Insights into origins of the natural gas in the Lower Paleozoic of Ordos basin, China

Based on natural gas compositions, carbon isotope ratios, and geological analysis, the genesis and origin of gases in the Lower Paleozoic of Ordos basin are discussed. Due to differences in distribution and genesis, the gases in Lower Paleozoic were divided into five types, namely western margin gas, weathered crust gas, pre-salt gas, Ordovician deep gas, and Cambrian gas. The results show that the delta C-13(1) and delta C-13(2) of western margin gas range from -35.6 to -38.9 parts per thousand and from -27.2 to -35.9 parts per thousand, respectively, indicating oil-type gas and mainly from the O(1)k and O(2)w marine source rocks. The delta C-13(1) of weathered crust gas varies from -31.2 to -37.8 parts per thousand, but delta C-13(2) is mostly around -25.0 parts per thousand, which indicates weathered crust gas originates from the Carboniferous-Permian coal-measure source rocks. In contrast, delta C-13(1) and delta C-13(2) in the pre-salt strata are mostly less than -30.0 parts per thousand. Yet its distribution of Delta(delta C-13(2)-delta C-13(1)) is from -1.7 to 13.6 parts per thousand, and C-1/C-2 + C-3 is of 6.68-4372.50. These indicate that the pre-salt gas is mainly high-over mature oil cracking gas from the O(1)m marine source rock. The delta C-13(1) of Ordovician deep gas is averaging -39.4 parts per thousand, and delta C-13(2) varies from -25.4 to -33.0 parts per thousand, which imply coexistence of oil-type and coal-type gas. It is presumed that the C-P coal-type gas may migrate into and accumulate at Ordovician deep reservoirs through unconformity and fault. The delta C-13(2) and Delta(delta C-13(2)-delta C-13(1)) of Cambrian gas are both the most negative, respectively, averaging -34.9 parts per thousand and averaging -3.2 parts per thousand, indicating over-mature oil-type gas. Formation and evolution of the Qingyang paleo-uplift result in that the Cambrian gas mainly comes from the Cambrian or Ordovician marine source rocks in the southern basin.