Regional synthesis of the AUTHORS O. V. Pinous ϳ Climate Change Research productive Neocomian complex Center, Institute for the Study of Earth, Oceans and Space, University of New of West : Sequence Hampshire, Durham, New Hampshire, 03824; [email protected] stratigraphic framework Oleg Pinous is a research scientist at the Institute of Earth, Oceans and Space. He O. V. Pinous, M. A. Levchuk, and D. L. Sahagian received his M.S. degree in geology from State University in 1993 and a Ph.D. in geology from the University of New Hampshire in 1997. For the last five years he ABSTRACT has worked in several research projects on stratigraphic analyses of several basins in We have developed a regional sequence stratigraphic framework and Kazakhstan. His research interests for the productive Neocomian complex of central West Siberia. include petroleum geology of the CIS basins, Sixteen depositional sequences have been identified on the basis of basin modeling, sequence stratigraphy, and analysis of regional seismic lines and well logs. A dip-oriented well- sea level change. log transect that was constructed across the and arches reveals detailed features of the stratigraphic architec- M. A. Levchuk ϳ Institute of Oil and Gas ture and depositional history of the Neocomian section in two- Geology, Russian Academy of Sciences, dimensional view. Integration of ammonite biostratigraphic data led Novosibirsk 630090, Russia; to development of a reliable chronostratigraphic framework for our [email protected] sequence interpretation. Mikhail Levchuk is a senior research scientist The Neocomian marine complex ranges in thickness from 350 at the Institute of Oil and Gas Geology to 700 m and consists of the clinoform (lower part) and topset (Novosibirsk, Russia). He obtained his M.S. packages. Deposition of the clinoform package occurred during a degree in sedimentary geology from period of at least 9 m.y. when clinoforms prograded more than 550 Novosibirsk State University in 1971 and Ph.D. in lithology and sedimentology from the km from east to west. Progradation occurred through lateral shelf Institute of Oil and Gas Geology in 1985. He outbuilding mostly during lowstand periods when sandstone units has more than 25 years of experience in accumulated in shelf-edge deltas, shoreline-shelf systems at the exploration/development and research shelf-break zone, and submarine fans in basinal parts (Achimov For- projects in the West Siberian basin. His mation). Periods of progradation were commonly interrupted by interests include petroleum geology, lithology, regional transgressions with significant retreat of depocenters land- facies analysis, and basin modeling. ward over the shelf (up to 200 km). The transgressive systems tracts on the shelf are laterally extensive shale horizons that represent D. L. Sahagian ϳ Climate Change Research useful markers for correlation. The overlying highstand deposits are Center, Institute for the Study of Earth, interpreted as thin but broad sand-prone packages with generally Oceans and Space, University of New Hampshire, Durham, New Hampshire, 03824; progradational appearance. The lowstand systems tracts on the shelf [email protected] are locally present as fluvial sandstones that are incised into under- lying highstand deposits. Dork Sahagian is the executive director of the Global Analysis, Integration, and Modeling Task Force of the International Geosphere- Biosphere program. Having received his Ph.D. INTRODUCTION in epeirogeny and sea level from the University of Chicago in 1987, he has since The West Siberian basin is one of the largest sedimentary basins in conducted a many-faceted research program 2 6 ן the world with an area of 2 10 km . As the largest hydrocarbon in sea level, basin analysis, tectonics, global change, and volcanology. His stratigraphic Copyright ᭧2001. The American Association of Petroleum Geologists. All rights reserved. research has lately focused on reconstructions Manuscript received June 24,1999; revised manuscript received September 11,2000; final acceptance of Mesozoic and Cenozoic eustasy. November 9,2000.

AAPG Bulletin, v. 85, no. 10 (October 2001), pp. 1713–1730 1713 province of Russia (and the former ), the West Sibe- ACKNOWLEDGEMENTS rian basin has been extensively explored and exploited for oil and gas during the second half of the 20th century. More than 90% of Thanks go to Victor Zakharov, Boris Shurygin, the total oil production in West Siberia is associated with the Neo- and Yuri Karogodin for informative discus- comian (Berriasian, Valanginian, and Hauterivian) stratigraphic in- sions that added value to our interpretation. We are grateful to S. M. Kamenetskaya for terval (Kontorovich et al., 1994). The best known examples of the technical work. The comments of AAPG re- productive Neocomian plays of West Siberia include those in the viewers R. Mitchum, D. Campion, and A. Don- giant fields of Samotlor, Salymskaya, Ust-Balyk, Mamontovskaya, ovan led to substantial improvement of the Fedorovskaya, and others (Rovenskaya and Nemchenko, 1992). For original manuscript. This work was supported example, the Samotlor field (fifth largest of the world) alone pro- by NSF (EAR9618945). vided at least 45% of the West Siberian oil during the 1970s (Rive, 1994). The majority of production in these fields was associated with large structurally trapped siliciclastic reservoirs (Peterson and Clarke, 1991). Exploration in the West Siberian basin began in the early 1950s after the first gas was discovered in Cretaceous reservoirs of the Berezovskaya well (1953). The Surgutskaya well was the first in central West Siberia (Middle ’ region) where oil producing units were encountered in the Neocomian interval. Intensive exploration activities during the 1960s led to discovery of many giant fields such as (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 initial detection of anticline structures by seismic surveys followed by ex- tensive exploration drilling (Kunin et al., 1993). This procedure has become a common explorational practice and has led to more than 1000 successful discoveries of hydrocarbon accumulations. By the mid 1980s most of the large structural plays had already been discovered, and many mature reservoirs had been significantly depleted (Peterson and Clarke, 1991; Prezhentsev, 1992; “ Region,” 1998). This caused a continuous and steady decline of oil production that continued through the late 1980s until the present (Rive, 1994). Production decline led to partial reorientation of the exploration priorities from the Neocomian to other stratigraphic intervals 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 include smaller plays in stratigraphic and combination traps that collectively represent a significant volume (Karogodin et al., 1996). These plays include sandstones of the Achimov Formation (a collective term for deep-marine sandstones of the Neocomian), as well as previously overlooked small reservoirs in the shelf zone (Naumov and Oni- shuk, 1992; Karogodin et al., 1996). Initial exploration in the Priobskoe and Prirazlomnoe fields in the early 1980s revealed very complex internal structure and zoning of 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 was previously 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

