Discussion on the method for determining BET specific surface area in argillaceous source rocks

Samples of argillaceous source rocks were taken at different depth in Dongying Sag and pretreated with different methods. Contrastive analysis to those samples by using N2 adsorption, pyrolysis, X-ray diffraction detection has been made, so as to discuss the validity and credibility of the detection method for specific surface area (SSA), and work for the study on shale gas preservation characteristics. The source rocks were treated with hydrogen peroxide and chloroform. It is found that the average TOC content in the source rock samples treated with hydrogen peroxide decreased than that of the original sample by 61.3% on average for the removal of organic matter in particle state and surface adsorption state, whereas decreased by 20.9% in the samples treated with chloroform for the dissolved organic matter (organic matter in intergranular pores and physical adsorbed) removal, accordingly, the average BET specific surface area increased by 77.5% in the samples treated with hydrogen peroxide than that of the original sample, whereas increased by 22.0% in the samples treated with chloroform, which shows that the organic matter on the specific surface of source rocks can be effectively removed and BET specific surface area extended both with chloroform and hydrogen peroxide treatment. Nonetheless, further analysis shows that the samples treated with chloroform contain the same mineral compositions as the original samples, while the samples treated with hydrogen peroxide have considerably different mineral compositions from the original samples, thus breaking up the original rock structure and the mineral content of the source rocks. Based on the study and analysis above, it is the best choice to select the chloroform treatment to the bulk rock samples for detecting the BET specific surface area, which is possible not only to effectively remove organic matter but also keep the original mineral structure of the rock, thus obtaining true in situ external specific surface area of source rocks, and providing reliable parameters for the study of shale gas occurrence state.