Reservoir Characteristics And Their Controlling Factors Of The Lower Cambrian Niutitang Formation Black Shales In Southeast Chongqing

To correctly recognize reservoir characteristics and their controlling factors ofthe Lower Cambrian black shales of Niutitang Formation in southeast Chongqing, thefollowing tests are systematically performed on the core samples collected from WellYuke1and Well Youke1, including clay minerals and whole rock X-rays diffractionanalysis, and analyses on common petrophysical properties, pore size and specificsurface area, organic carbon in rock, vitrinite reflectance (Ro) and maceralcomposition.The test results show that the average content of detrital minerals in the shalereservoirs of Niutitang Formation is51.34%, and quartz and feldspar are the maincomponents; the average content of clay minerals is32.74%, and illite, chlorite andandreattite are the main components, but the major component is illite; the content ofcarbonate minerals is less than the former, and carbonate minerals include calcite,dolomite and siderite; the content of pyrite is very little. The average effectiveporosity and permeability is1.2%and8.0×10-3mD respectively, demonstrating thesereservoirs are of ultra-low porosity and permeability. Moreover, mesopores as themain type account for73%of the total pore volume, macropore volume is less, andmicropore volume is the least. The average BET (Brunauer-Emmett-Teller) specificsurface area is7.75m2/g, and the pore diameter is5.3nm; the total content of organiccarbon (TOC) has an average of2.29%, and the average Rois3.12%, indicating thatthe shale reservoirs are in the over-mature stage.After observation research of core samples by field emission scanning electron microscopy(FE-SEM), it is found that the major minerals in the shales are quartz,plagioclase, K-feldspar, allite, andreattite, dolomite and ankerite, and the minorminerals are pyrite, apatite and anatase. There are4pore types in the shales includingintergranular pore, intragranular pore, organic pore and microfracture. Intergranularpores can be divided into pores between rigid particles, pores between plastic particlesand pores between rigid particles and plastic particles. Intragranular pores can bedivided into intragranular pores in sheet clay minerals, intracrystalline pores inframboidal pyrite concretion and dissolved pores.Further statistical analysis shows that TOC is positively correlated withmicropore volume, while the total content of clay minerals is negatively correlatedwith it; TOC, the total content of clay minerals and the content of carbonate mineralsare positively correlated with mesopore volume; TOC and the content of carbonateminerals are positively correlated with macropore volume, and the content of detritalminerals has weakly positive correlation with it, whereas the total content of clayminerals is negatively correlated with it.The TOC and porosity of shales in Niutitang Formation are calculated usingconventional logging method. It is discovered that the porosity calculated according tothe neutron curve fits well with core analysis results in both Well Yuke1and WellYouke1. However, in the shallow layers without gas and oil, gamma-ray curve has noobvious correlation with TOC obtained by core analysis.