The Factors Controlling The Distribution Of Sand Bodies In The Paleogene And The Role Of Sand Bodies In Controlling The Formation Of Oil And Gas Accumulation In The Southern Lufeng Depression

The study on the distribution characteristics of sand bodies in the southern part of the Lufeng depression and their controlling effect on the oil reservoir is of great significance for guiding the next exploration in the study area.However,there are still some problems in the study area,such as unclear distribution law of sand body,incomplete research on the main controlling factors of sand body and the reservoir-controlling effect of sand body characteristics.Therefore,based on core observation,logging data interpretation,seismic facies analysis,rock analysis and testing and other relevant data,this paper determines the type and distribution of the sedimentary facies of the target layer in the study area,analyzes the genetic type,development characteristics and distribution law of sand bodies,and further explore the controlling effects of paleoclimate,fault activity and paleogeomorphology on the distribution of sand bodies.On this basis,in combination with the oil and gas distribution law in the study area,the controlling effect of sand body characteristics and distribution law on oil and gas accumulation in the southern Lufeng depression is summarized.The main conclusions of this paper are as follows:(1)The scale of sand bodies in southern Lufeng depression is controlled by paleoclimate and fault activity.With the climate change of "semi-arid--humid--semi-arid",the scale of sand bodies is in a "large-small-large" cycle,and the lake basin type shows the variation law of "shallow lake--deep lake--shallow lake".The total sand body area shows a "small-large-small" variation rule with the change of "weak-strong-weak" on fault activity.(2)The development type of sand body is affected by the catchment unit area,uplift slope angle and height difference of the paleogeomorphology.The larger the catchment unit area,the larger the development scale of sand bodies;when the water body in the basin is shallow,the provenance slope area is dominated by large-scale braided river delta sand bodies;when the water body is deep,the high-angle steep slope zone develops nearshore underwater fans and fan delta sand bodies,while low-angle gentle slope belt develops near-source braided river delta sand bodies.(3)Sand body types have controlling effects on reservoir physical properties and oil and gas accumulation.In the southern part of Lufeng depression,the porosity of the sand bodies,at plain distributary channel,frontal distributary channel,and river mouth bar of fan delta,plain distributary channel,frontal distributary channel,mouth bar,sheet sand of braided river delta,slump fan as well as shore-shallow beach bar,is greater than 8% with the permeability being greater than 0.1 m D,which can meet the physical property conditions for oil and gas storage.Among them,the plain distributary channel and front distributary channel of braided river delta have the best physical properties and can be served as the main sand body types for hydrocarbon accumulation in the study area.(4)The scale of sand bodies has a certain influence on oil and gas accumulation.The frequency of oil layers explored by drilling is positively correlated with the total area of sand body distribution,the frequency of oil layers explored by drilling signifying “high-low-high-low”regularly with the change of sand body area.Moreover,the oil layers are mainly distributed in the 0-2 m thin layer,the 2-5 m medium-thick layer and the 5-10 m thicker layer in the single sand body formed by the joint forces of braided river delta front,shore-shallow lake and the lacustrine mudstone.(5)The connectivity of sand bodies has a controlling effect on oil and gas migration.The braided river delta sand body of the Wenchang section Ⅴ develops contiguously from the structural high point to the center of the lake basin into an effective transport system,and the hydrocarbon generation zone is connected with the transport lattice of sand body with a sand-ground ratio greater than 0.5,which provides important lateral transport conditions for hydrocarbon migration from hydrocarbon generation zone to the surrounding uplift zone.In contrast,the braided river delta of Wenchang section Ⅲ is small in scale.Since the center of the lake basin is dominated by shallow lacustrine thin sand bodies,and the hydrocarbon generation zone is in the range of the sand-to-ground ratio less than 0.5,which fails to be connected with the transport lattice of sand body,leading to that the oil and gas are mainly concentrated around the hydrocarbon generation zone in the sag.