Organic geochemistry and oil-generating potential of the Oligocene–Early Miocene Dingqinghu Formation sediments in the Lunpola Basin, Tibet
The hydrocarbon source rock potential of shales from the Oligocene–Early Miocene Dingqinghu Formation was evaluated using organic geochemistry (Rock-Eval pyrolysis, total organic carbon (TOC) and bitumen extraction) and organic petrology. Forty-one outcrop samples from Lunbori and Jiangria Co shales in the Lunpola Basin were analysed. At Lunbori, in the central part of the basin, the shales have very good source rock potential, and show TOC contents ranging from 3.72 to 11.2 wt%, with an average TOC value of 7.36 wt%, S2 values of 33.4 – 99.5 mg HC/g rock (average 61.8 mg HC/g rock), and extractable organic matter contents varying from 513 to 4525 ppm (average 2105 ppm). The shales have very good source rock potential. Rock-Eval Tmax values of 433 – 437°C (average 435°C) and high carbon preference index (CPI) values (4.76 – 6.70) indicate that the organic matter is immature to early mature for oil generation. The yellow fluorescence of kerogen under UV light is in good agreement with Tmax and CPI data. High hydrogen indices (HI) (635 – 1057 mg HC/g TOC) and H/C ratios (1.60 – 1.72) indicate that the Lunbori shales contain oil-prone organic matter and are characterized by high yellow fluorescing amorphous sapropelite content. The Lunbori shales contain a high content of hydrogen-rich Type II and minor Type I oil-prone kerogen, and have dominant medium to long molecular weight n-alkanes (maximizing at C25 or C27) with marked odd carbon number preference in the n-C23 – n-C31 range, pristine/phytane (Pr/Ph) ratios of 0.06 – 0.14, high phytane/n-C18 ratios (10.95 – 30.63), abundant gammacerane (gammacerane/C31 hopane: 0.44 – 1.80), and predominant regular sterane C27. These data indicate deposition in stratified, anoxic and saline depositional conditions with a kerogen predominatly formed from a lacustrine biota. The Jiangria Co shales in thewestern part of the Lunpola Basin contain some hydrogen-rich Type I and II oil-prone kerogens at a marginally mature level. They have TOC contents of up to 2.62 wt%, S2 values of up to 17.2 mg HC/g rock and extractable organic matter contents of up to 1731 ppm, indicating good source rock potential for oil generation. This is supported by their high HI values (420 – 720 mg HC/g TOC) and H/C ratios (1.05 – 1.50). The biomarker distributions of the extracts are characterized by odd carbon number predominance (C23–C31), low Pr/Ph ratios (0.05 – 0.09), high concentrations of phytane and high gammacerane content, indicating lacustrine reducing and saline conditions.