| Shale gas system is a continuous self-contained source-reservoir petroleum system.Hydrocarbon generation process and diagenesis jointly control the nanopore structure evolution,and further affect shale gas potential as well as gas occurrence.Taking marine shale in Wufeng-Longmaxi Formation from southern Sichuan Basin as an example,nanopore structure characteristic and its controlling factors are ascertained,and pore structure evolution during hydrocarbon generation and diagenesis is clarified.Furthermore,quantitative characterization method of the influence of diagenesis on shale porosity is established,and the evolution model of shale gas occurrence is ultimately constructed.Wufeng-Longmaxi shale is typical overmature organic-rich shale,with mineral composition mainly of quartz,clays and carbonates.Four types of shale lithofacies are recognized based on mineralogy and grain origin.Among these,siliceous shale was deposited in anoxic-dysoxic condition,silty and calcareous shales were deposited in a relatively dysoxic-oxic condition,while argillaceous shale was deposited under oxic condition.Furthermore,pores in Wufeng-Longmaxi shale are mainly composed of organic-matter(OM)hosted pores and intraparticle pores within clay minerals.Pore volume distribution of shales exhibits multipeak feature,with pore volume ranging from1.411 cm~3/100g to 3.028 cm~3/100g.Micro-(<2 nm),meso-(2 nm-50 nm)and macropore(>50 nm)account for 23%,58%and 19%of the total pore volume,respectively.Additionally,the isolated kerogen has markedly larger pore volume and surface area compared to its corresponding bulk shale.Pores that are smaller than 20 nm in size in shale are provided by both OM and minerals,while almost all the pores with size ranging from 20 nm to 80 nm are hosted within the OM.Overall,TOC content and clay minerals show the positive effect on nanopore development.Based on the pyrolysis of low-matured Xiamaling marine shale,hydrocarbon generation process can be divided into four main stages:the biochemical gas generation(bitumen generation),hydrocarbon generation caused by thermal degradation(oil window),wet gas generation caused by thermal cracking,and deep high temperature gas generation(dry gas generation).Evolution of pore morphology and structure shows a good correspondence to hydrocarbon generation.During OM conversion,mineral-related pores decreased or disappeared,while OM-related pores changed from nondevelopment,shrinkage OM pores,bubble-like OM pores to spongy OM pores.Quantification of pore structure shows that:(1)the presence of shrinkage OM pores and micro-fractures within OM at bitumen generation stage makes a contribution to the increased volume of meso-and macropores,while declined micropore volume is likely related to compaction and/or residual bitumen infilling;(2)the presence of abundant bubble-like OM pores at oil window makes the total pore volume increase;(3)the conversion of large-size bubble-like OM pores to small-size spongy OM pores at wet gas generation stage makes a contribution to increased micropore volume,and decreased meso-and macropores volume;and(4)the development of abundance of spongy OM pores at dry gas generation stage makes the total pore volume increase.Furthermore,evolution of gas adsorption capacity is mainly controlled by micropore structure,and the shale gas micropore loading potential gradually decreased during thermal maturation.The diagenetic types of the Wufeng-Longmaxi shale reservoir mainly consist of mechanical compaction,cementation,metasomatism,dissolution,and thermal evolution of OM.Porosity decline in siliceous and argillaceous shale is mainly caused by cementation and compaction,respectively,while this in calcareous shale is controlled by both cementation and compaction.Process of diagenesis and hydrocarbon generation jointly controls nanopore structure evolution.Early diagenesis controls the distribution of mineral-related pores,and pore status at oil window determines distribution of migrated OM,and further controls secondary OM pores development.Differences of detrital component in various shale lithofacies control the difference in diagenetic pathway and reservoir quality.Gas adsorption capacity of the Wufeng-Longmaxi shale ranges from 1.149 mg/g to2.442 mg/g,which is mainly controlled by temperature,TOC and clay content.Based on Langmuir excess adsorption model and the correlation between fitted adsorption parameter and temperature,a computation scheme has been established to calculate the gas adsorption capacity.With increasing depth,gas adsorption capacity increases initially,and then decreases.Free gas content increases progressively,and the proportion of adsorbed gas decreases.Gas occurrence evolution model of shale in Wufeng-Longmaxi Formation can be divided into three major stages.From the early sedimentary to the Late Permian,gas adsorption capacity increased rapidly and then decreased slightly.During this stage,shale was at low maturity,and there was only limited biogenic gas generation,resulting in gas adsorption under an under-saturated status.From the Late Permian to the Middle Cretaceous,gas adsorption capacity decreased continuously with increasing depth,while the shale reached a maximum gas generation.During the Middle Jurassic,free gas began to emerge,and its content increased with increasing depth.From then on,the formation entered into an uplifted period,leading to the end of gas generation and an increase in gas adsorption capacity.In addition to OM characteristic and expulsion efficiency during hydrocarbon generation,the amount of free gas lost(i.e.,preservation condition)during formation uplift controls present gas content of shales in the Wufeng-Longmaxi Formation. |
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