Lateral lithologic distribution of Triassic formations in the Algerian Oued Mya basin using combined petro-elastic and seismic inversion attributes

Abstract

This paper presents a combined litho-discrimination approach based on petro-elastic attributes and simultaneous seismic inversion. The analysis adopted attempts to characterise the Triassic fluvial reservoirs, in the Oued Mya basin, Algeria, in terms of lateral distributions of sandstone, shale, and silt. The Triassic sandstones suffer from the presence of silt, which significantly increases the compaction degree of the reservoir parts and decreases the quality of petrophysical properties. The litho-discrimination analysis improves seismic inversion accuracy, providing a more detailed understanding of subsurface rock properties. It separates the effects of different rock types, such as sandstone and shale, leading to better interpretations while reducing noise. It avoids misinterpreting noise as geological signals and, thus, yields more reliable inversion results. It enables better differentiation of complex lithologies, which is particularly crucial in regions with subtle variations in rock properties. For the purpose of an efficient litho-discrimination, a variety of petro-elastic attributes have been carefully examined to find the best attributes that can respond to lithology change. S- and P-impedances are found to be adequate litho-discrimination models with the best prediction capabilities. They have a high correlation coefficient and slightly linear lithology regressions for sand, silt, and shale, showing a good agreement with gamma ray well log responses and analysis results of core data. The litho-discrimination analysis can assist us in the prospect generation and risk assessment of the optimum locations for drilling wells in the region. Seismic pre-stack inversion is used to predict variations away from the well location. The applied workflow enables better discrimination of silt, sandstone, and shale within the T2 reservoir, where the presence of silt is mostly associated with compaction. The seismic analysis revealed a random non-uniform laterally distributed silt over the upper part of the T2 reservoir, which is consistent with the geological and sedimentological descriptions that support the existence and absence of compact intervals from well to well in the studied region. The later compaction (silty levels) is the main possible cause of the petrophysical parameter alteration, resulting in differences in the production results (oil, water, etc.) of the drilled wells in the area of interest.