Stratigraphic Framework of the Mafraq Formation, Northern Oman

Hamad Al-Shuaily¹*, Omar Al-Ja’Aidi², Henk Droste², and John Aitken^1
1PDO
²Shell, Oman
*[email protected]

The mixed clastic-carbonate Mafraq Formation (Late Triassic-Middle Jurassic) is the lowermost unit of the Sahtan Group in Oman. The formation comprises a fluvial to shallow-marine succession that onlaps the tilted Akhdar Group (Permo-Triassic) unconformity from the northwest to the southeast. Despite producible gas and oil indicators, the Mafraq Formation has remained largely underexplored and is poorly understood in Oman. A regional stratigraphic framework for the Mafraq Formation has been developed, which incorporates seismic, well, outcrop, core and biostratigraphic data. This permitted an assessment of the depositional and structural controls on the distribution of the Mafraq Formation. Improved understanding of the Mafraq play suggests potential for both conventional and stratigraphic trapping configurations. The Mafraq Formation is a complex diachronous depositional system including offshore, shallow-marine, coastal-plain, fluvial and alluvial-plain environments with stratal termination patterns visible on seismic. Several transgressive-regressive cycles can be identified including: (1) a Late Triassic regression that resulted in the deposition of fluvial sandstones in the northwest; 2) a Late Toarcian regression leading to the deposition of a fluvial system in the central part of northern Oman; (3) an Early Aalenian flooding that deposited shallow-marine oolitic limestone in the northwest; and (4) a Mid- and Late Bajocian flooding resulting in the deposition of marine sandstone in the southeast. Two principle trap types have been identified, namely conventional structural traps (either fault-bounded or salt-induced) and stratigraphic traps including isolated channel, pinch-out, truncation and onlap traps. The play is largely dependent on reservoir and seal thickness variations controlled by primary depositional processes and halokinesis and fault-related accommodation.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain