HARRIS, PAUL M. (MITCH), and RAYMOND A. GARBER, Chevron Petroleum Technology Company; AIGOUL TYSHKANBAEVA, SAULE BIRMANOVA, and MICHAEL E. CLARK, Tengizchevroil

Abstract: Geologic Framework for Tengiz Field, Kazakhstan

Introduction

The Tengiz Field, located in western Kazakhstan along the northeastern shore of the Caspian Sea, produces a 47' gravity oil from a thick (1500+ m) section of Carboniferous isolated platform carbonates. Cross sections and maps constructed from core, log and biostratigraphic data from this giant reservoir indicate important aspects of the geologic framework.

Depositional Model

We use a sequence/depositional model to interpret depositional environments for Tengiz. Environments include deep platform, shallow platform, shoal, platform margin (reef), and slope. The long-lived slopes to the platform were quite steep.

The platform top of Tengiz was subaerially exposed during lowstands and subjected to nondeposition, the formation of thin calcrete-cemented layers, or karst-related dissolution and brecciation at different positions across the platform. Depending on the actual slope of the platform edge and the magnitude of any sea-level drop, it is likely that a small carbonate factory formed on the slope at a position where it was intersected by sea level. Minor deposition on the slope would include carbonate sediment shed from this small upslope factory and any coarse debris weathered from the carbonate margin immediately above. During transgression, the top of the Tengiz platform was flooded but the sediment factory did not kept up with the rate of sea-level rise, therefore producing a deep platform top.We envision deposition across the entire platform top at these times to be principally packstones with crinoids, brachiopods, and locally reef mounds. Deposition on the slope would be of material shed off the platform top sediment factory. During highstand, the platform top remains flooded, but the sediment factory has had ample time to locally catch up to sea level, thereby producing a fully developed platform top, margin, and slope suite of environments.

Stratigraphic and Facies Framework

Cross sections help begin to understand the spatial variability of the depositional environments recognized in the Tengiz; cores. We have relied on core descriptions, biostratigraphic information, outcrop analogs (Cook and others, 1997, Kenter and others, 1997), and log correlation to develop cross sections.

Several sequence boundaries form the basis for our correlation. Sequence boundaries were determined by a variety of methods: changes in depositional environment (facies), changes in the thickness of successive cycles (stacking pattern), diagenesis associated with subaerial exposure, equivalency to a biostratigraphic pick, and correlation based on log character. We recognize four boundaries for the Bashkirian (B1-4), two for the Serpukhovian (S1-2), three for the upper Visean (V1-3), seven for the lower Visean (V4-10), four for the Tournaisian (T1-4), and one for the Devonian (Fammenian) (D-1).

Timing of Platform Growth.

The buildup (actual thickening) of the Tengiz platform was initiated by the Tournaisian, and was accentuated greatly during Visean time, as is shown by correlations from interior to flank wells. Subsequent deposition further added to the relief of the platform margin.

The timing for upbuilding of the Tengiz platform was also suggested to begin in the Visean by Lisovsky and others (1992).This timing of major upbuilding coincides with the last major cycle of growth for the Tengiz structure as discussed by Pavlov and others (1988).

Variations in Platform Size

The area covered by the Tengiz platform appears to have diminished through time as the younger margins show some amount of backstepping. This style of growth is consistent with long-lived carbonate platforms that have a more difficult time keeping up (filling accommodation space during each successive increment of time related to sealevel changes) as their platform margins increase in height.

The suggestion of smaller platform size with upward growth was also indicated by Lisovsky and others (1992) in their comparison of time-slice maps from Tengiz seismic data.

The reduced platform size may be one reason that the platform top during Bashkirian time is dominated by highenergy grainstones; the smaller platform size would facilitate a more widespread high-energy environment.

The ultimate demise of a carbonate platform with a long-term history of backstepping is drowning; based on the depositional environments recognized in core, we feel it is possible that the uppermost Bashkirian sequence boundary (B1) is a drowning unconformity.

Platform Interior Environments

Platform interior depositional environments are variable in Tengiz, but somewhat organized within the sequence framework. A deep to shallow platform change was consistently recognized vertically within a sequence.

Evidence for deepening from north to south across the platform top is present within some sequences.There is the progressive change in depositional environment from shoal to shallow platform or from shallow platform to deep platform.

Reef Environment

Although critical from the standpoint of development of the carbonate platform and sourcing sediment to the slope, the platform margin reef of Tengiz is volumetrically small in comparison with other environments. Following the outcrop studies of Kenter and others (1997), we view the reef as growing on the uppermost slope.

Reef mounds have not been recognized in platform top or slope wells, but their occurrence is predicted based on outcrop studies of Cook and others (1997).

Slope Environment

Like the platform margin, the slope depositional environment is greatly restricted in extent in a dip direction.

The slope environment is poorly understood in Tengiz due to the lack of core control and the complicated stratigraphy that results from reworking material downslope.

Our correlation across the flanks of the structure is simplified in the sense that faulting is not considered. Pavlov and others (1988) show seismic data that clearly indicates significant faulting of the flank strata. We remain unsure of the timing of any such faults, so have chosen to disregard them until analysis of the new Tengizchevroil 3-D seismic data provides more insight into the structure.

Karst

Karst zones were interpreted from cores in the Tengiz-8 well (Harris and others, 1998) and from lost circulation zones in several wells.

Although not accounting for all of the porosity development in the Tengiz-8 well, karst zones associated with several sequence boundaries equate with higher porosity zones, and one karst horizon is related directly to a high productivity zone in the well (Chambers and others, 1997).

Although karst seems the most likely interpretation, breccia and zones of enhanced porosity could also form from dissolution along major faults. Karst zones would develop discontinuously parallel to the stratigraphy, whereas fault-related dissolution would crosscut stratigraphy.

Spatial Variability

Simplified maps (Fig. 1) for several stratigraphic horizons showing the depositional environment at each cored well location in Tengiz serve as an important starting point for projecting environmental trends throughout the field.

The depositional environment maps reinforce and further illustrate many of the conclusions of the cross sections. In particular, the maps:

Confirm the changing configuration and general decrease in size of the platform through time: the outline of the field on the maps can be compared to illustrate changes relative to the stratigraphy;
Illustrate the abruptness of the platform edge: several maps show the relatively rapid change from a platform top environment (shoal, shallow platform, deep platform) to platform margin or slope environment;
Show a relatively homogeneous platform interior for some sequences and dramatic variation of depositional environments within others; and Suggest deepening to the south in some horizons.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England