Sequence-based lithofacies and paleogeography of Carboniferous Huanglong Formation in Linshui (eastern Sichuan)-northern Chongqing area

Abstract：The Linshui (eastern Sichuan)-northern Chongqing area only keeps unsound Upper Carboniferous Huanglong Formation because of intense structural uplift and erosion in early Hercynian period，and the Huanglong Formation of the study area is one of the most important natural gas reservoirs in eastern Sichuan area and one of the most favorable horizons in search for new gas sources. Based on core description and thin section analysis，combined with sedimentary lithologic marks，palaeontology and logging analysis，the authors have divided the Huanglong Formation into three main sedimentary facies, i.e., sabkha，gulf shelf and shore，and further divided these facies into many subfacies and microfacies. Using the classic sequence stratigraphy principle and technology represented by Vail，the authors have divided the Huanglong Formation into one sequence of Ⅰtype and three system tracts：LST，TST and HST. On the basis of a detailed analysis of sedimentary facies and sequence stratigraphy，the authors have compiled and described the sequence-paleogeographic maps which take the system tracts as the mapping units. Combined with the reservoir characteristics of all sorts of sedimentary facies，the barrier grain beach and the offshore grain beach are thought to be the most favorable reservoir facies.