1714 Neocomian Complex of West Siberia quality and resolution of the stratigraphic models; Trushkova (1980) suggested a similar clinoform model however, detailed stratigraphic modeling in West Si- for southern regions of West Siberia. Most geologists, berian fields is commonly hampered by disagreement however, continued to assume flat-lying stratal corre- among regional experts regarding depositional mech- lations through most of the 1970s and 1980s. anisms and stratigraphic architectures. This disagree- Extensive regional seismic research that began in ment has impeded the development of a unified re- the late 1970s put an end to the traditional approach gional framework of the Neocomian in West Siberia. of “flat” geology. A series of regional common depth- The controversies regarding regional modeling can be point profiles revealed an apparent clinoform structure resolved by application of sequence stratigraphic of the Neocomian strata of the central part of the basin. techniques. This interval has been called the “clinoform Neocom- The primary goal of the research presented here ian” ever since. was to develop a regional stratigraphic framework for Two main approaches have been developed for the Neocomian interval of the central part of the West stratigraphic interpretation of the clinoform units on Siberian basin on the basis of sequence stratigraphy. the basis of the new seismic data. According to the first To accomplish this goal it was necessary to (1) recon- approach, seismic units are used as a basis for strati- struct the stratigraphic architecture of the Neocomian graphic subdivision and correlation. Kunin et al. sequences, (2) develop a chronostratigraphic frame- (1993) identified a series of 33 formal seismic units work, (3) improve reconstruction of depositional his- (seismopackets) within the west-dipping clinoform tory, and (4) examine the main factors that controlled package of West Siberia on the basis of reflection con- sedimentation (eustasy, tectonics, and sediment supply figurations, terminations, and geometrical shapes. A rates). To describe our sequence stratigraphic model similar methodology was applied by Mkrtchyan et al. we focused on broadscale definition of depositional (1990) who subdivided the west-dipping clinoforms features and architectural elements rather than on spe- into 15 “seismocyclites.” The second approach is cific details. “lithmo,” or cycle, stratigraphy based on the assertion that lithological cyclicity of stratigraphic units provides means to subdivide a section into standard units of dif- PREVIOUS RESEARCH ferent ranges termed “cyclites” (Karogodin and Nezh- danov, 1988; Nezhdanov, 1990; Nezhdanov et al., The stratigraphic analyses in the West Siberian basin 1992). By matching the major lithological cycles to the during the 1960s and 1970s were conducted primarily seismic sections, Karogodin and Nezhdanov (1988) on the basis of geophysical methods (well log and seis- identified 24 “zonal cyclites” that comprise the Neo- mic). Resolution of seismostratigraphic analysis during comian clinoforms of the central West Siberia basin. that period allowed recognition of only the most pro- Although these approaches provide the means for for- nounced and continuous reflecting surfaces such as the mal stratigraphic subdivision and correlation, they do Bazhenov (Upper Jurassic), Alym (lower Aptian), and not account for the genetic relationship of stratal units Kuznetsov (upper Cenomanian–lower Turonian) shale as a result of dynamic development of depositional sys- horizons. The identification of productive sandstone tems within a basin. This lack can be alleviated by ap- units and correlation of strata was primarily performed plication of sequence stratigraphy as done in our pres- by well-log correlation and core control. The strati- ent study. graphic model that had evolved during this period was based on the assumption that the Neocomian complex consists of generally flat-lying subparallel strata (Re- REGIONAL OVERVIEW sheniya, 1969). In latitudinal cross section, sand layers grade westward to shales, and anticline structures act Basin Formation and Fill as petroleum traps. This model of Neocomian sedi- mentation was widely accepted by regional experts be- The basement underlying West Siberia consists of cause of its simplicity and apparent success for explo- folded Paleozoic and Precambrian rocks and was gen- ration in simple structural plays. Naumov et al. (1977) erally consolidated by the Middle Triassic (Milanovsky, published a model that was based on correlation of 1987). During the Early and Middle Triassic, the sys- closely spaced well logs from several fields that re- tem of north-south–oriented rifts developed across the vealed a clinoform geometry of the Neocomian units. basin concurrently with trap volcanism on the East

Pinous et al. 1715 Siberian craton (Zapivalov et al., 1996). The active rift- sin margins. Progradation of the clinoforms occurred ing stage terminated in the Late Triassic and was mostly from eastern and western directions until they followed by tectonic subsidence and deposition of met in the axial part of the basin in the Barremian (Fig- predominantly siliciclastic sediments of Jurassic, ure 2). The sediments were derived from the East Si- Cretaceous, Paleogene, Neogene, and Quaternary age berian Highlands and the Urals, with most of the sed- (Aleinikov et al., 1980; Cramer et al., 1999). The iment influx coming from the east. This resulted in thickness of Mesozoic–Cenozoic sedimentary cover in- asymmetrical basin fill, with thicker bodies with creases northward from an average 3–5 km in the study coarser sediments on the eastern flanks of the basin, area up to 5–7 km along the coast of the Kara Sea (Pe- where accommodation was filled by the westward- terson and Clarke, 1991). Lithospheric thinning asso- prograding clinoforms. In plan view the clinoforms are ciated with rifting would have led to cooling and con- relatively large, lenticular bodies that trend basinward comitant subsidence; however, the relatively narrow and overlap each other in the direction of the sediment rift basin would have been influenced by the unrifted source. These lenticular bodies are oriented parallel marginal areas that provided flexural support, thus ex- with the paleoshoreline and are laterally continuous for panding the area of subsidence beyond the rift bound- hundreds of kilometers (Mkrtchyan et al., 1990). Dur- aries. This slow flexural subsidence that occurred dur- ing the Barremian, the final marine sedimentation oc- ing the postrift period was accompanied by occasional curred in a long (about 2000 km) and narrow (50–150 reactivation of the extinct Triassic rift structures km) submeridionally oriented zone in the central part through the vertical movement of the basement of the basin (Figure 2). This zone has been termed the blocks. For example, the Novomolodezhnaya and Tag- “terminal channel zone” by regional geologists (Kunin rinskaya uplifts formed during the Late Cretaceous as et al., 1993). a result of inversions (uplift) that occurred in the Urengoi-Koltogor rift zone (Figure 1). At present, the Location main tectonic elements of the study area are the Ni- zhnevartovsk and Surgut arches. These structural units The area of study includes the central part of West account for the majority of oil production in the West Siberia along the Ob’ River that is conventionally re- Siberian basin, although the surrounding troughs and ferred to as the Middle Ob’ region (Figure 1). This depressions contain numerous important oil fields as region contains the majority of mature oil fields and well. is one of the most densely drilled regions in the Neocomian sedimentation was preceded by dep- world. osition of black shales of the Bazhenov Formation. In the latest Kimmeridgian–Volgian a major regional sub- Stratigraphy sidence episode coincided with a eustatic highstand that induced an extensive marine transgression in West The Neocomian section in the study area consists of Siberia (Cramer et al., 1999; Pinous et al., 1999c; west-dipping clinoforms overlain by a topset package. Shurygin et al., 1999). A large deep-marine basin cov- The base of the Neocomian (Berriasian) throughout ered an area of more than 2 million km2 where depo- most of the territory is within the black shales of the sition of organic-rich Bazhenov shales occurred as a re- Bazhenov Formation. Bazhenov sedimentary rocks are sult of a pelagic sedimentation in sediment-starved organic-rich bituminous shales that contain abundant conditions during the Volgian–Berriasian (Krylov and fossils. In some locations the shales are interbedded Korzh, 1984; Braduchan et al., 1986; Kliger, 1994; with layers of siltstones and fine sandstones and in Gavshyn and Zakharov, 1996). The Bazhenov For- these cases are referred to as anomalous Bazhenov sec- mation is the main source interval of West Siberia tions (Yasovich, 1981; Nezhdanov, 1985). Dark gray (Ͼ85% of West Siberian oil) and is one of the largest shales of the Podachimov Formation overlie Bazhenov oil-generating systems in the world (Peters et al., strata in most locations (Figure 3). According to re- 1993). gional observations, the Podachimov unit was depos- The accommodation that developed during the ited in deep-marine conditions prior to turbidite sedi- period of Bazhenov deposition was subsequently filled mentation from approaching clinoforms (Vyachkileva during the Neocomian regression. In the Berriasian the et al., 1990). The overlying Achimov Formation con- sediment supply rates dramatically increased, and dep- sists of a series of sandstone layers interbedded with osition of prograding clinoforms commenced at the ba- hemipelagic shales. Achimov strata generally represent

1716 Neocomian Complex of West Siberia roductive well-log correlation lines. ס regional seismic profiles; 4 ס oil fields; 3 ס boundary of the basin; 2 ס 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 p Figure 1. oil fields. Description of legend: 1

Pinous et al. 1717 Figure 2. Generalized map of Neocomian sedimentation of the West Siberian basin. (adapted from Trushkova, 1980; Kunin et al., 1993). The arrows delineate dip directions of the clinoforms. Note the asymmetri- cal structure of the basin fill. 1 ס boundary of the basin; 2 ס maximum transgression (Vol- -approximate shore ס gian); 3 line in the beginning of the Va- terminal ס langinian; 4 seismic ס channel zone; 5 lines, dip directions of the clinoforms.

turbidite deposits that accumulated at the central parts replaces C. As a result, the Surgut bed packages are and toes of the clinoforms (Prezhentsev, 1992; Trush- named BC14 through BC1 and AC12 through AC7. kova et al., 1992). The overlying strata are composed Beds BV4 and BV0 of the Nizhnevartovsk region cor- mostly of shales and siltstones with some minor sand- relate to the Surgut BC14 and BC10 beds, respectively stone beds representing slope deposits according to po- (Nezhdanov and Kornev, 1984; Braduchan, 1987b; sition in the section (Figures 3, 4). The slope deposits Mkrtchyan et al., 1990). The sandstone packages are are overlain by shelf sandstones that comprise the up- interbedded with shale horizons such as Samotlor- per parts of clinoforms. skaya, Urievskaya, Sarmanovskaya, Pimskaya, and oth- The overlying strata of the topset part of the Neo- ers. These units consist of fine-grained sediments with comian section contain significant sandstone beds and abundant organic content and microfossils that formed shale horizons deposited on the shelf. The shelf sand- during transgressive episodes when depocenters shifted stone bed packages are indexed as BV14 through BV3 landward over large distances on the shelf (Nezhdanov, (the B group of beds) and AV8 through AV1 (the A 1984). For example, the Pimskaya transgression (late group of beds) in the region of the Nizhnevartovsk arch Hauterivian) induced a shoreline shift of more than (Figures 1, 3). Individual sand beds are labeled with an 180 km eastward and flooded vast areas of coastal additional number: BV4-1 or AV1-3. In the Surgut plains. The shale horizons represent useful markers for arch region a similar indexing system is used, but V detailed local and regional correlation because of the

