Abstract: Influence of Sedimentary Discontinuities on Carbonate Reservoirs (Mabruk Field, Libya)

Machhour, L. and P. Raingeard - Total Paris; C. Lemoy - Total Libya; N. Audebert - Total Paris

Sedimentary discontinuities can have a real impact on underlying sediments and especially on carbonate reservoir qualities. The carbonate interval of the Heira Formation (Paleocene) in Mabruk Field (Sirt Basin, Libya), includes abundant discontinuities and provides a good illustration of their effects because abundant well and core data are available. The Sirt Basin was formed by intracratonic rifting, with possible phases of stability or structural reversals. Sedimentation was controlled by tectonic and eustatic influences, locally inducing high sedimentation rates, and with distribution of the various lithologies governed by ridge and trough paleotopographies.

Discontinuities within the interval are distributed in 3 zones: 1 ) A southern zone of dolomitized inner platform deposits, probably intermittently emerged; 2) a central zone, dominated by protected middle shelf deposits; and 3) a northern zone where peri-reef deposits prevail. Discontinuities are mostly developed in the southern area and decrease in frequency northwards as the sequence thickens. Most of the recorded discontinuities are related to temporary subaerial exposure, with local solution processes notable within the northerly build ups. Discontinuities always record breaks in sedimentation and related processes can significantly improve reservoir potential of underlying sediments. However, diagenetic processes are not all coeval and effects of early and late diagenesis must be distinguished.

Early diagenesis typically results in syn-sedimentary lithification of the sediment which may preserve molds by external casting but which may also form a hardened barrier layer directly below the discontinuity surface. These early processes thus may lead to the preservation of some porosity but they may also result in a sharp decease in permeability. Solution processes can also occur and provide early secondary porosity with good connectivity in the upper meters of sediment below discontinuities, although local cementation and internal sediments can reduce this significantly. Similarly, discontinuity-related early dolomitization can improve reservoir potential by providing intercrystalline porosity in affected horizons.

Later diagenetic processes are related to the circulation of mesogenetic fluids and commonly include widespread cement precipitation and a dramatic decrease in reservoir potential. However, as with the early phase, late dolomitization can also provide good intercrystalline porosity. Stylolitization is seen at some discontinuities, usually with related lowering of porosity, though often these horiozons have potential to be used as reservoir drains.

The impact of discontinuities on carbonate reservoir qualities is clearly evidenced in Mabruk Field, where the data sets show that they must be considered as primary factors within the reservoir and that they require particular attention during reservoir characterization and modeling.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil