Deep basin gas trap in the Upper Paleozoic of Ordos basin

The giant gas accumulations in Ordos basin found in low- permeability Paleozoic Paleozoic sandstones provides some of the best available information on basic characteristics of deep basin gas traps. The deep basin gas reservoir mainly consists of deltaic sandstone with varying amount of coarser sandstone and conglomerate. The numerous and mature coalbeds and organic-rich shales associated intimately with reservoir rocks throughout the region provide a prerequisite for deep basin gas accumulation. Gas generated in adjacent source rocks, moved vertically into the sand layers and then slowly migrated updip. This process lasted from late Triassic to late Cretaceous. This special form of deep basin gas trap defies conventional concepts of gas entrapment by turning them virtually upside down. Gas is trapped in the deeper part of the basin and is located in the downdip of an aquifer which outcrops mainly in northeast Ordos. The gas/water contact is really a wide belt, which marks the transition from downdip gas to updip water. Original gas accumulation pressures lie below the regional formation water pressure gradient. Gas-productive units in the deep basin are not controlled by local structure. An estimated 1. 47 × 1012 to 10. 5 × 1012m3 of gas in place are contained in the 15,000 km2 Upper Paleozoic gas accumulation.