1718 Neocomian Complex of West Siberia Figure 3. Stratigraphic subdivision of the Neocomian section of the Pokachevskaya 41 well (western margin of the Nizhnevartovsk highstand ס transgressive systems tract; HST ס lowstand systems tract; TST ס arch). Adapted from Vyachkileva et al. (1990). LST ;condensed section ס maximum flooding surface; CS ס transgressive surface; mfs ס sequence boundary; TS ס systems tract; SB ס bituminous shale; 5 ס shale; 4 ס siltstone; 3 ס sandstone; 2 ס sequence indexes; 1 ס prograding complex; K-7 ס PGC .plant detritus ס Teichichnus; 10 ס chondrites; 9 ס fish bones; 8 ס bivalves; 7 ס ammonites; 6 effect of their distinct and continuous nature on well West Siberian sequences on the well-log cross section logs and seismic sections (Figures 4, 5). somewhat resembles that of late Cenozoic sections of The marine units in the Neocomian topsets are the Gulf of Mexico (e.g., figures 17, 21 in Mitchum et overlain by various paralic and continental strata. In the al. [1993]). This led us to accept the Mitchum et al. Nizhnevartovsk region, deposition of the whole set of (1993) model with some modifications for our se- AV units occurred in various coastal plain environ- quence stratigraphic interpretation. ments such as lagoons, lakes, and river systems (Erv’e, Sixteen depositional sequences (labeled K-1 1972; Ezhova, 1978). The most distinctive feature of through K-16 from oldest to youngest) have been iden- these deposits is the greenish tint of the sandstones and tified within the prograding package of west-dipping shales as generally observed in cores (Erv’e, 1974; Neocomian clinoforms of the Middle Ob’ region (Fig- Nezhdanov et al., 1992). The greenish continental ures 4, 5). The total thickness of the marine parts of strata are capped by the laterally extensive and uniform the sequences gradually increases to the west from 350 Alymskaya horizon that was deposited as a result of an to 400 m at the Novomolodezhnaya and Ershovaya Aptian marine transgression (Karogodin et al., 1996). fields to 700 m at the Salymskaya field in the western The Alymskaya is conventionally used as a datum sur- part of the Surgut arch. Each sequence consists of low- face to hang the Neocomian sections (Figures 4, 5). stand, transgressive, and highstand systems tracts (HSTs) that are identified on the basis of their strati- graphic architecture and bounding surfaces. Lowstand SEQUENCE STRATIGRAPHY systems tracts (LSTs) are interpreted to consist of low- stand fans (LSFs) and lowstand wedges (or prograding The sequence stratigraphic framework developed in complexes), although it was not always possible to dis- this analysis is based on 7 regional seismic profiles and tinguish these features on seismic and well-log sections. the wire-line data of more than 300 wells from 32 oil We chose to use the term “prograding complex” (PGC) fields. The wire-line logs used in the correlation are rather than “lowstand prograding wedge” because the mostly of the spontaneous potential type, supple- upper parts of the lowstand units on the sections do mented by resistivity logs, and in a few cases by not commonly exhibit wedge-shaped geometries. We gamma-ray logs. To cross-check our wire-line interpre- abandoned the subdivision of the lowstand systems tation and place the wells in depositional context we tract into basin floor fan and slope fan because the in- conducted sedimentological analyses of 17 cores. sufficient quality of the seismic data did not make it Analysis of seismic data was conducted to deline- possible to distinguish these features. In addition, ap- ate the main morphological elements of the Neocom- plication of these terms as time-stratigraphic units has ian strata and define their spatial relationship. Despite been widely questioned in the sedimentological liter- the mediocre quality of some seismic sections, the ba- ature (Kolla and Perimutter, 1993; Reading and Rich- sic reflection configuration provided adequate infor- ards, 1994; Emery and Myers, 1996). Following are the mation for a large-scale regional framework (Figure 5). detailed procedures for sequence and systems tracts In contrast, wire-line log data with support from cores recognition for both seismic and well-log analyses. provided the highest-resolution data and were used in our study to substantially improve the seismic inter- Seismic Analysis pretation as well as to delineate fine-scale internal fea- tures of the sequences (Figure 4). To match our seismic Figure 5 demonstrates the example of sequence inter- and well-log interpretation we examined a series of pretation on the dip-oriented R-13 regional seismic wells tied to the seismic sections from the internal profile. This is one of the most representative regional technical reports of the Institute of Oil and Gas Ge- lines to be released for publication. Although it is lo- ology. In those cases, the tying procedure involved gen- cated about 80–100 km north of the well-log transect, erating synthetic seismograms, as is common practice it contains the same sequences and is characterized by in West Siberia (Mkrtchyan et al., 1990). similar stratigraphic architecture. Thus, the separate The concepts of sequence stratigraphic analysis location of well-log and seismic lines does not interfere that we applied in our study were introduced by Van with the main goal of broadscale definition of deposi- Wagoner et al. (1990), Vail et al. (1991), Mitchum et tional features. al. (1993), Emery and Myers (1996), Miall (1996), and At the base of the Neocomian section, the Bazh- Posamentier and Allen (2000). The architecture of the enov Formation generates a distinct peak-trough-peak

1720 Neocomian Complex of West Siberia sandstone units (shoreline-shelf and ס Figure 4 Regional wire-line correlation line across the Nizhnevartovsk and Surgut of Alymskaya Formation (1500–2100 m). 1 -paralic and conti ס shales (shoreline-shelf and deep-marine); 3 ס arches. The western and eastern lines are joined at the Asomkinskaya 22 well and deep-marine); 2 -trans ס sequence boundaries; 5 ס together represent a continuous line from N-Molodezhnaya to V-Shapshinskaya. All the nental deposits (sandstones, shales, siltstones); 4 .indexed sandstone beds ס sequence indexes; 7 ס wire lines are spontaneous potential (red) and resistivity (blue). The datum is the base gressive surfaces; 6 st. The western highstand systems indexed sandstone ס ס sequence indexes; BV3-4 ס transgressive systems tract; HST ס prograding complex; K-7 ס lowstand systems tract; TST ס the most pronounced sequence boundaries; PGC ס shale horizons. ס healing stage deposit; SB Interpretation of the regional seismic line R-13. Common depth-point data are zero phase with trough equal to the negative acoustic impedance contra ס tract; HSD and eastern lines are joined between the Kogolymskaya andbeds; V-Konitlorskaya Sarmanovskaya fields. LST Figure 5.

