Pore structure typing and fractal characteristics of lacustrine shale from Kongdian Formation in East China

Abstract

Nine as-received core samples were selected from four wells drilled in the lacustrine Kongdian Formation, which is a source rock located in Cangdong Sag, Bohai Bay Basin, China, and their geochemistry and pore structure characteristics were investigated by Rock-Eval pyrolysis, low-pressure nitrogen physisorption (LNP), mercury intrusion porosimetry (MIP), and spontaneous imbibition (SI) tests. Various models were employed to investigate the fractal characteristics of pore systems, and discuss the relationships between pore structure parameters and fractal dimension (Df). The results show that the samples can be divided into two groups (A and B) according to the different types of hysteresis loops from LNP isotherms, as well as geochemistry and pore structure parameters of samples. Negative relationship among BJH (Barrett-Joyner-Halenda) pore volume and S1 value, and obviously higher average pore diameter of Group B samples with significantly higher S1 values, indicate a blockage of residual hydrocarbons on small pores. According to the different Df−MIP values from MIP tests, the pore-throats of samples can be divided into four categories of A to D in the following sequence: microscopic (diameters less than 50 nm), small (50 nm-8 μm), medium (8 μm–20 μm) and large (greater than 20 μm) pore-throats/pores. The difference of Df from LNP and MIP tests is probably due to different sample sizes used and pore types assessed, while the difference of Df between MIP and SI tests is attributed to the mixed wettability of pores in shale samples. In addition, from the results of LNP tests with a measurable pore size of 2–200 nm, obvious positive relationships exist between Df−LNP−C (LNP test-derived Df value for Region C) and BJH pore volume, BET (Brunauer-Emmett-Teller) surface area and average pore size, as well as microscopic pore volume and percentage. The Df−LNP−C can be considered as an effective parameter to reflect the degree of development of microscopic pores in samples.