Sequence stratigraphy and sedimentary facies in the lower member of the Permian Shanxi formation, northeastern Ordos Basin, China

The Lower Permian Shanxi (山西) Formation is one of the main gas-bearing stratigraphic units in northeastern Ordos (鄂尔多斯) Basin, China. Based on an integrated investigation of well logs, cores, and outcrop, we delineated the sedimentary facies of the lower member of the Shanxi Formation and divided the succession into three third-order sequences from base to top as SQ1, SQ2, and SQ3. The lower region of Shanxi Formation was deposited in the following sedimentary facies or subfacies: subaqueous braided channel, subaqueous interdistributary, mouth bar, swamp and shelf in the Daniudi (大牛地) Gas Field and braided channel, and shelf and lake at Heidaigou (黑岱沟) outcrop. Braided-river deposits form the lowstand systems tract (LST) in each sequence. Braided channels mark the sequence boundaries at Heidaigou outcrop. A shelf and lake depositional environment with dark gray mudstone forms the transgressive systems tract (TST). The location where dark gray mudstone first appears above the braided channel marks the first flooding surface (FFS), and the end of that marks the maximum flooding surface (MFS). The highstand systems tract (HST) deposits are fine-grained sediments with an aggradational parasequence at Heidaigou outcrop and swamp coalbed in the Daniudi Gas Field. Mouth-bar sand bodies in braided delta front, which form the LST in each sequence, form excellent reservoirs in the Daniudi Gas Field.