The origin of natural gas and the hydrocarbon charging history of the Yulin gas field in the Ordos Basin, China

The genetic type, source and charging history of natural gas in the Yulin gas field in the Ordos Basin have been studied by combining the carbon isotopic composition of natural gas and geochemical characteristics of light hydrocarbons with carbon isotope fractionation model results and fluid inclusion analysis. The carbon isotopic composition of methane and ethane in the Yulin gas field is relatively enriched in C-13 with delta C-13(1) values ranging from 35.3 parts per thousand to -29.8 parts per thousand (average value = 32.4 parts per thousand) and delta C-13(2) ranging from 26.3 parts per thousand to -23.5 parts per thousand (average value = -24.8 parts per thousand). The C-7 light hydrocarbons are predominated by methylcyclohexane, accounting for 65.8% to 80.9% (average value = 71.6%), which is characteristic of coal-derived gas. Furthermore, the gas geochemistry indicates that, although marine limestone source rocks are present in the region, the contribution of oil-associated gas from this source to the Yulin gas field is quite low. Based on the empirical relationship between delta C-13(1) and %Ro of gas source rocks, and the kinetic isotope fractionation predictions for gases generated in the Yulin gas field, the observed delta C-13 of methane is heavier than that of natural gas only originated from in-situ coal-measures. This result shows that gas in the Yulin gas field is contributed not only by the natural gas generated from local coal-measure source rocks, but also from the higher maturity natural gas sourced from the coal-measure source rocks to the south or southwest of this gas field. Fluid inclusion analysis proves that the accumulation of natural gas in the Yulin gas field has the characteristics of continuous charge. (C) 2009 Elsevier B.V. All rights reserved.