| The Jurassic Yan’an Formation reservoir in the southwest of Ordos Basin has gradually become an important economically beneficial exploration under the back-ground of low oil price due to its good reservoir physical properties,small reservoir size,high abundance of reserves,high single-well production and low saturation pressure.Although previous studies had carried out some work on the reservoir characteristics,distribution rules and reservoir control factors of Jurassic reservoirs in Ordos Basin,the research mainly focused on the central part of the basin,while the results for the target area were relatively small.There are still many scientific problems,such as the effectiveness of oil sources,the distribution of favorable reservoirs,the influence of tectonic action on reservoirs,water-rock interaction,the division of accumulation stages,and the characteristics of dredging system,which need to be further studied.This paper was based on drilling core,logging,experimental analysis and production dynamic data,etc.Multi-theoretical analysis methods of sedimentary geology,sedimentary geochemistry and petroleum geology were selected.Focused on the reservoir formation mechanism as the main line of research,the systematic research idea of "basic elements of reservoir-reservoir characteristics-factors controlling hydrocarbon accumulation-accumulation control factors-accumulation model" was clarified.For the first time,this paper systematically and comprehensively studied the accumulation factors,effects and periods of Jurassic reservoirs in the southwest of Ordos Basin,and made clear the main controlling factors of accumulation,and clarified the accumulation mechanism of "hydrocarbon source control distance,supercharged migration,continuous filling,facies belt-paleowater sealing,and landform-structure controlling accumulation".In addition,taking accumulation factors and accumulation function as the breakthrough point,two kinds of accumulation models in time and space scales were established.It will provide scientific basis for the development of Mesozoic Jurassic sedimentary evolution history,migration and accumulation history in Ordos Basin,and also promote the next beneficial exploration in study area.The mainly conclusion of this paper are as follows:Firstly,the geological factors of Jurassic reservoir in the southwest of the basin were specified..According to the the analysis of organic matter abundance,type,maturity and distribution characteristics of dark mudstone in Chang 7 member of Yanchang Formation in the study area,this source rock that can effectively form oil and gas were systematically proved to have a certain thickness and a wide distribution in the plane of the research area,but its hydrocarbon generation capacity was limited due to the low degree of evolution.Meanwhile,the analysis of the distribution characteristics of n-alkanes,biomarkers and isotopic characteristics of crude oil also shows that the oil comes from the underlying the source rock of Chang 7 member.It was clear that the fluvial facies deposits are developed in Yan 10 member and Fuxian Group,and the lacustrine delta deposits are developed in Yan 9 ～ Yan 7 Member.The reservoir characteristics of fluvial deposits in Yan 10 member were studied innovatively according to different paleogeomorphology units.The results showed there are significant differences among geomorphic units.On the whole,regardless of river or delta deposition,The Yan’an Formation reservoir had the characteristics of uniform rock,mineral components,coarse rock size and good pore-throat structure,which showed high reservoir physical properties under low permeability background,and the causes of that were discussed,which was considered that the dominant factors of good reservoir formed are sandstone particle size and sand body composition,sedimentary system,weak compaction and strong dissolution.Secondly,the genesis of special structural form and reservoir fluid characteristics in Tianhuan depression were found out.The structural characteristics of the top of Yan’an formation in today’s Tianhuan depression were finely described,That is,to the east of the axis of the depression,a row of banded nosing structures are developed,and multiple dome-shaped low amplitude structures are developed in the central region of the axis.The causes of this special structural form were discussed,and it was considered that it is formed by the superposition of the NE stress under the extensional structure and the north-south stress caused by the Weibei uplift.Clarified the crude oil of Jurassic Yan’an Formation was characterized by low density,low viscosity,light weight and good quality,and formed in brackish-fresh water environment under weak reduction conditions.Six reservoir types were summarized according to trap type and occurrence,and the distribution of oil reservoirs was characterized by "more in the lower strata and less in the upper strata,more in the north area and less in the south area".The geological characteristics of formation water in Jurassic Yan’an Formation were discussed for the first time.The physical and chemical properties of formation water were clarified,and the genetic types were discussed in detail.It was mainly high evolution brine,and the geochemical characteristics were zoning.Affected by high salinity formation water,low resistivity reservoirs were developed in the area.Thirdly,the migration characteristics and accumulation time of Jurassic reservoirs in the southwest of the basin were illustrated.The complex three-dimensional translocation system(conformities,high permeability sand bodies and fracture systems)developed from the bottom to the top during migration was been identified by making full use of the geological section,core observation,logging and other geological data.The equilibrium depth method was used to recover the excess fluid pressure in the Chang 7 member of the study area,and its plane distribution was largely affected by the distribution of pre-Jurassic ancient rivers.The Oil transport and accumulation should be driven by residual pressure and buoyancy.By using the method of joint analysis of Kasimov migration coefficient,carbon isotope of crude oil and nitrogenous compound,it was identified that the hydrocarbon generation center is west,northwest and southwest,and the hydrocarbon is filled and accumulated in Yan’an Formation through lateral migration.Based on the analysis of fluid inclusions,the characteristics of laser Raman spectroscopy were innovatively applied to jointly analyze the reservoir forming time and filling characteristics of Yan’an Formation reservoir.It was considered that the filling time in the north is earlier than that in the south,and then the reservoir forming period was defined,that is,from the end of the late Jurassic,there was a continuous filling and accumulation process,and charging strength from weak to strong and then from strong to weak.Lastly,the accumulation mechanism and the accumulation model of time and space scale of Jurassic reservoirin the southwest of the basin were constructed.On the basis of the analysis of reservoir forming conditions and the study of reservoir forming process,the main controlling factors of reservoir accumulation in Yan’an Formation were summarized,and the restriction of source rock distribution of Yanchang Formation on reservoir was emphasized,that is,the closer to the center of hydrocarbon generation depression,the more developed the distribution characteristics of Jurassic reservoirs were.It was proposed that the reservoir of Yan 10 member was controlled by palaeogeomorphology,and the source of pre-Jurassic ancient river and gentle slope area of palaeogeomorphology slope were favorable positions for reservoir formation,while low-amplitude structure,sedimentary facies and formation water preservation conditions controlled the distribution of reservoirs above Yan 9 member.It was clarified for the first time that the accumulation mechanism of Jurassic reservoir is "source control distance,pressurized migration,continuous charging,facies belt--palaeo-water storage,geomorphology--structure control reservoir".On this basis,two reservoir forming models on the spatial and temporal scales were constructed: "Palaeogeomorphology controlled lithology-structural trap reservoir in Yan 10 member" and "low-amplitude structure controlled lithology-structural trap reservoir in Yan 9 member". |
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