Laboratory investigation of permeability, pore space and unconfined compressive strength of uplifted Jurassic mudstones: The role of burial depth and thermal maturation

In this study, organic-lean mudstones that have undergone variable maximum burial were investigated to assess potential effects of diagenesis on permeability, pore space and unconfined compressive strength. This is particularly relevant for mudstone formations, such as encountered in the Lower Saxony Basin, where local subsidence and uplift periods caused a strongly heterogeneous basin development. The studied sample set provides a unique opportunity for such a study insofar that over a relatively short distance of 50 km (Hils area) and with a relatively homogeneous mineral composition, thermal maturities increase from 0.48 to 1.45% vitrinite reflectance. This corresponds roughly to temperatures and maximum burial depths of 75 to 165 degrees C and 1,300 m to 3,600 m, respectively. The results demonstrate that permeabilities and porosities decrease strongly up to a maximum burial depth of approximately 2,500 m, from 2.7 to 0.21 x 10-21 m2 and approximately 14 to 8%, respectively. Reduction of porosity occurs predominantly in the macro- and mesopore range as evidenced by lowpressure nitrogen adsorption measurements. Further burial to a maximum depth of approximately 3,600 m leads only to negligible change of these properties. Initial reduction is due to mechanical compaction, whereas further mechanical compaction is possibly counteracted by the generation of secondary porosity via cracking of solid bitumen and/or kerogen, similar to findings reported for various organic-rich shales but much less pronounced. Specific surface areas (approximately 15 to 33 m2/g) gradually decrease with increasing maximum burial depth. Unconfined compressive strength (UCS) values increase from 25 MPa at a maximum burial depth of 1,300 m to 40 (Liassic) and 50 MPa (Doggerian) at 2,600 m. Decreasing UCS values were observed for specimen that experienced larger burial depths up to 3,600 m (31 MPa for Liassic and 36.5 for Doggerian specimen). Whether the properties obtained correspond to those encountered at the proposed maximum burial depths is questionable, since severe uplift, storage of the cores (for over 30 years in wooden boxes at ambient conditions), and sample preparation may have resulted in changes in these properties. However, data on mineralogy and comparison of petrophysical properties obtained in this study with those from mudstones that have not been uplifted indicate that at least the trends observed and to some extent the absolute values observed are representative of maximum burial conditions.

Automated summary

This study investigates the potential effects of diagenesis on permeability, pore space, and unconfined compressive strength in organic-lean mudstones that have undergone different maximum burial depths. The results show that permeabilities and porosities significantly decrease up to a maximum burial depth of approximately 2,500 m, while further burial to 3,600 m has negligible impact on these properties. The findings provide important insights into the physical properties of mudstones and their relevance in oil and gas exploration and development.