Effect of gypsum rock on oil-gas accumulating process and its implication for exploration: a case of the ordovician Maigaiti slope, southwest of Tarim Basin

Regional gypsum rock is developed in the Lower-Middle Cambrian of Maigaiti slope, southwest depression of Tarim Basin. Its controlling effect on the accumulation of oil and gas has been seldom concerned yet. Process of the oil-gas accumulation and main controlling factors haven't been recognized clearly, which make exploratory target be restricted near the crust of Ordovician, and progress of the oil-gas exploration has been restrained seriously. According to the genetic analysis of the oil, gas and water with geochemical parameters, it's considered that gypsum rock of the Lower-Middle Cambrian played an important role in the genesis of oil, gas and water. Natural gas with high dryness and carbon isotopes is oil-cracked origin which has undergone thermochemical sulfate reduction reaction under the gypsum rock, and the abnormal Na2SO4water is formed due to the mixing of the formation water with natural gases which have migrated to the Upper Ordovician traps. Based on the analysis of the structure evolution of the top layer of the gypsum, the fault active stages and the coupling with oil-gas accumulation, it's concluded that the gypsum also played an important role in the accumulation of the oil and gas. The discovered oil-gas fields are formed by the paleo oil-gas pools of Cambrian, which were sealed up by the gypsum rock, differentially adjusted by the structure and separately transported by the fault. Finally the oil-gas accumulation model has been built by combining the factors of source rock, reservoir and cap rock etc. It's predicted that the area under the Lower-Middle Cambrian gypsum rock of Maigaiti slope could be a new domain for exploration. © 2015, Science Press. All right reserved.