Evaluation On The Generation And Accumulation Conditions And Prediction Of Favourable Areas Of Lower Cretaceous Shale Oil And Gas In Liupanshan Basin

Liupanshan Basin developed in the special tectonic position,controlled by multiphase tectonic units,it,s under a complex tectonic background,is located in the joint of southwestern margin of North China block and Qilian-qinling structure belt.The basin is rich in hydrocarbon resources,good oil and gas geological conditions,shows distribution of oil and gas of Lower Cretaceous,Tertiary,Jurassic and Carboniferous are more in the early geological survey,mainly concentrated in Lower Cretaceous,especially in the latest drilling,shows of shale oil and gas of Lower Cretaceous are more.In this thesis,multiple techniques and methods are comprehensively combined to characterize Cretaceous Madongshan and Naijiahe Formations,including field survey,experiment analysis,on-site desorption,isothermal adsorption,standard comparison,geophysical prospecting,and mathematical method.Various geological conditions are systematically analyzed,including basin tectonic setting,gravity-magnetic-electrical characteristics,sedimentary characteristics,formation development,geochemical characteristics,reservoircharacteristics,formation mechanism and conditions,accumulation mechanism and conditions,oil and gas enrichment,main controlling factors,and accumulation ass emblages.Highlighting Madongshan Formation shales,this study comprehensively explores and evaluates the generation and accumulation conditions of shale oil and gas in the study area,optimizes the favourable areas for shale oil and gas exploration,and preliminarily assesses shale oil and gas resources.(1)Liupanshan Basin is characterized by special tectonic location and complex background.During Cretaceous,it was a typical faulted lacustrine basin,which is similar to the sedimentary and structural environments in the western margin of Ordos Basin.Onthe basis physical data analysis of gravity,magnetic,magnetotelluric sounding,there are three low resistance areas,four magnetic anomaly areas and five electrical layers within the basin.Delineating the Haiyuan and Guyuan Depressions are the key research areas,this result is in conformity with the conclusions of the geological data.A total of three first-order tectonic units and 10 second-order tectonic units can be divided.(2)According to previous research and field research,Upper Cretaceous is absent in the Liupanshan Basin.The Lower Cretaceous consists of Sanqiao,Heshangpu,Liwaxia,Madongshan,and Naijiahe Formations from the bottom to top.In the early Cretaceous,two sedimentary-subsidence center of the basin-Haiyuan Depression and Guyuan Depression,the whole basin: north high,south low,high on both sides of the east and the west,low center.There are four types of system tracts,namely the low stand system tract,high stand system tract,transgressive system tract and depletion system tract.From the bottom to top,there exists an up-fining and deepening retrogradation sequence from alluvial fan to braided river and then to lake.Two sedimentary patterns can be recognized: one is the alluvial fan-fan delta-lake pattern in the Sanqiao and Liwaxia Formations,and the other is the braided river delta-lake pattern in the Madongshan and Naijiahe Formations.(3)The source of Madongshan and Naijiahe Formations is terrestrial.Shales are characterized by huge accumulative thickness,most of the large single layer thickness(about 40 m),and good continuity.During the depositional period,there were multiple sedimentary centers,among which Haiyuan and Guan Depressions were the most developed.The organic matters are of high abundance,all kerogen types(mainly Types II and III),and low maturity to maturity.Horizontally and vertically rules can be found in the oil and gas occurrence in the the basin:horizontally,shale oil gradually turns into shale gas from the sedimentary centre to the margin or slope;vertically,shale gas-shale oil and gas-shale oil-shale gas occur in a regular pattern from the deep to the shallow part of the basin.(4)According to the identification of rock minerals,the content of brittle minerals in the study area is more than 40%,which is relatively high,the content of clay minerals is 13.2%,which is low,and thus favourable for shale oil and gas development.There are many types of reservoir spaces,with main forms of micro-pores and micro-fractures.Micro-pores are of complex structures and mostly filled by clay and carbonate minerals.There develops milli-micro-nano multiple scale pore-throat systems.Shale reservoir are classified as low-porosity and low-permeability type.Gas content varies between 0.9 to 2.03 m3/t,with that average of 1.51 m3/t in Madongshan Formation and that average of 1.33 m3/t in Naijiahe Formation,hydrocarbon showings is better.Oil content is not high either,but the carbocoal is of high yield.The potential of oil generation(S1 + S2)is between 0.23mg/g and 2.78 mg/g,which is also low.(5)According to the results of previous research and the research,the most typical shale oil and gas accumulation mechanism is the comprehensive role of shales contributing to "generation-reservoir-accumulation”.Shale gas is featured by mixture of multiple gases,among which adsorption gas is the main occurrence state in the study area,while shale oil is characterized by combination of source and reservoir,residual accumulation and continuous distribution.Conditions such as geochemistry,physical properties,mineral composition and pore-throat system,decide the potential of oil and gas generation of shales,the quality of reservoir rocks,the enrichment of oil,and the content of gas.To be specific,the enrichment of shale gas is mainly controlled by burial depth,mineral composition,organic matter abundance,physical properties,and gas content,while that of shale oil is determined by organic matter abundance,maturity,and hydrocarbon generation and expulsion effects.(6)The cap rock,trap and preservation conditions are analyzed,accumulation assemblages are matched temporarily and spatially.Self-generating,self-reservoiring,and self-sealing assemblage is the most favourable for shale oil and gas enrichment.Traps related to Lower Cretaceous are mainly lithologic and structural traps,characteristics of the rows and zonal.The development of normal/inverse faults,fault gouge and mylonite are conducive to oil and gas preservation.The evaluation standard of oil and gas parameters is built for shales in Lower Cretaceous,Liupanshan Basin,and the shale oil and gas assessment method is also established.A total of three favourable areas are optimized in terms of Madongshan Formation,namely the eastern part of Haiyuan Depression,the middle part of Guyuan Depression,and the northwest part of Well Panzhong 2.Moreover,there are two favourable areas in terms of Naijiahe Formation,the middle part of Haiyuan Depression and the northeast part of Guyuan Depression.Finally,shale gas resources are preliminarily evaluated.The research of this paper has a certain guidance and reference for the the whole Liupanshan basin and other Chinese shale oil and gas areas,in the late research,exploration,and development of the shale gas and oil.