Abstract: Lithofacies, Porosity Types, and Bitumen Distribution in the Super Giant Tengiz Reservoir, Kazakhstan: A Preliminary Assessment

Anthony J. Lomando, K. Suisenov

The seismic expression of the Tengiz reservoir can be interpreted as a large carbonate buildup complex. This geometry is a product of combined constructional, structural, and erosional processes. The reservoir at Tengiz ranges in age from Famennian to Bashkirian. During this 60 m.y., a broad spectrum of carbonate facies types was deposited and modified by both early and late diagenetic processes. Particle types are typical for shallow-marine settings and reservoirs of this age range. Grain-supported textures contain primary interparticle and intraparticle porosity. Early secondary porosity, including moldic and solution enlarged interparticle pores, commonly occurs in association with short-term exposure events. At the top of the reservoir, a major unconformity that lasted into the Pe mian created a karst overprint on the uppermost several hundred meters of the reservoir. Additional secondary porosity formed during burial and may have been generated in association with the hydrocarbon migration-entrapment system. Solution-enlarged fractures, leaching along stylolites, and dissolution of earlier cement phases reflect this process. All pore types are negatively impacted by calcite, dolomite, and gypsum cements. The occurrence of gypsum at 4000+ m may be controlled by the overpressured conditions encountered in the reservoir.

Bitumen in the Tengiz reservoir occurs most commonly in the upper 400 m. Bitumen lines and fills primary and secondary pores, reducing porosity and permeability, but cannot be identified with normal log suits. Interpretation of current geochemical data indicates a two-stage migration history. The first stage, which partially filled the reservoir, was deasphalted causing the precipitation of bitumen. The second stage was responsible for filling the reservoir with a 1000+ m hydrocarbon column. Future work will focus on bitumen characteristics and distribution in order to model bitumen occurrence within the constraints of a deasphalting mechanism.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994