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Changes in Acoustic Properties and Velocity Anisotropy in a Very Organic-Rich Marine Shale With Increasing Temperature

Abstract

The organic content of marine source rock shales has a major control on their petrophysical properties, and the change in seismic rock properties with increasing organic richness forms the basis for identification of source rocks on seismic data. With increasing thermal stress, a proportion of the particulate organic matter and associated bitumen will be converted to liquid/gas. However, the very complex physiochemical changes (phase changes, volume changes, pressure development and migration/expulsion of mobile phases) occurring associated with mature source rocks are still poorly understood. Very organic-rich (40-49 wt.% TOC) samples of the Blackstone Oil-shale in the Kimmeridge Clay Formation (Dorset, UK) have been analyzed in the laboratory as a function of increasing temperature. The motivation for the study is to quantify changes in seismic properties associated with mature source rocks. Identification of mature source rocks on seismic data would provide important information about the basin temperature history. By itself it would also be a unique source rock identification criterion. The performed analyses include ultrasonic measurements, mineralogy, geochemical analysis, CT-scanning, µCT-scanning and petrographical analyses by environmental scanning electron microscopy (ESEM) techniques. The studied oil-shale samples are calcareous with abundant biogenic carbonate (coccolithophores). The micro-fabric is characterized by a finely laminated to lenticular texture of organic matter, biogenic carbonate and siliciclastic minerals. Velocity measurements have been performed at 2000 psi confining pressure and increasing temperatures up to 235°C. These measurements were performed at plugs oriented 0°, 45° and 90° relative to the primary lamination. Both compressional velocity (Vp) and shear velocity (Vs) are gradually reduced by 30-40% when the temperature is increased from 25°C to 235°C. Changes in the rock fabric are observed and documented by detailed ESEM analyses. The samples show Transverse Isotropic (TI) symmetry and the anisotropy parameters (ε, γ & δ) show strong temperature dependence. The results suggest that seismic attributes potentially may be used to identify the change from non-mature to mature source rocks.