Main controlling factors of gas pooling in Ordovician marine carbonate reservoirs in the Ordos Basin and advances in gas exploration

Industrial gas flow was obtained in Shancan-1 and Yu-3 wells in the Ordos Basin in 1989 and the Jingbian Gas Field was discovered by then as the largest marine carbonate gas field in China,which confirms the huge potential of gas exploration in the lower Paleozoic marine carbonate reservoirs in this basin.In order to promote the early buildup of another Daqing in West China and provide robust technical support for further exploration in the Jingbian field,a summary was made of the main controlling factors of gas pooling in this study area as well as the involved technical approaches to gas exploration.Through the integrated analysis of depositional characteristics,source rock conditions,reservoir types and reservoir forming assemblages,three exploration domains in this Ordovician marine carbonate-developed basin were determined including the middle and eastern Ordovician weathering crust,dolomites in eastern Central paleouplifts,karst fractures and caverns in the west.After recent years' efforts in geological research and technological breakthrough,the main controlling factors of gas pooling in the above exploration domains have been well understood and a complete set of techniques have been formed such as a fine description of paleokarst landforms,forecast of weathering crust reservoirs,seismic prediction of dolomite reservoirs and detection of their gas-bearing property,recognization of fractures and caverns and reservoir prediction,and so on.Thus,new progress has been made in gas exploration in this study area.The discovered gas-bearing area in the Jingbian field has been increasing and gas reserves there nearly doubled,which is a realistic goal for the PetroChina Changqing Company to enhance its reserves and production.Multiple gas enrichment zones have been discovered in the dolomite reservoirs in the eastern Central paleouplift with natural gas in place of close to 100 billion m3,which is a realistic alternative domain.New signs of gas pooling have been found in the fractures and caverns in the western basin,which is a new area of further gas exploration.