Depositional Environments of Upper Triassic Sandstones, El Borma Oil Field, Southwestern Tunisia

BENTAHAR, HEDI, and FRANK G. ETHRIDGE, Earth Resources Department, Colorado State University, Fort Collins, CO

El Borma oil field in southwestern Tunisia is located on the Algerian border and produces from five Upper Triassic sandstone reservoirs at depths ranging from 2300 to 2400 m. The 250 sq km field has recoverable reserves of 770 mm bbl of equivalent oil. Reservoir sandstones rest unconformably on south-dipping Lower Devonian clastic deposits. Silurian shale represents the major oil source rock and the field is capped by 550 m of shale, carbonate, and evaporite.

Hercynian topography below the reservoir sandstones comprises an 18 km wide, northeast-oriented paleovalley. Each of the four lower reservoir sandstones, bounded by a lower scour surface and a basal lag deposit, is commonly discontinuous and separated by lenticular shale beds. These 5 to 15 m thick sandstones display cylindrical shaped log profiles and were probably deposited in channels flowing to the northeast. The overlying 12 m thick transgressive marine dolomitic shale contains carbonized bivalves and is capped by a paleosoil with root structures and siderite cement indicating subaerial exposure. The clay-rich and locally bioturbated uppermost reservoir sandstone was probably deposited in a tidally influenced estuary.

Overall, the Upper Triassic reservoirs at El Borma consist of valley-fill estuary deposits that were formed during transgression of the sea from the northeast.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)