HYDROCARBON POTENTIAL AND PALAEO-DEPOSITIONAL ENVIRONMENT OF LACUSTRINE SOURCE ROCKS: MIDDLE JURASSIC SHIMENGOU FORMATION, NORTHERN QAIDAM BASIN, NW CHINA

Q. T. Meng ^a,b,*, A. Bechtel^c, R. F. Sachsenhofer^c, Z. J. Liu^a,b, D. Gross^c, P. C. Sun^a,b, F. Hu^a,b, L. Li^a,b, K. B Wang^a,b, C. Xu^a,b, L. L. Chen^a,b, and W. R. Zeng^a,b

^a College of Earth Sciences, Jilin University, 130061 Changchun, China.

^b Key Laboratory for Oil Shale and Paragenetic Energy Minerals, Changchun, 130061, Jilin Province, China.

^c Department Applied Geosciences and Geophysics, Montanuniversität Leoben, Peter-Tunner Str.5, A-8700 Leoben, Austria.

^* Corresponding author, email: mengqt@jlu.edu.cn

The Middle Jurassic Shimengou Formation in the Qaidam Basin, NW China, includes coals and lacustrine source rocks which locally reach oil shale quality (i.e. yielding >3.5 % oil on low-temperature distillation). In the present study, the palaeo-depositional environment and hydrocarbon potential of the 84.5 m thick Shale Member of the Shimengou Formation are investigated based on bulk geochemical parameters, organic petrographic data, biomarker analysis, and stable isotope geochemistry of 88 core samples.
The Shale Member was deposited in an anoxic freshwater lake which formed following the drowning of a precursor low-lying mire. Variations in bulk geochemical parameters allow four informal units to be identified, referred to (from the base up) as Units 1 to 4. These contain intervals of oil shale of varying thicknesses. In Unit 1, mudstones in the interval referred to as oil shale Layer 1 (true thickness [TD]: 2.06 m) are OM-rich as a result of algal blooms and photic zone anoxia, and correspond to an initial flooding event. Subsequently, productivity of aquatic organisms decreased, resulting in the deposition of organic-lean mudstones in Unit 2. Oil shale Layers 2 (TD: 2.03 m) and 3 (TD: 8.03 m) near the base of Unit 3 were deposited during maximum water depths. As with Layer 1, high productivity by algal blooms resulted in photic zone anoxia in a stratified water column. The shales in the upper part of Unit 3 are characterized by high TOC contents and a gradual increased input of terrigenous OM, and were deposited in a stable semi-deep lake. Finally, organic-lean mudstones in Unit 4 were deposited in shallow lacustrine conditions. The reconstruction of depositional environments in thick, non-marine shale-rich successions by mineralogical, petrographic and inorganic geochemical methods may be challenging as a result of the homogenous composition of component mudstones. The results of this study indicate, however, that sub-division and basin-wide correlation of such intervals can be achieved by organic geochemical analyses.
Oil shales and organic-rich mudstones in Units 1 and 3 of the Shimengou Formation Shale Member are excellent oil-prone source-rocks with a Source Potential Index of 3.2 t HC/m². Considering the large area covered by the Shimengou Formation in the northern Qaidam Basin (~34,000 km²), the results of this study highlight the regional significance for future petroleum exploration. They indicate that variations in organic productivity and dilution by minerals are key factors controlling the abundance and type of organic matter in the formation. An understanding of these factors will assist with the identification of exploration targets.

Key words: Biomarkers, lacustrine source rocks, depositional environment, hydrocarbon potential, oil shale, alginite, Shimengou Formation, Qaidam Basin, China.

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