Petroleum Geology of the Cretaceous Isis Field, Offshore Tunisia

Jeffrey J. Dravis, Jeffrey E. Walter

Isis Field, located on a paleohigh in the Pelagian Sea 140 kilometers offshore of Tunisia, contains oil and gas in the Cenomanian Isis Limestone Member, a unit dominated by rudist reef-derived debris, and up to 85 meters thick. The Isis reservoir is encased in deeper-marine chalks of the Fahdene Formation; the trap is combination structural and stratigraphic. Seven wells and 3-D seismic define the limits of this reservoir, which is buried to depths between 2400-2650 meters; a horizontal well is planned for late summer, 1994.

Four major depositional facies comprise the Isis limestones: (1) rudist rudstones to floatstones (proximal reef); (2) rudist low-mud packstones/grainstones (proximal to distal reef debris); (3) more micritic peloidal-molluscan (rudist) packstones with large benthic foraminifers (protected shallow subtidal); and (4) more basinal mixed rudist and planktic microfossil packstones/wackestones.

Reservoir quality exists in all four facies but is best developed in facies types 2 and 4. Preserved porosity is predominantly secondary moldic and vuggy; microporosity is very common as well. Primary interparticle and intraparticle pores are variably occluded by carbonate cements. Above the oil-water contact, all porosity is totally saturated with hydrocarbons and water saturations are low. Early secondary porosity resulted from dissolution of former aragonitic rudist grains. However, widespread dissolution of calcitic rudists, planktic microfossils and micritic fabrics reflect a phase of deeper-burial dissolution which created abundant microporosity in micritic limestones normally considered to have very low reservoir potential. Fracturing is not common in the reservoir.

Upward-shoaling depositional cycles exist in this reservoir. However, the continuity of these cycles and their depositional facies, remain uncertain because of patchy core control and prolific shedding of sediment by rudist reef complexes. Some depositional cycle contacts are vertical permeability barriers; others are not because of the later effects of deeper-burial dissolution.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995