Dolomitisation and Dedolomitisation of Shallow Marine, Upper Albian-Lower Turonian Carbonates of the Jeffara Escarpment, Southern Tunisia

Reservoir architecture of mid-Cretaceous carbonate reservoirs in Tunisia is in a large part controlled by dolomitisation. Despite this fact, very little work has been conducted to determine the extent and timing of dolomitisation and its impact on reservoir quality.

This study examines a superbly exposed section along the Jeffara escarpment of southern Tunisia, which provides an excellent opportunity to study the extensively dolomitised Upper Albian-Lower Turonian shallow water carbonates of the Zebbag Formation (Rhadouane, Kerker and Gattar Members), which are potential outcrop analogues for reservoirs currently under production and appraisal in North Africa.

During the mid-Cretaceous, Tunisia was situated around 11○N, and covered by a shallow epeiric sea that formed as part of the southern Tethyan margin. Anhydrites form lateral equivalents of the upper parts of the Gattar Member and are common within the Kerker Member, implying an arid climate throughout deposition.

Facies analysis of the Rhadouane (Upper Albian-Cenomanian) and Kerker (Cenomanian) members reveals stacked upward-shallowing packages consisting of bioturbated skeletal packstone-grainstones to algal laminated wackstones-grainstones, with rare ooid shoals and tidal bars. Within the Kerker Member, a thick (~1m thick) gypsum layer is an excellent regional correlative marker. Facies analysis within the Gattar (Lower Turonian) Member has proved difficult due to the pervasive and fabric destructive nature of the dolomitisation.

Dolomitisation within the lowermost Rhadouane (Albian-Cenomanian) and Kerker (Cenomanian) Members is incomplete and non-fabric destructive. Planar dolomite fabrics suggest dolomitisation occurred at temperatures <50○C, and lack of compaction features suggest a limited amount of burial. Later fractures provided conduits for hotter dolomitising fluids, recrystallising planar-e dolomite to non-planar dolomite. Dedolomite and calcitisation is common within the Zebbag Formation occurring in thick, ~1.5m beds, within large non-strata bound fractures and as replacement of cores of dolomite rhombs by calcite. Dedolomite layers are laterally discontinuous, and can return to pure dolomite compositions over a distance of centimetres. Preliminary data suggests that dedolomitisation took place during uplift of the platform, potentially from downward percolating meteoric porewaters.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California