimentary geology fromNovosibirsk State University in 1971 and Ph.D.in lithology and sedimentology from theInstitute of Oil and Gas Geology in 1985. Hehas more than 25 years of experience inexploration/development and researchprojects in the West Siberian basin. Hisinterests include petroleum geology, lithology,facies analysis, and basin modeling.

D. L. Sahagian � Climate Change ResearchCenter, Institute for the Study of Earth,Oceans and Space, University of NewHampshire, Durham, New Hampshire, 03824;[email protected]

Dork Sahagian is the executive director of theGlobal Analysis, Integration, and ModelingTask Force of the International Geosphere-Biosphere program. Having received his Ph.D.in epeirogeny and sea level from theUniversity of Chicago in 1987, he has sinceconducted a many-faceted research programin sea level, basin analysis, tectonics, globalchange, and volcanology. His stratigraphicresearch has lately focused on reconstructionsof Mesozoic and Cenozoic eustasy.

1714 Neocomian Complex of West Siberia

province of Russia (and the former Soviet Union), the West Sibe-rian basin has been extensively explored and exploited for oil andgas during the second half of the 20th century. More than 90% ofthe total oil production in West Siberia is associated with the Neo-comian (Berriasian, Valanginian, and Hauterivian) stratigraphic in-terval (Kontorovich et al., 1994). The best known examples of theproductive Neocomian plays of West Siberia include those in thegiant fields of Samotlor, Salymskaya, Ust-Balyk, Mamontovskaya,Fedorovskaya, and others (Rovenskaya and Nemchenko, 1992). Forexample, the Samotlor field (fifth largest of the world) alone pro-vided at least 45% of the West Siberian oil during the 1970s (Rive,1994). The majority of production in these fields was associatedwith large structurally trapped siliciclastic reservoirs (Peterson andClarke, 1991).

Exploration in theWest Siberian basin began in the early 1950safter the first gas was discovered in Cretaceous reservoirs of theBerezovskaya well (1953). The Surgutskaya well was the first incentral West Siberia (Middle Ob’ region) where oil producing unitswere encountered in the Neocomian interval. Intensive explorationactivities during the 1960s led to discovery of many giant fields suchas Megion (1961), Ust-Balyk (1961), Z-Surgut (1962), and Samo-tlor (1965) (Rovenskaya and Nemchenko, 1992). During that pe-riod of time, the exploration procedure generally included an initialdetection of anticline structures by seismic surveys followed by ex-tensive exploration drilling (Kunin et al., 1993). This procedure hasbecome a common explorational practice and has led to more than1000 successful discoveries of hydrocarbon accumulations.

By the mid 1980s most of the large structural plays had alreadybeen discovered, and many mature reservoirs had been significantlydepleted (Peterson and Clarke, 1991; Prezhentsev, 1992; “TyumenRegion,” 1998). This caused a continuous and steady decline of oilproduction that continued through the late 1980s until the present(Rive, 1994). Production decline led to partial reorientation of theexploration priorities from the Neocomian to other stratigraphicintervals such as the Jurassic and Paleozoic. Nevertheless, the Neo-comian remains the most important stratigraphic interval for ex-ploration in the basin. New targets in the Neocomian includesmaller plays in stratigraphic and combination traps that collectivelyrepresent a significant volume (Karogodin et al., 1996). These playsinclude sandstones of the Achimov Formation (a collective term fordeep-marine sandstones of the Neocomian), as well as previouslyoverlooked small reservoirs in the shelf zone (Naumov and Oni-shuk, 1992; Karogodin et al., 1996).

Initial exploration in the Priobskoe and Prirazlomnoe fields inthe early 1980s revealed very complex internal structure and zoningof the stratigraphically trapped reservoirs (Shpilman et al., 1994;Karogodin et al., 1996; Pinous et al., 1999a). This highlighted se-rious limitations associated with the traditional approach that waspreviously used for exploration and development in structural traps.The case of the Priobskoe field demonstrated that explorational suc-cess in Neocomian stratigraphic traps primarily depends on the

ACKNOWLEDGEMENTS

Thanks go to Victor Zakharov, Boris Shurygin,and Yuri Karogodin for informative discus-sions that added value to our interpretation.We are grateful to S. M. Kamenetskaya fortechnical work. The comments of AAPG re-viewers R. Mitchum, D. Campion, and A. Don-ovan led to substantial improvement of theoriginal manuscript. This work was supportedby NSF (EAR9618945).

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quality and resolution of the stratigraphic models;however, detailed stratigraphic modeling in West Si-berian fields is commonly hampered by disagreementamong regional experts regarding depositional mech-anisms and stratigraphic architectures. This disagree-ment has impeded the development of a unified re-gional framework of the Neocomian in West Siberia.The controversies regarding regional modeling can beresolved by application of sequence stratigraphictechniques.

The primary goal of the research presented herewas to develop a regional stratigraphic framework forthe Neocomian interval of the central part of the WestSiberian basin on the basis of sequence stratigraphy.To accomplish this goal it was necessary to (1) recon-struct the stratigraphic architecture of the Neocomiansequences, (2) develop a chronostratigraphic frame-work, (3) improve reconstruction of depositional his-tory, and (4) examine the main factors that controlledsedimentation (eustasy, tectonics, and sediment supplyrates). To describe our sequence stratigraphic modelwe focused on broadscale definition of depositionalfeatures and architectural elements rather than on spe-cific details.

PREVIOUS RESEARCH

The stratigraphic analyses in the West Siberian basinduring the 1960s and 1970s were conducted primarilyon the basis of geophysical methods (well log and seis-mic). Resolution of seismostratigraphic analysis duringthat period allowed recognition of only the most pro-nounced and continuous reflecting surfaces such as theBazhenov (Upper Jurassic), Alym (lower Aptian), andKuznetsov (upper Cenomanian–lower Turonian) shalehorizons. The identification of productive sandstoneunits and correlation of strata was primarily performedby well-log correlation and core control. The strati-graphic model that had evolved during this period wasbased on the assumption that the Neocomian complexconsists of generally flat-lying subparallel strata (Re-sheniya, 1969). In latitudinal cross section, sand layersgrade westward to shales, and anticline structures actas petroleum traps. This model of Neocomian sedi-mentation was widely accepted by regional experts be-cause of its simplicity and apparent success for explo-ration in simple structural plays. Naumov et al. (1977)published a model that was based on correlation ofclosely spaced well logs from several fields that re-vealed a clinoform geometry of the Neocomian units.

Trushkova (1980) suggested a similar clinoformmodelfor southern regions of West Siberia. Most geologists,however, continued to assume flat-lying stratal corre-lations through most of the 1970s and 1980s.

Extensive regional seismic research that began inthe late 1970s put an end to the traditional approachof “flat” geology. A series of regional common depth-point profiles revealed an apparent clinoform structureof the Neocomian strata of the central part of the basin.This interval has been called the “clinoform Neocom-ian” ever since.

Two main approaches have been developed forstratigraphic interpretation of the clinoform units onthe basis of the new seismic data. According to the firstapproach, seismic units are used as a basis for strati-graphic subdivision and correlation. Kunin et al.(1993) identified a series of 33 formal seismic units(seismopackets) within the west-dipping clinoformpackage of West Siberia on the basis of reflection con-figurations, terminations, and geometrical shapes. Asimilar methodology was applied by Mkrtchyan et al.(1990) who subdivided the west-dipping clinoformsinto 15 “seismocyclites.” The second approach is“lithmo,” or cycle, stratigraphy based on the assertionthat lithological cyclicity of stratigraphic units providesmeans to subdivide a section into standard units of dif-ferent ranges termed “cyclites” (Karogodin and Nezh-danov, 1988; Nezhdanov, 1990; Nezhdanov et al.,1992). By matching the major lithological cycles to theseismic sections, Karogodin and Nezhdanov (1988)identified 24 “zonal cyclites” that comprise the Neo-comian clinoforms of the central West Siberia basin.Although these approaches provide the means for for-mal stratigraphic subdivision and correlation, they donot account for the genetic relationship of stratal unitsas a result of dynamic development of depositional sys-tems within a basin. This lack can be alleviated by ap-plication of sequence stratigraphy as done in our pres-ent study.

REGIONAL OVERVIEW

Basin Formation and Fill

The basement underlying West Siberia consists offolded Paleozoic and Precambrian rocks and was gen-erally consolidated by theMiddle Triassic (Milanovsky,1987). During the Early and Middle Triassic, the sys-tem of north-south–oriented rifts developed across thebasin concurrently with trap volcanism on the East


Siberian craton (Zapivalov et al., 1996). The active rift-ing stage terminated in the Late Triassic and wasfollowed by tectonic subsidence and deposition ofpredominantly siliciclastic sediments of Jurassic,Cretaceous, Paleogene, Neogene, and Quaternary age(Aleinikov et al., 1980; Cramer et al., 1999). Thethickness of Mesozoic–Cenozoic sedimentary cover in-creases northward from an average 3–5 km in the studyarea up to 5–7 km along the coast of the Kara Sea (Pe-terson and Clarke, 1991). Lithospheric thinning asso-ciated with rifting would have led to cooling and con-comitant subsidence; however, the relatively narrowrift basin would have been influenced by the unriftedmarginal areas that provided flexural support, thus ex-panding the area of subsidence beyond the rift bound-aries. This slow flexural subsidence that occurred dur-ing the postrift period was accompanied by occasionalreactivation of the extinct Triassic rift structuresthrough the vertical movement of the basementblocks. For example, the Novomolodezhnaya and Tag-rinskaya uplifts formed during the Late Cretaceous asa result of inversions (uplift) that occurred in theUrengoi-Koltogor rift zone (Figure 1). At present, themain tectonic elements of the study area are the Ni-zhnevartovsk and Surgut arches. These structural unitsaccount for the majority of oil production in the WestSiberian basin, although the surrounding troughs anddepressions contain numerous important oil fields aswell.

Neocomian sedimentation was preceded by dep-osition of black shales of the Bazhenov Formation. Inthe latest Kimmeridgian–Volgian a major regional sub-sidence episode coincided with a eustatic highstandthat induced an extensive marine transgression inWestSiberia (Cramer et al., 1999; Pinous et al., 1999c;Shurygin et al., 1999). A large deep-marine basin cov-ered an area of more than 2 million km2 where depo-sition of organic-rich Bazhenov shales occurred as a re-sult of a pelagic sedimentation in sediment-starvedconditions during the Volgian–Berriasian (Krylov andKorzh, 1984; Braduchan et al., 1986; Kliger, 1994;Gavshyn and Zakharov, 1996). The Bazhenov For-mation is the main source interval of West Siberia(�85% of West Siberian oil) and is one of the largestoil-generating systems in the world (Peters et al.,1993).

The accommodation that developed during theperiod of Bazhenov deposition was subsequently filledduring the Neocomian regression. In the Berriasian thesediment supply rates dramatically increased, and dep-osition of prograding clinoforms commenced at the ba-

sin margins. Progradation of the clinoforms occurredmostly from eastern and western directions until theymet in the axial part of the basin in the Barremian (Fig-ure 2). The sediments were derived from the East Si-berian Highlands and the Urals, with most of the sed-iment influx coming from the east. This resulted inasymmetrical basin fill, with thicker bodies withcoarser sediments on the eastern flanks of the basin,where accommodation was filled by the westward-prograding clinoforms. In plan view the clinoforms arerelatively large, lenticular bodies that trend basinwardand overlap each other in the direction of the sedimentsource. These lenticular bodies are oriented parallelwith the paleoshoreline and are laterally continuous forhundreds of kilometers (Mkrtchyan et al., 1990). Dur-ing the Barremian, the final marine sedimentation oc-curred in a long (about 2000 km) and narrow (50–150km) submeridionally oriented zone in the central partof the basin (Figure 2). This zone has been termed the“terminal channel zone” by regional geologists (Kuninet al., 1993).

