Geochemical characteristics and source of Cambrian natural gas in the Longdong area, Ordos Basin, China

In recent years, significant progress has been made in Cambrian natural gas exploration in the Longdong area of the Ordos Basin. However, the genetic types and sources of Cambrian gas remain unclear, severely constraining the exploration and development of deep natural gas in the basin. This study presents a detailed analysis of the composition and carbon isotope characteristics of Cambrian natural gas in the Longdong area and compares them with Upper Paleozoic coal-type gas from the Sulige area and Taiyuan Formation bauxite gas in the Longdong area. The genetic types and maturity characteristics of Cambrian natural gas are investigated, and its sources are analyzed in combination with the characteristics of potential source rocks. The results show that the Cambrian natural gas in the Longdong area is predominantly oil-type gas. Only Well L20 exhibits higher carbon isotope values of ethane and propane, indicating distinct coal-type gas characteristics, whereas other Cambrian gas samples show lower ethane and propane isotope values, characteristic of typical oil-type gas. Except for Well L20, methane, ethane and propane carbon isotopes, together with compositional characteristics, further demonstrate that Cambrian natural gas in the Longdong area mainly represents secondary cracking gas of oil and wet gas. All Cambrian natural gas in the Longdong area has reached the over-mature dry gas stage and shows significant carbon isotope reversal features. Based on geochemical characteristics, including genetic types and maturity of natural gas, combined with the distribution of potential source rocks, it is concluded that Cambrian oil-type gas in the Longdong area primarily originates from high-abundance marine source rocks in the Cambrian Dongpo Formation of the Fuping–Luochuan Gulf in the southern part of the basin, whereas the coal-type gas in Well L20 is derived from overlying Permian coal source rocks.