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The Evolution from Depositional Geometry to Trap Definition by Rapid Multi-layer Mapping

Michelle Allum
Technical Team Leader, PetroCom Technologies

Abstract

Cretaceous sandstone reservoirs off the west coast of Africa were deposited on a basin floor setting. These sandstones were historically identified by seismic attributes that contrasted with the surrounding rocks. The traps are stratigraphic and the terminations are upslope. The geometry of the lows in which the sands accumulated is different from the geometry of the traps, because through time the shape of the beds was modified by compaction or by fluid mobilisation in the underlying substratum. To successfully identify the optimal location for exploration and appraisal wells, the evolution from depositional geometry to the current trap geometry must be mapped to understand the fill history and where to place the critical pinch-out edge. This can best be done by mapping numerous layers in the overlying strata. While this process has traditionally been very time consuming, there are modern tools that can help significantly.

Case studies on the shelf in Trinidad show how the geometry of sandstones bodies transported by turbidites, were controlled by the paleobathemetry. In these cases, the basin floor responded to fluid movement in the underling strata and so the changes in bathemetry were dynamic and served to highlight the topographic control on sandstone distribution. The change from depositional geometry to trap architecture is gradual and is caused by compaction and mud mobilisation. Layer by layer mapping illustrates how the sandstone bodies migrated in response to the changes in bathemetry and how the changes in the underlying strata concerted those bodies into potential traps.

AAPG Search and Discovery Article #90203 © AAPG Geoscience Technology Workshop, Trinidad and Tobago Deep Horizon and Deep Water Frontier Exploration in Latin America and the Caribbean, March 9-11, 2014, Port of Spain, Trinidad