--> --> History of Oil and Gas Generation from the Silurian Qusaiba Shales in the Saudi Arabian Basin: Implications for Gas in Place Assessments

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History of Oil and Gas Generation from the Silurian Qusaiba Shales in the Saudi Arabian Basin: Implications for Gas in Place Assessments


Organic matter rich black shales are represented in the Qusaiba member of the Qalibah Formation of the early Silurian (Rhuddanian to Telychian). A hot shale unit makes up the lower part of the Qusaiba shale member and represents a rapid deposition on a maximum flooding surface; due to rapid sea level rise because of glacial melting. Type II kerogen containing Qusaiba shales of Silurian age are wide spread and are known to have sourced oil and gas reservoirs in the Paleozoic Petroleum System in the Kingdom of Saudi Arabia. Accurate assessment of shale gas in place requires multidisciplinary evaluation of many key variables that play a role in both shale gas reservoir quality, as well as shale gas production quality. Organic matter type and quantity, level of thermal maturity, intergranular porosity and intra-kerogen porosity are just few of many essential properties of a given shale to qualify it as a potential gas reservoir. What really makes a shale a commercially viable gas-sorbed reservoir depends on the proper burial history and timing of hydrocarbon generation, which determines the amount of gas that will be expelled or retained, both as free and adsorbed gas. Burial and thermal histories of the Qusaiba shales have been modeled by using a Petroleum System and Basin Modeling software. Results suggest that the Qusaiba shales from the Arabian Basin have attained sufficient maturity for oil and gas generation. The main oil and gas generation in the West Arabian Basin took place during late Devonian/early Carboniferous prior to the Middle Carboniferous uplift and erosion episode that resulted in uplift, cooling and pressure drop in the Qusaiba shales, leading to partial degassing. Most of the gas leaving the Qusaiba shale must have entered the underlying Upper Ordovician Sarah sands, where pressured gas is encountered in most of the drill holes intersecting this unit. Further reburial of the shales in the Mesozoic and Cenozoic were not sufficient to reset the thermal maturity attained during the late Paleozoic. In the East-central Arabian basin, oil and gas generation started in the late Jurassic-Cretaceous and reached its peak (mainly due to oil to gas cracking) in the Tertiary. In the majority of the East-central Arabian Basin, 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.