Geochemical characteristics and genesis of Paleozoic natural gas in the Ordos Basin

The geochemical characteristics and genesis of Paleozoic natural gas in the Ordos Basin are complex, with some disputes on the gas source of Lower Paleozoic. In order to better grasp the overall geochemical characteristics and variation laws of Paleozoic natural gas in the basin, the geochemical data of more than 700 natural gas samples from the whole basin are comprehensively analyzed based on the systematic collection, collation and analysis of previous data, and the genesis of natural gas is explored by combining the analysis of key elements such as natural gas formation, hydrocarbon formation and reservoir formation. The results show that the main body of Upper Paleozoic natural gas is coal-based gas controlled by maturity, with maturity increasing from northeast to southwest, and the southern part of the basin has a mixture of Upper and Lower Paleozoic gas. The Lower Paleozoic gas is dominated by oil-type gas from marine carbonate formations, of which the upper combination is characterized by a two-source composite reservoir of Upper Paleozoic coal-type gas and Lower Paleozoic oil-type gas; the middle-lower combination is a relatively independent oil-type gas reservoir, subject to different degrees of secondary transformation based on geological conditions and the degree of thermal evolution of organic matter, resulting in significant anomalies in its carbon isotope composition. The research has proved that marine hydrocarbon sources including carbonate rocks exist in the Lower Paleozoic of the basin, and the natural gas in the Upper and Lower Paleozoic has mixed to varying degrees in different locations of the basin.