| Pengyang area located in the western margin of Ordos Basin,where spans the thrust belt and Tianhuan depression.During the deposition of Yan'an formation,braided river delta plain facies was developed on the plane,and vertically,the channel sand body and swamp mudstone overlapped each other.The oil and gas of Yan'an formation in Pengyang area comes from Triassic Yanchang formation.Yanchang formation is a typical lithologic hydrocarbon reservoir with overall dense and poor physical properties.The reservoir sand body of Yan'an formation is well developed and the physical property is good.But the reservoir scale is small and the reservoir is often not in the area with the best physical properties.The reason is that the oil escapes upward due to the lack of caprocks with strong sealing ability in the upper part of the good reservoirs.Therefore,the determination of a good reservoir-cap assemblage is very important for finding favorable zones for oil and gas enrichment.This paper will discuss its influence on oil and gas accumulation from the perspective of reservoir-cap assemblages.In this paper,based on a large number of geological data and completed project information and combined with logging interpretation data,the reservoir-caprock system is vertically identified,the caprock mudstone thickness and mud ratio are calculated,the sedimentary microfacies are divided horizontally,and the position of reservoir-cap rock is determined in the plane.In addition,the logging interpretation model of reservoir and caprock porosity and displacement pressure is established by using advanced test and analysis methods combined with core analysis data,and the differential parameters of porosity and displacement pressure between reservoir and caprock are calculated.The reservoir-cap assemblage in the study area is comprehensively evaluated and classified by using the five parameters of storage coefficient,mudstone thickness,mud ratio,porosity and displacement pressure difference,and then the influence of reservoir-cap assemblage on oil enrichment is discussed.Main achievements are summarized as follows:(1)The distributary channel of Yan 6 to Yan 9 is the main area of reservoir development,and the inter-distributary depression is a good regional caprock.The average thickness of caprock mudstone is 20 to 23 meters,and the average ratio of mud-to-horizon is between 54%and64%.(2)Based on the existing measured data and logging interpretation data,a logging interpretation model is established.The average porosity of reservoir sandstone is 13%to15%,and the that of caprock mudstone is about 11.87%to 13%.The average permeability of reservoir sandstone is 16×10-3?m2 to 249.6×10-3?m2,and that of caprock mudstone is 5.5×10-3?m2 to 139.3×10-3?m2.The average displacement pressure of reservoir sandstone is 0.09MPa to 0.18MPa,and that of caprock mudstone is 0.16 MPa to 0.32MPa.(3)The average porosity difference between reservoir and caprock is 2.7%in Yan 7 member,2.4%in Yan 8 member,and 1.8%in Yan 9 member.The average displacement pressure difference between reservoir and caprock is 0.18 MPa in Yan 7 member,0.38 MPa in Yan 8member,and 0.2 MPa in Yan 9 member.(4)The types II and III of reservoir-cap assemblages are dominant in Yan 7 and Yan 8members,followed by the types I,while Yan 9 member is mainly composed of types III,and the area of types I reservoir-cap assemblage is relatively small.Most of the high-yield wells in the study area are distributed in the types I and II reservoir-cap assemblages,and sporadically distributed in the types III.According to the above research results,the three well areas of Z188,Y244 and Z178 were selected as favorable zones for production,while the three well areas of M13,Y74 and Y114 were selected as predicted favorable areas. |
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