Genesis and accumulation of natural gas in the Upper Palaeozoic strata of north‐eastern Ordos Basin, China

Abstract

The north‐eastern Ordos Basin in Central China has become an exploration target in recent years due to the potential of natural gas resources in the Upper Palaeozoic strata. In this study, by analysing the geochemical characteristics of the source rocks and the natural gas, the genesis of the Upper Palaeozoic natural gas was identified. In addition, the accumulation process was discussed by combining studies of hydrocarbon generation and burial/thermal history. The results reveal three sets of source rocks developed in the study area—Carboniferous Benxi (C2b), Permian Taiyuan (P1t), and Shanxi (P1s) formations—consisting of mudstones, carbonaceous mudstones, and coals. The source rocks contain abundant Type III organic matter and have entered mature to high‐mature stage, while source rocks in the south‐eastern part of the study area have entered the overmature stage due to the activity of Zijinshan magmatic rocks. The natural gas has a high content of methane (average 93.99%). The stable carbon isotope of methane (δ13C1) value (average −37.05‰) and that of ethane (δ13C2) value (average −26.4‰) and the composition of the light hydrocarbons reflect the idea that the natural gas is organic in origin, thermogenic, and mainly terrigenous‐sourced. The evolution of a transitional to continental depositional environment of the source rocks results in the variation in the geochemical characteristics of the Upper Palaeozoic gas in the study area. Analysis of fluid inclusions indicates a long and continuous period of natural gas charging in the study area from 165 to 65 Ma. Sandstone bodies close to or interbedded with the source rocks made the gas charging from the source rocks to the reservoirs over a short distance with high efficiency, while faults and fractures provided paths for the gas migrating to reservoirs vertically far from the source rocks. The huge gas generation potential of the source rocks and continuous distribution pattern of sandstone bodies provides favourable conditions for the development of a large‐scale natural gas reservoir.