ORGANIC GEOCHEMISTRY OF POTENTIAL SOURCE ROCKS IN THE TERTIARY DINGQINGHU FORMATION, NIMA BASIN, CENTRAL TIBET

Licheng Wang1,2, Chengshan Wang*1,2, Yalin Li1,2,

Lidong Zhu3 and Yushuai  Wei1,2

1 School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China.

2 State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China.

3 Institute of Sedimentary Geology, Chengdu University of  Technology, Chengdu 610059, China.

* Corresponding author, email: chshwang@cugb.edu.cn

The Tertiary Nima Basin in central Tibet covers an area of some 3000 km2 and is closely similar to the nearby Lunpola Basin from which commercial volumes of oil have been produced. In this paper, we report on the source rock potential of the Oligocene Dingqinghu Formation from measured outcrop sections on the southern and northern margins of the Nima Basin.

In the south of the Nima Basin, potential source rocks in the Dingqinghu Formation comprise dark-coloured marls with total organic carbon (TOC) contents of up to 4.3 wt % and Hydrogen Index values (HI) up to 849 mg HC/g TOC. The organic matter is mainly composed of amorphous sapropelinite corresponding to Type I kerogen. Rock-Eval  Tmax (430–451oC) and vitrinite reflectance (Rr ) (average Rr = 0.50%) show that the organic matter is marginally mature. The potential yield (up to 36.95 mg HC/g rock) and a plot of S2 versus TOC suggest that the marls have moderate to good source rock potential. They are interpreted to have been deposited in a stratified palaeolake with occasionally anoxic and hypersaline conditions, and the source of the organic matter was dominated by algae as indicated by biomarker analyses.

Potential source rocks from the north of the basin comprise dark shales and marls with a TOC content averaging 9.7 wt % and HI values up to 389 mg HC/g TOC. Organic matter consists mainly of amorphous sapropelinite and vitrinite with minor sporinite, corresponding to Type II–III kerogen. This is consistent with the kerogen type suggested by cross-plots of HI versus Tmax and H/C versus O/C. The Tmax and Rr results indicate that the samples are immature to marginally mature. These source rocks, interpreted to have been deposited under oxic conditions with a dominant input of terrigenous organic matter, have moderate petroleum potential. The Dingqinghu Formation in the Nima Basin therefore has some promise in terms of future exploration potential.

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