Progress in Paleozoic coal-derived gas exploration in the Ordos Basin, West China

Based on practices of gas exploration and development in the Ordos Basin, this paper analyzes the controlling factors of large-scale accumulation conditions and distribution characteristics of coal-derived gas, and makes conclusions on the exploration progress of coal-derived gas based on present exploration situations. The approach has proven coal-derived gas reserves of 5.24×1012 m3 in the Ordos Basin. Twelve coal-derived gas fields have been found so far, mainly distributed in Upper Paleozoic Carboniferous-Permian clastic rocks and Lower Paleozoic Ordovician marine carbonate reservoirs. Tight sandstone gas reservoirs are developed in the Upper Paleozoic. The features of widely-distributed hydrocarbon-generation, interlayered with large-scale distribution of sandstones, where the reservoirs have become tight and followed by natural gas accumulation later, close range migration and efficient hydrocarbon accumulation, has resulted in a large area of distribution of tight sandstone gas zones. The Sulige large gas field has been found with proven coal-derived gas reserves of 3.49×1012 m3, and two large-scale gas reserve replacement fields have been developed and implemented in the eastern part of the basin and in the Longdong area. Marine carbonate gas reservoirs are developed in the Lower Paleozoic. Gas generated from overlying source rocks have migrate downwards and accumulated in Ordovician weathering crust karst and dolomite reservoirs. The Jingbian gas field was discovered, with proven coal-derived gas reserves of 7000×108 m3, meanwhile, several gas-rich regions have been found in the eastern side of the dolomite. Key words: Ordos Basin, Paleozoic, coal-derived gas, tight gas, carbonate gas, accumulation conditions, exploration progres