Flow-through extraction of oil and gas shales under controlled stress using organic solvents: Implications for organic matter-related porosity and permeability changes with thermal maturity

• Strong increase of porosity and permeability upon solvent extraction
• Experiments reveal compositional differences between oils in large and small pores.
• Extraction time series reveal large differences in organic geochemical composition.
• Petrophysical properties of Posidonia Shale strongly change with thermal maturity.

Four core plugs from the Lower Jurassic Posidonia Shale of the Hils Syncline in northern Germany have been subjected to flow-through extraction tests with dichloromethane (DCM) under controlled stress conditions in a tri-axial flow cell. The samples represent a maturity sequence from 0.53 to 1.45% VRr. The bitumen sequentially extracted from the natural pore space of the shale plugs was analyzed for its geochemical composition. Changes in rock matrix density, porosity and permeability resulting from the removal of soluble organic matter were determined. The relative porosity increase of the plugs after extraction ranged from 6 to 13% and correlated with the extract yield. Klinkenberg-corrected permeability coefficients measured with helium increased by a factor of 17.0 for the immature/early mature sample and 26.6 for the overmature sample. Petrographical investigations after extraction indicate that fluid flow occurred predominantly parallel to bedding as evidenced by open fractures and fractures bearing residues that apparently precipitated from the DCM solution. Compositional variations of the extracts over time are interpreted in terms of the organic geochemical inventory of bitumen associated with the natural pore system and its accessibility at different maturity levels. These patterns deviate strongly from the bulk rock extracts of the powdered samples.