A petroleum system and basin modeling study of Northwest and East-Central Saudi Arabia: Effect of burial history and adjacent rock lithology on the gas potential of the Silurian Qusaiba Shales

Organic-rich hot shales of the Silurian Qusaiba Formation are known to have sourced oil and gas reservoirs in the Paleozoic Petroleum System in Saudi Arabia. Recently, these shales have attracted attention in terms of unconventional oil/gas reservoirs. Shale gas assessment requires evaluation of many key variables that play a role in both shale gas reservoir quality and shale gas production quality. We have reassessed all the key factors for shale gas reservoir quality by benchmarking with world-class gas-sorbed reservoirs and found that the Qusaiba hot shales' properties fulfill almost all criteria, and therefore the Qusaiba hot shales were qualified as high-quality gas-sorbed reservoirs. Two key factors, namely burial history and adjacent rock lithology, were determined to be very critical and worth a detailed evaluation by using a petroleum system and basin modeling approach. For better use and interpretation of modeling results, we revisited many assumptions on oil/gas generation and retention mechanisms in organic-rich shale source rocks and the model was input accordingly. The evolution of maturity and resultant oil and gas generation from and retention/oil-gas cracking within the Qusaiba hot shales have been modeled for estimations of present-day gas potential. Calibrated modeling results suggest that main gas generation in the northwest part of the Arabian Basin took place prior to the middle Carboniferous uplift and erosion episode that caused fracturing of the organic-rich shales, but gas loss has been negligible due to the sealing effect of the overlying thick warm shales. In the east-central Arabian Basin, the Qusaiba shales reached peak gas generation maturity in the Tertiary, and present-day burial depths and temperatures are at, or very close to, maximum throughout the burial history, which enhanced the preservation of gas in the Qusaiba shales. Although this study has focused on shale gas potential, our results suggest that Qusaiba hot shales, and to some extent the overlying warm shales, have also great potential for shale oil resources in the wider area where the Qusaiba shales are found at oil generation maturity level.