Print this pageTwo-Stage Hydrocarbon Migration Model for the Tengiz Field, Kazakstan
BASKIN, DAVID K., RAY A. GARBER, PAUL M. HARRIS, JEFFREY L. WARNER, WILLIAM S. HALLAGER and KUBENTAY SUISINOV
The Tengiz Field, located in western Kazakstan bordering the Caspian Sea, produces a light 42 gravity oil from a >1500 m section of Carboniferous and Devonian platform carbonates. In the upper 500 m, about one-third of the porosity (0-24 percent, average 7 percent) is filled with solid organic matter consisting of bitumen and thermally mature (spent) kerogen. Paleotemperature in the uppermost reservoir (Bashkirian) is estimated at 130 degree C from solid bitumen reflectances and spore coloration indices. Maximum temperature in the overlying Permian Artinskian shale is about 100 degree C from spore coloration. This 30 degree C difference across a 42 Ma unconformity supports a separate migration for oil which generated the solid bitumen and present-day producible crude as suggested by Soviet workers.
Solid bitumen likely formed via deasphalteening of an originally emplaced Carboniferous oil; subsequent burial matured the bitumen to observed reflectances of 1.4-1.6 percent. 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 from a more-mature, Permian source which migrated into the Bashkirian reservoir during the present cycle of burial in a 18 degree 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.