Sedimentary Characterization of the Upper Paleozoic Coal-Bearing Tight Sand Strata, Daniudi Gas Field, Ordos Basin, China

The coal-bearing strata of the Upper Paleozoic(from the Taiyuan Formation to the lower member of the Shanxi Formation) are the most important units that have high gas production in the Daniudi gas field, which is a typical tight-sandstone reservoir with high heterogeneity in the Ordos Basin, China. Based on an integrated investigation of well logs, cores, SEM and 3-D seismic data, we delineated the sedimentary facies of the coal-bearing strata and divided the succession into sequenced stratigraphic units of different depositional systems. A sedimentary hiatus was documented for the first time in the study area and forms the sequence boundary between the Lower Pennsylvanian Carboniferous Taiyuan Formation(Ct1) and the Upper Pennsylvanian Carboniferous Taiyuan Formation(Ct2). The coal-bearing strata in Ct1 are indicative of a barrier coastal deposition system. Tidal channels are identified by their fine-grained, cross-stratified character. The sands in the tidal channels are well sorted, and the quartz content is above 95%. The coalbed located beside the sandstone is thought to be a lagoon. Gas-bearing, coarse-grained sandstone in the coal-bearing strata spanning from the Ct2 to the lower members of the Shanxi Formation(P1s) is interpreted as a fluvial-dominated braided delta that is divided into four third-order sequences. The coal-bearing strata are composed of sandstone, mudstone and coalbed from base to top in each sequence. Braided-river deposits form the lowstand system tract(LST) within each sequence. A shelf and lake depositional environment containing dark gray mudstone forms the transgressive systems tract(TST). The highstand systems tract(HST) deposits form the swamp coalbed in each sequence.