Origin of Natural Gas from the Ordovician Paleo-Weathering Crust and Gas-Filling Model in Jingbian Gas Field, Ordos Basin, China

Quanyou Liu¹, Mengjin Chen², Liu Wenhui1, Li Jian³, Pinlong Han², and Yanru Guo^3
1Exploration & Production Research Institute, Beijing, China
²Research Institute of Petroleum Exploration and Development, Beijing, China
³Langfang Cranch of Research Institute of Petroleum Exploration and Development, Langfang, China

The Ordos basin is one of the most tectonically stable sedimentary basins favorable to natural gas generation and preservation. As a result, it becomes the second largest natural gas producing basin in China. The origin and potential source of producing natural gas from the Ordovician paleo-weathering crust in the Jingbian gas field, one of the five giant gas fields in the basin remain unclear. The main argument is whether the gas is generated from the Carboniferous-Permian coal or the Ordovician carbonate. This study provides intensive gas geochemical evidences to clarify its origin and potential source. 82 natural gases collected from the Ordovician paleo-weathering crust in the Jingbian gas field are dominated by C1-C4 gaseous hydrocarbons with low CO₂, H₂S and N₂ non-hydrocarbon components. The content of C1~C4 ranges from 79.26 to 99.93% with a gas dryness (C1/C1-4) of 0.981~0.999, suggesting that natural gases are of the characteristics of dry gas and possibly originate from high to over mature source. The carbon isotopic compositions of methane and its homologues in natural gases from the Ordovician paleo-weathering crust are widely varied, and the ∂13C1, ∂13C2 and ∂13C3 are -38.9‰ ~ -29.1‰, -37.5‰ ~ -22.2‰ and -30.0 ~ -20.9‰, respectively. Apparently the ∂13C2 and ∂13C3 in the Jingbian gas field are much more negative than those of typical coal-type gas origin in Sulige, Wushenqi, Yulin, Daniudi, Kela 2 gas fields. The carbon isotopic signature suggests that the natural gas in the Ordovician paleo-weathering crust in the Jingbian gas field most likely originated from the Ordovician carbonate source with sapropelic organic matter. The ∂D value of methane ranges from -177 to -155‰ and supports our speculation about the natural gases from the marine sapropelic organic matter.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas