Distinct petroacoustic signature in heterozoan and photozoan carbonates resulting from combined depositional and diagenetic processes

Abstract

The acoustic properties of heterozoan and photozoan carbonates are compared and interpreted in the light of their distinct patterns of diagenetic evolution. The combined role of depositional and diagenetic processes on the petroacoustic signatures of both heterozoan and photozoan carbonates has been assessed on the basis of a multi-disciplinary and multi-scale study of Oligo-Miocene limestone outcrops from the Cape Range peninsula (NW Australia). This study integrates a detailed diagenetic analysis (using sedimentology, petrography and stable isotopes geochemistry), as well as physical properties characterization (through acoustic velocity laboratory and porosity measurements, petroacoustic modelling and 3D CT-scan imaging). The three formations studied are characterized by distinct patterns of diagenetic and petroacoustic evolution: heterozoan, non-tropical carbonates from the Mandu and Tulki formations are characterized by significant development of microporosity and pressure-solution compaction features which led to decreasing the pore network stiffness. Photozoan, sub-tropical to tropical carbonates from the Trealla formation are dominated by moldic, vuggy and partially cemented intergranular pores which tends to increase the pore network stiffness. The distinct petroacoustic signatures of heterozoan and photozoan limestones have been shown to be mainly related to (1) differences in compaction during burial resulting from an emersion phase and (2) differences in the mineralogy of dominant carbonate producers.Temperate High-Mg calcite dominated carbonate typically poorly cemented allowed for greater compaction degree, reducing the ratio of intergranular macropore. Furthermore, these dominant calcite producers (large benthic foraminifers, red algae …) from the heterozoan Mandu and Tulki formations favoured the development of intragranular microporosity whereas less-stable aragonite dominated biota from the photozoan Trealla Limestone favoured the development of molds and vugs, during early meteoric diagenesis. Finally, petroacoustic modelling approaches such as αK inversion have proved to be efficient for quantifying the impact of diagenetic processes and associated pore types on acoustic properties. These results, emphasizing the strong variability in depositional and diagenetic features for distinct carbonate fabrics could have interesting implications in terms of reservoir properties 3D modelling and upscaling approaches.