Location

The area of study includes the central part of WestSiberia along the Ob’ River that is conventionally re-ferred to as the Middle Ob’ region (Figure 1). Thisregion contains the majority of mature oil fields andis one of the most densely drilled regions in theworld.

Stratigraphy

The Neocomian section in the study area consists ofwest-dipping clinoforms overlain by a topset package.The base of the Neocomian (Berriasian) throughoutmost of the territory is within the black shales of theBazhenov Formation. Bazhenov sedimentary rocks areorganic-rich bituminous shales that contain abundantfossils. In some locations the shales are interbeddedwith layers of siltstones and fine sandstones and inthese cases are referred to as anomalous Bazhenov sec-tions (Yasovich, 1981; Nezhdanov, 1985). Dark grayshales of the Podachimov Formation overlie Bazhenovstrata in most locations (Figure 3). According to re-gional observations, the Podachimov unit was depos-ited in deep-marine conditions prior to turbidite sedi-mentation from approaching clinoforms (Vyachkilevaet al., 1990). The overlying Achimov Formation con-sists of a series of sandstone layers interbedded withhemipelagic shales. Achimov strata generally represent

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Figure 1. Map of West Siberia with area of study. In the left panel the heavy dashed line delineates the boundary of the basin. Numerals 1–15 indicate individual productiveoil fields. Description of legend: 1 � boundary of the basin; 2 � oil fields; 3 � regional seismic profiles; 4 � well-log correlation lines.


Figure 2. Generalized map ofNeocomian sedimentation ofthe West Siberian basin.(adapted from Trushkova, 1980;Kunin et al., 1993). The arrowsdelineate dip directions of theclinoforms. Note the asymmetri-cal structure of the basin fill. 1� boundary of the basin; 2 �maximum transgression (Vol-gian); 3 � approximate shore-line in the beginning of the Va-langinian; 4 � terminalchannel zone; 5 � seismiclines, dip directions of theclinoforms.

turbidite deposits that accumulated at the central partsand toes of the clinoforms (Prezhentsev, 1992; Trush-kova et al., 1992). The overlying strata are composedmostly of shales and siltstones with some minor sand-stone beds representing slope deposits according to po-sition in the section (Figures 3, 4). The slope depositsare overlain by shelf sandstones that comprise the up-per parts of clinoforms.

The overlying strata of the topset part of the Neo-comian section contain significant sandstone beds andshale horizons deposited on the shelf. The shelf sand-stone bed packages are indexed as BV14 through BV3(the B group of beds) and AV8 through AV1 (the Agroup of beds) in the region of the Nizhnevartovsk arch(Figures 1, 3). Individual sand beds are labeled with anadditional number: BV4-1 or AV1-3. In the Surgutarch region a similar indexing system is used, but V

replaces C. As a result, the Surgut bed packages arenamed BC14 through BC1 and AC12 through AC7.Beds BV4 and BV0 of the Nizhnevartovsk region cor-relate to the Surgut BC14 and BC10 beds, respectively(Nezhdanov and Kornev, 1984; Braduchan, 1987b;Mkrtchyan et al., 1990). The sandstone packages areinterbedded with shale horizons such as Samotlor-skaya, Urievskaya, Sarmanovskaya, Pimskaya, and oth-ers. These units consist of fine-grained sediments withabundant organic content and microfossils that formedduring transgressive episodes when depocenters shiftedlandward over large distances on the shelf (Nezhdanov,1984). For example, the Pimskaya transgression (lateHauterivian) induced a shoreline shift of more than180 km eastward and flooded vast areas of coastalplains. The shale horizons represent useful markers fordetailed local and regional correlation because of the

Figure 3. Stratigraphic subdivision of the Neocomian section of the Pokachevskaya 41 well (western margin of the Nizhnevartovskarch). Adapted from Vyachkileva et al. (1990). LST � lowstand systems tract; TST � transgressive systems tract; HST � highstandsystems tract; SB � sequence boundary; TS � transgressive surface; mfs � maximum flooding surface; CS � condensed section;PGC � prograding complex; K-7 � sequence indexes; 1 � sandstone; 2 � siltstone; 3 � shale; 4 � bituminous shale; 5 �ammonites; 6 � bivalves; 7 � fish bones; 8 � chondrites; 9 � Teichichnus; 10 � plant detritus.


effect of their distinct and continuous nature on welllogs and seismic sections (Figures 4, 5).

The marine units in the Neocomian topsets areoverlain by various paralic and continental strata. In theNizhnevartovsk region, deposition of the whole set ofAV units occurred in various coastal plain environ-ments such as lagoons, lakes, and river systems (Erv’e,1972; Ezhova, 1978). The most distinctive feature ofthese deposits is the greenish tint of the sandstones andshales as generally observed in cores (Erv’e, 1974;Nezhdanov et al., 1992). The greenish continentalstrata are capped by the laterally extensive and uniformAlymskaya horizon that was deposited as a result of anAptian marine transgression (Karogodin et al., 1996).The Alymskaya is conventionally used as a datum sur-face to hang the Neocomian sections (Figures 4, 5).

