Influence of Soxhlet-extractable bitumen and oil on porosity in thermally maturing organic-rich shales

Porosity in shales is important for storage of shale gas in reservoirs. As organic-rich shale thermally matures and enters the oil window, generated bitumen and oil can fill pore spaces, block pore connectivity, and reduce porosity. Low-pressure N-2 and CO2 adsorption techniques were used to quantify mesoporosity (pore size 2-50 nm, accessible to N-2 and CO2) and microporosity (pore size <2 nm, accessible to CO2 only) in New Albany Shale samples of Devonian and Mississippian age from Indiana and Illinois ranging from marginally mature (vitrinite reflectance R-0 = 0.55%) to post-mature (R-0 = 1.41%). After measuring their original porosity, the shale samples were Soxhlet-extracted in refluxing dichloromethane (DCM, boiling temperature 39.6 degrees C) to remove soluble oil/bitumen, vacuum-dried, and then re-measured for meso and microporosities. Subsequently, the same samples were Soxhletextracted in toluene (boiling temperature 111 degrees C, with enhanced solubility of oil/bitumen), vacuum-dried, and again characterized porosimetrically. The maturation sequence of the original, non-extracted shales expresses a higher mesoporosity in lower maturity samples (vitrinite reflectance R-0 = 0.55%, and 0.65%), and an intermittent decrease in mesoporosity in samples of post-mature stage (R-0 = 1.15%) in two size fractions (4-mesh and 60-mesh). The intermittent decrease in mesoporosity is consistent with partial filling of pore spaces with bitumen and oil until secondary cracking reclaims some of the lost open pore space from liquid hydrocarbon phases. Organic matter (OM) transformation is thus a pivotal cause for the observed evolution of mesoporosity in original, non-extracted shales. Micropore volumes display a varying trend throughout thermal maturation, and are significantly controlled by total organic carbon content. Compared to 4-mesh sample fractions, a reduction in grain size of 60-mesh fractions for gas adsorption porosimeny prominently enhances mesopore volumes, whereas the effects on micropore volumes are variable. These findings may be associated with the fact that for smaller particles it is easier to attain equilibrium during gas adsorption porosimetty. Solvent extraction of soluble bitumen and oil from the shale samples generally opens additional pore space for N-2 and CO2 adsorption, although the specific effects on mesoporosity and microporosity depend on maturity, total organic carbon (TOC) content, type of solvent, and grain size of the Soxhlet-extracted shales. The mesopore volume increases more in extracted samples with higher maturity, whereas the strongest gain in micropore volume is observed at elevated TOC content and highest maturity. Comparative porosities of original and Soxhlet-extracted shale samples constrain the evolution of porosity along maturation, as well as the effect of partial oil/bitumen filling and blocking of pores. This study also employs FTIR analyses of DCM and toluene Soxhlet extracts to differentiate low-temperature DCM-extractable, mostly aliphatic OM from higher-temperature toluene-soluble OM containing condensed cross-linked polyaromatic structures. (C) 2014 Elsevier B.V. All rights reserved.