Controls on organic matter enrichment of the early-middle Miocene lacustrine oil shale, central Tibet (SW China): New insights from clay minerals and geochemistry

Abstract

The Lunpola oil shale is the largest lacustrine oil shale deposit in Tibet (SW China), which deposited in a Cenozoic lacustrine rift basin and contains abundant organic matter (OM), whereas the crucial factors that controlled the OM enrichment remain elusive. Here, we present new datasets of total organic carbon (TOC) contents, clay minerals and whole-rock geochemistry of the early-middle Miocene Middle Dingqinghu Formation oil shale from the Lunpola basin, central Tibet, in order to elucidate the paleoclimate, paleo-environment condition and the controlling factors on the OM enrichment of the lacustrine oil shale.Oil shales from the Lunpola East section have high TOC contents (avg. 9.60%), while the contiguous mudstones contain low TOC contents (avg. 0.44%). During the oil shale deposition, high smectite and kaolinite contents, low illite contents and low Sr/Cu ratios suggest strong chemical weathering and hence warm and humid climate, which may probably have close relationship with the mid-Miocene climatic optimum (MMCO). However, during the mudstone deposition, the relatively high illite and low smectite contents and higher Sr/Cu ratios suggest weak chemical weathering, indicating relatively arid climatic condition. The paleo-environment indexes (Corg/P, MoEF and UEF) and MoEF-UEF covariation reflect an anoxic bottom water condition during the oil shale deposition, while the mudstone formed in oxic-dysoxic water environment. The weak and moderate positive relationship between bio-productivity proxies (P/Ti and Cu/Ti ratios) and TOC contents reveal that the bio-productivity condition is not the crucial factor for the OM enrichment. Moreover, the water column environment was brackish and stratified during the oil shale deposition, and the sedimentation rate was moderate, which were also conducive to the OM preservation. The positive correlations between the TOC and the paleoclimate and paleoenvironment proxies demonstrate that warm and humid climate during the MMCO and anoxic water conditions play predominant roles in OM enrichment of the Lunpola lacustrine oil shale. Besides, modest sedimentation rate can prevent the dilution of OM, which can also promote the OM enrichment. This study can not only deepen the understanding of OM enrichment mechanism in lacustrine rift basin but also provide important information for predicting the distribution of the oil shale in the Lunpola Basin.