SEQUENCE STRATIGRAPHY

The sequence stratigraphic framework developed inthis analysis is based on 7 regional seismic profiles andthe wire-line data of more than 300 wells from 32 oilfields. The wire-line logs used in the correlation aremostly of the spontaneous potential type, supple-mented by resistivity logs, and in a few cases bygamma-ray logs. To cross-check our wire-line interpre-tation and place the wells in depositional context weconducted sedimentological analyses of 17 cores.

Analysis of seismic data was conducted to deline-ate the main morphological elements of the Neocom-ian strata and define their spatial relationship. Despitethe mediocre quality of some seismic sections, the ba-sic reflection configuration provided adequate infor-mation for a large-scale regional framework (Figure 5).In contrast, wire-line log data with support from coresprovided the highest-resolution data and were used inour study to substantially improve the seismic inter-pretation as well as to delineate fine-scale internal fea-tures of the sequences (Figure 4). Tomatch our seismicand well-log interpretation we examined a series ofwells tied to the seismic sections from the internaltechnical reports of the Institute of Oil and Gas Ge-ology. In those cases, the tying procedure involved gen-erating synthetic seismograms, as is common practicein West Siberia (Mkrtchyan et al., 1990).

The concepts of sequence stratigraphic analysisthat we applied in our study were introduced by VanWagoner et al. (1990), Vail et al. (1991), Mitchum etal. (1993), Emery andMyers (1996), Miall (1996), andPosamentier and Allen (2000). The architecture of the

West Siberian sequences on the well-log cross sectionsomewhat resembles that of late Cenozoic sections ofthe Gulf of Mexico (e.g., figures 17, 21 in Mitchum etal. [1993]). This led us to accept the Mitchum et al.(1993) model with some modifications for our se-quence stratigraphic interpretation.

Sixteen depositional sequences (labeled K-1through K-16 from oldest to youngest) have been iden-tified within the prograding package of west-dippingNeocomian clinoforms of the Middle Ob’ region (Fig-ures 4, 5). The total thickness of the marine parts ofthe sequences gradually increases to the west from 350to 400 m at the Novomolodezhnaya and Ershovayafields to 700 m at the Salymskaya field in the westernpart of the Surgut arch. Each sequence consists of low-stand, transgressive, and highstand systems tracts(HSTs) that are identified on the basis of their strati-graphic architecture and bounding surfaces. Lowstandsystems tracts (LSTs) are interpreted to consist of low-stand fans (LSFs) and lowstand wedges (or progradingcomplexes), although it was not always possible to dis-tinguish these features on seismic andwell-log sections.We chose to use the term “prograding complex” (PGC)rather than “lowstand prograding wedge” because theupper parts of the lowstand units on the sections donot commonly exhibit wedge-shaped geometries. Weabandoned the subdivision of the lowstand systemstract into basin floor fan and slope fan because the in-sufficient quality of the seismic data did not make itpossible to distinguish these features. In addition, ap-plication of these terms as time-stratigraphic units hasbeen widely questioned in the sedimentological liter-ature (Kolla and Perimutter, 1993; Reading and Rich-ards, 1994; Emery andMyers, 1996). Following are thedetailed procedures for sequence and systems tractsrecognition for both seismic and well-log analyses.

Seismic Analysis

Figure 5 demonstrates the example of sequence inter-pretation on the dip-oriented R-13 regional seismicprofile. This is one of the most representative regionallines to be released for publication. Although it is lo-cated about 80–100 km north of the well-log transect,it contains the same sequences and is characterized bysimilar stratigraphic architecture. Thus, the separatelocation of well-log and seismic lines does not interferewith the main goal of broadscale definition of deposi-tional features.

At the base of the Neocomian section, the Bazh-enov Formation generates a distinct peak-trough-peak

Figure 4 Regional wire-line correlation line across the Nizhnevartovsk and Surgutarches. The western and eastern lines are joined at the Asomkinskaya 22 well andtogether represent a continuous line from N-Molodezhnaya to V-Shapshinskaya. All thewire lines are spontaneous potential (red) and resistivity (blue). The datum is the base

of Alymskaya Formation (1500–2100 m). 1 � sandstone units (shoreline-shelf anddeep-marine); 2 � shales (shoreline-shelf and deep-marine); 3 � paralic and conti-nental deposits (sandstones, shales, siltstones); 4 � sequence boundaries; 5� trans-gressive surfaces; 6 � sequence indexes; 7 � indexed sandstone beds.

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Figure 5. Interpretation of the regional seismic line R-13. Common depth-point data are zero phase with trough equal to the negative acoustic impedance contrast. The westernand eastern lines are joined between the Kogolymskaya and V-Konitlorskaya fields. LST � lowstand systems tract; TST � transgressive systems tract; HST � highstand systemstract; HSD � healing stage deposit; SB � the most pronounced sequence boundaries; PGC � prograding complex; K-7 � sequence indexes; BV3-4 � indexed sandstonebeds; Sarmanovskaya � shale horizons.


(doublet) seismic reflection referred to as the B hori-zon. It is easily identified throughout the study areaand represents the most important marker for seismicinterpretation. The lower part of the Neocomian sec-tion displays distinct large-scale clinoforms. The in-ternal configurations of these units show typicaloblique and sigmoidal reflection patterns. In the updipdirection the topset reflections demonstrate generallyconcordant subparallel reflection configurations thatgradually become discontinuous to chaotic eastwardwhere marine units change to their transitional andcontinental equivalents. These generally aggradationalshelf and nonmarine patterns comprise the upper partof the Neocomian section and progressively thicken tothe east (Figure 5). The datum in Figure 5 was set onthe base of the Alymskaya Formation that capsthe Neocomian section and displays the strong nearlycontinuous seismic reflection referred to as the Mhorizon.

In the clinoform part of the Neocomian the LSTsare defined by (1) distinct onlap onto a previous clino-form slope (sequence boundary), (2) a downlappingrelationship with the Bazhenov reflection, and(3) oblique toplap in the upper part. Presence of thelocal zones with mounded configuration (e.g., K-12,K-7, and K-10) that downlap the Bazhenov in bothdirections indicates the presence of LSFs. We expectthat better seismic quality would lead to recognitionof these features in the other Neocomian sequences.The overlying deposits of PGC are characterized bytheir onlapping relationship with the slope of the pre-vious highstand deposits and downlap termination ofthe internal reflection patterns with the Bazhenov ho-rizon and the mounds with bidirectional downlap(LSFs). A shingled appearance of the downlap ter-minations of the PGC in most sequences indicates thepresence of interbedded or “shingled” turbidites. Theupper parts of the PGC commonly demonstrateoblique toplap reflection terminations that occurredas a result of rapid progradation at the shelf edge.These patterns are particularly evident in lowstandunits of K-8, K-12, and K-15 sequences. A shingledappearance of such units on seismic sections suggeststhe presence of a series of prograding sandstone unitsthat pinch out landward at their preceding equivalentsand represent separate compartments. Identificationof such features in the Povkhovskoe field significantlyimpacted the field development process (Orlinsky andFaizullin, 1993).

The PGC units are separated from transgressivesystems tracts (TSTs) by a transgressive surface that

marks a transition from an underlying clinoform in-terval to an interval of mostly topsets. The TSTs con-tain the shale units Cheuskinskaya, Sarmanovskaya,Pimskaya, and others that are commonly expressed byfairly parallel, strong reflections in the topset parts ofsequences. The HSTs generally alternate with trans-gressive deposits on the shelf (topset part) where theylocally demonstrate small-scale progradational config-uration of reflection patterns. The thickness of high-stand deposits may increase considerably at the offlap-break zone where they exhibit a lens morphology(e.g., K-11 and K-12). Sequence K-16 contains awedge-shaped unit that onlaps the subjacent lowstandclinoform and represents a basinal part of the trans-gressive systems tract (Figure 5). We previously iden-tified this feature as a “healing phase” deposit in thesame sequence of the Priobskoe field (Pinous et al.,1999a). A healing-phase wedge forms during trans-gression as fine-grained materials are transported sea-ward, forming a wedge-shaped unit seaward of theshelf break (Posamentier and Allen, 1993a, b).

Well-Log Analysis

The regional well-log transect was constructed acrossthe Nizhnevartovsk and Surgut arches from the No-vomolodezhnaya field to the Verkhne-Shapshinskayafield (Figure 4). Detailed bed-by-bed correlation wasconducted for marine units, whereas individual bedswere not differentiated in continental and paralicstrata because of their discontinuous nature (shown ingreen in Figure 4). In the eastern part of the studyarea (K-1 to K-6) the thickness of marine Neocomianis 350–450 m, which gradually increases to the westreaching 700 m in Salymskaya zone.

The highly bituminous Bazhenov Formation atthe base of the Neocomian is a classic condensed sec-tion. It shows a distinctive response on wire-line logsas a significant increase of resistivity values. At the topof the Neocomian the base of the Alymskaya For-mation represents a distinct correlation marker withits uncommonly low resistivity. The clinoform andtopset parts of the Neocomian section contain a va-riety of depositional environments that are character-ized by specific well-log trends.

Lowstand Systems TractSequence boundaries in the clinoform part of theNeocomian section were identified at the bases ofsandstone units of the Achimov Formation. As men-tioned previously, the LSTs contain discrete units of

Pinous et al. 1723

LSFs that are overlain by PGC deposits. The well-logdata alone, however, did not provide adequate meansto unambiguously separate LSF units from the over-lying PGCs. As a result, we marked all lowstand de-posits on the cross section as LSTs without subdivid-ing them into LSF and PGC.

The Achimov sandstones comprise the lowerparts of LSTs in the basinal parts of the clinoforms.These units were deposited mostly as turbidites insubmarine fan systems and are characterized by a sig-nificant increase in spontaneous potential and resistiv-ity. The well-log trends may vary from cylindrical tofining up, bow trends, and irregular serrated patterns(Figures 3, 4). Cylindrical and bow trends are inter-preted as large individual sandstone sheets of fanlobes. In some specific cases they may delineate amal-gamated massive sandstone units of submarine fansand ramps (Pinous et al., 1999a, b). The fining-upward patterns are interpreted to be the deposits offeeding channels of levee channel zones. The serratedtrends are most common for the Achimov sandstoneunits and depict turbidite deposits that can be relatedto different parts of submarine fans. High-resolutionanalysis of the well logs and cores from the Priobskoefield showed that the similar Neocomian turbiditeunits consist of thinly interbedded sandstones andshales that range in thickness from 0.2 to 2 m (Pinouset al., 1999a). The slope deposits that overlie theAchimov sands commonly show subparallel sponta-neous potential and resistivity lines; however, in somewells the resistivity trends may exhibit frequent ser-rated patterns (e.g., Prirazlomnaya, Salymskaya, andPokamasovskaya). Core studies showed that these sec-tions commonly contain numerous layers of siltstonesthat are in places interbedded with thin beds of veryfine sandstones and display significant variations of re-sistivity (Mkrtchyan et al., 1990; Vyachkileva et al.,1990).

