A Study On Differential Enrichment Of Shale Oil In Lucaogou Formation,Jimsaer Sag

The Middle Permian Lucaogou Formation in the Jimusar Sag,as a hot spot for shale oil exploration in China,has been explored for more than ten years and has been developed successively in recent years.However,under the background of complex saline lacustrine deposition,the enrichment conditions and configuration relations of shale oil are complicated due to the interbedded source and reservoir and the general oil-bearing reservoirs,which leads to the obvious spatial differences in the enrichment mechanism and mode of shale oil,thus affecting the selection of favorable sweet spot.Aiming at the above problems,this paper starts with the hydrocarbon source and reservoir conditions of shale oil enrichment and their combination characteristics,and systematically analyzes the source,reservoir quality and their coupling relationship in different source-reservoir combinations in the isochronous stratigraphic framework.Through dissection,its spatial differences were clarified,and different types of shale oil enrichment models were established to provide geological support for the exploration and development of shale oil in the study area.Through the comprehensive comparative analysis of drilling core observation,thin section identification,whole-rock X-ray diffraction,molecular and elemental geochemistry,logging and other data,the Lucaogou Formation is divided into 8isochronous source-reservoir-sedimentary units from top to bottom.The differences of each source-reservoir unit were analyzed.Based on the analysis of petrology,organic petrology,molecular geochemistry and hydrocarbon-generating thermal simulation experiments,the main hydrocarbon-generating parent materials-Oscillator and Dunaliella,which belong to the structural algae,were discovered for the first time in the source rocks of the saltwater lacustrine facies in the study area.The water salinity of Oscillator development is relatively low,the oil generation window is narrow,the initial activation energy of hydrocarbon generation is high,and the hydrocarbon expulsion efficiency is lower than that of Dunaliella.Its products have the characteristics of pre-peak n-alkane,high C₂₈ regular sterane,low gamma cerane and noβ-carotane.Dunaliella development in relatively high water salinity,with wide oil-generating window,and can be produced early and discharged early;the products have the characteristics of post-peak n-alkane,highβ-carotane,C₂₉ regular sterane and slightly higher gammacerane.According to the comprehensive study of petrology,organic and inorganic geochemistry,isotope,molecular geochemistry and other data,the influence of hydrothermal fluid on the development of hydrocarbon-generating parent material in source rocks has been clarified for the first time.During hydrothermal（volcanic）activity,the climate is relatively warm and humid,and the water salinity decreases.the relative development of lamellar algae.According to molecular geochemistry,organic carbon and pyrolysis（Rock-Eval）experiments,etc.,the thresholds of C₂₉ββ/（αα+ββ）and C₂₉ααα20S/（20S+20R）for the expulsion source rocks of the Lucaogou Formation are determined to be about 0.28 and0.44,respectively,which are the vertical and plane divisions of the effective source rocks.According to the comprehensive analysis of reservoir rock slices,cast slices,high-pressure mercury intrusion,particle size and physical property analysis,and biomarker parameters,the differences in controlling factors of salinity changes on reservoir performance under different salinity backgrounds are clarified.The sandstone in the low salinity background is dominated by tractive flow,with few suspended components,and the reservoir space is dominated by remaining intergranular pores and intergranular dissolved pores,followed by intragranular dissolved pores.The power is weakened,the particle sorting performance becomes poor,the content of dissolved pores between particles decreases,and the storage properties become worse.The sandstone in the high salinity background is dominated by suspended sediments,rich in a large amount of syn-sedimentary carbonate,the reservoir space is dominated by intragranular dissolved pores,and the intergranular dissolution is weak;with the relative increase of salinity,the carbonate content increases,the particle size becomes smaller,the particle size and mineral composition are gradually uniform,the dissolved pores of the particles are uniformly distributed,the connectivity becomes better,and the reservoir performance is enhanced.Regardless of the depositional background of high salinity or low salinity,the reservoir rock develops secondary dissolution pore zones below the peak oil generation depth of the source rock,which significantly improves the physical properties of single sand body reservoirs at this depth.Through the comparative analysis of crude oil physical properties and thermal evolution of source rocks,it is found for the first time that the density and viscosity of crude oil generated from source rocks increase first and then decrease with the increase of thermal evolution degree,and the maximum density and viscosity correspond to the peak of oil generation.According to the fine oil source comparison,the Lucaogou Formation shale oil is divided into two types:near-autogenous and self-reservoir type and far-source lateral migration type.The enrichment of shale oil from near-genesis and self-storage is mainly controlled by the amount of hydrocarbon generation and expulsion in the source rock,the quality of crude oil and the physical properties of the reservoir.However,at the peak of oil generation,the density and viscosity of the crude oil generated by the source rock are the highest,which is not conducive to the enrichment and development of shale oil.Therefore,the source rock below the peak depth of oil generation is the source-reservoir integrated shale oil sweet spot.Vertically,the section with the highest salinity has the characteristics of“three excellent and one stable”（hydrocarbon generation and expulsion intensity,crude oil physical properties and reservoir physical properties are optimal,and the lateral distribution of source-reservoir combination is stable）,and it is a horizontal well.Optimal segment for fracturing.The far-source lateral migration shale oil is mainly controlled by the amount of hydrocarbon generation and expulsion of the source rock,the structural location,and the physical properties and scale of the reservoir.Three enrichment modes are divided on the above.In the first mode,the sand body is thinned toward the structural high,and the thick sand is migrated and enriched.The sandstone is enriched in the direction of thinning;Parallel to the structural contour line,the crude oil accumulates in the high position parallel to the provenance line,and does not migrate along the provenance direction;in the third mode,the sandstone thins towards the low part of the structure,and when the crude oil migrates to the high position,it also Adjusted along the provenance direction of the sand body,and enriched in the secondary dissolution pore development zone.