High-resolution sequence stratigraphy and sedimentary characteristics of Benxi Formation in Yan’an area of Ordos Basin

The oil and gas resources in Yan’an area of Ordos Basin are abundant, and Benxi Formation is one of the important exploration intervals. However, its sedimentary environment is complex, and the distribution of sand bodies and sedimentary evolution characteristics are not clear, which restricts the process of natural gas exploration and development in this area. Therefore, strengthening the study of geology of Benxi Formation in Yan’an area, especially the study of sequence stratigraphy, sedimentary characteristics and evolution, will provide favorable theoretical support for paleogeographic reconstruction, oil and gas exploration and development in this area. Based on the theories of high-resolution sequence stratigraphy, sedimentary petrology, logging geology and sedimentology, the stratigraphic and sedimentary characteristics of the late Paleozoic Benxi Formation in Yan’an area of Ordos Basin were studied, and the sequence stratigraphic unit division, isochronous stratigraphic framework, sedimentary facies types and evolution process were determined. The results show that in terms of sequence stratigraphy, one long-term base-level cycle, two medium-term base-level cycles and multiple short-term cycles can be identified in Benxi Formation. In terms of sedimentary facies types, 12 types of lithofacies and 7 lithofacies combinations were identified. According to this, it is considered that tidal flat and barrier island-lagoon sedimentary facies were developed in Benxi Formation in Yan’an area, and sedimentary microfacies such as tidal channel, sand flat, mixed flat, mud flat, peat flat, barrier island and lagoon sediments were identified. In terms of sedimentary facies distribution, tidal flat deposits are mainly developed in the southwest and south, and barrier island-lagoon deposits are mainly developed in the northeast and north. The sedimentary patterns of Ben 1 section and Ben 2 section are basically consistent.