--> Abstract: Jurassic Petroleum System of the Northern Qaidam Basin, NW China, by B. D. Ritts; #90937 (1998).

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Abstract: Jurassic Petroleum System of the Northern Qaidam Basin, NW China

RITTS, BRADLEY D., Department of Geological Sciences, Stanford University, Stanford, CA 94305-2115. [email protected]

Jurassic nonmarine strata crop out around much of the northern margin of the Qaidam basin, where they consist primarily of fluvial and lacustrine facies. These rocks contain sequences of freshwater lacustrine to terrestrial type III to type II source rocks in excess of 350 m. Basin analysis of these outcrops, coupled with sparse subsurface data suggest that Jurassic sedimentary rocks were deposited in a regionally-extensive, south-facing, intracontinental foreland basin related to contractional deformation in the Qilian Shan. This basin experienced maximum subsidence during the Middle Jurassic on its NE margin (adjacent to the Qilian Shan), and accumulated more than 1000 m of Pliensbachian-Bajocian nonmarine strata. These strata lap out to the southwest and are mostly absent along the present SW margin of Qaidam.

Biomarker characteristics of most produced oils from the NE Qaidam basin indicate that they were derived from Jurassic freshwater lacustrine source rocks. These oils have high pristane:phytane ratios, odd over even n-alkane preference and bimodal distribution of n-alkanes, absence of ·-carotane, g-carotane and gammacerane, low concentrations of C34 and C35 homohopanes, dominance of C29 steranes, and high concentrations of diasteranes. These indicators are consistent with an oil derived from a shale source rock that was deposited in a freshwater (not saline) lacustrine system with an abundant input of higher plant organic material. Furthermore, biomarker analysis of Lower-Middle Jurassic source rock extracts demonstrates that these source rocks have the same molecular characteristics as the inferred Jurassic oils.

Basin modeling indicates that these oils were generated and expelled from the Jurassic source section beginning in Miocene. By that time, both Mesozoic and Tertiary deformation had encroached on the basin, resulting in abundant structural traps and laterally-restricted drainage areas. Furthermore, nonmarine Tertiary strata provided additional reservoir and seal intervals.

No Jurassic oils have been reported from producing fields in the NW Qaidam basin; and exploration has been limited, with no significant discoveries, in the central part of northern Qaidam. However, the presence of Jurassic source facies in outcrop in NW Qaidam, and their inferred extension into the subsurface of north-central Qaidam suggests that unexplored Jurassic plays may remain throughout much of the northern Qaidam basin.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah