ABSTRACT: Geologic Framework for the Tengiz and Korolev Fields, Kazakhstan-Carboniferous Isolated Carbonate Platform
HARRIS, PAUL M. (Mitch) , Chevron Petroleum Technology Company, Houston, TX
The supergiant Tengiz field exhibits depositional features typically observed in isolated carbonate platforms but rare for the Carboniferous: extensive inner and outer platform deposits, a raised rim feature, steep platform margins, and thick flank deposits. Many of the same platform and slope features observed in Tengiz are inferred from much less well data for Korolev.
Platform Growth and
The Tengiz and Korolev platforms formed during the Carboniferous on the Primorskian Arch, one of several structural highs in the southeast portion of the PriCaspian Basin in western Kazakhstan. The platform edges are abrupt, showing a relatively rapid change from the platform top (platform, shoal, or platform margin) to slope environment. Platform deposition includes cycles shoaling upward from open-marine packstones to shoal grainstones. A reef (microbial boundstone with scattered megafossils) is localized to a very narrow belt along the platform margin. Boundstones give way downslope to breccias and finally argillaceous lime mudstone beds. A variety of open porosity types are present; karst zones are best developed in the rim and fractures characterize both the rim and flanks.
A hierarchy of cycles, sequences, and composite sequences is developed by integrating core and well logs with newly acquired 3-D seismic data. The stratigraphic framework, although not finalized, explains many aspects of the reservoir quality. The buildup (actual thickening) of the platforms was seemingly initiated in the Devonian and accentuated greatly during Visean time. The position of the platform margins is controlled, in part, by deep-seated faults. The areal extent of both platforms generally diminished during the Carboniferous as evidenced by backstepping of the younger margins.
Key Platform Well
Tengiz well T-220, because of the great amount of continuous core taken in the well, is invaluable from the perspective of understanding the nature of deposition, stratigraphy, diagenesis, reservoir quality, and log response for the interior portion of the platform.
The upper contact of the Bashkirian is significant as it represents the demise of the shallow-water platform. Three sequences of similar grainstone-dominated deposits form a composite sequence that comprises most of the Bashkirian. Four sequences recognized within the Serpukhovian form another composite sequence. The boundary between the Bashkirian and Serpukhovian is significant as it represents a major change in the style of deposition on the top of the Tengiz platform and equates with a time gap. During the Serpukhovian, the facies reflect slightly deeper water and more skeletal-rich environments. Three composite sequences are tentatively identified within the Visean (Oksky) interval; each contains three sequences consisting of grainstone-packstone cycles that are similar to those of the Serpukhovian. A portion of one sequence recognized within the upper part of the Visean (Tula) contains packstone-grainstone cycles. The difference in the character of the cycles between the Visean (Oksky)-Serpukhovian and the Bashkirian reflects changes in the nature of sea level variations between the lower and upper Carboniferous.
Porosity loss in T-220, and other Tengiz platform wells, is by calcite cementation and minor compaction. The variety of open primary and secondary pore types is a principal reason for the permeability variation that is observed within the well. Core samples are dominantly limestone with local partial dolomitization and silicification associated with tuffs or subaerial exposure.
T-220 displays more or less constant 15% porosity with 1–10 md permeability, separated by thin tighter intervals at sequence and cycle boundaries. Where bitumen occurs it displays an inverse correlation with porosity (that is, bitumen is more abundant in rocks with low porosity). Samples with bitumen occur in short intervals, almost always at sequence or cycle boundaries, with long intervals in between with zero bitumen. The downhole porosity logs show an excellent match to core porosities; gamma-ray response increases where tuffs and shales are present.
Key Flank Well
Tengiz well T-47 is a flank well located off the southeast edge of the platform; it is an important well from the perspective of understanding the nature of deposition, diagenesis, reservoir quality, and log response in the flanks of the platform.
The Bashkirian through Visean (Oksky) consists of two general rock types: (1) coarse-grained (rudstones and grainstones rich in boundstone intraclasts), and (2) finer grained (grainstones, packstones, and wackestones). Both rock types represent debris that was shed down the slope from the platform margin. The Visean (Tula) is predominantly fine-grained slope packstones. The Devonian (Famennian) consists of shallow platform grainstones and packstones.
T-47 penetrated a generally limestone stratigraphy, with three important exceptions: (1) dolomite-rich intervals with 50% to 70% dolomite in the basal Serpukhovian, and lower Visean (Tula) through top Devonian; (2) a silica-rich interval in the Visean (Tula); and (3) a barite-rich interval with 0% to 26 wt. % (17 vol. %) barite near the top Devonian. Bitumen fills 10–15% of the potential porosity (defined as core measured porosity plus bitumen) in most samples.
The Bashkirian through Visean (Oksky) slope deposits have open porosity of three types: isolated vugs, pores between clasts, and microfractures. The Visean (Tula) packstones are varyingly silicified, and a few small, open vugs are the only visible porosity. Wireline logs indicate a porosity distribution that is generally consistent with the limited core data. Small, isolated, bitumen-stained vugs, fenestral pores, and microfractures remain open in a few Devonian grainstone and packstone samples.
Wireline logs indicate a porosity distribution that is generally consistent with the limited core data. Porosity from 0% and 9% typifies the section above the Devonian; porosity generally <4% is representative for the Devonian. The Visean (Oksky) slope deposits appear on an FMI log as irregular layers dipping up to 30 that are rarely continuous across the image, and thin continuous layers gently dipping away from the platform characterize the Visean (Tula) slope deposits. Irregular continuous layers with dips <5 typify the Devonian platform deposits.
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