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ABSTRACT: Genetic features of oil source rocks in the tertiary lacustrine evaporites in the Western Qaidam Basin

Jin, Qiang , The University of Petroleum, Dongying, Shandong, China

Evaporites and oil source rocks were recognized previously as typical sediments of dry and wet environments respectively. However, the dark colored calcareous shales or marls in the Tertiary lacustrine evaporite sequences (E3 and N1) were defined as the oil source rocks in the western Qaidam basin. The shales and marls were mostly in low TOC (TOC < 0.6%) and small hydrocarbon potential. Investigations of sedimentation and geochemistry on the evaporites and source rocks, the author found that some perspective source rocks (TOC > 1.5%) which interbedded with gypsum or halite layers. A genetic model for the sequences was presented which was described as "a small deep-water brine lake surrounded by a large shallow-water saline lake". The deep-water brine lake was located at the depocenter and extended along a deep-cut fault, in which sulphate, chlorite and the perspective source rocks were deposited in an anoxic condition. The anoxic condition was formed in deep part of the brine lake by layered water column. But the large saline lake was an oxidic to semi-anoxic environment, in which poor-quality source rocks (TOC < 0.6%) and marls were deposited. Organic matter in the perspective and poor-quality source rocks was quite different. This research believed that the perspective source rocks were major contributors to the oils discovered in the western Qaidam basin, though they were in thin-layer and only distributed in the depocenter area. The genetic model for oil source rocks in the lacustrine evaporites was an useful tool for hydrocarbon assessment in the Qaidam basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia