Ranking Influence of Microfabrics and Microporosity on Acoustic Propagation in Unconventional Tight Carbonate Reservoirs

Many researchers reported that porosity and, to less extent, sedimentary fabric are the main controlling aspects of acoustic wave propagation in carbonate reservoirs. In this study, we focused on microporous microfacies of shallow marine, deeply-seated tight carbonate reservoir from the Middle Eastern Permian Khuff-C Member of Khuff Formation. The Khuff Formation is known as the main Paleozoic target of non-associated gas in the Arabian Platform. The main objective of this work is to shed more light on aspects influencing acoustic wave propagation in tight carbonate reservoirs. In particular, the strength and related microfabric properties of these rocks. Three microfabrics (MFs) were targeted in this study; those are MF-1: microcrystalline mudstone, MF-2: Intraclastic skeletal peloidal grainstone, and MF-3: Thinly-laminated microbialites. These samples were picked from a well core of the topmost part of Khuff-C reservoir. High-resolution profiles of acoustic (p-wave) and (s-wave) were recorded under ambient conditions. SEM and petrography were used to detect MFs and visual microporosity (MP); the latter was calibrated with log-derived porosity. Impulse hammer tests were run on these samples to correlate MFs strength with acoustic wave propagations. MF-1 is fine mudstone to dolomitic mudstone, occurred frequently with inclusions of organic matter. MF-1 showed p-wave and s-wave velocities of (3147-4868m/s) and (1657-2551 m/s), respectively, and 17-19%MP. MF-2 is composed of skeletal grains (bivalves dominate), pellets, and various-sized mud intraclasts. MF-2 is a well-cemented grainstone with p-wave and s-wave velocities of (4055-6029m/s) and (2129-3010m/s), respectively, and 9-11%MP. MF-3 is intercalation of fine organic-rich microbial mats and coarser skeletal MF. MF-3 revealed P-S velocities of (2619-5143m/s) and (1514-2808m/s), respectively, and 12-15%MP. Visual MP showed a dominance of intercrystalline micropore type in all MFs, and highly conform with log-derived MP. All MFs showed a weak to moderate revered correlation between MP and each ultrasonic velocities. Comparatively, same quality of correlation was obtained between ultrasonic velocities and MFs elastic strength. Even in highly cemented grainstone (MF-3), the strength test revealed low values when faced by fractures and heterolithic components. In this study, the organic content and rarely-occurred fractures observed in these microfabrics have their impact on ultrasonic propagation with the incorporation of the microporosity and strength factors.