Study On The Main Controlling Factors Of The Reservoir Composition Of Chang 8 Oil Layer In Chenghao Area, Ordos Basin

The early exploration of Chenghao area was mainly focused on Chang 6 and other shallow layers.In recent year,the activity of exploring the petroleum has increased,which has led to change in ground levels from Chang 6 to Chang 8 and other deep stratum.There is a huge difference about the property of oiliness between Chang 81 and Chang 82.According to the data of the exploration at different wells,the hydrocarbon distribution of petroleum at Chang 81 is complex and the chemical property varies considerably between thick bodies of sand.However,what factors lead to the above phenomenon?In order to figure out the reason for the above phenomenon,we need to figure out the main factors controlling the hydrocarbon accumulations in Chang 8 level of Chenghao area.Based on the previous research results performed in relevant regions,this paper combines sedimentary petrology,petroleum geology.It also combines basic theories,preliminary work including core observations,collection and interpretation of well logging data,the observation and arrangement of rock slice.Further,we have also included our study on scan of electron microscope pictures,the collection of reservoir physical properties and mercury injection permanents.We have also analyzed geochemical data,(analyzed on the aspect of sandstone distribution),reservoir characteristics,characteristics of hydrocarbon accumulation.The main purpose of the study is to check not only the key factors of petroleum entrapment in Chenghao area but also provide theoretical support for further exploration.The research indicates that shallow water delta is developed in Chang 8 of Yanchang formation in Chenghao area,and most of the sand bodies are underwater distributaries channel sands.The sand bodies of Chang 81 are large scale and thick horizontally,at the same time,the sand bodies of Chang 82 have a smaller scale and thinner thickness.In addition,the sand body connectivity of Chang 81 sand body is better than that of Chang 82 sand body.The type of reservoir rocks mainly composed of lithic arkoses,followed by arkosic arenite.Moreover,interstitial materials include hydromica,chlorite,and ferrocalcite.As for sandstones,there are many fine sandstones and most of them are mid-fine grained,furthermore,the type of reservoir space is mainly composed of inter-grain pores and telepathic pore,some microcracks are also developed in the local area.The physical property of Chang 8 reservoir is poor,belonging to the ultra-low pore-extra low permeability reservoir,in which the average porosity is 7.04%,and the range of porosity is from 5%to 9%,and most of the permeability is less than 0.1 × 10-3 ? m2,plus,the physical property of Chang 81 is better than that of Chang 82.The reservoir of Chang 8 experienced three digenesis including compaction,cementation,and dissolution.Development of Chang 7 mudstones in the study area is extensive,with a large distribution area and strong continuity.Geochemical experiments found out that mudstone of Chang 7 in the study area has a large potential for hydrocarbon generation and provides a good source of petroleum in the environment for Chang 8 reservoir.The oil and gas transporting force in the study area is because of the underpressure formed by the stress of the Chang 7 source rock and the abnormally high pressure generated by the hydrocarbon generation pressurization.Through the homogenization temperature logging of fluid inclusions and the thermal history of source rocks,it is shown that the oil and gas in this area has undergone two migrations,including 150-132 Ma and 118-100 Ma.Interconnected sand bodies and partially developed fractures provide a passageway for hydrocarbon migration.Overall,the distribution of Chang 8 oil and gas in Chenghao area is mainly affected by the differences in reservoir physical properties,sand body size,and connectivity,sand structure internal structure and crack development degree.Because Chang 81 contact with the Chang 7 source rock directly,and as a result of the development of local crack,the good reservoir property,the strong sand body connectivity,and the changes of internal structure in the sand body,the oil and gas distribution on the plane is complex and the reservoir distributions are rich.The reservoir of Chang82 has poor physical properties,and the sand body size is much smaller than that of Chang 81.Moreover,the connectivity is poor,and the reservoir distribution is infrequent.