ANALYSIS OF NITROGEN SOURCES FOR LOWER CAMBRIAN SHALES WITH COMPLEX TECTONIC BACKGROUNDS

The Lower Cambrian shales have always been the main target of marine shale gas exploration in Southern China. However, in some areas with complex tectonic backgrounds , the methane content in the Lower Cambrian shale reservoirs is low, but the nitrogen content is very high. In order to figure out this problem, the Lower Cambrian shales from the Upper Yangtze Southeast Chongqing region and the Lower Yangtze Xiuwu basin are selected as the research object. First, the gas components in the Lower Cambrian shales are analyzed; then, nitrogen isotope analysis is used to identify the sources of nitrogen. According to the parallel and perpendicular permeability experiment, the FIB-SEM experiment and the FIB-HIM experiment, combined with the regional geological backgrounds, seismic interpretation and core description, the reasons for high nitrogen content in the Lower Cambrian shale from the Upper Yangtze Southeast Chongqing region and the Lower Yangtze Xiuwu basin are clarified. The results show that the gas in the Lower Cambrian shale of the Upper Yangtze Southeast Chongqing region is mostly nitrogen derived from the atmosphere and high-maturity evolution of organic matter; the shale bedding surfaces, the detachment layers at the bottom of the Lower Cambrian, the widespread deep faults, and the thermal evolution of organic matter are the causes of high nitrogen and low hydrocarbons in shale gas. Further, the gas in the Lower Cambrian shale of the Lower Yangtze Xiuwu basin is mostly nitrogen derived from the superdeep crust, upper mantle and atmosphere, and the shale bedding surfaces, the detachment layers at the bottom of the Lower Cambrian, the widely developed faults and the volcanic activity during the Jurassic period are the reasons for the high nitrogen and low hydrocarbons in shale gas.