Paleozoic petroleum system elements were reviewed for the potential of pervasive gas accumulations (basin-centered gas) in Saudi Arabia. Detailed analysis of a basin located southwest of the Ghawar field was conducted as a regional case study.

Mapping the subcrop of the Hercynian regional unconformity revealed several deep Paleozoic basins comprising the crucial element for forming a pervasive gas accumulation: an active, gas mature source rock (the Silurian Qusaiba shale). A detailed assessment of the unconventional gas resource play in the southwest Ghawar basin focused on a Silurian-Permian clastic sequence that is in close proximity to the Qusaiba shale. The critical interaction between the thermal maturity of the Silurian Qusaiba shale, the low permeability nature of the Silurian-Permian clastic sequence, and their relationship to the regional pressure system and fluid type distribution was analyzed to confirm and map the potential extent of pervasive gas. Water was found to breach the pervasive gas accumulation on the flanks of the basin and locally around faults. The water in the basin flanks is from meteoric water recharge that took place 10,000 years ago, whereas the water around faults is from deep aquifer systems.

This case study has established a new evaluation process for generating unconventional gas resource play concepts. The process involves integrating the analysis of six critical elements:

  1. the thermal maturity of the source rock,

  2. proximity of reservoirs to the source rock,

  3. reservoir quality,

  4. abnormality of reservoir pressure,

  5. regional fluid distribution, and

  6. the effectiveness of interactions among these five elements.

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