Pinous et al. 1721 (doublet) seismic reflection referred to as the B hori- marks a transition from an underlying clinoform in- zon. It is easily identified throughout the study area terval to an interval of mostly topsets. The TSTs con- and represents the most important marker for seismic tain the shale units Cheuskinskaya, Sarmanovskaya, interpretation. The lower part of the Neocomian sec- Pimskaya, and others that are commonly expressed by tion displays distinct large-scale clinoforms. The in- fairly parallel, strong reflections in the topset parts of ternal configurations of these units show typical sequences. The HSTs generally alternate with trans- oblique and sigmoidal reflection patterns. In the updip gressive deposits on the shelf (topset part) where they direction the topset reflections demonstrate generally locally demonstrate small-scale progradational config- concordant subparallel reflection configurations that uration of reflection patterns. The thickness of high- gradually become discontinuous to chaotic eastward stand deposits may increase considerably at the offlap- where marine units change to their transitional and break zone where they exhibit a lens morphology continental equivalents. These generally aggradational (e.g., K-11 and K-12). Sequence K-16 contains a shelf and nonmarine patterns comprise the upper part wedge-shaped unit that onlaps the subjacent lowstand of the Neocomian section and progressively thicken to clinoform and represents a basinal part of the trans- the east (Figure 5). The datum in Figure 5 was set on gressive systems tract (Figure 5). We previously iden- the base of the Alymskaya Formation that caps tified this feature as a “healing phase” deposit in the the Neocomian section and displays the strong nearly same sequence of the Priobskoe field (Pinous et al., continuous seismic reflection referred to as the M 1999a). A healing-phase wedge forms during trans- horizon. gression as fine-grained materials are transported sea- In the clinoform part of the Neocomian the LSTs ward, forming a wedge-shaped unit seaward of the are defined by (1) distinct onlap onto a previous clino- shelf break (Posamentier and Allen, 1993a, b). form slope (sequence boundary), (2) a downlapping relationship with the Bazhenov reflection, and Well-Log Analysis (3) oblique toplap in the upper part. Presence of the local zones with mounded configuration (e.g., K-12, The regional well-log transect was constructed across K-7, and K-10) that downlap the Bazhenov in both the Nizhnevartovsk and Surgut arches from the No- directions indicates the presence of LSFs. We expect vomolodezhnaya field to the Verkhne-Shapshinskaya that better seismic quality would lead to recognition field (Figure 4). Detailed bed-by-bed correlation was of these features in the other Neocomian sequences. conducted for marine units, whereas individual beds The overlying deposits of PGC are characterized by were not differentiated in continental and paralic their onlapping relationship with the slope of the pre- strata because of their discontinuous nature (shown in vious highstand deposits and downlap termination of green in Figure 4). In the eastern part of the study the internal reflection patterns with the Bazhenov ho- area (K-1 to K-6) the thickness of marine Neocomian rizon and the mounds with bidirectional downlap is 350–450 m, which gradually increases to the west (LSFs). A shingled appearance of the downlap ter- reaching 700 m in Salymskaya zone. minations of the PGC in most sequences indicates the The highly bituminous Bazhenov Formation at presence of interbedded or “shingled” turbidites. The the base of the Neocomian is a classic condensed sec- upper parts of the PGC commonly demonstrate tion. It shows a distinctive response on wire-line logs oblique toplap reflection terminations that occurred as a significant increase of resistivity values. At the top as a result of rapid progradation at the shelf edge. of the Neocomian the base of the Alymskaya For- These patterns are particularly evident in lowstand mation represents a distinct correlation marker with units of K-8, K-12, and K-15 sequences. A shingled its uncommonly low resistivity. The clinoform and appearance of such units on seismic sections suggests topset parts of the Neocomian section contain a va- the presence of a series of prograding sandstone units riety of depositional environments that are character- that pinch out landward at their preceding equivalents ized by specific well-log trends. and represent separate compartments. Identification of such features in the Povkhovskoe field significantly Lowstand Systems Tract impacted the field development process (Orlinsky and Sequence boundaries in the clinoform part of the Faizullin, 1993). Neocomian section were identified at the bases of The PGC units are separated from transgressive sandstone units of the Achimov Formation. As men- systems tracts (TSTs) by a transgressive surface that tioned previously, the LSTs contain discrete units of

