Technical Paper
Technical Paper

Rock Physics Modeling of Silicious and Carbonate Facies: Conceptual AVO Models Applied to Reservoirs Around the Globe

Written by: Dr Alan Mur and Dr Lev Vernik

We assembled and expanded a database of siliciclastic log data and published laboratory measurements on dry carbonate samples to demonstrate micromechanics-based methods of rock physics modeling of carbonate rocks coming from diverse depositional and diagenetic environments. By focusing on the effects of mineralogy, porosity, pore shapes, and effective stress on elastic properties of limestones and dolomites in a wide 2-45% porosity range, we show that the Vernik-Kachanov rock physics model (RPM), previously developed for siliciclastics, can be successfully used in seismic reservoir characterization of carbonates worldwide. This rock physics model adheres to the strict micromechanics principles (effective field theory) and allows us to account for realistic pore shapes and separate them from the effects of cracks. Because of the very diverse pore geometries typically observed in carbonates, we use thin section image analysis yielding pore perimeters and areas, which allow us to constrain the pore stiffness represented by the pore shape factors. We subdivide, wherever feasible, the database into textural and mineralogical facies and analyze differences and similarities between them in terms of elastic modeling, which may be utilized in AVO inversion-based reservoir characterization efforts worldwide.

A key difference between carbonate and siliciclastic rock physics modeling and feasibility for prospect de-risking is in (1) the relatively greater stiffness implied by the carbonate mineralogy, (2) the more advanced diagenetic cementation, and (3) the more complex pore microstructure (Rafavich et al., 1984; Eberly et al., 2003; Baechle et al., 2008). Our ability to account for, let alone model, these effects is still quite limited. Mur and Vernik (2019) discussed calibration techniques and implications of the contact theory and ellipsoidal inclusion based models in siliciclastic sands and shales and concluded that the use of any “effective pore geometry” (e.g., ellipsoid aspect ratio) often leads to confusion and/or misinterpretation when actual data on pore geometries are available (Vernik and Kachanov, 2010). Mur and Vernik (2020) extended the Vernik and Kachanov (2010) model to carbonates and showed the model effectively handles tasks for carbonate rock physics modeling and feasibility for prospect de-risking.

We use image analysis workflow (Mur et al., 2011) to constrain pore shape factor and then use the calibrated models for scenario testing at key Asia-Pacific reservoirs, further establishing the readiness of the model for log repair and forward modeling for AVO inversion feasibility and post inversion reservoir characterization.

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