Characteristics And Controls On The Mesopore Structre Of The Shales In The Lower Part Of Shahejie Formation In The Dongpu Depression

Recent studies have focused on the potential of the Paleogene Shahejie Formation in Bohai Bay Basin as an unconventional shale oil reservoir.This study characterizeed the pore structure of the lower part of the third member of the Shahejie Formation(Es3x shale oil reservoir in the Dongpu Depression of Bohai Bay Basin.To better understand the major controls on mesopore structure in prospective lacustrine sediments in China,we compareed it with other basins.A series of experiments and comparative analyses were conducted in thisstudy.The experiments were implemented on 22 shale core samples from the Dongpu Depression,which include total organic carbon measurements,rock-eval pyrolysis,vitrinite reflectance analysis,X-ray diffraction analysis and low-pressure nitrogen adsorption-desorption experiment(LPNA).Based on the data of LPNA,the total pore volume(TPV),specific surface area,average pore diameter,pore-size distribution,and fractal dimension of the samples were calculated.These data were then compared with three major Chinese continental basins(Songliao,Ordos and Bohai Bay).The results of experiments and comparative analyses revealed that the average TPV and SSA of samples from the Dongpu depression are 10.7 × 10-3 ml/g and 5.1 m2/g,which show a smaller pore size than other lacustrine shales from Songliao and Ordos basin.The mesopore volume is mostly contributed by pores of diameter between 3.1 nm and 4.5 nm,and pore structures of Dongpu Shales are more complex than those of other basins,with surface fractal dimension varying from 2.4 to 2.7 and pore structural fractal dimension varying from 2.4 to 2.9.The characteristics of mesopore structure of Dongpu shales are determined dominantly by the degree of thermal maturity.The differences in pore characteristics between the Dongpu shales and others were caused by differences in kerogen types and thermal maturity.In addition,the small pore size and complex pore structure of the Dongpu shales is likely to be characteristic of Es3x lacustrine calcareous shales in the whole Bohai Bay Basin,implying the exploitation of in-situ shale oil in Dongpu may be difficult.The similarities between the Dongpu Depression and the Jiyang Depression within the Bohai Bay Basin indicate that data from the Jiyang Depression,which has been studied in more detail,may be preliminary extrapolated to the Dongpu Depression as a first order approximation.