The Sedimentary Facies And The Accumulation Mechanism Of Petroleum And Gas In Chang4+5and Chang6Reservoirs From Zhongshanchuan Region Of Ordos Basin

After largely exploration in recent20years, the pressure underneath is seriously insufficient, which leads to the significantly decrease of production in Chang4+5and Chang6reservoir of Yanchang formation in oil region of Zhongshanchuan in Ordos Basin. Therefore, it requires us to expand the areas with water flooding in oil region of Zhongshanchuan and use secondary oil recovery to increase the oil production and stabilize the high oil recovery. In this thesis, based on the studies of geology in Zhongshanchuan region, we used geophysical well logging and lithofacies palaeogeography methods to divide and compare Chang4+5and Chang6reservoirs. Together with the petrologic study and understandings of the property and the controls of the reservoirs, we attempt to reveal the mechanism of oil and gas accumulation..Referred to the method to divide and compare the Triassic Yanchang Formation in Ordos Basin, through using the mark layer and comparing the cycling and the thickness, we divide Chang4+5from oil region of Zhongshanchuan into two groups, Chang4+51and Chang4+52, the latter of which is further divided into Chang4+521and Chang4+522sub-group, while Chang6oil group is divided into Chang61, Chang62and Chang63sands group. The oil-bearing reservoir is mainly distributed in Chang4+52and Chang61; locally indicated in Chang62.Through comprehensive geologic study, Chang4+5and Chang6are thought to reflect delta sedimentary facies. Specifically, Chang61can be defined as delta front sub-facies, where subaqueous distributary channel and inter-distributary bay are developed. To its east and north, two NE-SW banded distributary channels are developed, with distributary channel sandbodies-1.2-4.5km wide, and separated by inter-distributary bay. Chang4+5, as delta plain sub-facies, developed two micro-facies, distributary channel and flood plain, composed of six NE-SW distributary channel sandbodies, about1.0-4.0km wide. The studied reservoir is mainly composed of medium to fine-grained feldspar sandstone. The physical property of the reservoir is relatively good, with micro-fine throat-like and fine porosity structure.It can bedefined as a low to extra-low permeable reservoir, in which oil is widely present and even superimposed as flakes, and mainly controlled by sedimentary micro-facies, showing a good relationship of source-reservoir-caprock.