Paleokarst reservoirs and gas accumulation in the Jingbian field, Ordos Basin

The Jingbian gas field in central Ordos Basin, with a proven initial in place gas reserve of approximately 11 trillion cubic meters, is the largest paleokarst carbonate gas field in China. Paleokarst in Ordovician strata of central Ordos most commonly occurs in the paleo-weathering surface of the O1m5 member of the Majiagou Formation. The karst intervals are generally proximal to the sub-Upper Paleozoic unconformity; however, dissolution features in strata well below that exposure surface are possibly related to intra-Majiagou Formation unconformities. The quality of gas reservoirs are initially controlled by sedimentary facies, with anhydrite-bearing dolomite flat facies being the most favorable zones for dissolution and dolomitization to form karst and large/small dissolution cavities. The gases are generally dry, derived dominantly from the overlying Carboniferous–Permian coal measures. The gases are accumulated in stratigraphic traps related to karst paleo-geomorphology and lithologic traps associated with late diagenetic features of carbonate rocks. Although the precise timings of the thermal events during the evolution of the Ordos Basin are still subject to considerable debate, there is a general consensus that events occurring during the Yenshanian orogeny (150–115 Ma) were the most important for the Paleozoic source rocks. It appears that two episodes of hydrocarbon charge have occurred in the Ordovician gas reservoirs in the Jingbian field.