GEOCHEMICAL INVESTIGATION OF LACUSTRINE OIL SHALE IN THE LUNPOLA BASIN (TIBET): IMPLICATIONS FOR PALEOENVIRONMENT AND PALEOCLIMATE

Abstract

The Dingqinghu Formation oil shale, located in the centre of the Lunpola basin, represents a potential large lacustrine oil shale resource in Tibet. A geochemical investigation of the oil shale was performed to reconstruct paleoenvironment and paleoclimate during deposition. The total organic carbon (TOC) contents (1.46-11.85%), S2 values (4.79-115.80 mg HC/mg rock) and HI (328-1040 mg HC/mg TOC) of oil shale samples are high, and indicate that the oil shale has a good oil source rock potential. The thermal maturity assessed from PI (0.01-0.09) and Tmax (429-440 °C) shows an immature to early mature stage of the organic matter. The oil shale exhibits characteristics of odd-over-even predominance, maximum n-alkanes peak at nC25 or nC23, a higher proportion of C29 sterane, low δ 13 Corg values (-29.9 to -26.7‰), a low Pr/Ph ratio (0.03-0.40), high values of the gammacerane index (up to 25.24), and presence of β-carotane, which is consistent with a reducing, stratified and hypersaline palaeo-lake with the main contribution of algae and bacteria to the organic matter (OM). The development history of palaeo-lakes from the Oligocene to the Early Miocene indicates that the climate of the Lunpola basin region during the deposition of oil shale was arid.