Comprehensive Study On Tight Reservoirs Of Chang8 Member From Upper Triassic Yanchang Formation In Fuxian Exploration Area,Ordos Basin

Combined with the analysis of core observation, lithological thin section observation, scanning electron microscopy, mercury data micron CT analysis, the sedimentary facies, reservoir characteristics, reservoir and diagenesis densification process system of Chang 8, Yanchang formation were studied in Fuxian Exploration Area, Ordos Basin. The features of tight oil reservoir and the source rocks were analyzed in detail. Finally, the further exploration and favorable areas were noted.The main conclusions are as following:The reservoir of chang 8 member of Yanchang formation were controlled by the microfacies of delta front underwater distributary channel, underwater distributary mouth bar and interdistributary bay. The member of chang 82 were mainly underwater distributary channel sand body sandwiched between the shunt Bay mudstone till that the member of chang 81 evolved into underwater distributary channel which deposited with lake transgressive cycle characteristics of the sand back on the vertical.The sandstone reservoir components of chang8 shows the feature of high feldspar endmember lower end of the quartz element, andnterstitial material is mainly calcite and illite. Reservoir average porosity of 8.0% and permeability averaging 0.26mD,shows the typical of tight sandstone reservoir. The pore type are mainly intergranular pores, intergranular dissolved pore, intragranular dissolved pore among which the primary intergranular porosity and feldspar dissolved pore are dominated.Pore structure characteristics are so complex and unevenly distributed, for sub-micron holes and small amounts of nanoscale pore throat are dominated.The diagenies of chang 8 member are strong when the most has entered to the diagenetic stage A of diagenesis experienced including mechanical compaction, cementation, metasomatism, dissolution and fracturing etc.And the base of tight sandstone is muddy debris, broken biotite plastic components and other debris, the key of density are strong compaction and cementation of carbonate minerals sandstone and a number of secondary porosity feldspar debris forms soluble improved reservoir properties, resulting in formed part of tight sandstone reservoirs.The reservoir properties are effecred by depth, lithology, sedimentary facies, carbonate cements based on which the Chang 8 reservoir is divided into four types, type I of high quality reservoir, II middle class reservoir, ?class poor reservoir, ? class non-reservoir,among which the type ? are major contributors for production.Geochemical parameters show that Chang 8 member developed both source rocks and reservoirs. The potential oil and gas accumulation area are the thick sandstones with good continuity and pore structures which developed in underwater distributary channels. According to above research, three favorable zones of Class I and six favorable zones of Class ? are predicted in Fuxian exploration area.