--> ABSTRACT: Biomarker Characteristics of Crude Oils from the Qaidam Basin, NW China, by Bradley D. Ritts, Andrew D. Hanson, J. Michael Moldowan, and Andrew Ekuan; #91019 (1996)

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Biomarker Characteristics of Crude Oils from the Qaidam Basin, NW China

Bradley D. Ritts, Andrew D. Hanson, J. Michael Moldowan, and Andrew Ekuan

The Qaidam basin, northwest China, has producing oil fields in the northwestern and northeastern corners of the basin. However, the petroleum systems in the basin and molecular organic geochemistry of the oils are undocumented.

Jurassic and Tertiary petroleum source rocks occur throughout the northern Qaidam basin, and in surrounding mountain belts. The nonmarine Tertiary section contains thick sections of carbonate and evaporite-rich, hypersaline lacustrine mudstones, interbedded with alluvial and fluvial sandstones and conglomerates. In contrast, nonmarine Jurassic source rocks consist of laminated, black, lacustrine shales and related coals that were deposited in humid fluvial environments.

Oils from the northwestern Qaidam basin have consistent biomarker and non-biomarker molecular organic geochemistry parameters, including low pristane:phytane ratios, high levels of gammacerane and C34 and C35 homohopanes, and the presence of ß and ^ggr-carotane. These parameters indicate the source rock was deposited in a hypersaline, anoxic, reducing lacustrine environment, and probably had a significant carbonate or evaporite component. Based on geologic and geochemical data, the source rocks for these oils were in the Tertiary lacustrine section.

Some oils from the northeastern part of the basin are similar geochemically to those from the northwest Qaidam basin, and also are derived from Tertiary hypersaline lacustrine source rocks. However, other oils from the northeastern part of the basin are distinct in geochemical character. These oils have high pristane:phytane ratios, and do not contain hypersaline lacustrine indicators, as do northwestern Qaidam oils. The high pristane:phytane ratio, low relative abundance of C34 and C35 homohopanes and high terpane:sterane ratio, suggest a nonmarine origin for the northeastern Qaidam oils. These data indicate that Jurassic lacustrine shales were source rocks for oils in the northeastern Qaidam basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California