Characteristics of matrix-related pores associated with various lithofacies of marine shales inside of Guizhong Basin, South China

摘要

Matrix-related pore characteristics of between Lower Carboniferous shale and Middle Devonian shale in Guizhong Basin were investigated to explore the reservoir capacity. Via the combination of organic geochemistry analysis, X-ray diffraction, gas adsorption, mercury intrusion porosimetry, helium porosimetry and focused ion beam-scanning electron microscopy, material basics and physical properties were studied. Five lithofacies were divided as comparable units in two formations according to thermal maturity, OM abundance and mineral components, namely organic-moderate argillaceous shale with high thermal maturity (OMAS-H) and organiclean argillaceous shale with high thermal maturity (OLAS-H) in Lower Carboniferous Formation, and organiclean mixed shale with over thermal maturity (OLMS-O), organic-rich mixed shale with over thermal maturity (ORMS-O) as well as organic-rich calcareous shale with over thermal maturity (ORCS-O) in Middle Devonian Formation. Results showed that (1) OMAS-H and OLAS-H performed better porous features than OLMS-O, ORMS-O and ORCS-O, in terms of PV, PSA and porosity, (2) pores developed better in OMAS-H and OLAS-H than those in OLMS-O, ORMS-O and ORCS-O, in terms of pores of rigid grains, pores associated with clay flakes and OM-hosted pores, (3) specific PVs of OM were dramatically larger than those of mineral components, while specific PVs of OM in both OMAS-H (0.348 ml/g x 10(-6)) and OLAS-H (0.248 ml/g x 10(-6)) were about an order of magnitude higher than those in OLMS-O (0.055 ml/g x 10(-6)), ORMS-O (0.025 ml/g x 10(-6)) and ORCS-O (0.011 ml/g x 10(-6)). The ability of pore contribution largely depends on whether corresponding components are suited in the appropriate evolution stage with suitable thermal and pressure circumstance. Then sufficient degree of pore carriers (matrixes) need to be thought about, especially OM and clay as the secondary pore providers. Also, the demand for pore preservation highlights the importance of brittle minerals which can establish rigid frameworks. Therefore, in the case of controlling factors on pore characteristics, the importance ranking should be taken into consideration.