The Lithofacies of Sandstones Interbedded with Shales: Implication for Organic Matter Accumulation of Triassic Deep Lacustrine Setting, Southern Ordos Basin

The sandstones interbedded with shales in the lacustrine black shale have great potential to become important targets for oil and gas exploration, but there has been a lack of systematic research regarding their types and genesis. This study focused on the investigation of the Triassic Chang 73 member deep lacustrine sandstone. Eleven lithofacies are identified and classified into three different types of deposits: ash fall and intra- and extra-basinal turbidite deposits. Vitric tuff, pumice-bearing shale, and ash are the main ash fall lithofacies. The presence of reverse grading and a significant concentration of plant fragments/micas suggest extra-basinal turbidite deposits. However, the collapse of deltaic intrabasinal sediments has been well sorted and does not contain plant debris and low-density materials. These three different types of sediments combined with a humid climate are the main causes for the deposition of a large number of sandstone layers in the deep lacustrine environment. Furthermore, the abundant volcanic eruptions are associated with increased organic matter accumulation and promoted the bloom of algae. In addition, the generated oil in the black shales would then migrate and accumulate in the interbedded sandstones. This research provides geological evidence for the prospective prediction of lacustrine shale oil accumulations.