Abstract: Two-Stage Hydrocarbon Migration Model for the Tengiz Field, Kazakstan
ALLAGER, WILLIAM S., KUBENTAY SUISINOV, DAVID K. BASKIN, RAY A. GARBER, PAUL M. HARRIS, and JEFFREY L. WARNER,
The Tengiz Field, located in western Kazakstan bordering the Caspian Sea, produces a light 42 degrees API gravity oil from a ^rang1500m section of Carboniferous and Devonian platform carbonates. About one-third of the average 7% porosity in the upper 500m is filled with solid organic matter consisting of solid bitumen and thermally mature (spent) kerogen. Paleotemperature in the upper-most reservoir (Bashkirian) is estimated at 130 degreesC from solid bitumen reflectances and spore coloration indices. Maximum temperature in the overlying Permian Artinskian shale is about 100 degrees C from spore coloration. This 30 degrees C difference across a 42 Ma unconformity supports suggestions by Soviet workers of separate migration for oil which generated the solid bitumen and present-day producible crude.
Solid bitumen likely formed via deasphalteening of an originally emplaced Carboniferous oil that was possibly partially self-sourced from platform peloid/algal/brachiopod packstones. Subsequent burial matured the bitumen to observed reflectances of 1.4-1.6%. Solid bitumen post-dates burial calcite cementation and dissolution, and compaction-related fracturing and stylolitization. Pre-Artinskian uplift and erosion breached the Bashkirian reservoir losing the liquid crude not converted to solid bitumen. Associated diagenetic features include solution-enlarged porosity and formation of low-temperature, calcite spar veins that cross-cut and overlie solid bitumen.
Present day oil was likely generated off-structure from a more-mature, Permian source which migrated into the Bashkirian reservoir during the present cycle of burial in a 18 degrees C/km thermal gradient. This oil, which represents essentially all of the producible petroleum, formed secondary fluid inclusions in calcite spar cements along with a distinct, tarry bitumen in vugs and veins.
AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria