BIB-SEM pore characterization of mature and post mature Posidonia Shale samples from the Hils area, Germany

Organic-matter porosity is an important component in mature organic-rich shales providing storage capacity for liquids and gases. This study presents 2D microstructural characterization of porosity and in particular organic-matter porosity of two organic-rich Posidonia Shale samples with different maturity using Broad Ion Beam milling and Scanning Electron Microscopy (BIB-SEM). The organic-matter porosity is investigated in a mature sample and a postmature sample. Results show that both samples show a similar trend in pore size distributions, though the pores in the oil-mature sample are less frequent, resulting in a lower visible porosity of 0.82% compared to 2.47% in the postmature sample. This difference is interpreted to be a combination of enhanced cementation and compaction in the mature sample and to secondary organic-matter porosity generation in the gas window for the postmature sample. The organic-matter pores as found in the postmature samples are mostly absent in the mature sample. The latter shows only crack-type porosity at the organic-matter–mineral interface which is interpreted to be related to either shrinkage because of devolatilization or hydraulic fracturing because of hydrocarbon generation. The transition from the crack-type porosity along organic-matter into intra-organic-matter pores is related to thermal maturation. The total BIB-SEM visible porosity is compared with bulk Mercury Intrusion Porosimetry (MIP) porosity to assess the pore connectivity and evaluated using Wood's Metal Injection (WMI). Pore orientations suggest, and WMI demonstrates, that the preferred transport pathways are sub-parallel to bedding. Comparison between MIP porosity and BIB-SEM inferred porosity indicate that most of the pores are connected with pore throats below 10nm. Overall, crack-porosity is minor, although it may be important on a larger scale as suggested by previous studies. •Mature Posidonia Shale shows crack-type porosity at organic-matter – mineral interface.•A part of these cracks are associated with devolatilization of the organic matter.•Organic matter in the overmature Posidonia Shale sample contains an average organic matter porosity of about 2.5%.•This organic-matter porosity is formed by hydrocarbon generation and is much greater than in the oil mature sample.•A porosity evolution model from early mature to post-mature is deduced.