Evolution Of Inorganic Pore Structure And Its Controlling Factors In Longmaxi Formation Shale

Physical and chemical conditions of different geological periods in the process of shale formation are different,leading to the change of mineral accumulation patterns and even to the generation of new minerals?cements?,which causes continuous transformation of the inorganic pore structure of the shale,while the migration and reservation of shale oil/gas are closely related to the changes of shale inorganic pore structure in the same period.Reconstructing the evolution process of inorganic pore structure of shales is of great significance for understanding the migration and storage of gas and oil in shales and helps petroleum geologists to understand the inorganic pore structure characteristics of shale.The Longmaxi Formation shale is an important shale gas reservoir in China,and its exploration and development level represent the highest level of shale gas reservoir study and industrial production in China.In recent years,studies on the Longmaxi Formation shales have accumulated various data,which lays a foundation for a further study on the inorganic pore structure of the shales.The purpose of this study is to find specific characteristics and controlling factors of the inorganic pore structure of the Longmaxi Formation shales in different diagenesis stages.Samples of the Longmaxi Formation shale were collected in a stratigraphic section in Lucheng Village,Xishui County,Guizhou Province.Based on the previous studies,the evolution process of the inorganic pore structure of the Longmaxi Formation shale was divided into four stages,according to their diagenetic order:?1?Mineral size sorting under various hydraulic conditions during the stage of sedimentation;?2?The formation of primary inorganic pores through mineral accumulation during the early diagenetic stage;?3?The formation of cements and modification of the primary inorganic pores structure in the middle diagenetic stage;?4?The formation of a stable but anisotropic inorganic pore structure seen at the precent.By means of mathematical and experimental simulations and modern instruments measurments,an intensive study on the inorganic pore structure characteristics of the Longmaxi Formation shales during each diagenesis stages had been applied,and the evolution and controlling factors of the inorganic pore structure of the Longmaxi Formation shales revealed.After nearly four years of study,the following results have been achieved.1.After analyzing the mineral composition,of major and trace elements concentration and separating minerals by means of ultrasonic disaggregation of 10shale samples of the Longmaxi Formation,the size statistics of minerals particles and their morphological characteristics were obtained by laser particle size analyzer and field emission scanning electron microscope?FE-SEM?observation separately.Based on the results of the particle size distribution of samples,the nonuniformity coefficient of the quartz particle size in different layers of the section was calculated to be less than 2.1,showing a very high degree of sorting.And the difference of hydrodynamic conditions of the Longmaxi Formation is quantitatively deduced to be about 12.5%and have no obvious difference between each layer.2.At present,the production of shale artificial core basically requires cement,and the use of cement artificially increases the number of uncontrollable variables in the test,making it difficult to directly show the influence of mineral particle size and content on the pore structure.Accordingly,a new way of preparing cement-free artificial core by quartz and smectite grains with different content and particle size ratio has been invented to simulate shale inorganic pore structure.24 artificial cores were made,simulating the inorganic pore structure characteristics of shale with different particle ratio of brittle minerals to clay minerals.It is found that when the brittle mineral particle size is larger or smaller than the clay mineral particle size,the brittle and clay minerals in shale are arranged in different methods.As the content of brittle minerals increases,both porosity and permeability of shale continue to rise.When the ratio of the particle size of brittle minerals to clay minerals increases,the permeability increases and the porosity decreases.Based on the artificial core test data and previous studies,a mathematical theoretical model between shale mineral particle size,content and permeability is established to quantitatively predict the influence of shale brittle mineral content and particle size on shale permeability.3.According to the results obtained through field emission scanning electron microscopy,electron probe microanalysis and other techniques,it is confirmed that cements of the Longmaxi formation shale consist mainly of quartz and ankerite.Filling modes of two types of cements on the primary pore structure of Longmaxi Formation shales is deduced by comparison of results of samples with two different types of cements separately.Ankerite cements tend to fill micrometer-sized pores,while quartz cements tend to fill nanoscale pores between clay minerals.the amount of formation water required for the formation of ankerite cements were calculated based on the method of Land et al.,1985,and the formation of ankerite cements is little earlier than quartz cements.Quantitative mathematical parameters were obtained by further processing the qualitative cements distribution image data by the multi-fractal method to characterize the spatial distribution of different shale cements.The distribution of ankerite and quartz cements is completely different,and the presence of ankerite cements also affects the distribution pattern of the subsequently formed quartz cements.4.Based on the fact that potassium is transported with the formation water in the connected pores of shales,and that it participates in the illitization of smectite and enrichs in the illite/smcetite mixed layer and in illite,the electron probe microanalysis was used to restore the morphological characteristics of primary connected inorganic pore structure during the middle diagenetic stage of Longmaxi Formation shales.It is found that the morphology of primary pore structure of the upper and lower section of the Longmaxi Formation are basically consistent,based on a quantitatively comparison of multifractal data.The primary pore structure and morphological characteristics of the SQ₁ and SQ₂ sections of the Longmaxi Formation have a continuous distribution range of tens of micron scale and the pore diameter is close to 1 micron.5.The pore structure of the Longmaxi Formation shale was tested by rock sections and scanning electron microscopy.It is found that the inorganic pore structure of the Longmaxi Formation shale consists of nanoscale and micron pores in the SQ₁ part,while only nanoscale pores exist in the SQ₂ part.The heterogeneity of the pore structure of the Longmaxi Formation shale was also studied.The uniformity extension distance of the inorganic pore structure of shale along the stratum is obviously larger than the vertical direction of the stratum.6.The inorganic pore structure of the Longmaxi Formation shale has undergone mineral sorting in the sedimentary stage;the loose pore structure controlled by the particle size and content of the brittle mineral in the early diagenetic stage;and the compact pore structure caused by cements in the middle diagenetic stage.The way of accumulation of brittle minerals and clay minerals directly affects the pore structure characteristics of primary inorganic pores and the heterogeneity of inorganic pore structures in different directions of shales.Cement is the most important controlling factor for inorganic pores structure in shales.This study reveals the characteristics and main controlling factors of inorganic pore structure in each diagenetic stage of Longmaxi Formation shale.At the same time,through the application of mineralogy and geochemical methods,the characteristics of samples today are related to the geological parameters of the Longmaxi Formation shale in the past diagenetic stages,which effectively reduces the research cost and provides a reference for future shale reservoir studies.