The sandstones units in the upper parts of LSTs(top of PGC) demonstrate typical coarsening-upwardtrends. These deposits formed as a result of rapidprogradation of shelf-edge deltas and the associatedshoreline-shelf systems (Pinous et al., 1999a). Asmentioned in the seismic description, the obliquetoplap patterns at this interval suggest the presenceof a series of small prograding compartments thatpinch out landward at their preceding equivalents.Consequently, we correlated these units as a series ofprograding sandstone packages where seismic controlwas present. If the seismic control was lacking, how-ever, we correlated the sandstone beds as continuous

and flat-lying units, as traditional methods of well-logcorrelation (visual comparison of geometrical shapesof the logs) suggest. Sands associated with deltas andprograding shorelines were deposited in the inner andmiddle neritic zone. At the same time, significantvolumes of sand passed beyond the deltas onto theslope and basin floor. In places, some gravity-drivensands rest on the slope, whereas the bulk of coarse-grained materials was deposited at the base of theslope as shingled turbidites. Each shingled turbiditemay represent a small-scale LSF of a higher-ordersequence.

In the landward position the LSTs contain fluvialchannels that may downcut into underlying marinedeposits (e.g., K-11 and K-6 of Mamontovskaya andPotochnaya fields). These features are interpreted bytypical cylindrical and fining-upward well-log trends.

Transgressive and Highstand Systems TractThe LST deposits are overlain by shale horizons suchas Pimskaya, Sarmanovskaya, and Samotlorskaya.These units represent TSTs that formed during promi-nent regional transgressions when shelf depocentersshifted landward dramatically. Regional studies andcore analysis of these intervals revealed their fine-grained composition, widespread areal distribution,and abundance of fauna, confirming their transgressivenature (Nezhdanov, 1984). On the well logs thetransgressive deposits show generally subparallel shalyintervals with occasional thin sandstone beds (on theshelf). Transgressive surface at the base of each TSTis marked by a sharp transition from deltaic andshallow-marine sandstones to transgressive shales onthe shelf. In several cases, the intervals with low re-sistivity values may be interpreted as maximum flood-ing surfaces at the top of the TST (Figure 3). Theoverlying HST deposits accumulated on the shelf asrelatively thin but broad sandstone prone units. Theirdeposition occurred as a result of rapid progradationof deltaic and shoreline-shelf depositional systems.

The intervals with uncommonly low resistivityvalues (maximum flooding surfaces) within the shalehorizons (e.g., Pimskaya, Pravdinskaya, Cheuskin-skaya) can be traced to the distal parts of the shelf andslope where they represent condensed sections. Thecore studies reveal their hemipelagic nature and abun-dance of fauna, confirming the interpretation (Bor-odkin et al., 1978; Nezhdanov, 1984). The correlationof condensed sections on well logs was particularlyuseful to reveal slope geometries of the clinoforms(e.g., Pravdinskaya horizon).


Regional Examples

To describe the most noticeable stratigraphic featuresand episodes of depositional history we selected threeareas (oil-producing zones): Agansko-Potochnaya,Ma-montovskaya, and Salymskaya (Figures 1, 4).

The Agansko-Potochnaya ZoneThe Agansko-Potochnaya zone is located in the cen-tral and western parts of the Nizhnevartovsk arch andincludes the Aganskaya and Potochnaya fields. Thethickness of marine Neocomian ranges from 400 to500 m in this area. The section includes the clinoformpart of depositional sequence K-6 as well as topsetparts of K-7 and K-8 (Figure 4). In the basinal part,the base of K-6 (sequence boundary) is identified atthe base of Achimov sandstones, although it could notbe unambiguously traced on the slope in the easternflank of the Aganskaya field because of insufficientwell control. Deposition of the deep-water Achimovsandstones in the lower part of the K-6 sequence wasinitiated by a major relative sea level fall that triggeredsediment supply to the submarine fans. The shelfbreak was located between Aganskaya and V-Cher-nogorskaya fields at that time. Deposition of subma-rine fans of the K-6 sequence was accompanied byfluvial incision and fluvial sedimentation in the Cher-nogorskaya and Rubinovaya zones (Figure 4). Depo-sition of BV8 sandstones (PGC) occurred in proximityof the shelf edge in shallow-marine and deltaic envi-ronments when relative sea level was at the lowstandor slowly rising phase. Outbuilding of the slope acrossthe entire Agansko-Potochnaya zone was accompa-nied by simultaneous deposition of Achimov sand-stones as shingled turbidites. Subsequent increase inrise of relative sea level led to transgression and dep-osition of the Samotlorskaya horizon that acts as a sealfor the BV8 shelf reservoirs. The lower part of theSamotlor unit consists mostly of shales with abundantorganic content, provides an excellent correlationmarker, and is interpreted as a TST. During the trans-gression the shoreline shifted at least 125 km east-ward, reaching the eastern flanks of the Samotlor andChernogorskaya fields. The transgressive and overlyingHSTs comprise a thick unit that consist mostly ofshales with occasional thin sandstone layers that areconventionally indexed as BV7. The boundary be-tween TST and HST is not clearly expressed on thewell logs. The overlying part of the section containsthe lowstand shallow-marine BV6 bed that is cappedby the transgressive Urievskaya horizon. In turn, the

Urievskaya unit is overlain by a series of thick fluvialchannels and other nonmarine deposits that comprisethe continental part of the K-8 sequence. B-8 andBV6 represent the most important pay intervals of theAgasko-Potochnaya zone and account for the majorityof oil production in the western part of the Nizhne-vartovsk arch (“Tyumen Region,” 1998).

