Reservoir Characteristics And Hydrocarbon Enrichment Law Of Chang 8 Oil-bearing Group In Ansai Area, Ordos Basin

The degree of exploration and development of the Chang 6,Chang 4+5 and Chang 2 reservoir groups in Ansai area is quite high,however the research on Chang8 is seldom involved.Recent years,with the further exploration and development in Ansai,a considerable oil controlled reserves have been found in Chang8.As the new domain of searching reserves,the reservoir characteristics and hydrocarbon accumulation characteristics of Chang8 lack systematic study,which is directly restricts us on exploration and development.Therefore,in order to indicate the direction of exploration for this area,it is necessary to strengthen the study of Ansai Chang8,deepen the geological understanding,clear reservoir enrichment regularity.Based on the correlative literature and research fruit of domestic and foreign research scholars,guided by petroleum geology,Sedimentary petrology,Petroleum geochemistry,Synthetically and Applying log data,core observation,rock slices identification,SEM,this paper made a research on the reservoir characteristics,oil sources,carrier system,reservoir-forming dynamic and hydrocarbon accumulation model of Chang 8 in Ansai area,which can provide scientific basis and direction for the further exploration and development in Ansai.The results have shown that the reservoir lithology of Chang8 formation in study area is mainly feldspar sandstone,lithic arkose and lithic feldspar sandstone,the major interstitial matters are ferrocalcite cement and chlorite,the reservoir space are mainly intergranular pore and feldspar dissolved pore,in addition to a small amount of fracture developed.The porosity of Chang8 formation are mainly distributed in the range of 6%to 8%and the main distribution of permeability are in the range of less than 0.1 mD.In the vertical,the physical properties of Changs1 is better than Chang82,but they both belong to extra-low porosity and permeability reservoirs.The Chang8 formation came through strong compaction,cementation,which is the major factors of influencing the low-permeability reservoir in the area.The sand body of Chang82 was better developed and had larger thickness than Chang81,and the lateral stability of sand body was good along the direction of source.The thick sand bodies of Chang82 was distributed in east area,by contrast,the thick sand bodies of Chang81 was distributed in central area.The dark mudstones were most developed in Chang9,followed by Changs,the scale of dark mudstones of Chang7 was the smallest and its lateral connectivity was the worst.Oil-source correlation shows that source the crude oils of the Chang8 are derived from Chang9 source rocks.The main driving force for hydrocarbon accumulation is abnormally high pressure produced by mudstone undercompaction and hydrocarbon generation;The fracture was the major flowing channels between source rocks and reservoir.In summary,the main controlling factors on hydrocarbon accumulation in study area are mainly in the following aspects the distribution of reservoir characteristics,the scale of sand-body,source rocks distribution,and the distribution of fracture.