Study On Sedimentary Facies Of The Lower Part Of 8th Member Of Lower Shihezi Formation In Ordos Basin

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 south of Yimeng uplift,north of Yan'an-Gaoqiao,east of Tianhuan depression,and in the west of Iraq Shaanxi slope.Regional tectonic position belongs to the northwest part of Iraq Shaanxi 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.The formation of Sulige gas field is mainly controlled by the river delta sands distribution and the basin regional tectonic setting of the Upper Paleozoic.Sulige gas developed large sandstone lithologic traps.The main gas reservoir is the 8th Member of Lower Xiashihezi Group in Permian,lithology is mainly coarse-grained quartz sandstones in the delta plain distributary channel microfacies sediments.Years of exploration and development of production practices confirmed that the sand thickness of the 8th Member changes in a wide range,so the reservoir is strongly non-homogeneous.Focus on the sedimentary environment,reservoirs and other areas of sciences,the Research Institute of Changqing Oil Field,as well as a number of institutions and research units have carried out several rounds of researches and explorations,and their cognitions on the deposition and reservoir are still not consistent.Most researchers believe that the 8th Member sands in Sulige area are(braided)fluvial and(braided)delta distributary channel sands,while some other scholars raised that they are mainly shore lake sand dam sands and so on.Therefore,from a new angle of departure,emphasis on structural anatomy and depositional systems of the main reservoir sandstone body awareness,and for the successful completion of the planning objectives,to figure out the sedimentary facies types and their spatial distribution patterns of the 8th Member of Lower Xiashihezi Group,which is the main part of natual gas reservoir in Sulige area,has a great significance to find more high-quality reservoir,accelerate the pace of gas exploration,development and production.The 8th Member of Xiashihezi Group in Sulige area formed during the North China Basin translated from the remnants of the epicontinental-the offshore lake basin declining and falling stage to the coastal island depression stage,and the terrigenous clastic depositional systems is well developed.Source analysis is an important basis to distinguish the tectonic setting of sedimentary basins and restore paleoclimate and paleoenvironment,as well as to reshape the ancient geomorphological features of the study area,to reproduce the ancient river system and to determine provenance parent rock nature.The study based on regional sedimentary background,combined with the type of sandstone detrital composition,light and heavy mineral distribution characteristics,quartz cathodoluminescence analysis techniques and geochemical aspects of laboratory analysis,we explore the provenance of the 8th Member of Xiashihezi Group.We believe that the matter source and paleocurrent mainly came from the north and the northeast of the basin.In this study area,there are some provenance differences between the east and the west of the basin.There are more newer Devonian-Carboniferous granites and volcanic rocks in the provenance of quartz sandstone in the western region.While part of the provenance in the eastern region came from Archean metamorphic rocks of the northeast of Yinshan block.Sandstone rock types are quartz sandstone,lithic quartz sandstone,lithic sandstone,and the main type is lithic quartz sandstone.Rocks with fine gravel-coarse structure usually have high quartz content,and the thinner the granularity is,the higher the lithic content is.About the detrital composition of sandstone,single crystal quartz is mostly granite type,then the metamorphic type,while the polycrystalline quartz is mainly metamorphic rock type with less feldspar.Interstitial material in sandstone is mainly hydromica,kaolinite,siliceous and carbonate cements,while part of the layers contain more chlorite film and tuffaceous hybrid base.Sandstone generally has a coarse granularity,and the Sorting is medium,better,and the detrital roundness is medium.Make full use of the drilling core data,logging data and indoor tests analysis data of the study area,based on sedimentology signs,logging facies indicators,geochemical,paleontological and other signs,combined with many previous studies,We have established the deposition system classification scheme of the 8th Member of Xiashihezi Group.The 8th Member of Xiashihezi Group mainly developed the alluvial plain depositional system,lacustrine delta depositional system,and the lake system.The north of the study area developed the alluvial plain system,and in this area we can identify heart beaches,natural levee,crevasse splay and some other depositional microfacies.The central and the north of the study area developed the lake delta depositional system,further divided into braided river delta plain deposits and delta front deposits.The agvantages of delta plain facies are more obvious,and the distributary channel,the natural levee,the river mouth bar,the distal dam and some other sedimentary microfacies are all identifiable.The southern edge of the study area and on both sides of the delta front can be further divided into Lakeside and shallow lacustrine subfacies.Due to the frequent swing and the characteristics of the rapid migration of the braided channel of the 8th Member of Xiashihezi Group,the sand bodies are generally north-south distributed,and the sand-carrying capacity of rivers reduced,the sedimentation reduced,and the stability of the river becomes week.Sand bodies lateral migration and cross-compound is more frequent,and developed with a large-scale in the planar,patchy distributed.In the vertical,sand bodies interconnected to form a "common-connected component",in which the channel sand bodies stacked each other.According to the result of the deposition simulation,the main control factors of the patchy distribution of sand bodies in a large area include terrain slope,lake level,the proportion of sand to mud,provenance supply and so on.In low slope conditions,the delta distributary channels are well developed,and the lateral migration and aggradation are common,forming a large area of sandstones.In conditions of the stable lake level,the distributary channels lateral migration is common,forming a large area of sandstones.In conditions of the high proportion of sand to mud,sands mainly deposited in estuary,forming a flower-shaped sand bar and sand bar migration superposed to form a contiguous sand body.Multi-source supply resulted in the pooling of water and flowers contiguous,forming a large area of sandstones.