Diagenetic controls on the elastic velocity of the early Triassic Upper Khartam Member (Khuff Formation, central Saudi Arabia)

Abstract

The Permo-Triassic intervals of the Arabian Plate host the most important gas reservoirs in the world, the Khuff Formation and its equivalents. Understanding the main controls on the elastic properties of the Khuff carbonates is crucial for hydrocarbon exploration and quantitative interpretation of sonic and seismic data. In total, 88 samples were collected from the Upper Khartam Member of the Khuff Formation (central Saudi Arabia) to investigate the controls and variations in petrophysical properties and the associated acoustic velocities. Samples were analyzed using optical microscopy, XRD, and SEM to identify their texture, mineralogy and pore types. Porosity, permeability, and compressional and shear wave velocities were measured for all samples. A significant scatter is observed in the velocity-porosity relationship of the studied samples. Depositional environment, lithofacies and minerology show no clear impact on such observed scatter. Our results show that variations in pore types, predominantly of diagenetic origin, are the main factor controlling variations of velocity at any given porosity. Samples characterized by vuggy and non-cemented moldic porosity have a higher velocity when compared to samples characterized by infilled molds, interparticle and microporosity. The impact of pore types on velocity is investigated by means of velocity-porosity trajectories obtained by the differential effective medium (DEM) modeling, which incorporates variable equivalent pore aspect ratio (EPAR) values. Three main diagenetic processes, all being non-EPAR-preserving, contribute to the evolution of velocity and porosity: 1) dolomitization creating connected pore system between the euhedral dolomite rhombs which increases permeability (best reservoir unit) but decreases pore aspect ratio and thus velocity; 2) dissolution of microporous ooids and skeletal grains creating high EPAR molds and vugs at the expense of low EPAR micropores, 3) blocky calcite cementation that does not preserve the rounded shape of open oomolds reducing EPAR from more than 0.4 to less than 0.1 for extensively infilled molds. The established impedance-porosity trajectories in this study suggest that elastic properties can provide important insights about diagenetic overprints, pore types and reservoir quality in Khuff carbonates.