Study On Stratigraphic Division And Sedimentary Facies Of The 8th Member Of Lower Shihezi Formation Of Su5 & Su20 Region In Sulige Gas Field

Upper Paleozoic of Ordos Basin contains very rich natural gas resources, while Sulige gas field has been found that the largest natural gas-rich regions.Currently the main gas-bearing blocks found are located in the Yikezhaomeng Wushenqi,Etuokeqi Etuokeqianqi of Inner Mongolia Autonomous Region and in Dingbian territory,Yulin region of Shaanxi Province.Region surface is mainly desert and marsh,with the ground altitude arranges from 1250 to 1350 meters.Sulige gas field is located in the nouth of Hahantumiao and Taoli,northwest of Wushenqi.Regional tectonic position belongs to the northwest part of Yishan slope in Ordos Basin.Upper Paleozoic gas source rocks are the paralic coal rocks and mudstones from the lower part of Carboniferous-Permian.The delta plain (front) distributary channel sands, delta front estuarine sands,marine and coastal sands and tidal channel sands developed between and up the gas source rocks constitute the reservoir rock.The floodplain lacustrine mudstones in the upper part of Permian form the ideal regional seal of the Upper Paleozoic gas reservoir.Make a fine contrast to the stratigraphic division of individual-well and multi-well evaluating is an important step of the description of reservoir studies and also the premise to the further research of reservoir space distribution and variation characteristics,has a great significance of subsequent exploration and development of oil and gas fields in this region.The entire deposition period has not undergone large-scale tectonic activity in Su 5&Su 20 area,the stratums thickness is uniform,there is only north-south directional trend in thickness change whithin the scope of entire area.The Upper Paleozoic statum in show obvious characteristics of logging data in the area, we are able to identify many regional marker beds clearly and established a new principle of stratigraphic division. There are more than 500 finished drilling wells in the study area and they were all divised and contrasted under the new principle.The logging data of those finished drilling wells shows that the GR and SP curves can directly reflect the sandstone of the 8th member of Shihezi formation. The GR curve showed a box-shaped, at the low value segment has a slight tooth-shaped characteristics, sometimes bell-shaped and box/bell-shaped characteristics.The mudstone is showing a clear baseline characteristic. The main types of sedimentary structures of the 8th member of Shihezi formation in Su5&Su20:Trough cross-bedding and Tabular cross-bedding.The water flow energy is strong during the sedimentary period,so the cross-beddings are well developed in various sizes.Combined with previous conclusions that have been given and make full use of the research area core drilling data, logging data. Based on sedimentology flag, logging facies flag to define the Sedimentary microfacies program of the 8th member of Shihezi formation in Su5&Su20. Braided river delta is the main type of sedimentary system in the study area.The subfacies is typical braided river delta plain deposition,and can easily find obvious sediments of branches of rivers, swamps and river islands.During the deposit period of the 8th member of Shihezi formation in Su5&Su20,the sand bodies main distributed in north-south direction and following the direction the carrying capacity and the stability of the braid river decreased, lateral migration and river crossover phenomenons are frequent to see. The sand bodies of the 8th member of Shihezi formation are developed in single flake,and with the deposition and the river swing,sand bodies kept stacking that make the better connectivity in the vertical directionAccording to the single-well and multi-well sedimentary facies analysis and the regular pattern of the sand bodies distribution,that can established a braided river delta sdimentary facies model.The branch of river is the main subfacies of the braided river deposition,many sand bodies kept stacking in vertical direction and there is a lack of a clear "dual" structure. Due to the sediment supply is adequate and strong water flow energy condition with low slope, a large number of bifurcation developed during the river moving forward, and that make the bottom has a strong scouring effect and the sand bodies extend farther.In the plane,sand bodies developed in pieces and in the vertical, sand bodies interconnected to from a "common-connected component", in which the channel sand bodies stacked each other.