The Enrichment Mechanism Of Inter-salt Shale Oil

Dongpu depression is a typical inland salt-lake basin,and several sets of extremely thick and widely distributed gypsum-salt layers are developed in the Paleogene Shahejie Formation.A high yield heavy oil of 100 m³/day has been obtained from the inter-salt shale in the upper sub-member of Es₃ in Well PS18-1,which has promoted the exploration and development of inter-salt extra-heavy shale oil in Dongpu Depression.Compared to ordinary shale oil,the biggest feature of inter-salt shale oil is that the strong sealing effect of gypsum-salt layers on shale oil has led to widespread development of over-pressure,and its enrichment mechanism is of great significance for the exploration and development of inter-salt shale oil.In this thesis,a series of experimental work has been carried out on shale samples from the upper sub-member of the Shahejie Formation in the Liutun sag,including major and trace element analysis,carbon and oxygen isotope analysis,TOC analysis,X-ray diffraction mineral analysis,rock extraction and extract component analysis,pyrolysis before and after extraction,high-pressure mercury injection,low-temperature nitrogen adsorption,argon ion profiling,scanning electron microscopy,micro CT,and fluid inclusion analysis.The organic matter（OM）sedimentary environment in organic-rich shale has been studied,and three OM enrichment mechanisms have been summarized.The microscopic pore structure has been comprehensively characterized and classified.The impact of mineral types,OM characteristics,and pore structure on the occurrence state of shale oil and the main occurrence space of shale oil have been discussed.The characteristics of intra-source migration and accumulation for inter-salt shale oil have been clarified.Combined with fluid inclusions and organic geochemistry,verify their migration and accumulation characteristics,identify their enrichment intervals,and summarize the enrichment mechanism of inter-salt shale oil.The major and trace elements and rock pyrolysis indicate that the studied interval mainly develops Type I and Type II OM,with relatively high OM abundance and good preservation conditions（between reduction and strong reduction）.On the whole,it is in cold and dry climatic conditions,with relatively fast deposition rates,and the water-mass restriction is constantly changing from strong restriction to semi-opening.By analyzing the variation of parameters such as paleo-productivity,redox conditions,paleo-salinity,paleo-climate,sedimentation rate,input of terrigenous debris,and water-mass restriction with depth.It is found that the OM enrichment mechanism in the studied interval includes three types:production-based Type,combination-based Type,and combination-based Type.The production-based Type is characterized by that the primary productivity determines the OM enrichment(the correlation coefficient（R²）between productivity index Ba _EF and TOC is 0.76).The combination-based Type is characterized by that the primary productivity and preservation condition jointly determine the OM enrichment(the R² between productivity index Ba _EF and TOC is 0.5,and the R² between redox index and TOC is 0.37).The preservation-based Type is characterized by that the preservation condition is the first factor for OM enrichment（the R² between redox index and TOC is 0.77）.Argon ion profiling,scanning electron microscopy,and micro CT were used to explore the microscopic pore structure and its connectivity.It was found that the pores in the studied interval can be divided into three types:intragranular pores,intergranular pores,and microfractures.At the same time,a large number of corrosion pores,bedding fractures,and pore systems（consisting of interconnected micropores）are developed,and the pores connectivity gradually becomes better with the enhancement of OM thermal evolution.By using the splicing method to integrate the high-pressure mercury injection and low-temperature nitrogen adsorption to get the full-pore size distribution curve of shale oil occurrence in the studied interval,and it was found that a large number of pores less than 100 nm were developed.Referring to domestic and foreign classification schemes and combining with the distribution characteristics of full-pore size,the pores are divided into nano-scale pores（1-10 nm）,micro-nano scale pores（10-100 nm）,and micron scale pores（greater than 100 nm）.According to the basic model of high-pressure mercury intrusion,the capillary bundle model,the surface area formula of the capillary bundle is calculated using geometric formulas,and then the pore specific surface area distribution is calculated by using the pore volume distribution（the mercury intrusion method）.Combined with the specific surface area distribution measured by low-temperature nitrogen adsorption,the full-pore specific surface area distribution is spliced.Using the FHH model to calculate the fractal dimension to characterize the complexity of pores.Specifically,using the expectations and variances in mathematics to calculate the pores average parameters and homogeneity parameters at all levels（nano-scale,micro-nano scale,and micro-scale）based on the full-pore size distribution,to characterize the pores size and homogeneity at all levels.At the same time,the pore volume and pore specific surface area were calculated to explore the pore structure and its main controlling factors.The results show that micro-nano scale pores are the most developed and unevenly distributed,which is mainly controlled by the OM type,abundance and maturity;micro-scale pores