West Siberian Examples

 


Regional synthesis of the productive Neocomian complex of West Siberia: Sequence Stratigraphic Framework

O. V.Pinous, M.A. Levchuk, and D.L. Sahagian, AAPG Bulletin, v. 85, no.10 (October 2001), pp . 1713–1730

The Neocomian section in the study area consists of west-dipping clinoforms overlain by a topset package. The base of the Neocomian (Berriasian) throughout most of the territory is within the black shales of the Bazhenov Formation. Bazhenov sedimentary rocks are organicrich bituminous shales that contain abundant fossils. In some locations the shales are interbedded with layers of siltstones and fine sandstones and in these cases are referred to as anomalous Bazhenov sections (Yasovich, 1981; Nezhdanov, 1985). Dark gray shales of the Podachimov Formation overlie Bazhenov strata in most locations (Figure 3). According to regional observations, the Podachimov unit was deposited in deep marine conditions prior to turbidite sedimentation from approaching clinoforms (Vyachkileva et al., 1990). The overlying Achimov Formation consists of a series of sandstone layers interbedded with hemipelagic shales. Achimov strata generally represent turbidite deposits that accumulated at the central parts and toes of the clinoforms (Prezhentsev, 1992; Trushkova et al., 1992). The overlying strata are composed mostly of shales and siltstones with some minor sandstone beds representing slope deposits according to position in the section (Figures 3, 4). The slope deposits are overlain by shelf sandstones that comprise the upper parts of clinoforms.

The overlying strata of the topset part of the Neocomian section contain significant sandstone beds and shale horizons deposited on the shelf. The shelf sandstone bed packages are indexed as BV14 through BV3 (the B group of beds) and AV8 through AV1 (the A group of beds) in the region of the Nizhnevartovsk arch. Individual sand beds are labeled with an additional number: BV4 1 or AV13. In the Surgut arch region a similar indexing system is used, but V replaces C. As a result, the Surgut bed packages are named BC 14 through BC 1 and AC 12 through AC7. Beds BV4 and BV0 of the Nizhnevartovsk region correlate to the Surgut BC 14 and BC 10 beds, respectively (Nezhdanov and Kornev, 1984; Braduchan, 1987b; Mkrtchyan et al., 1990). The sandstone packages are interbedded with shale horizons such as Samotlorskaya, Urievskaya, Sarmanovskaya, Pimskaya, and others. These units consist of fine-grained sediments with abundant organic content and microfossils that formed during transgressive episodes when depocenters shifted landward over large distances on the shelf (Nezhdanov, 1984). For example, the Pimskaya transgression (late Hauterivian) induced a shoreline shift of more than 180 km eastward and flooded vast areas of coastal plains. The shale horizons represent useful markers for detailed local and regional correlation because of the effect of their distinct and continuous nature on well logs and seismic sections. The marine units in the Neocomian topsets are overlain by various paralic and continental strata. In the Nizhnevartovsk region, deposition of the whole set of AV units occurred in various coastal plain environments such as lagoons, lakes, and river systems (Erv'e, 1972; Ezhova, 1978). The most distinctive feature of these deposits is the greenish tint of the sandstones and shales as generally observed in cores (Erv'e, 1974; Nezhdanov et al., 1992). The greenish continental strata are capped by the laterally extensive and uniform Alymskaya horizon that was deposited as a result of an Aptian marine transgression (Karogodin et al., 1996). The Alymskaya is conventionally used as a datum surface to hang the Neocomian sections.

At the base of the Neocomian section, the Bazhenov Formation generates a distinct peak-trough-peak (doublet) seismic reflection referred to as the B Horizon. It is easily identiifed throughout the study area and represents the most important marker for seismic interpretation.The lower part of the Neocomian section displays distinct largescale clinoforms. The internal configurations of these units show typical oblique and sigmoidal reflection patterns.In the updip direction the topset reflections demonstrate generally concordant subparallel reflection configurations that gradually become discontinuous to chaotic eastward where marine units change to their transitional and continental equivalents. These generally aggradational shelf and non-marine patterns comprise the upper part of the Neocomian section and progressively thicken to the east. In the clinoform part of the Neocomian the LSTs are defined by (1) distinct onlap onto a previous clinoform slope (sequence boundary), (2) a downlapping relationship with the Bazhenov reflection, and (3) oblique toplap in the upper part. Presence of the local zones with mounded configuration (e.g., K12, K7, and K10) that downlap the Bazhenov in both directions indicates the presence of LSFs. We expect that better seismic quality would lead to recognition of these features in the other Neocomian sequences. The overlying deposits of the "Prograding Complex (PGC) are characterized by their onlapping relationship with the slope of the previous highstand deposits and downlap termination of the internal reflection patterns with the Bazhenov horizon and the mounds with bidirectional downlap (LSFs). A shingled appearance of the downlap terminations of the PGC in most sequences indicates the presence of interbedded or “shingled” turbidites. The upper parts of the PGC commonly demonstrate oblique toplap reflection terminations that occurred as a result of rapid progradation at the shelf edge. These patterns are particularly evident in lowstand units of K8, K1 2, and K15 sequences.A shingled appearance of such units on seismic sections suggests the presence of a series of prograding sandstone units that pinch out landward at their preceding equivalents and represent separate compartments. Identiifcation of such features in the Povkhovskoe field significantly impacted the field development process (Orlinsky and Faizullin, 1993).

