Formation And Evolution Of Pores In Organic-rich Shale And Its Controlling On Shale Gas Content

The Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation of Lower Paleozoic in Sichuan Basin and its surroundings is the most favorable target of shale gas exploration and exploitation in China.At present,large-scale commercial development has been successfully realized in Southeastern Sichuan Basin.The study of pore evolution of shale reservoir and its relationship with gas bearing property is the key to the evaluation of shale gas accumulation and the basis of favorite spot prediction.In this paper,shale samples from Wufeng-Longmaxi Formation and Xiamaling Formation are collected.Based on the integrated analysis of organic geochemistry,mineralogy,pore structure system as well as gas bearing characteristic,the hydrocarbon generation process and shale gas formation mechanism is clarified,and the pore evolution characteristic within the whole hydrocarbon generation process as well as formation mechanism of different pore types is revealed,furthermore,the coupling relationship between pore evolution and gas bearing characteristic is also illustrated.For the high-over mature marine shale in South China,gas generation of kerogen pyrolysis and crude oil and/or bitumen crake are matched in terms of the gas origin material and timing of gas generation,forming a combined gas supply model,which ensures the efficiency gas supplement.There are two main origin of shale gas supply,one is from kerogen cracking at the end of maturation stage-high maturation stage with Ro<1.6%(mainly 1.1%Ro-1.6%Ro),the second is from crude oil/bitumen cracking with Ro?2.5%-3.2%,produced large amount of natural gas,which is the main source of shale gas supply.Besides,the heavy hydrocarbon gas can also crake at relatively high maturation,suppling methane gas content,but only has a little bit contribution to gas content.The pore evolution of shale reservoir is controlled by diagenesis together with hydrocarbon generation.At lower maturation stage,with Ro mainly less than 0.7%,pore development is critically controlled by diagenesis,while in high-over maturation stage,porosity change is mainly controlled by hydrocarbon generation function.Generally,the pore volume and specific surface area firstly increase and then decrease with increasing thermal maturation.During the whole process of hydrocarbon generation,two peaks of pore development exist,the first one response to the end period of kerogen cracking to gas,while the second is the peak period of crude oil and/or bitumen cracking to gas.The formation mechanism and evolution of different types of pore in shale reservoir are different.The formation and evolution of organic matter pore,which is the most important pore for shale gas storage,is controlled by hydrocarbon generation.Organic matter(OM)pores within kerogen resulted from oil generation and kerogen cracking to gas.OM pores within migrated organic matter/bitumen,is derived from crude oil/bitumen cracking to gas at high-over mature stage,and most of which are bubble-like or sponge-like pores.Dissolution pores mainly occur within carbonate minerals,which is associated with organic acids generated in the process of hydrocarbon generation in the mature stage.They are transported with water to the surface of easily soluble minerals calcite/dolomite,and local dissolution function occurs within mineral grains,resulting in dissolution pores.Interparticle pores or intercrystalline pores are the residual inter-mineral pores after compaction and migrated organic matter filling.The evolution of pores is accompanied by changes of gas content and occurrence state.The occurrence state of shale gas experienced four evolution processes: biochemical free gas stage,pyrolysis adsorbed gas stage,cracking free gas stage and free gas adsorption stage.Adsorbed gas and free gas content showed different changing trend with pore structure evolution.In the early stage,adsorbed gas experienced fast and slow increase process.Free gas increases slowly and adsorbed gas content is more than free gas content.With large amount of gas generation by migrated organic matter cracking,free gas content increased dramatically.In the late stage,the formation encountered uplift,and adsorption occurred with part of free gas transferred to adsorbed gas.free gas content has a little bit decrease trend,but is also the dominant shale gas content.The pore development and structure characteristics control the gas-bearing property,and the abnormal high pressure caused by the gas-bearing is positive to the pore preservation.Pore evolution and the gas-bearing property have a coupling effect in space and time.