Abstract: Abnormal Pressure in the Precaspian Basin: Transition Zones Distribution

ANISSIMOV, LEONID, KONSTANTIN GOLOVIN, and MARINA KALINNIKOVA

The formation pressure database for the Precaspian Basin and adjoining areas includes more than 10000 cases of well testing. The pressure distribution and characteristics of abnormal pressure zones in the Basin can be classified by the depth and stratigraphy. For this territory the abnormal pressure zones are situated at a depth of more than 4000 m. If for normal pressure reservoirs the difference of formation fluid potential is 2-3 MPa, than for abnormal pressure zones it is 20 - 30 MPa. The stratigraphic position of transition zones corresponds to Upper Devonian for outer parts of the Basin and to Lower Permian formations for the inner parts. The top of undercompacted zones is presented by indurated and carbonate-bearing shales. But it is difficult to assume that clay mineralogy is the cause of abnormal pressure generation at such narrow intervals. The water / hydrocarbon ratio in shales is the most important factor making it possible to stand great pressure gradients and the source rocks in oil-window zones differ from water-saturated shale by its sealing characteristics. This approach explains the distribution of hydrocarbons and formation pressure within the sedimentary section and assesses the expulsion rate using the clay porosity distribution.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria