UPPER TRIASSIC POTENTIAL SOURCE ROCKS IN THE QIANGTANG BASIN, TIBET: ORGANIC GEOCHEMICAL CHARACTERISTICS
Article first published online: 19 JUN 2013
© 2013 The Authors. Journal of Petroleum Geology © 2013 Scientific Press Ltd
Journal of Petroleum Geology
Volume 36, Issue 3, pages 237–255, July 2013
How to Cite
Zeng, Y. H., Fu, X. G., Zeng, S. Q. and Du, G. (2013), UPPER TRIASSIC POTENTIAL SOURCE ROCKS IN THE QIANGTANG BASIN, TIBET: ORGANIC GEOCHEMICAL CHARACTERISTICS. Journal of Petroleum Geology, 36: 237–255. doi: 10.1111/jpg.12554
- Issue published online: 19 JUN 2013
- Article first published online: 19 JUN 2013
- Qiangtang Basin;
- Upper Triassic;
- Tumengela Formation;
- Zangxiahe Formation;
- thermal maturity;
- source rock potential;
Upper Triassic coal-bearing strata in the Qiangtang Basin (Tibet) are known to have source rock potential. For this study, the organic geochemical characteristics of mudstones and calcareous shales in the Upper Triassic Tumengela and Zangxiahe Formations were investigated to reconstruct depositional settings and to assess hydrocarbon potential. Outcrop samples of the Tumengela and Zangxiahe Formations from four locations in the Qiangtang Basin were analysed. The locations were Xiaochaka in the southern Qiangtang depression, and Woruo Mountain, Quemo Co and Zangxiahe in the northern Qiangtang depression.
At Quemo Co in the NE of the basin, calcareous shale samples from the Tumengela Formation have total organic carbon (TOC) contents of up to 1.66 wt.%, chloroform bitumen A contents of up to 734 ppm, and a hydrocarbon generation capacity (Rock-Eval S1+ S2) of up to 1.94 mg/g. The shales have moderate to good source rock potential. Vitrinite reflectance (Rr) values of 1.30% to 1.46%, and Rock-Eval Tmax values of 464 to 475 °C indicate that the organic matter is at a highly mature stage corresponding to condensate / wet gas generation. The shales contain Type II kerogen, and have low carbon number molecular compositions with relatively high C21−/C21+ (2.15–2.93), Pr/Ph ratios of 1.40–1.72, high S/C ratios (>0.04) in some samples, abundant gammacerane (GI of 0.50–2.04) and a predominance of C27 steranes, indicating shallow-marine sub-anoxic and hypersaline depositional conditions with some input of terrestrial organic matter.
Tumengela and Zangxiahe Formation mudstone samples from Xiaochaka in the southern Qiangtang depression, and from Woruo Mountain and Zangxiahe in the northern depression, have low contents of marine organic matter (Type II kerogen), indicating relatively poor hydrocarbon generation potential. Rr values and Tmax data indicate that the organic matter is overmature corresponding to dry gas generation.