Evaluation of geochemical characteristics and source of natural gas in Lower Paleozoic, Daniudi area, Ordos Basin

In recent years, great progress has been made in the exploration of natural gas in the Paleozoic in the Daniudi Gas Field in the northern Ordos Basin, especially the discovery of high yield industrial gas flow in the Ordovician weathering crust. However, the study of the origin and source of natural gas in the Lower Paleozoic in the Daniudi area is relatively limited, which directly restricts the next exploration project in the Lower Paleozoic in the northern Ordos Basin. The systematic analyses of gas components and carbon, hydrogen and noble gas isotopes showed that natural gas of the Paleozoic in the Daniudi area is dominated by methane and contains a certain amount of heavy hydrocarbons, and the carbon dioxide and nitrogen content is relatively high. It is not only dry gas generated at high to over maturity, but also wet gas originated from the mature stage. The natural gas of the Upper Paleozoic in the Daniudi area is a typical coal-type gas, while the natural gas of the Lower Paleozoic is both coal-type gas and oil-type gas. The carbon dioxide in the natural gas of the Paleozoic is mainly organic and with a small amount in the Lower Paleozoic being inorganic, which may be produced by the pyrolysis of carbonate minerals in the Lower Paleozoic source rocks. Geochemical analyses such as the carbon isotopes of source rock desorption gas, natural gas and kerogen, the hydrogen isotope of methane, the argon isotope dating, and the end-member gas mixed simulation experiments demonstrated that the coal-type gas in Upper and Lower Paleozoic in the Daniudi area in the northern Ordos Basin originated from the Upper Paleozoic coal-bearing source rocks, while the oil-type gas in the Lower Paleozoic was mixed with 0-45% coal-type gas, and the oil-type gas was predominant in the natural gas, which mainly originated from the source rocks of the Majiagou Formation in the Lower Paleozoic.