Hydrocarbon Retention Mechanism During Thermal Maturation Of Shale And Evaluation Of Oil And Gas Potential

Scientific questions:Due to increasingly deep understanding of petroleum system and growing energy demand,unconventional hydrocarbon,especially shale oil and gas,has become a hot topic.In recent years,huge shale oil and gas resources have been discovered in China,which have shown bright prospects.However,great difficulties challenge the exploration and exploitation of shale oil and gas in China.As petroliferous basin in China are mainly terrestrial with complex geological conditions,shale strata are characterized by complex facies variation,wide ranges of burial depth and hydrocarbon thermal maturity.A variety of geological elements control the efficiency of shale oil and gas development.Under such background,several key scientific issues remain unsolved,including main controlling factors and mechanisms of hydrocarbon retention in heterogeneous shales,characteristics of hydrocarbon retention in shales during thermal maturation,evaluation method of the amount of hydrocarbon retained in shales,etc.These issues are of great significance to the establishment of reasonable method for shale oil/gas resource evaluation,prediction of shale oil and gas distribution,effective exploration and development of shale oil and gas.Research procedures:Focusing on scientific problem related to mechanisms of hydrocarbon retention in shales and characteristics of oil/gas-bearing properties of shales during thermal maturation,this thesis performed a series of experimental studies on the Es₃ lacustrine shales from the Dongpu Depression in Bohai Bay Basin,China.First,study the geological features of shales,characterize retained hydrocarbon in shales,and then reveal the main controlling factors and genetic mechanisms of hydrocarbon retention in shales.Second,through semi-closed pyrolysis experiment,discuss the evolution trend of amounts and compositions of retained hydrocarbon in shales during thermal maturation,and establish an evaluation method for shale oil and gas.Third,apply the research results to evaluate shale oil and gas resource potential and their distribution of the Es₃ Member in the Dongpu Depression,and select favorable targets for shale oil/gas exploration.Main research results:The occurrences of retained hydrocarbon in shales include free hydrocarbon,sorbed hydrocarbon and dissolved hydrocarbon.The classical method for evaluation of oil/gas-bearing properties using pyrolysis S1 and bitumen content cannot accurately gain information of sorbed and light hydrocarbon.Therefore,it is necessary to calibrate retained hydrocarbon content in shales.Hydrocarbon retention in shales are influenced by different factors,and these factors are not independent.In this study,main controlling factors are summarized as lithofacies,pore structure,surrounding rocks and burial depth.Lithofacies controls hydrocarbon retention by mineral composition and organic matter properties.Organic-rich lithofacies with high content of authigentic minerals are favorable for hydrocarbon retention.Micropores developed in shales are effective space for hydrocarbon storage.Pores with diameter larger than 10nm and fractures are main contributors for retention of free-and sorbed-oil,while contribution from pores with diameter smaller than 10nm are secondary.Surrounding rocks can influence sealing conditions of shale oil/gas system.In the northern Dongpu Depression,shales are surrounded by thick salt layers with excellent sealing conditions,therefore the retained hydrocarbon content are higher that shales in the south.Burial depth can control hydrocarbon retention by tightness of shales and thermal maturity of organic matter.High retained hydrocarbon content can occur in moderate burial depth when porosity evolution and hydrocarbon generation process are in good match.Semi-open system pyrolysis and hydrocarbon compositional analysis were performed on an immature Es₃ shale sample.It is indicated that retained oil content and composition change gradually during thermal maturation.Retained oil is dominated by sorbed oil,the ratio of free oil increases with increasing thermal maturity.Accoding to SARA analysis,retained oil has higher content of NSO compounds than expelled oil,and compositional differences between retained and expelled oil become more obvious as thermal maturity increases.Based on experimental results,a mathematical method for evaluation of shale oil/gas-bearing properties was established.By using this method,oil/gas-bearing properties of shales can be reasonably evaluated and the influence from ignoring light hydrocarbon loss and sorbed-hydrocarbon content can be minimized.Application:Evolution of hydrocarbon-bearing properties of shales in the north and south of the Dongpu Depression differ from each other,therefore phase distribution of oil and gas in the south and north are quite different.By applying the above mentioned mathematical method for evaluation of shale oil/gas-bearing properties to the Es₃ Shale in the Dongpu Depression for resource evaluation,considering free,sorbed and light hydrocarbon in shales,this study calculates resources of total retained hydrocarbon and movable hydrocarbon of the Es₃Shale in the Dongpu Depression using“volume method”,which are 76.20×10⁸t and 31.76×10⁸t,respectively.Based on the above,the study selects favorable targets for exploration of shale oil and gas in the Es₃ Shale in the Dongpu Depression,the favorable shale oil/gas-bearing area of the Es₃^L,Es₃^M,and Es₃^U are184.95km²、413.58km² and 230.28km².