Sequence Framework of the Shu'aiba Reservoir, Lower Cretaceous Aptian Shaybah Field, Saudi Arabia

Nasser M. Al-Ghamdi and J. Fred Read
Virginia Tech, Blacksburg, VA

Fourteen cores up to 450 ft long through the Lower Cretaceous (Aptian) Shu'aiba Formation were logged in Shaybah field. Facies include, from deep to shallow:glauconitic argillaceous Palorbitolina packstone-wackestone(deeper water);Palorbitolina planktic foram chalky lime mudstone; Lithocodium wackestone-packstone-bindstone; fragmental skeletal packstone-grainstone with rudists and/or platy and branching corals (bank flank); Glossomyophorus rudist floatstone-rudstone; Offneria rudist floatstone-rudstone (bank crest); and very fine skeletal peloid packstone-grainstone (with Agriopleura in Upper Shu'aiba Formation; shallow lagoon). The Shu'aiba is an overall upward shallowing succession composed of five 3rd order sequences ranging from 40 to 100 ft , dipping gently seaward to the north. Sequence 1 consists of Orbitolina deeper platform facies up into Lithocodium carbonates. Sequence 2 has planktic foram chalky mudstone up into Lithocodium and Palorbitolina carbonates, capped by local rudist banks. Sequence 3 has flooding facies of Palorbitolina mudstone/wackestone, and highstand rudist banks flanked coral facies, and capped locally by rounded rudstone gravels. Sequence 4 starts with a local, rudist floatstone banks capped by more regional Offneria rudist rudstone banks that pass updip into lagoonal carbonates; the sequence is capped updip by thin shale, and elsewhere by gravel beach facies. Sequence 5 is dominated by shallow lagoonal with Agriopleura-bearing fine packstone with local, thin transgressive and highstand rudist banks, overlain by the regional unconformity beneath the Nhr-Umr Shale. The facies development is complicated by localized bank relief within the buildup interior, subtle structural warping and eccentricity driven sea-level changes superimposed on 3rd order relative sea level cycles.