The PGC units are separated from transgressive systems tracts (TSTs) by a transgressive surface that marks a transition from an underlying clinoform interval to an interval of mostly topsets.The TSTs contain the shale units Cheuskinskaya, Sarmanovskaya, Pimskaya, and others that are commonly expressed by fairly parallel, strong reflections in the topset parts of sequences. The HSTs generally alternate with transgressive deposits on the shelf (topset part) where they locally demonstrate small-scale progradational configuration 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., K1 1 and K12). Sequence K16 contains a wedge-shaped unit that onlaps the subjacent lowstand clinoform and represents a basinal part of the transgressive systems tract (Figure 5). We previously identified this feature as a “healing phase” deposit in the same sequence of the Priobskoe Field (Pinous et al., 1999a). A healing-phase wedge forms during transgression as fine-grained materials are transported seaward, forming a wedge-shaped unit seaward of the shelf break (Posamentier and Allen, 1993 a, b).

   

 

Well-Log Analysis

The highly bituminous Bazhenov Formation at the base of the Neocomian is a classic condensed section. It shows a distinctive response on wire-line logs as a significant increase of resistivity values. At the top of the Neocomian the base of the Alymskaya Formation represents a distinct correlation marker with its uncommonly low resistivity. The clinoform and topset parts of the Neocomian section contain a va­riety of depositional environments that are characterized by speciifc well-log trends.

Lowstand Systems Tract

Sequence boundaries in the clinoform part of the Neocomian section were identified at the bases of sandstone units of the Achimov Formation. As mentioned previously, the LSTs contain discrete units of LSFs that are overlain by PGC deposits. The well-log data alone, however, did not provide adequate means to unambiguously separate LSF units from the overlying PGCs. As a result, we marked all lowstand deposits on the cross section as LSTs without subdividing them into LSF and PGC. The Achimov sandstones comprise the lower parts of LSTs in the basinal parts of the clinoforms. These units were deposited mostly as turbidites in submarine fan systems and are characterized by a significant increase in spontaneous potential and resistivity. The well-log trends may vary from cylindrical to fining up, bow trends, and irregular serrated patterns. Cylindrical and bow trends are interpreted as large individual sandstone sheets of fan lobes. In some specific cases they may delineate amalgamated massive sandstone units of submarine fans and ramps (Pinous et al., 1999a, b). The fining-upward patterns are interpreted to be the deposits of feeding channels of levee channel zones. The serrated trends are most common for the Achimov sandstone units and depict turbidite deposits that can be related to different parts of submarine fans. High-resolution analysis of the well logs and cores from the Priobskoe Field showed that the similar Neocomian turbidite units consist of thinly interbedded sandstones and shales that range in thickness from 0.2 to 2 m (Pinous et al., 1999a). The slope deposits that overlie the Achimov sands commonly show subparallel spontaneous potential and resistivity lines; however, in some wells the resistivity trends may exhibit frequent serrated patterns (e.g., Prirazlomnaya, Salymskaya, and Pokamasovskaya). Core studies showed that these sections commonly contain numerous layers of siltstones that are in places interbedded with thin beds of very fine sandstones and display signiifcant variations of resistivity (Mkrtchyan et al., 1990; Vyachkileva et al., 1990). The sandstones units in the upper parts of LSTs (top of PGC) demonstrate typical coarsening-upward trends. These deposits formed as a result of rapid progradation of shelf-edge deltas and the associated shoreline-shelf systems (Pinous et al., 1999a). As mentioned in the seismic description, the oblique toplap patterns at this interval suggest the presence of a series of small prograding compartments that pinch out landward at their preceding equivalents. Consequently, we correlated these units as a series of prograding sandstone packages where seismic control was present. If the seismic control was lacking, however, we correlated the sandstone beds as continuous and flat-lying units, as traditional methods of well-log correlation (visual comparison of geometrical shapes of the logs) suggest. Sands associated with deltas and prograding shorelines were deposited in the inner and middle neritic zone.At the same time, significant volumes of sand passed beyond the deltas onto the slope and basin floor. In places, some gravity-driven sands rest on the slope, whereas the bulk of coarse-grained materials was deposited at the base of the slope as shingled turbidites. Each shingled turbidite may represent a small-scale LSF of a higher-order sequence. In the landward position the LSTs contain fluvial channels that may downcut into underlying marine deposits (e.g., K-11 and K-6 of Mamontovskaya and Potochnaya fields). These features are interpreted by typical cylindrical and fining-upward well-log trends.

Transgressive and Highstand Systems Tract

The LST deposits are overlain by shale horizons such as Pimskaya, Sarmanovskaya, and Samotlorskaya. These units represent TSTs that formed during prominent regional transgressions when shelf depocenters shifted landward dramatically. Regional studies and core analysis of these intervals revealed their fine-grained composition, widespread areal distribution, and abundance of fauna, confirming their transgressive nature (Nezhdanov, 1984). On the well logs the transgressive deposits show generally subparallel shaly intervals with occasional thin sandstone beds (on the shelf). Transgressive surface at the base of each TST is marked by a sharp transition from deltaic and shallow-marine sandstones to transgressive shales on the shelf. In several cases, the intervals with low resistivity values may be interpreted as maximum flooding surfaces at the top of the TST. The overlying HST deposits accumulated on the shelf as relatively thin but broad sandstone prone units. Their deposition occurred as a result of rapid progradation of deltaic and shoreline-shelf depositional systems.

The intervals with uncommonly low resistivity values (maximum flooding surfaces) within the shale horizons (e.g., Pimskaya, Pravdinskaya, Cheuskin­skaya) can be traced to the distal parts of the shelf and slope where they represent condensed sections. The core studies reveal their hemipelagic nature and abundance of fauna, confirming the interpretation (Borodkin et al., 1978; Nezhdanov, 1984). The correlation of condensed sections on well logs was particularly useful to reveal slope geometries of the clinoforms (e.g., Pravdinskaya horizon).

West Siberia Seismic Sequence Stratigraphic Cross-Section (Pinous et al, 2001): To download, right click on image; Save target as:

 


 


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