PORE STRUCTURE CHARACTERISTICS AND THEIR CONTROLLING FACTORS OF LACUSTRINE SHALE: A CASE STUDY FROM CANGDONG SAG IN BOHAI BAY BASIN, CHINA

The pore structure characteristics and their controlling factors of the lacustrine shales in the 2^nd member of Kongdian Formation of Cangdong Sag are elaborated based on fluorescent thin-section, field emission scanning electron microscope (FE-SEM) and low temperature N₂ adsorption combined with X-ray diffraction and organic geochemical analysis. The results show that the shale samples mainly consist of felsic minerals (quartz + feldspar) with an average of 33.2%, carbonate minerals (calcite + dolomite) with an average of 34.7%, clay minerals with an average of 13.0%, and analcite with an average of 14.7%. It is characterized by high brittle minerals content , low thermal maturity, and Type Ⅱ₁ and Ⅱ₂ organic matter. Three different kinds of pore spaces are developed in the shale samples: intergranular pores, dissolution pores, and microfractures, while organic pores are rarely found. The BET specific surface area of shale samples was in the range of 0.5256-1.4758m²/g with an average of 0.8078m²/g, and BJH total volume is 0.00235-0.00707cm³/g with an average of 0.00428cm³/g. The mesopore specific surface area in shale accounts for 63.31%-93.10% of total specific surface area, averaged at 72.51%. The pore volume of mesopore in shale accounts for 35.61%-66.73% of total pore volume, averaged at 49.99%, along with macropores make the greatest contribution to the total pore volume. 3nm and 50nm are the two peak value which exist in the pore size distribution of shale samples. The isothermal curves from N₂ adsorption experiments mainly contain H2-H3 types, which indicates that wide-open pore and microfratures in relatively large-scale are the main types. Compared with marine shales, macropores and mesopores make great contribution for free hydrocarbon accumulation, while micropores make less contribution on account of low maturity and low clay minerals content. The results show that intergranular pores within brittle minerals, dissolution pores within unstable minerals (especially within dolomite and analcite), and interlayer microfractures between different laminae are the primary reservoirs for oil. Overall, mineral composition and sedimentary structure determine the pore structure parameters of shale in the study area.