The Mamontovskaya ZoneThe Mamontovskaya zone is located in the southernpart of the Surgut arch and includes the Mamontov-skaya and Teplovskaya fields (Figures 1, 4). The K-11and K-10 sequences comprise the marine clinoformsin the area. The K-11 sequence contains the most im-portant shelf and deep-marine sandstone bodies. Itwas formed under similar conditions as those of thepreviously described K-6 sequence of the Agansko-Potochnaya zone and displays a generally similar in-ternal architecture. The entire group of BC10 beds isinterpreted as an LST that contains important oil res-ervoirs in both shallow-marine/deltaic units and pos-sibly in fluvial channels. Transgressive deposits of theCheuskinskaya horizon act as a regional seal for theBC10 beds. The marine part of the overlying BC8-9beds comprise the highstand systems tract. The rela-tive sea level fall that followed the deposition of K-11led to significant erosion of the shallow-marine anddeltaic sedimentary rocks of BC8-9 with deposition ofnonmarine sandstones in fluvial depositional systems,as suggested by their well-log configuration. Thesenonmarine units are underlain by the sequenceboundary and represent an LST of the K-12 sequence.Like BC10, BC8-9 beds contain good reservoirs thatare capped by transgressive deposits of the Sarmanov-skaya horizon. A thick aggradational stack of contin-uous sandstone beds (BC6, BC5, BC3, BC4, BC2-3,and BC1) that are interbedded with shale depositsoverlies the Sarmanovskaya. Deposition of the sand-stone beds in the eastern part of the Mamontovskayazone occurred in paralic and continental environmentswhere they represent various channels as suggested bycylindrical and fining-up well-log patterns. In the east-ern part of the zone, the same beds are mostly rep-resented by shallow-marine deposits as suggested bycoarsening-upward well-log trends (Figure 4). TheBC1 bed is capped by the Pimskaya shale horizon thatformed during the most prominent transgression andis the most areally extensive unit within the Neocom-ian. Eastward of Mamontovskoe field, the Pimskayarepresents a distinct marine wedge within the sur-rounding continental strata.

Pinous et al. 1725

The Salymskaya ZoneThe Salymskaya zone is located nearly 60 km west ofthe Mamontovskaya field and includes the Salymskayaoil field (Figures 1, 4). The thickness of themarineNeo-comian reaches 700 m in this area. The K-13 sequencedisplays a thick clinoform part in which the majority ofsandstone units were deposited in submarine fans of theLST (Achimov Formation). The shelf beds of BC5 areconsiderably thinner than shelf equivalents of the Ma-montovskaya zone and are not cut by channels of theoverlying sequence. The outer slope of the sequence iswell expressed by the continuously dipping condensedsectionof thePravdinskayahorizon that is characterizedby a low-resistivity interval. In the shelfal part, Pravdin-skaya is overlain by highstand deposits that contain theBC4 and BC2-3 marine sandstone beds in the easternpart of the field. Deposition of the Pimskaya transgres-sive horizon marks the beginning of a prolonged periodof fine-grained sedimentation during which more than100 m of shales were deposited. A condensed sectionwithin the Pimskayahorizon is clearly identifiedby low-resistivity patterns onwell logs. Another noticeable fea-ture of the Salymskaya zone is a thick transgressivewedge of AC9, AC8, and AC7 beds in the K-15 andK-16 sequences. These units formedduring aprolongedtransgression when the shoreline shiftedmore than 200km eastward from the Priobskoe and V-Shapshinskayafields, reaching the center of the Salymskaya field (Pi-nous et al., 1999a).

BIOSTRATIGRAPHY

The biostratigraphy of Mesozoic marine strata in WestSiberia is based on reference fossils such as ammonites,bivalves, and belemnites, as well as microfossils andpalynomorphs (Vyachkileva et al., 1990; Zakharov etal., 1996). The Middle Jurassic–Lower Cretaceous ofthe region includes some of the most fossiliferous rocksin the boreal realm. Detailed zonal scales for Neocom-ian strata have been developed on the basis of marginaloutcrop sections in West Siberia, and good correlationhas been established with western European chrono-stratigraphic standards (Krymholts et al., 1988). Bio-stratigraphic investigation of cores from central WestSiberia has shown that the Neocomian sections of theregion are characterized by the same biostratigraphiczonal successions as those from the key outcrop sec-tions of the eastern slope of the subarctic Urals and theKhatanga depression (Zakharov et al., 1996). Ammon-ites and bivalves provide the means for the most pre-

cise possible age determination and correlation in theboreal Neocomian. In our study area, however, thesefossils are dispersed in thick sedimentary packages andare relatively uncommon in cores. Nevertheless, ex-perience shows that at least one out of five Neocomiancores in the Middle Ob’ area contains ammonites.Thus, the abundance of drilling data makes it possibleto alleviate the problem of macrofossil rarity andachieve zonal resolution for age dating of stratigraphicunits and correlation. The data on Neocomian am-monites from the Middle Ob’ area have been exten-sively analyzed and revised over the last 20 yr to im-prove regional stratigraphic schemes (Golbert et al.,1971; Braduchan, 1982; Nezhdanov and Kornev,1984; Braduchan, 1987a; Vyachkileva et al., 1990; Pi-nous, 1993). To establish a firm chronostratigraphicframework for our sequence stratigraphic model wesummarized the data on ammonite biostratigraphyfrom West Siberian cores on the basis of published lit-erature and our own results (Figure 6). Most biostrati-graphic zones contain at least several depositional se-quences (up to six in the Neotollia zone), whichsuggest high rates of sedimentation in the Neocomian.

DISCUSSION

The depositional architecture of the Neocomian com-plex of West Siberia is widely accepted as having beenshaped by frequent fluctuations of relative sea levelduring a period of overall regression (Binshtok, 1980;Mkrtchyan et al., 1990; Nezhdanov et al., 1992; Kuninet al., 1993). Although some investigators attributethis to regional factors such as tectonics or changes inthe sedimentation regime (Gurari, 1994), others arguethat these variations are purely eustatic in origin (Go-gonenkov et al., 1988; Nezhdanov, 1990; Pavlova andSmirnov, 1993). These differences in interpretationpoint to a fundamental problem because evaluation ofthe relative contributions of tectonics, sedimentation,and eustasy during deposition is critical for understand-ing the depositional mechanism and history of the cli-noform Neocomian complex.

