A Mixed Clastic/Carbonate Reservoir Example: The Albian Deposits from Offshore Congo

Virgone, Aurélien ¹; Poli, Emmanuelle ¹; Mabille, Cedric ¹; Nely, Georges ¹; Duval, Christine ¹; Tarrouilly, Loïc ¹; Gatignol, Benjamin ¹; Bernard-Graille, Céline ¹
(1)TOTAL SA, Pau, France.

Lateral variability of stratigraphic sequences under changing conditions of subsidence and/or sediment supply was well studied during last decade but the application of these concepts for reservoir modeling is a challenge.

Our study at field scale for an Albian case history from offshore Congo provides insights as to how manage the reservoir heterogeneities in a mixed clastic /carbonate system influenced by soft salts deformations (rafting). In this kind of reservoirs, the challenges for production include the presence of many vertical barriers, rapid lateral facies change, complex geometries (onlaps, toplaps, and downlaps) and complex lateral connection through faults. An integration of geophysics, tectonic, sedimentologic, either diagenetic events, help to propose a new coherent and relevant geological model at field and regional scale (facies / petrophysical heterogeneities) for better well location and production optimization (including for mature fields).

A geological model at field scale is detailed (through seismic picking, core analyses, electrofacies maps) and we can demonstrate that:

-“Good” Carbonate reservoirs are deposited during a warm period (ooids, green algae and corals assemblages) and preserved during HST on the topographic highs

-Meteoric weathering (microkarst?) and / or erosion in apex occurred during LST

-“Good” Clastic reservoirs are well developed during LST in topographic lows if the clastic supply is sufficient, and are more or less affected by early dolomitization process (reflux). If the clastic supply is low, topographic lows are filled by anoxic mudstone

-“Bad” Carbonate reservoirs are mainly deposited during the TST, in cooler periods (oncoïds, red-algae, echinoids assemblages)

Rapid eustatic variations during Albian time is proposed to explain this reservoir architecture and lateral facies changes, but the development of the good or bad carbonate fabrics are amplified, stopped or eroded by salt deformation.

The impact of the structural framework during the sedimentation appears crucial. Also, the presence of “extrados” fault filled by clastic (in LST) or colonized by good carbonate was shown. Furthermore, most of the dolomitic/anhydritic cement is a late hydrothermal process localized on the western part of the raft.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.