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

Abstract

The geochemical characteristics of the Paleozoic natural gas in the southern part of the Ordos Basin shows significant differences from those in the northern part. These differences lead to the increase in dryness coefficient and a heavier carbon isotope composition of methane attributed to the increase in organic matter maturity of the source rocks. Additionally, the Upper Paleozoic natural gas in the southern basin contains a higher carbon dioxide (CO₂) gas content, and exhibits a common phenomenon of methane and ethane carbon isotope composition inversions. This paper employs gas geochemistry as the principal analytical method to systematically compare the north-south differences in the Paleozoic natural gas composition and to explore its origin and source. The findings indicate that the Upper Paleozoic natural gas in the southern basin mainly composed of highly over-mature coal-type gas. However, certain gas geochemical indicators suggest the presence of lower paleomarine hydrocarbon sources in specific areas. The observed inversion of methane and ethane carbon isotope composition in the Upper Paleozoic natural gas in the southern basin is attributed to the mixing of different types of natural gas. Specifically, the varying degrees of mixing with the Lower Paleozoic oil-type gas—characterized by a higher ethane content and lighter ethane carbon isotope values—are identified as the primary cause of the inversion of carbon isotopes. Furthermore, geochemical indicators of natural gas in the lower Paleozoic in the southern basin strongly reflect typical marine hydrocarbon source characteristics. While these gases predominantly originate from marine source rocks, a minor contribution from the Upper Paleozoic coal-type gas cannot be entirely ruled out.