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Biofacies and Palaeoenvironments of the Mishrif Formation in Saudi Arabia

Hughes, Geraint W.*1
(1) Exploration Technical Services, Saudi Aramco, Dhahran, Saudi Arabia.

The Mishrif is the youngest Formation of the Wasia Group, dated on planktonic and benthic foraminiferal evidence as Late Cenomanian, and is encountered only in the subsurface of Saudi Arabia. Three biofacies are evident and have been used to subdivide the formation into four carbonate depositional cycles, of which the shallowest beds are sufficiently porous to provide intra-formational reservoirs.

The deepest biofacies contains planktonic foraminifera Hedbergella delrioenisis, Heterohelix moremani and rare Rotalipora appenninica with calcareous dinocysts Calcisphaerula innominata and Pithonella ovata. Fragments of the pelagic crinoid Roveacrinus are well represented. This biofacies, together with rare costate nodosarids and lenticulinids and the fine-grained packstone fabric, suggest deposition in a very low energy, deep marine setting, typical of an intra-shelf basin. This biofacies is accompanied by species of the complex-walled miliolid Praealveolina, considered as allochthonous and derived from an up-slope shallow lagoon setting. The overlying radiolitid and caprinid rudist fragments form a distinctive, coarse-grained biofacies and can be interpreted as a shallow marine, rudist bank complex. A Praealveolina-dominated bioclastic biofacies is located at the shallowest part of the depositional cycles and represents lagoon biofacies that prograded basinwards.

By integrating the four shallowing-upwards biofacies with their lateral biofacies variations, regional palaeoenvironmental evolution of the intra-shelf basin margin can provide rock-based indicators to predict the distribution of the reservoir lithofacies.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain