Molecular differences in sequential extracts obtained by core flooding of the early mature ultra-tight Posidonia Shale

Abstract

The here analyzed Posidonia Shale (PS) samples have relatively high total organic carbon (TOC) contents greater than 2 wt% and are thermally early mature. They, thus, already experienced some generation of petroleum and contain abundant bitumen in the pore space. Solvent core flooding extraction was performed on five clay- and carbonate-rich early-mature Posidonia Shale plugs using dichloromethane (DCM) as permeating fluid under controlled confining pressure in a tri-axial flow cell. Molecular characteristics and differences were observed in original, sequential and residual extracts obtained by core flooding. Furthermore, porosity and permeability changes due to removal of soluble organic matter were recorded. Our results show that core flooding extraction of the clay-rich shales failed due to their low permeability (3.0–5.0 nD), while sequential extraction was successfully performed on the carbonate-rich shales. Acidic compounds were flushed out first, which indicate that these compounds might be preferentially expelled during the early stage of primary migration. Pristane and phytane are preferentially expelled in the whole process of expulsion as compared to n-alkanes. Composition of the other aliphatic (including n-alkanes, hopanes and steranes) and aromatic (including methylnaphthalenes, phenanthrenes, dibenzothiophenes and methyldibenzothiophenes) compounds of the sequential, original and residual extracts do not exhibit much difference. Statistical data indicate that the original and residual extracts have higher similarity and show lower apparent maturity than the sequential extracts indicating that expelled oil might show higher maturity than the related source rock. This observation might be important for interpreting oil/source rock correlations based on geochemical parameters. Porosities of the carbonate-rich shales were slightly smaller than those of the clay-rich rocks, while the permeability is much higher in the carbonate-rich lithologies. It is worth noting that porosity and permeability did not increase during the extraction of soluble organic matter. On the contrary, due to counteracting compaction effects that reduce pore volume extraction lead to an overall porosity and permeability decrease.