Research On The Accumulation Characteristics And Controlling Factors Of Chang 8 In Huachi Area, Ordos Basin

Chang 8 oil-bearing formation of the Yanchang Formation reservoir in the Ordos Basin is rich in low-permeability oil and gas resources,but it is difficult to explore and develop.In recent years,under the guidance of unconventional oil and gas geology theory,many new breakthroughs have been made with the regard of oil and gas exploration of Chang-8 formation,and shows a good prospect of exploration and development.However,there is a difference of oil enrichment in low-permeability oil resources of Chang 8 oil-bearing formation,and the controlling factors of the regularity of differential accumulation are not yet clear.This paper,taking the Chang 8 oil-bearing formation in Huachi area as the research object,based on the previous research with the core observation,thin slice identification,mercury pressure,physical property analysis,geochemical analysis experiment and so on.This paper is focused on the sandstone distribution,reservoir characteristics,source rock distributions,migration systems and hydrocarbon accumulation dynamics of Chang-8 formation in Huachi area,Ordos Basin,in order to find the main controlling factors of the difference between the eastern and western parts of Huachi area and establish the accumulation model.The distribution of sandstones and the study of reservoir characteristics shows that the Chang-8 skeleton sandstones in Huachi area belongs to underwater distributary channel sandstones in delta front.But the development scale of sandstones is different between various regions.The development scale and continuity of the western sandstones in Huachi is obviously better than that in the eastern part of Huachi.Reservoir rocks are mainly composed of lithic feldspar sandstones and feldspar lithic sandstones.While interstitial material type varies and relatively high in content in the studied area including hydromica,chlorite membrane,ferrocalcite,and silicon.Reservoir of Huachi area possesses fine particle,bad roundness,and good sorting.The main reservoir spaces are intergranular pores and dissolution pores.The average porosity and permeability is respectively 8.06%and 0.61×10-3?m2 of Chang 8 reservoir belongs to the ultra-low pore-ultra-low permeability reservoir.The average porosity and permeability is respectively 10.81%and 1.42×10-3?m2 in the western Huachi area,which are higher than the Eastern Huachi area as 9.70%and 1.11 ×10-3?m2 respectively.Chang-7 distributing widely has high abundance of organic material,large thickness and tremendous hydrocarbon generation ability.It has strong hydrocarbon generation ability,which belongs to the category of good source rocks.The study of oil source correlation shows that the oil from Chang 8 of Huachi area mainly originates from overlying Chang 7 source rocks.The reservoir types of the Chang-8 reservoir in Huachi area are mainly belonged to lithologic oil-gas reservoirs.Sandstones with good internal connectivity and fractures are the major migration systems of crude oil.The oil and gas transporting force in this area is mainly from under compaction of source rocks and abnormal high pressure from hydrocarbon generating pressurization.Huachi area is located at the center of the source rock,Chang-7 source rock is welldistributed with abundant oil sources,and the oil-generating potential is not the decisive factor controlling the oil-water differential accumulation in this area.The scale of sandstones,the physical property and heterogeneity of reservoir affect the enrichment degree of oil.Restricted by the reservoir-forming power,the contact relationship between Chang-7 source rock and Chang-8 reservoir is the premise of downward migration of oil,and the matching relationship of muddy partition and fracture between source and reservoir determines whether oil can be poured upside-down.On those premise,oil distribution is controlled by factors such as sandstones development scale,sandstones internal structure,reservoir physical properties and heterogeneity.Argillaceous interbeds are developed between source rocks and reservoirs in the western part of Huachi,which prevents the effective migration of oil.Thick sand with large scale,good physical properties,strong homogeneity and no obvious internal structural changes is not conducive to oil accumulation,which turns into a migration pathway for oil.The interlayer between source rock reservoirs in eastern Huachi area is relatively rare,this is beneficial to the pour irrigation of oil.And the thin sandstones of eastern Huachi have strong heterogeneity,which are easy for limited crude oil to seal and form reservoir under lithological and physical conditions.Based on the above-mentioned reservoir control factors,a reservoirforming model for the reservoir enrichment of the Chang 8 oil-bearing reservoirs in the study area was established.