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Abstract

During the Early Cretaceous, the Lucula Sandstone was deposited along the platform margin of the Malongo Subbasin in offshore Cabinda, Angola. This subbasin is one of a series of lake-filled syn-rift basins that formed along the West African continental margin at an early stage of the opening of the Atlantic Ocean. The Lucula Sandstone is subdivided into a shallow-water facies association, deep-water facies association, and dolostone facies. The shallow-water facies association has beach and nearshore facies and fluvio-deltaic facies. Beach and nearshore facies are characterized by fossiliferous, planar- to cross-laminated sandstones arranged in cyclic, upward-fining packages. Fluvio-deltaic facies are characterized by large-scale cross-bedding and deformed, well-sorted sandstones. The deep-water facies association has turbidite- and slump-dominated facies. Turbidite-dominated facies consist of shales and fine-grained, planar-laminated turbidite sandstones. Slump-dominated facies consist of highly bioturbated, very fine-grained sandstones and siltstone with slump structures. Dolostone facies contain sandy, highly recrystallized dolomite.

Two orders of cycle frequency occur in the Lucula Sandstone. Low-frequency cycles are represented by the interstratification of shallow- and deep-water cycles. These cycles are generated by longer-term, climate-forced lake-level changes. High-frequency cycles are represented by the cyclic pattern of beach and nearshore facies. Cycles are produced by low-amplitude lake-level changes caused by short-term climatic changes.

Sandstones of the beach-nearshore and fluvio-deltaic facies have the best reservoir quality in the Lucula Sandstone. The distribution of potential shoreline and deltaic reservoirs like the Lucula Sandstone is generally predictable with use of depositional models of rift lake systems. But because of the complex nature of rift systems, stratigraphic prediction will always be somewhat problematic.

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