Ordovician Lithostratigraphic Sequence and Sedimentary Evolution in the Middle Section of the Western Ordos Basin

In recent years, many theoretical studies and production practices have been carried out around the exploration of oil and gas resources in the western margin of Ordos Basin. In order to deepen the geological understanding, this paper studies the Ordovician lithostratigraphic sequence and sedimentary evolution in the middle section of the western Ordos Basin. The sequence boundaries of Ordovician were summarized by field outcrops and drilling, logging and logging data. Based on the results of stratigraphic division, the Ordovician sedimentary evolution was comprehensively studied according to the facies indicators such as lithologic association, sedimentary structure and fossil assemblage. The lower Paleozoic Ordovician in the study area can be divided into Sandaokan Formation, Zhuozishan Formation, Kelimoli Formation, Wulalike Formation and Lashizhong Formation from bottom to top. The main sequence boundaries are regional unconformity and sedimentary facies catastrophe. The Sandaokan Formation and Zhuozishan Formation were formed in shallow sea carbonate platform sedimentary environment. The Kelimoli Formation was formed in the environment of carbonate platform margin slope. The Wulalike Formation and Lashizhong Formation mainly develop abyssal plain deposits, with intermittent low-density turbidity current deposits. In the study area, obvious sea-level rise and fall cycles can be found, and the maximum water depth is in the sedimentary period of graptolite shale of Wulalike Formation. In this paper, the stratigraphic sequence and sedimentary facies sequence were comprehensively determined, and conditions of natural gas accumulation under the combination of facies sequence were analyzed. The research results can provide sufficient geological basis for subsequent exploration in this area.