Control actions of sedimentary environments and sedimentation rates on lacustrine oil shale distribution, an example of the oil shale in the Upper Triassic Yanchang Formation, southeastern Ordos Basin (NW China)

Oil shale is an important form of unconventional resources. It is significant to determine the control factors on oil shale distribution. Abundant oil shale resources are hosted in the Member Chang 7-3 (Ch7-3), Triassic Yanchang Formation in southeastern Ordos Basin (NW China). This study was conducted to investigate the sedimentary environment, distribution and organic matter source of the Ch7-3 Oil shale by a combined study of outcrops, core data, organic and element geochemical indices. The controlling factors of the oil shale distribution were determined by an analysis of sedimentary environments and geochemical features of the oil shale. Sedimentary facies analysis based on outcrops and core data indicated that the Ch7-3 Oil shale was deposited in lacustrine-delta environments. Warm and moist climates and reducing conditions indicated by major and trace elements were favorable to organic matter production and preservation. Rock-Eval pyrolysis showed that the organic matter of the Ch7-3 Oil shale is from both lacustrine phytoplankton and terrigenous higher plants. The organic matter from terrigenous higher plants was probably transported into the deep lake by gravity flows. Sedimentary facies and (La/Yb)(N) values show that the spatial distribution of oil shale was controlled by sedimentary environments and sedimentation rates. In the deep lake area near to delta-turbidite fan deposition systems, terrestial organic matter input was bounteous, and sedimentation rates were moderate, which were beneficial to organic matter accumulation and preservation, providing favorable conditions for oil shale formation. In delta-turbidite fan deposition systems, high sedimentation rates led to organic matter dilution. In the deep lake area far from delta-turbidite fan deposition systems, sedimentation rates were relatively low, and organic matter was easily degraded, in addition only lacustrine organic matter input with no terrestial organic matter input inhibited organic matter enrichment.