1722 Neocomian Complex of West Siberia LSFs that are overlain by PGC deposits. The well-log and flat-lying units, as traditional methods of well-log data alone, however, did not provide adequate means correlation (visual comparison of geometrical shapes to unambiguously separate LSF units from the over- of the logs) suggest. Sands associated with deltas and lying PGCs. As a result, we marked all lowstand de- prograding shorelines were deposited in the inner and posits on the cross section as LSTs without subdivid- middle neritic zone. At the same time, significant ing them into LSF and PGC. volumes of sand passed beyond the deltas onto the The Achimov sandstones comprise the lower slope and basin floor. In places, some gravity-driven parts of LSTs in the basinal parts of the clinoforms. sands rest on the slope, whereas the bulk of coarse- These units were deposited mostly as turbidites in grained materials was deposited at the base of the submarine fan systems and are characterized by a sig- slope as shingled turbidites. Each shingled turbidite nificant increase in spontaneous potential and resistiv- may represent a small-scale LSF of a higher-order ity. The well-log trends may vary from cylindrical to sequence. fining up, bow trends, and irregular serrated patterns In the landward position the LSTs contain fluvial (Figures 3, 4). Cylindrical and bow trends are inter- channels that may downcut into underlying marine preted as large individual sandstone sheets of fan deposits (e.g., K-11 and K-6 of Mamontovskaya and lobes. In some specific cases they may delineate amal- Potochnaya fields). These features are interpreted by gamated massive sandstone units of submarine fans typical cylindrical and fining-upward well-log trends. and ramps (Pinous et al., 1999a, b). The fining- upward patterns are interpreted to be the deposits of Transgressive and Highstand Systems Tract feeding channels of levee channel zones. The serrated The LST deposits are overlain by shale horizons such trends are most common for the Achimov sandstone as Pimskaya, Sarmanovskaya, and Samotlorskaya. units and depict turbidite deposits that can be related These units represent TSTs that formed during promi- to different parts of submarine fans. High-resolution nent regional transgressions when shelf depocenters analysis of the well logs and cores from the Priobskoe shifted landward dramatically. Regional studies and field showed that the similar Neocomian turbidite core analysis of these intervals revealed their fine- units consist of thinly interbedded sandstones and grained composition, widespread areal distribution, shales that range in thickness from 0.2 to 2 m (Pinous and abundance of fauna, confirming their transgressive et al., 1999a). The slope deposits that overlie the nature (Nezhdanov, 1984). On the well logs the Achimov sands commonly show subparallel sponta- transgressive deposits show generally subparallel shaly neous potential and resistivity lines; however, in some intervals with occasional thin sandstone beds (on the wells the resistivity trends may exhibit frequent ser- shelf). Transgressive surface at the base of each TST rated patterns (e.g., Prirazlomnaya, Salymskaya, and is marked by a sharp transition from deltaic and Pokamasovskaya). Core studies showed that these sec- shallow-marine sandstones to transgressive shales on tions commonly contain numerous layers of siltstones the shelf. In several cases, the intervals with low re- that are in places interbedded with thin beds of very sistivity values may be interpreted as maximum flood- fine sandstones and display significant variations of re- ing surfaces at the top of the TST (Figure 3). The sistivity (Mkrtchyan et al., 1990; Vyachkileva et al., overlying HST deposits accumulated on the shelf as 1990). relatively thin but broad sandstone prone units. Their The sandstones units in the upper parts of LSTs deposition occurred as a result of rapid progradation (top of PGC) demonstrate typical coarsening-upward of deltaic and shoreline-shelf depositional systems. trends. These deposits formed as a result of rapid The intervals with uncommonly low resistivity progradation of shelf-edge deltas and the associated values (maximum flooding surfaces) within the shale shoreline-shelf systems (Pinous et al., 1999a). As horizons (e.g., Pimskaya, Pravdinskaya, Cheuskin- mentioned in the seismic description, the oblique skaya) can be traced to the distal parts of the shelf and toplap patterns at this interval suggest the presence slope where they represent condensed sections. The of a series of small prograding compartments that core studies reveal their hemipelagic nature and abun- pinch out landward at their preceding equivalents. dance of fauna, confirming the interpretation (Bor- Consequently, we correlated these units as a series of odkin et al., 1978; Nezhdanov, 1984). The correlation prograding sandstone packages where seismic control of condensed sections on well logs was particularly was present. If the seismic control was lacking, how- useful to reveal slope geometries of the clinoforms ever, we correlated the sandstone beds as continuous (e.g., Pravdinskaya horizon).

Pinous et al. 1723 Regional Examples Urievskaya unit is overlain by a series of thick fluvial channels and other nonmarine deposits that comprise To describe the most noticeable stratigraphic features the continental part of the K-8 sequence. B-8 and and episodes of depositional history we selected three BV6 represent the most important pay intervals of the areas (oil-producing zones): Agansko-Potochnaya, Ma- Agasko-Potochnaya zone and account for the majority montovskaya, and Salymskaya (Figures 1, 4). of oil production in the western part of the Nizhne- vartovsk arch (“Tyumen Region,” 1998). The Agansko-Potochnaya Zone The Agansko-Potochnaya zone is located in the cen- The Mamontovskaya Zone tral and western parts of the Nizhnevartovsk arch and The Mamontovskaya zone is located in the southern includes the Aganskaya and Potochnaya fields. The part of the Surgut arch and includes the Mamontov- thickness of marine Neocomian ranges from 400 to skaya and Teplovskaya fields (Figures 1, 4). The K-11 500 m in this area. The section includes the clinoform and K-10 sequences comprise the marine clinoforms part of depositional sequence K-6 as well as topset in the area. The K-11 sequence contains the most im- parts of K-7 and K-8 (Figure 4). In the basinal part, portant shelf and deep-marine sandstone bodies. It the base of K-6 (sequence boundary) is identified at was formed under similar conditions as those of the the base of Achimov sandstones, although it could not previously described K-6 sequence of the Agansko- be unambiguously traced on the slope in the eastern Potochnaya zone and displays a generally similar in- flank of the Aganskaya field because of insufficient ternal architecture. The entire group of BC10 beds is well control. Deposition of the deep-water Achimov interpreted as an LST that contains important oil res- sandstones in the lower part of the K-6 sequence was ervoirs in both shallow-marine/deltaic units and pos- initiated by a major relative sea level fall that triggered sibly in fluvial channels. Transgressive deposits of the sediment supply to the submarine fans. The shelf Cheuskinskaya horizon act as a regional seal for the break was located between Aganskaya and V-Cher- BC10 beds. The marine part of the overlying BC8-9 nogorskaya fields at that time. Deposition of subma- beds comprise the highstand systems tract. The rela- rine fans of the K-6 sequence was accompanied by tive sea level fall that followed the deposition of K-11 fluvial incision and fluvial sedimentation in the Cher- led to significant erosion of the shallow-marine and nogorskaya and Rubinovaya zones (Figure 4). Depo- deltaic sedimentary rocks of BC8-9 with deposition of sition of BV8 sandstones (PGC) occurred in proximity nonmarine sandstones in fluvial depositional systems, of the shelf edge in shallow-marine and deltaic envi- as suggested by their well-log configuration. These ronments when relative sea level was at the lowstand nonmarine units are underlain by the sequence or slowly rising phase. Outbuilding of the slope across boundary and represent an LST of the K-12 sequence. the entire Agansko-Potochnaya zone was accompa- Like BC10, BC8-9 beds contain good reservoirs that nied by simultaneous deposition of Achimov sand- are capped by transgressive deposits of the Sarmanov- stones as shingled turbidites. Subsequent increase in skaya horizon. A thick aggradational stack of contin- rise of relative sea level led to transgression and dep- uous sandstone beds (BC6, BC5, BC3, BC4, BC2-3, osition of the Samotlorskaya horizon that acts as a seal and BC1) that are interbedded with shale deposits for the BV8 shelf reservoirs. The lower part of the overlies the Sarmanovskaya. Deposition of the sand- Samotlor unit consists mostly of shales with abundant stone beds in the eastern part of the Mamontovskaya organic content, provides an excellent correlation zone occurred in paralic and continental environments marker, and is interpreted as a TST. During the trans- where they represent various channels as suggested by gression the shoreline shifted at least 125 km east- cylindrical and fining-up well-log patterns. In the east- ward, reaching the eastern flanks of the Samotlor and ern part of the zone, the same beds are mostly rep- Chernogorskaya fields. The transgressive and overlying resented by shallow-marine deposits as suggested by HSTs comprise a thick unit that consist mostly of coarsening-upward well-log trends (Figure 4). The shales with occasional thin sandstone layers that are BC1 bed is capped by the Pimskaya shale horizon that conventionally indexed as BV7. The boundary be- formed during the most prominent transgression and tween TST and HST is not clearly expressed on the is the most areally extensive unit within the Neocom- well logs. The overlying part of the section contains ian. Eastward of Mamontovskoe field, the Pimskaya the lowstand shallow-marine BV6 bed that is capped represents a distinct marine wedge within the sur- by the transgressive Urievskaya horizon. In turn, the rounding continental strata.

