Pore Structure Characteristics Of The Lower Paleozoic Marine Shale In The Sichuan Basin And Their Relationships With Burial Depth Of Shale

The Lower Cambrian Qiongzhusi Formation and Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formation marine shales are the most important shale gas exploration and development target in the Sichuan Basin.In this paper,FESEM–PCAS?Field emission scanning electron microscopy-Pores and cracks analysis system?quantitative analysis,multifractal analysis and low pressure N₂,CO₂adsorption experiments are conducted to quantitatively characterize the pore morphology,heterogeneity and connectivity of the Qiongzhusi and Wufeng-Longmaxi shales from twenty-six wells and three profiles in different burial depth,which purpose is to reveal the influence of burial depth of shale,overpressure and etc.to the pore characteristic of shale.The results are as follows:Organic matter?OM?pores,mineral matrix pores and fracture pores are developed in the Qiongzhusi and Wufeng-longmaxi shales,Sichuan Basin.The development degree of OM pores and mineral matrix pores,pore morphology,pore heterogeneity,pore connectivity and etc.of shales are controled by OM components,burial depth,strata pressure coefficient and minerals composition.Six OM components including pyrobitumen,spherical kerogen,algal fragment,bacterial-like aggregates,graptolite and micrinite are indentified under optical microscope and FESEM,and the PCAS soft is applied to quantitatively study the pores characteristics for different OM components.The results find that the pyrobitumen-hosted pores are the most common in our samples with the high plane porosity and form factor.Moreover,pores within pyrobitumen are closely associated with the mineral components of shale.We affirm that the quartz-rich brittle shale will provide more visible SEM pyrobitumen-hosted pores compared to the clay-rich ductile shale.In addition,the preservation effect of quartz on pyrobitumen-hosted pores is more obvious than that of carbonate and pyrites.Box counting multifractal analysis is performed to study the heterogeneity and connectivity of shale.It is concluded that the Qiongzhusi shales in superdeeply burial depth have the low micropore heterogeneity,high meso-macropore heterogeneity and good connectivity of micropore and meso-macropore.And,quartz can increase the micropore heterogeneity of these shales,whereas the TOC can decrease the meso-macropore heterogeneity,to a certain.Based on the ideas of constraining variable,contrastive analysis is conducted to reveal the pores characteristics of shlaes from different burial depth,pressure coefficient,shale lithofacies and testing production?shale gas content?.Results show that:?1?The compaction associated with burial depth may result into the homogenization of pore structure of shale in super-deeply burial depth and the increase of average pore diameter.Meanwhile,the compaction can compress the pores of shales to reduce the pore size and change the pore shape from round or elliptical-shaped to slit-shaped.In addition,in contrast with the pore structure parameters of shale in superdeeply,deeply and middle-shallow burial depth under overpressure or high-pressure,it is found that the superdeeply buried shales have the largest average pore diameter,mesopore/micropore special surface area and mesopore/micropore pore volumn,which reflects that mesopores are dominant in superdeeply buried shales;?2?Overpressure or high-pressure can resist the compaction of overburden to preserve the pores in shale,especially for OM pores.Shales under overpressure have the higher plane porosity and OM pores development degree compared with that under normal pressure shales.The difference of pore structures of shale under overpressure and normal pressure,are largely reflected in the OM pores development.And,pore pressure in shale is controlled by the sealing conditions.For the stable tectonic regions like Z#202 well area,the good sealing conditions are conductive to the preservation of OM pores and shale gas enrichment,which further maintain overpressure.The poor sealing condition will lead to the escape of shale gas and the disappearance of overpressure,which is reflected in the poor OM pores development and the extremely developed structural fractures;?3?the shale lithofacies with high quartz content have the higher plane porosity,OM pore development,special surface area and pore volume,which are conductive to the gas enrichment;?4?The bedding fracture and interlayer fracture are conductive to the shale gas enrichment whereas structural fractures can be the destruction.The structural fractures resulting from tectonic activity may be the main cause for the destruction of preservation condition of shale gas for the tectonic active regions.After the factors impacting pore structures including pressure coefficient and shale lithofacies are restricted,the relationship between pore structure parameters and burial depth of shale are made,and it is found that the three parameters including average pore diameter,mesopore/micropore special surface area,mesopore/micropore pore volume are moderate to highly correlated to burial depth of shale.Based on the shale gas reservoir characteristics and other geological conditions,6000 m is suggested to be used as the boundary of deeply buried shales and super–deeply buried shales for the Qiongzhusi Formation and Wufeng-Longmaxi Formation shales in the Sichuan Basin.Moreover,the burial depth for the shale gas exploration and development should be less than 4613 m and 5508 m,respectively,after evaluation.Pyrobitumen is the most abundant OM component in Qiongzhusi Formation and Wufeng-Longmaxi Formation shales.Pores within pyrobitumen are characterized by the high plane porosity and regularity and are hydrothermal genesis,which are not affected by the OM original type.The initial morphology of pyrobitumen-hosted pores is nearly round to elliptical.Thus,the nearly round to elliptical shape of pyrobitumen-hosted pores mean that the pores may contain amount of gas and high pore pressure.Therefore,pyrobitumen-hosted pores can used as an indicator to reflect the overpressure status of shale in theory.The statics of this thesis shows that?1?The medium degree between the gas content and the form factor of pyrobitumen-hosted pores of shales with the burial depth more than 2500 m,may effective characterize the micropore overpressure status-the more regular the shapes of pores in pyrobitumen are,the better the micropore overpressure status sustained and the higher shale gas content,and?2?the highly degree between the gas content and the logging pressure coefficient of shales with the burial depth less than 2500 m,may reflect that the pressure coefficient can represent the gas bearing capability of shale while the connectivity of shale in shallow burial depth is good.Therefore,the form factor of pyrobitumen-hosted pores may effective characterize the micropore overpressure status of shales with the burial depth more than 2500 mFinally,the geological factors influencing the gas content of the two shales contain the thermal maturity,roof and floor conditions and hydrocarbon expulsion,are studied.It is concluded that the Tongwan unconformity distributed in the bottom of Qiongzhusi Formation may be the critical factor resulting into the low gas content of the lower section of the Qiongzhusi Formation,which is reflected in that the gas content,special surface area,micro special surface area and pore volume of shales show a obvious decrease with the burial depth above the unconformity about 80 m.