The Study Of Sedimentary Microfacies And Reservoir Evaluation Of Yan'an Formation About The Northern Part Of West Block In Mahuangshan Area

The Mesozoic Yan'an fomation of Mahuangshan area has a fine hydrocarbon exploration potential in the western Ordos Basin.Up to now, Many Jurassic oil fields have been found around Li zhuang zi,Hong jing zi and Da shui keng regions,e.g. Bai Yan Jing and Huan Chi Jurassic oil fields.Recently, Yan 8 in ND3 and ND5 wells have achieved high-yield industrial oil flow without any water,and ND17-2 well got about 60m3 high-yield oil flow in Yan 5 by production test.All of those comfirm a fine hydrocarbon exploration potential of Yan'an formation in Mahuangshan area.However,with the advance of the exploration and development it can be found that many factors have directly curbed hydrocarbon exploration,e.g. lacks of knowledge of sedimentary microfacies and reservoir characteristics of the Yan'an formation.Therefore,it is essential to make a study for sedimentary microfacies,and reservoir characteristics,which have a important guiding significance for the future hydrocarbon exploration and development of Yan'an formation in Mahuangshan area.Based on absorbing the previous studies, this paper firstly make a comprehensive analysis of sedimentary microfacies for Yan'an formation in Mahuangshan area by using datum of core outcrops,loggings,drilling records,seismic,and slice etc..The results indicate that Yan'an formation mainly developed delta plain deposits.Yan10 to Yan8 members and Yan6 to Yan5 members all belong to delta plain deposits, and five microfacies types including distributary channel, natural levee, crevass-splay, swamp and interdistributary bay.Yan7 member belongs to delta front and prodelta deposits,its microfacies include underwater distributary channel,river mouth bar,distal bar and prodelta mud.The study of reservoir petrologic feature indicates that sandstone composition differ in layers.Yan10 to Yan9 members are mainly composed of coarse to medium feldspathic lithic sandstone and lithic arkose sandstone.Yan8,Yan6 and Yan5 members mainly developed fine arkose,and with minor quartz sandstone. From lower to upper layer,the partical size gets finer.Diagensis and diagenetic sequence of sandstone reservoirs vary a lot in the East and the West of Mahuangshan area owing to the difference in sandstone composition.In the western,sandstone composition is low in matrix and quartz overgrowth is weak,where quartz sandstone is prolific.Besides,the compaction is not prominent and the intergranular pore is developmental in this district;In the eastern,arkose sandstone and lithic arkose sandstone dominate.Dissolution of feldspar can be seen clearly under the microscope. Compaction is stronger than the western.Ferrocalcite,ankerite,anhydrite,and barite cementation vary a lot in this district and the area of low degree of cementation is quality reservoir developed zone.On the basis of physical analysis, mercury injection date, slice observation,we know that Yan'an formation exists kinds of pore types,such as primary intergranular pore,intragranular dissolved pore,moldic pore and dissolution fracture etc.The porosity mainly concentrate in 8%～18%,and average porosity is 12.87%.The permeability mainly lie in(0.1～1.5)×10-3μm2 and (>10)×10-3μm2,and the average permeability is 48.69×10-3μm2.Morever, the reservoir properties is get better from Yan 10 member to Yan 8 member.According to capillary pressure curve shape, reservoir can be divided intoⅠ,Ⅱ,Ⅲthree types.Simultaneously,using the saturation of capillary pressure value,the needed closed heights which would bring about oil-producting are calculated,it should believe thatⅠ,Ⅱreservoir require a small closed height (<10m)because of nice reservoir properties.Therefore,Yan'an formation enjoy the oil production capacity for that their reservoirs thickness are frequently greater than 10m.But reservoir properties ofⅢis poor so that they require a high degree of closed heights(>100m).And it might not have the capacity to produce oil,if any reservoir reconstructions are applied.Combined with oil-water relative permeability and the production capacity analysis,mercury penetration point that the low limit of Yan'an formatiom porosity is 8%,the low limit of permeability is 0.5×10-3μm2,irreducible water saturation is 35%,reminding oil saturation is 30%.Based on above,the criteria of reservoir evaluation classification is established to make reservoir evaluation of single well at last.Sedimentary microfacies,tectonic and diagenesis are main factors which control reservoir properties of Yan'an formation in Mahuangshan area.The high quality reservoir are developed in medium sandstone whose sandbody is thick,and accompanies with low compaction,weak cementation,strong dissolution and high structure.Finally,combined the distribution of oil and gas,the criteria of reservoir evaluation,this paper predict the favorable reservoir development area in different members by means of the above analysis.