1724 Neocomian Complex of West Siberia The Salymskaya Zone cise possible age determination and correlation in the The Salymskaya zone is located nearly 60 km west of boreal Neocomian. In our study area, however, these the Mamontovskaya field and includes the Salymskaya fossils are dispersed in thick sedimentary packages and oil field (Figures 1, 4). The thickness of the marine Neo- are relatively uncommon in cores. Nevertheless, ex- comian reaches 700 m in this area. The K-13 sequence perience shows that at least one out of five Neocomian displays a thick clinoform part in which the majority of cores in the Middle Ob’ area contains ammonites. sandstone units were deposited in submarine fans of the Thus, the abundance of drilling data makes it possible LST (Achimov Formation). The shelf beds of BC5 are to alleviate the problem of macrofossil rarity and considerably thinner than shelf equivalents of the Ma- achieve zonal resolution for age dating of stratigraphic montovskaya zone and are not cut by channels of the units and correlation. The data on Neocomian am- overlying sequence. The outer slope of the sequence is monites from the Middle Ob’ area have been exten- well expressed by the continuously dipping condensed sively analyzed and revised over the last 20 yr to im- section of the Pravdinskaya horizon that is characterized prove regional stratigraphic schemes (Golbert et al., by a low-resistivity interval. In the shelfal part, Pravdin- 1971; Braduchan, 1982; Nezhdanov and Kornev, skaya is overlain by highstand deposits that contain the 1984; Braduchan, 1987a; Vyachkileva et al., 1990; Pi- BC4 and BC2-3 marine sandstone beds in the eastern nous, 1993). To establish a firm chronostratigraphic part of the field. Deposition of the Pimskaya transgres- framework for our sequence stratigraphic model we sive horizon marks the beginning of a prolonged period summarized the data on ammonite biostratigraphy of fine-grained sedimentation during which more than from West Siberian cores on the basis of published lit- 100 m of shales were deposited. A condensed section erature and our own results (Figure 6). Most biostrati- within the Pimskaya horizon is clearly identified by low- graphic zones contain at least several depositional se- resistivity patterns on well logs. Another noticeable fea- quences (up to six in the Neotollia zone), which ture of the Salymskaya zone is a thick transgressive suggest high rates of sedimentation in the Neocomian. wedge of AC9, AC8, and AC7 beds in the K-15 and K-16 sequences. These units formed during a prolonged transgression when the shoreline shifted more than 200 DISCUSSION km eastward from the Priobskoe and V-Shapshinskaya fields, reaching the center of the Salymskaya field (Pi- The depositional architecture of the Neocomian com- nous et al., 1999a). plex of West Siberia is widely accepted as having been shaped by frequent fluctuations of relative sea level during a period of overall regression (Binshtok, 1980; BIOSTRATIGRAPHY Mkrtchyan et al., 1990; Nezhdanov et al., 1992; Kunin et al., 1993). Although some investigators attribute The biostratigraphy of Mesozoic marine strata in West this to regional factors such as tectonics or changes in Siberia is based on reference fossils such as ammonites, the sedimentation regime (Gurari, 1994), others argue bivalves, and belemnites, as well as microfossils and that these variations are purely eustatic in origin (Go- palynomorphs (Vyachkileva et al., 1990; Zakharov et gonenkov et al., 1988; Nezhdanov, 1990; Pavlova and al., 1996). The Middle Jurassic–Lower Cretaceous of Smirnov, 1993). These differences in interpretation the region includes some of the most fossiliferous rocks point to a fundamental problem because evaluation of in the boreal realm. Detailed zonal scales for Neocom- the relative contributions of tectonics, sedimentation, ian strata have been developed on the basis of marginal and eustasy during deposition is critical for understand- outcrop sections in West Siberia, and good correlation ing the depositional mechanism and history of the cli- has been established with western European chrono- noform Neocomian complex. stratigraphic standards (Krymholts et al., 1988). Bio- The main tectonic process related to Neocomian stratigraphic investigation of cores from central West sedimentation in the study area was slow flexural sub- Siberia has shown that the Neocomian sections of the sidence that occurred during the postrift phase (Haf- region are characterized by the same biostratigraphic izov, 1974; Aleinikov et al., 1980). Growth of the zonal successions as those from the key outcrop sec- modern Nizhnevartovsk and Surgut arches as well as tions of the eastern slope of the subarctic Urals and the formation of local uplifts through aulacogene inver- Khatanga depression (Zakharov et al., 1996). Ammon- sion took place after the Neocomian in the Late Cre- ites and bivalves provide the means for the most pre- taceous and early Cenozoic (Kontorovich et al.,

