Geological Controls On Shale Gas Enrichment And Evaluation Of Core Areas For The Marine Shales In The Eastern Upper Yangtze Region

Drilled wells in the marine shales in the upper Yangtze Region has shown both great potential of shale gas exploration and development and strong heterogeneity of shale gas content.The factors effecting the shale gas content include hydrocarbon generation potential,reservoir space,preservation conditions etc.Various techniques including geochemistry,tight gas reservoir analyses and shale gas content evaluation are used in this study to determine the geochemical composition,shale pore structure and gas content of the organic-rich shales in Lower Cambrian and Upper Ordovician(Wufeng Formation)-Lower Silurian(Longmaxi Formation),in the eastern Upper Yangtze Region,China.Based on the precise shale gas content evaluation,main controlling factors of shale gas enrichment were analysed.Finally,a evaluating method for core areas selection in complex tectonic region was proposed.The following are the main outcomes of this thesis.The hydrocarbon-generation potential of shales depends upon its geochemical features.The geochemical characteristic of shales in both Lower Cambrian and Upper Ordovician(Wufeng Formation)-Lower Silurian(Longmaxi Formation)show high TOC contents,while the shales in Lower Cambrian are thicker with high TOC content(TOC>2%).The kerogen carbon isotope data show that shale in Lower Cambrian has type I kerogen,whereas,it has both type I to II kerogens in Wufeng Formation-Longmaxi Formation and preferred to be type II.The equivalent vitrinite reflectance of the Lower Cambrian ranged from 2.92% to 4.62%(average 3.91%),and that for Wufeng Formation-Longmaxi Formation is 3.08%-4.29%(average 3.61%).Except for the higher maturity of shales in the Lower Cambrian,which is negative for shale gas preservation,the thickness of shales with high TOC content and the kerogen type of shales in the Lower Cambrian are even better than that in the Wufeng Formation-Longmaxi Formation for hydrocarbon generation.The pore structure of shales determines their storage capacity.Various techniques including focused ion beam scanning electron microscopy(FIB-SEM),high pressure mercury penetration,low temperature gas absorption and He porometer are used in this study to determine the pore structure of the studied shales.These analysis revealed that dissolved pores and intergranular pores are greatly developed in the Lower Cambrian Niutitang Formation.While pores in the organic matter show strong heterogeneity.The organic matter pores appear like tabular or pinhole,with pore radius from 3 to 25 nm,and the pore connectivity is poor.The organic matter porosity of Niutitang Shale is below 1.59% and has less to do with the TOC content,revealing that the mineral-matrix-pores contribute more than the organic matter pores to the porosity in the Niutitang Shale.For the Wufeng-Longmaxi Shale,microfractures,intragranular mineral pores and organic matter pores are greatly developed.Pores in sapropel detritus appear like honeycomb,with pore radius from 3 to 100 nm,and show a good connectivity.The porosity the organic matter calculated by FIB-SEM 3D reconstruction range from 9.13 to 18.42% and its contribution to the shale porosity are controlled by TOC in the Wufeng-Longmaxi Shale.The complex pore structure and pore network system of the graptolite periderm and its contribution to the high gas production of the Longmaxi Shale in the Upper Yangtze Region are firsted reported using various integrated methods involving organic petrology,paleobiology and Field emission scanning electron microscopy(FE-SEM).Pores in the graptolite periderm show strong directionality.Microfructures and nanometer-sized pores are developed between cortical fibrils along the longer axis of the graptolite periderm.While in the perpendicularly section of graptolite periderm,nanometer-sized pores between cortical fibrils appear like tube.Statistics of SEM images show that the porosity of graptolite periderms are 1.62-4.23% with an average of 2.51%.Each graptolite periderm can be considered as a inter-connected pore network,and the centimeter scale graptolite pore system has connected both the nanometer-sized organic matter pores and micrometer-sized mineral pores as well as microfructure.These pore networks might have played a vital role for the shale gas production and might be the key factor of the high gas production of the Wufeng-Longmaxi Shale.The shale gas content is a key parameter in shale gas evaluation and it also decides whether the shale gas can be produced economically.A new method and instrument that measures the residual shale gas content precisely was developed in this study.Traditional residual shale gas content was measured by a ball-milling instrument using displacement method,which had a lot of drawbacks that includes time wasting,large reading errors,sample wasting and hard to collect residual gas samples etc.The new invented method based on ideal gas equation,using electromagnetic shock to crush shale samples and measure the residual gas content through temperature and pressure variations before and after crush.This method can greatly save the measuring time,using fewer samples and making it easily for residual gas sample collection.Stable carbon isotope fractionation of shale gas during degassing has been simulated using isotope geochemistry method,which may offer a new geochemical method for calculating the proportion of free gas vs.adsorbed gas during shale gas production and as well as shale gas yield predicting.By long time desorption of the Shuijingtuo Shale in Lower Cambrain and the Longmaxi Shale in Lower Silurian at reservoir temperature and high temperature(90?),it is concluded that the stable carbon isotope of methane gets heavier logarithmically and appear similar phenomena at high temperature(90?).The combination of multi-component adsorption and double-pore flow model can well stimulate the isotope fractionation during degassing,revealing the process of degassing is effected by both diffusion and adsorption/desorption.Thus,the proportion of free gas vs.adsorbed gas and the prediction of shale gas yield could also be calculated according to the stable carbon isotope fractionation results.The matrix permeability of shale is an important parameter of shale gas development.An innovate method using cubic samples and 3D-print membrane was used to measure the anisotropic permeability of the Longmaxi Shale.In addition,SEM Maps observation and Micro-CT 3D-reconstruction of the cubic shale samples are used to analyse the anisotropic permeability.The test results of three shale samples show that the permeabilities along the bedding directions vary between 98.3 and 3042.4 nD,while permeability perpendicular to bedding varies between 5.5 and 17.3 nD.The permeability anisotropy ratio between the parallel and perpendicular to bedding direction varies from 6.35 to 51.47,showing a strong anisotropy especially between the parallel and vertical directions.SEM Maps observation and Micro-CT 3D reconstruction show that microfractures along bedding directions are widely developed,which greatly increase the permeability and also causes the anisotropic permeabilities of these shales.A mylonitized nanometer-sized organic matter/clay mineral intergranular pores was recognized in the Lower Cambrian Lujiaping Shale,which illustrates the microscopic mechanism for marine shale gas preservation in northeastern Chongqing that tectonic condition is complex and organic matter is over-matured.Continuous observations from outcrop,core samples,thin section to SEM combined with high pressure mercury penetration and low temperature gas adsorption analyses have found that a mylonitized pore network system from nanometer-sized intergranular pore spaces,aggregate pore spaces in clay mineral flakes to pore network are developed in the cleavage domains in the Lujiaping Shale.These nanometer-sized intergranular pore spaces make up the fundamental storage space for the Lujiaping Shale,and have large specific surface areas as well as high methane sorption capacity and high capillary pressure,which have contributed to the preservation of shale gas in this complex tectonic area.According to the location of the study area that with special complex tectonic condition and based on the analyses of hydrocarbon potential,reservoir characteristics and shale gas content of the two shales,18 indexes in geochemistry,topographic condition,reservoir features and shale gas content are used to evaluate the elements of shale gas core areas in the eastern Upper Yangtze Region.The Block northeast Chongqing and Block Fuling are used as case studies to analyze the elements of shale gas core areas using analytic hierarchy process(AHP).It is concluded that besides the hydrocarbon potential in Block northeast Chongqing,Block Fuling is better in integrity and closure,topographic condition,reservoir feature and shale gas content.Block Fuling is more suitable for shale gas core area.