The main tectonic process related to Neocomiansedimentation in the study area was slow flexural sub-sidence that occurred during the postrift phase (Haf-izov, 1974; Aleinikov et al., 1980). Growth of themodern Nizhnevartovsk and Surgut arches as well asformation of local uplifts through aulacogene inver-sion took place after the Neocomian in the Late Cre-taceous and early Cenozoic (Kontorovich et al.,


Figure 6. Biostratigraphicsummary. The data are summa-rized from Braduchan (1982,1987a), Golbert et al. (1971),Nezhdanov and Kornev (1984),Pinous (1993), Vyachkileva etal. (1990), and our recentresults.

Pinous et al. 1727

1994). Clinoform progradation in the study area oc-curred throughout the Valanginian and Hauterivianover an interval of at least 9 m.y. During this periodclinoforms prograded at least 500 km forming 16 dep-ositional sequences. The great thickness of the pro-grading units (450–750 m for the marine parts) indi-cates very high rates of sediment supply providedfrom the East Siberian craton.

One of the most noticeable features of the clino-forms is that most of them display a lateral continuityalong depositional strike. For example, the clinoformsof K-6, K-13, and K-15 extend hundreds of kilometersnorth and south of the study area as described in sev-eral articles (Mkrtchyan et al., 1990; Karogodin et al.,1996; Kunin, 1998). Transgressive units such as thePimskaya, Sarmanovskaya, and Savuiskaya are identi-fied in the Urengoi area that is 500 km north of theMiddle Ob’, further demonstrating the great areal ex-tent of the Neocomian units (Borodkin et al., 1978;Braduchan, 1982). We consider that it is unlikely thatfrequent regional base-level oscillations that led to adeposition of 16 sequences could be produced entirelyby broadscale tectonic variations (intraplate stress,etc.). Eustatic change is perceived to be a more rea-sonable factor to control regional sedimentation pat-terns in this case. The local tectonics, however, couldbe responsible for lateral variations in a few interpretedsequences. For example, the sequence K-9 demon-strates an considerably thinner pattern on R-13 (Figure5) compared with R-1, R-9, and well-log transects (Fig-ure 4). In addition, K-10 splits into two separate se-quences on R-13.

To assess the relative contributions of eustasy vs.other factors, we applied the quantified eustatic curvegenerated from the central Russian platform (Sahagianet al., 1996). On the basis of measurement of theshoreline shifts on the well-log transect (Figure 4) andthe adjacent seismic sections (R-1 and R-9), we con-structed a transgressive-regressive curve that may serveas a reasonable approximation of regional sea levelchange during deposition of the Neocomian complex(Figure 7). Comparison of the transgressive-regressivecurve to the quantified eustatic curve revealed a closecorrespondence for most events, suggesting at least asignificant influence of eustasy during deposition.

CONCLUDING REMARKS

Application of a sequence stratigraphic approach to theNeocomian complex of West Siberia has made it pos-

sible to integrate the results of multidisciplinary studiesinto a self-consistent regional model of sedimentation.This helps significantly in resolving conceptual contro-versies involved in previous stratigraphic studies on theregion. The most important aspect of our model (lack-ing in previous studies) is that it considers the geneticrelationships between the stratal units resulting fromdynamic development of depositional systems withinthe Neocomian. The well-log transect developed inthis study provides a regional sequence stratigraphicframework in two dimensions that can be subsequentlyenhanced by detailed models of depositional systemsand three-dimensional (3-D) considerations from theareas where detailed exploration and field develop-ment have been conducted for the Neocomian inter-val. For example, the stratigraphic model from thePriobskoe field that we developed in a previous study(Pinous et al., 1999a) may be used to introduce fine-scale details and 3-D considerations into the frame-work we present here.

The stratal architecture of the Neocomian sectionformed as a result of frequent fluctuations of relativesea level during the overall regression. Although tec-tonic subsidence provided copious accommodation,and high rates of sedimentation amplified this, leadingto thick basin fill, eustasy was a significant factor informing the sequence stratigraphic architecture of theNeocomian sections in West Siberia. Knowledge of ac-curate sea level history along with detailed 3-Dmodelsof sedimentation for the region may significantly aid instratigraphic predictions in less intensely drilled partsof the West Siberian Neocomian.

Clinoform depositional packages are among thetypical architectural elements revealed by seismic stud-ies in sedimentary basins. Large-scale series of progra-dational clinoforms similar to those of the West Sibe-rian Neocomian are present in many basins such as thePliocene deposits of Taranaki Basin offshore New Zea-land (Bally, 1987), the Lower Cretaceous of Exmouthplateau offshore Australia (Erskine and Vail, 1988),the Lower Cretaceous of Pletmos and Orange basinsoffshore South Africa (Brink et al., 1993; Muntinghand Brown, 1993), and many others. The West Sibe-rian clinoform complex, however, is the largest in theworld and is also the most densely drilled as a result ofenormous exploration activities during Soviet control(Kontorovich et al., 1994; Rive, 1994). Thus, the de-tailed modeling of the West Siberian Neocomian mayprovide useful means to improve our general under-standing of depositional mechanisms and internal ar-chitecture of clinoform packages.


Figure 7. Comparison of thetransgressive-regressive historyof the West Siberian Neocom-ian to the quantified eustaticcurve generated from Russianplatform stratigraphy (Sahagianet al., 1996). The transgressive-regressive curve was con-structed on the basis of mea-surement of the shoreline shiftson the well-log transect (Figure5) and adjacent seismic sections(R-9 and R-1). The good corre-spondence between the twocurves indicates that eustasywas an important factor duringNeocomian sedimentation inWest Siberia.

Pinous et al. 1729

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