Pinous et al. 1725 Figure 6. Biostratigraphic summary. The data are summa- rized from Braduchan (1982, 1987a), Golbert et al. (1971), Nezhdanov and Kornev (1984), Pinous (1993), Vyachkileva et al. (1990), and our recent results.

1726 Neocomian Complex of West Siberia 1994). Clinoform progradation in the study area oc- sible to integrate the results of multidisciplinary studies curred throughout the Valanginian and Hauterivian into a self-consistent regional model of sedimentation. over an interval of at least 9 m.y. During this period This helps significantly in resolving conceptual contro- clinoforms prograded at least 500 km forming 16 dep- versies involved in previous stratigraphic studies on the ositional sequences. The great thickness of the pro- region. The most important aspect of our model (lack- grading units (450–750 m for the marine parts) indi- ing in previous studies) is that it considers the genetic cates very high rates of sediment supply provided relationships between the stratal units resulting from from the East Siberian craton. dynamic development of depositional systems within One of the most noticeable features of the clino- the Neocomian. The well-log transect developed in forms is that most of them display a lateral continuity this study provides a regional sequence stratigraphic along depositional strike. For example, the clinoforms framework in two dimensions that can be subsequently of K-6, K-13, and K-15 extend hundreds of kilometers enhanced by detailed models of depositional systems north and south of the study area as described in sev- and three-dimensional (3-D) considerations from the eral articles (Mkrtchyan et al., 1990; Karogodin et al., areas where detailed exploration and field develop- 1996; Kunin, 1998). Transgressive units such as the ment have been conducted for the Neocomian inter- Pimskaya, Sarmanovskaya, and Savuiskaya are identi- val. For example, the stratigraphic model from the fied in the Urengoi area that is 500 km north of the Priobskoe field that we developed in a previous study Middle Ob’, further demonstrating the great areal ex- (Pinous et al., 1999a) may be used to introduce fine- tent of the Neocomian units (Borodkin et al., 1978; scale details and 3-D considerations into the frame- Braduchan, 1982). We consider that it is unlikely that work we present here. frequent regional base-level oscillations that led to a The stratal architecture of the Neocomian section deposition of 16 sequences could be produced entirely formed as a result of frequent fluctuations of relative by broadscale tectonic variations (intraplate stress, sea level during the overall regression. Although tec- etc.). Eustatic change is perceived to be a more rea- tonic subsidence provided copious accommodation, sonable factor to control regional sedimentation pat- and high rates of sedimentation amplified this, leading terns in this case. The local tectonics, however, could to thick basin fill, eustasy was a significant factor in be responsible for lateral variations in a few interpreted forming the sequence stratigraphic architecture of the sequences. For example, the sequence K-9 demon- Neocomian sections in West Siberia. Knowledge of ac- strates an considerably thinner pattern on R-13 (Figure curate sea level history along with detailed 3-D models 5) compared with R-1, R-9, and well-log transects (Fig- of sedimentation for the region may significantly aid in ure 4). In addition, K-10 splits into two separate se- stratigraphic predictions in less intensely drilled parts quences on R-13. of the West Siberian Neocomian. To assess the relative contributions of eustasy vs. Clinoform depositional packages are among the other factors, we applied the quantified eustatic curve typical architectural elements revealed by seismic stud- generated from the central Russian platform (Sahagian ies in sedimentary basins. Large-scale series of progra- et al., 1996). On the basis of measurement of the dational clinoforms similar to those of the West Sibe- shoreline shifts on the well-log transect (Figure 4) and rian Neocomian are present in many basins such as the the adjacent seismic sections (R-1 and R-9), we con- Pliocene deposits of Taranaki Basin offshore New Zea- structed a transgressive-regressive curve that may serve land (Bally, 1987), the Lower Cretaceous of Exmouth as a reasonable approximation of regional sea level plateau offshore Australia (Erskine and Vail, 1988), change during deposition of the Neocomian complex the Lower Cretaceous of Pletmos and Orange basins (Figure 7). Comparison of the transgressive-regressive offshore South Africa (Brink et al., 1993; Muntingh curve to the quantified eustatic curve revealed a close and Brown, 1993), and many others. The West Sibe- correspondence for most events, suggesting at least a rian clinoform complex, however, is the largest in the significant influence of eustasy during deposition. world and is also the most densely drilled as a result of enormous exploration activities during Soviet control (Kontorovich et al., 1994; Rive, 1994). Thus, the de- CONCLUDING REMARKS tailed modeling of the West Siberian Neocomian may provide useful means to improve our general under- Application of a sequence stratigraphic approach to the standing of depositional mechanisms and internal ar- Neocomian complex of West Siberia has made it pos- chitecture of clinoform packages.

Pinous et al. 1727 Figure 7. Comparison of the transgressive-regressive history of the West Siberian Neocom- ian to the quantified eustatic curve generated from Russian platform stratigraphy (Sahagian et al., 1996). The transgressive- regressive curve was con- structed on the basis of mea- surement of the shoreline shifts on the well-log transect (Figure 5) and adjacent seismic sections (R-9 and R-1). The good corre- spondence between the two curves indicates that eustasy was an important factor during Neocomian sedimentation in West Siberia.

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1730 Neocomian Complex of West Siberia