The Impact of Diagenesis on Rock Properties in Shales
Channel: GeoscienceAustralia
Date: 2022-04-21

Marcellus Shale and Canning Basin shale samples were used to study diagenetic effects on the mineral and organic matter and their impact on petrophysical and geomechanical response.

The Marcellus Shale contained an anomalously low electrical resistivity interval within high thermal maturity organic matter.  Multiple techniques were used to elucidate the micro- and nano-scale structure of the Marcellus Shale which showed highly porous bitumen residing in the original pore network of the rock.  This organic matter had been transformed to a partially ordered proto-graphite structure under temperatures in excess of 250°C, determined by multiple geothermometric lines of evidence, causing the low resistivity observed.

For the Canning Basin Ordovician shales, there are two shales, the Goldwyer and Bongabinni Formations, which have similar burial history (100m apart) but different depositional environments and diagenetic histories.  Early diagenetic precipitation of dolomite and anhydrite in the Bongabinni prevented grain rotation and compaction resulting in a relatively stiff rock and isotropic rock physics response.  The Goldwyer underwent later quartz overgrowth development and contained more well-aligned illite, giving a weaker, more compressible shale with stronger fabric orientation and significant velocity anisotropy.

These studies highlight the strong impact that organic matter, minerals and their transformation during diagenesis can have on the evolution of rock properties and emphasise that multi-disciplinary studies can be crucial to unravelling the often complex geological history in shale formations.

This seminar was first presented as part of the AAPG Foundation’s Distinguished Lecture Program in 2021-22.