Utilization of geochemical signatures for unconventional reservoir characterization, Saudi Arabia

Paleozoic tight sandstone and shale reservoirs in the Kingdom of Saudi Arabia are considered potential sources of natural gas that will meet the growing domestic demand. In order to enhance their exploration and exploitation, research focusing on the quality of these potential gas reservoirs has tremendously increased in the past few decades. Most of the Paleozoic reservoirs have similar lithologies and are poorly constrained biostratigraphically, making facies analysis, stratigraphic correlation, and geosteering problematic. This paper describes how chemostratigraphic markers are used to identify and separate individual lithologies. In this study, 320 data points were acquired from continuous core samples obtained from three Paleozoic formations. Raw elemental spectral data were acquired using a high-resolution tabletop µXRF system. The spectral data were used as geochemical signatures to provide a better understanding of the geochemical composition of each sample. Multivariate statistical analysis including principal component analysis (PCA), pairwise correlation and hierarchical clustering of principal components analyses (HCPC) were used to characterize the acquired geochemical signatures. Eleven chemofacies within three lithofacies were identified based on PCA and HCPC analyses of the geochemical signatures with clearly defined boundaries. Stratigraphic distribution patterns of the geochemical signatures (e.g. Si, Ca, Mn, Al, K, and Fe), such as sharp truncation at the upper and lower bounds, clearly defined each lithofacies. Thus, key marker elements for each lithofacies and chemozones within it were identified. Formation intervals were clearly defined based on the chemozones, even where the lithologies appeared homogenous. The overall results indicate sediment deposition under varied depositional settings, from fluvial to shallow marine environments. The results demonstrate that elemental composition analysis can be used as a discriminating tool for resolving stratigraphic uncertainties due to its ability to document subtle variations among samples that appear homogenous with other techniques. The definition of key marker elements for each chemofacies will serve a useful tool to support geosteering operations for better well placement.