GEOCHEMICAL CHARACTERIZATION AND HETEROGENEITY ASSESSMENT OF THE SILURIAN QALIBAH FORMATION, CENTRAL SAUDI ARABIA

Reservoir heterogeneity issues in unconventional reservoirs, such as shale gas, play a crucial role in reservoir properties. The Silurian Qalibah Formation is perhaps the most important unconventional reservoir in Saudi Arabia as it is the main source rock for the entire Paleozoic successions. It is composed of the mud-rich Qusaiba Member overlaid by the sandy Sharawra Member. This formation displays significant reservoir heterogeneity issues that needs to be addressed using multidisciplinary research. In this study, detailed sedimentological and geochemical characteristics are utilized to better explain the reservoir properties. One outcrop and one core were analyzed from the Qalibah Formation in Qassim Region, Central Saudi Arabia, and around 250 samples were systematically collected. All the collected samples were analyzed using XRF and XRD to get the elemental and mineralogical compositions, respectively, and the total organic carbon (TOC) was characterized using rock eval pyrolysis. Clay minerals constitute 50% of the bulk mineralogy in the Qusaiba Member and 28% in the Sharawra Member, with the rest comprising mica, quartz, and k-feldspars. The Qusaiba Member exhibits poor to fair organic matter quantity, with an average TOC of 0.3 wt% and immature type III to IV kerogen. Conversely, the Sharawra Member displays lower average TOC of 0.05 wt% indicating minimal organic matter preservation. Fluctuations in Ti/Al, Rb/Al, and Y/Al ratios suggest detrital fluxes during Qusaiba Member deposition and slow sedimentation rate. Conversely, higher sedimentation rate during Sharawra Member deposition is indicated by less fluctuation in these ratios. Proxies for paleoclimate (Rb/Sr and Sr/Cu), paleosalinity (Sr/Ba and Rb/K), and paleoredox (Ni/Co and V/Cr) exhibit fluctuations, pointing to a humid and warm climate, brackish to fresh water and suboxic to oxic depositional conditions in the Qusaiba Member. These conditions shift to semi-humid to arid climate, fresh and oxic water in the Sharawra Member. These fluctuations in the depositional conditions explain the heterogeneity in the Qalibah Formation and it can be utilized to subdivide it into chemozones that can be correlated spatially. The results of this work can help to better understand and evaluate the Qalibah Formation and its subsurface equivalent.