Microscopic Characteristics And Controlling Factors Of Saliferous Shales In The Dongying Sag

As the development of unconventional petroleum,the researches about shale as reservoir have been a hotspot in petroleum geology field.In our country,the researches about shale reservoirs are mainly focused on southern marine shales.As the process of terrestrial shale reservoirs,the studies about microscopic characteristics of lacustrine shale reservoirs become more and more.This paper studied the microscopic reservoir characteristics of saliferous shales in Es₄ of the Dongying Sag,and made a primary discussion on the controlling factors.The main study area is Mingfeng sub-sag in the north of Dongying Sag.Core samples are selected from the Well Feng-8,Fengshen-1,Fengshen-2 and Dongfeng-5.These core samples are carried on X-ray diffraction?XRD?for whole-rock and clay mineral,rock organic carbon analysis,Ar-polishing Field Emission Scanning Electron Microscopy?FESEM?,low temperature nitrogen adsorption experiment and a series of tests.The results of the mineral content analysis show the main minerals of saliferous shales of the Dongying Sag are clay minerals,quartz,feldspar and carbonate minerals.Followed by evaporite minerals such as anhydrite,gypsum and halite.The accessory mineral,pyrite,distributes unevenly and the mean value is lower.The main minerals of dark grey argillaceous shale are clay minerals,quartz and feldspar,next are carbonate minerals.The main minerals of calcilutite are carbonate minerals,clay minerals,quartz and feldspar.And carbonate minerals are mainly dolomite and ankerite,mean values of these are about 30%.Gypsiferous mudstones are main composed of evaporite,carbonate and clay minerals.The main evaporite minerals are anhydrite.And the average value of anhydrite is 34.3%,while 8.3%for halite.In addition,clay mineral content analysis shows the main clay minerals of study area are illite,illite/smectite and chlorite,non-kaolinite and smectite.Rock organic carbon analysis shows the content of TOC is from 0.07%to 4.54%,vary in a large range,which shows strong heterogeneity.The average of TOC is 1.68%.The value of vitrinite reflectance?R_o?is in the range of 0.60%-1.17%,reflecting the mature stage of organic matter.It is indicated that there are four pore types for the saliferous shales of study area by FESEM.The pore types are interparticle pores,intraparticle pores,organic-matter pores and fracture pores.Interparticle pores are common and mainly distributed between rigid particles,clay minerals and rigid particles,pyrite and pyrite,pyrite and clay minerals.Intraparticle pores are mainly dissolved pores in carbonate minerals and feldspar particles.There are a few rounded pores in organic matter.The organic-matter pores primarily presented as organic matter edge aperture gap.In addition,fracture pores are also widespread development.Adsorption-desorption curves are got through the low temperature nitrogen adsorption experiments and divided into Type-I,Type-II,Type-III and Type-IV four types according to the IUPAC classification in this paper.The main types in study area are Type-II and Type-I,followed by Type-III,Type-IV is few.This indicates the pore types of saliferous shales are mainly open parallel plate and ink-bottle shape,next are slit-and cylinder-shaped pores.The results of low temperature nitrogen adsorption experiments show that the values of BET specific surface area are in the range of 7.5609-52.1371m²/g,the mean value is 19.8302m²/g.The range of specific surface area values of micro pores is larger,from 0.2985 m²/g to12.4865m²/g,and the average value is 4.2080 m²/g.The percentage of specific surface area of micro pores for the whole is 1.10%-82.41%.The values of BJH pore volume are in the range of 0.01042-0.09572cm³/g and average value is 0.04332 cm³/g.The pore volume percentages of micro-,meso-and macro-pores for whole pore volume are 9.63%,77.82%and 12.55%respectively.As can be seen,most of pore volume is provided by mesopores,the following are macropores and micropores.In addition,pore diameter distribution of adsorption branch shows the range of pore diameter is wide and mainly in 2-20nm.The peak values of most samples are within 10nm.Further statistical analysis are made and it's indicated that there is no significant correlation between TOC with pore volume,specific surface area.R_o has a positive correlation with pore volume and specific surface area on certain degree.Clay minerals contents have positive correlation with pore volume and specific surface.And there is a positive correlation between clay mineral contents with pore volume,specific surface area of micropores and mesopores,while negative correlation with macropores.However,brittle minerals have negative correlation with pore volume,specific surface area of micropores and mesopores,while positive correlation with macropores.There is a negative correlation between evaporite minerals with pore volume,specific surface area to some extent.