Hydrocarbon Expulsion Mechanism Of Lower Cambrian Argillaceous Source Rocks In The Tarim Basin

The platform basin area of Tarim Basin is rich in marine oil and gas resources,the argillaceous source rocks of Lower Cambrian yuertus formation and xidashan formation-xishanbulake formation are a set of important source rocks in the basin,but current research mainly focuses on the evaluation of source rocks,and the research on how to expell oil and gas from the set of source rocks,that is,the hydrocarbon expulsion mechanism,is relatively weak.Solving this problem can deepen the understanding of the hydrocarbon expulsion mechanism of the set of source rocks,correctly understand the scope of source rocks,and then guide oil and gas exploration.Based on the principle of mass balance,this paper systematically studies the characteristics of hydrocarbon generation,residue and expulsion of source rocks,the amount of hydrocarbon generation and expulsion is calculated,and the hydrocarbon expulsion mechanism of source rock is clarified.The specific process is as follows:firstly,based on the principle of element balance,combined with the geological and geochemical laws followed by kerogen transformation to generate oil and gas,the quantitative research model is established,the optimal solution of the model is obtained by simplex method,and the hydrocarbon generation of source rock is calculated;then,according to the characterization model of residual gaseous hydrocarbon,the residual gaseous hydrocarbon of different forms is calculated,meanwhile,using many rock pyrolysis S₁data,the evolution process of residual oil is restored,and the residual hydrocarbon of source rock is calculated;then,based on the research results of hydrocarbon generation and residual hydrocarbon,the hydrocarbon expulsion amount is calculated,by the characterization model of hydrocarbon expulsion amount in different phases,the hydrocarbon expulsion amount of source rock in water-soluble phase,oil-soluble phase,diffusion phase and free phase is simulated;finally,by the characterization model of hydrocarbon expulsion force,the pressure strength,fluid rock thermal expansion force,hydrocarbon generation compatibilization force,diffusion force,clay mineral transformation and dehydration force and capillary force are calculated,by the relationship between different hydrocarbon expulsion force and hydrocarbon expulsion phase,the relative contribution of each force to hydrocarbon expulsion is evaluated,the hydrocarbon expulsion geological model is established,and the hydrocarbon expulsion mechanism of the source rocks is elucidated.The results show that the gas generation volume of this set of source rocks in the platform basin is 276×10¹²m³,and the oil generation quality is 1489×10⁸t,the residual gas volume is 34×10¹²m³,the residual oil quality is 152×10⁸t,the expelled gas volume is242×10¹²m³,the expelled oil quality is 1337×10⁸t,the threshold（R_o）of methane gas,C_2-4 gas and oil expulsion are 0.55%,0.65%and 0.8%respectively;the oil and gas expulsion process of the Lower Cambrian argillaceous source rocks in the Tarim Basin can be divided into three typical stages:in the first stage,the source rocks are buried in the middle and deep layers（about 3000 m）,and has not yet entered the hydrocarbon expulsion threshold,the phase state of oil and gas expulsion is dominated by water-soluble phase and diffusion phase,the clay mineral transformation and dehydration force and diffusion force are the main hydrocarbon expulsion forces,the amount of hydrocarbon expulsion is relatively small and the expelled oil and gas are difficult to accumulate,in the second stage,the source rock is buried deep（3000m～6000m）,and has entered the hydrocarbon expulsion threshold,the oil and gas expulsion and transportation are characterized by the coexistence of multi-phase and multi-dynamics,the amount of hydrocarbon expulsion is relatively large and the expelled oil and gas are easy to accumulate,in the third stage,the source rock is located in the ultra-deep layer（6000m）,the phase state of oil and gas expulsion is mainly free phase,and the difference of capillary force and the thermal expansion force of fluid rock are the main driving force for oil and gas expulsion,the natural gas expelled at this stage is easy to accumulate.