account for a relatively low proportion and have a relatively uniform distribution,mainly controlled by the OM abundance and maturity;while nano-scale pores account for a relatively low proportion and have the most uniform distribution,mainly controlled by the OM maturity and type.Using the pyrolysis data before and after extraction to maximize the identification accuracy of free oil and adsorbed oil,the effects of the OM characteristics（such as abundance,type,and maturity）,mineral composition,and pore structure on free oil and adsorbed oil were discussed,and the shale oil occurrence space was discussed.The results show that the adsorption capacity of type I OM on shale oil is greater than that of type II OM,and the OM maturity has little effect on the adsorption oil and free oil,while the OM abundance is crucial to the shale oil occurrence state;Fragile minerals such as quartz and feldspar can promote the shale oil flow and accumulation;The effect of pore volume on free oil is stronger than that on adsorbed oil,and the effect of pore specific surface area on the occurrence state of shale oil is very weak.This further indicates that mineral adsorption are not the main adsorption methods for the studied interval,and the pore structure complexity（fractal dimension）has a stronger effect on free oil than adsorbed oil;Micro-nano scale pore systems are the main storage spaces for shale oil,with a small amount occurring in micro-scale pores,while nano-scale pores mostly control the shale oil flow and adsorption.Based on a large amount of pyrolysis data,we have analyzed the entire-process of hydrocarbon generation and expulsion in inter-salt shale oil,and the determined threshold of hydrocarbon expulsion（OSI=158 mg/g TOC）is much larger than that of Chang 7 member（OSI=70 mg/g TOC）.This is consistent with the fact that the inter-salt extra-heavy shale oil in the studied interval requires greater hydrocarbon generating power to break through the barrier of gypsum-salt layer.So the threshold of hydrocarbon expulsion for inter-salt shale oil is much higher than that for ordinary shale oil.There are four small intervals a（3257-3260 m）,b（3262-3267m）,c（3273-3278 m）and d（3281-3282 m）meeting the conditions of hydrocarbon expulsion.Based on the oil inclusions analysis,the hydrocarbon expulsion process was verified and the migration distance was calculated.Combined with the shale oil component fractionation,to further verify the intra-source migration and accumulation characteristics.The results show that the studied interval met the conditions of hydrocarbon generation and expulsion for at least three times,and the maturity of discharged crude oil has changed significantly,including lower maturity crude oil with orange yellow fluorescent（29.2-30.2 Ma）,medium maturity crude oil with bright yellow fluorescent（27.8-28.8Ma）,and higher maturity crude oil with blue white fluorescent（0-1 Ma）.Interval a（3257-3260 m）has strong capacity for hydrocarbon generation and expulsion,and three stages of hydrocarbon expulsion occurred successively,with two hydrocarbon expulsion directions（upward and downward）in the longitudinal direction.The downward discharged crude oil is concentrated in the 3260-3261.5 m;The interval b（3262-3267 m）has relatively weak capacity for hydrocarbon generation and expulsion,and only one stage of hydrocarbon expulsion occurring,with a large amount of crude oil remaining in place,and a small amount of crude oil migrating to between 3261.5 m and 3262 m;Interval c also has strong capability for hydrocarbon generation and expulsion,and undergone three stage of hydrocarbon expulsion with a relatively large thickness（much thicker than interval a）,which have migrated downward and enriched at 3278-3285 m.The oil saturation index and micro-nano scale pore volume in this enriched interval have developed locally high values,and the content of light components is high,with good mobility;The OM in interval d has just reached conditions for hydrocarbon generation and expulsion,and no large amount of crude oil has migrated.Combined with the OM enrichment mechanism,it can be seen that the intervals where shale oil is most enriched belong to the preservation-based Type.This indicates that the shale oil enriched in the preservation-based Type has high abundance and good mobility.The accumulation of a large amount of OM in the production-based Type and combination-based Type has laid a material foundation for the generation of shale oil.The large development of micro-nano scale pores and high porosity（14.64～28.78%）provide good reservoir conditions for the enrichment of shale oil.The organic matter that reaches the threshold of hydrocarbon generation and expulsion begins to generate a large amount of hydrocarbons,but the barrier of the gypsum-salt layer prevents the hydrocarbons migration.With the further improvement of the OM thermal evolution degree,when the oil saturation index（OSI）reaches 158 mg/g TOC in the hydrocarbon expulsion interval,intense hydrocarbon generation causes tremendous pressure to break through the barrier of the gypsum-salt layer and hydrocarbon expulsion.After breaking through the blockade,the OSI in hydrocarbon expulsion interval gradually decreases while the OSI hydrocarbon enrichment interval gradually increases.Moreover,the components fractionation during migration can lead to lighter shale oil components and good mobility in the enriched interval.This enriched interval has a high content of light components,with high OSI and pore volume locally developed,which is the advantageous interval for exploration and development.