Lithofacies distribution characteristics and its controlling factors of shale in Wufeng Formation-Member 1 of Longmaxi Formation in the Jiaoshiba area

Abstract

It is essential to investigate shale lithofacies distribution and controlling factor of the shale for geological evaluation of shale gas exploration and development. Through comprehensive analysis of cores, thin sections, cathode luminescence, whole-rock X-ray diffraction, element capture spectroscopy, major/trace element and other data, three major types and eight sub-type shale lithofacies in the shale of Wufeng Formation-Member 1 of Longmaxi Formation in Jiaoshiba area are identified by the three-end-member method and shale lithological classification nomenclature, and the spatiotemporal distribution law and main development controlling factors of shale lithofacies are well studied. In the Jiaoshiba area, vertically, the marine shale develops siliceous shale, mixed shale and argillaceous shale from bottom to top. Besides, lateral distribution of the shale is different from north to south; the shale lithofacies in the north area changes rapidly, the mixed shale in the north area is much thicker than that in the south area, while the siliceous shale in the south area is relatively thicker. Difference in the shale lithofacies is controlled by special sedimentary geologic events; development of the siliceous shale is controlled by the Ordovician-Silurian global volcanic event to some extent, while the mixed shale is significantly influenced by effect of bottom current, and the argillaceous shale is mainly affected by supply of terrestrial clastic material.