--> Abstract: The Coquinas Sequence: Lacustrine Carbonate Reservoirs in Campos Basin, by M. D. Carvalho, R. J. Jahnert, and U. M. Praça; #90933 (1998).

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Abstract: The Coquinas Sequence: Lacustrine Carbonate Reservoirs in Campos Basin

Carvalho, M. D.; R. J. Jahnert and U. M. Praça - Petrobras/Cenpes/E&P

The Coquinas Sequence (Barremian), the most important sequence of the Lagoa Feia Formation, is the main oil and gas exploratory target in the pre-salt sediments of the Campos Basin, Brazil. It is composed by fluvial-alluvial deposits and expressive lacustrine carbonate bodies (coquinas), deposited in a complex lake system controlled by local environmental conditions and tectonics. Its thickness varies from 2,400 m in paleostructural lows to 0 m in paleostructural highs.

The main reservoir consists of coquinas lenses characterized by units over 10 m thick made up of amalgamation of relatively thin layers (20 to 100 cm thick) of bivalve-rich calcirudites, classified as grainstones, that locally show cross bedding stratifications, associated with thin layers of calcarenite or calcilutite and no siliciclastic rock intercalations. Succession of this facies sub-association form layers 20 to 550 m thick and are associated with paleohighs. The sedimentary features suggest high energy and shallow water depositional conditions. The coquinas have an average porosity around 15% and permeability about 120 mD. Porosity is secondary, characterized by the interparticle, vugular and moldic types.

Seven lacustrine facies association and environmental interpretation have been defined for the Coquinas Sequence: 1) calcirudite lenses, the main reservoir rock, interpreted as bioclastic bars, 2) calcirudites/siliciclastics, representing bioclastic sandy beaches; 3) calcarenites/calcilutites, representing bioclastic calcarenites beaches; 4) fine-grained siliciclastics/carbonates, representing the marginal lake environment; 5) calcirudite/siliciclastic sheets, interpreted as bioclastic sheets/bar fringes; 6) fossiliferous calcilutites/sandstones, representing the bioaccumulation banks and 7) laminated siliciclastics/ostracodal calcilutites, interpreted as deep lacustrine sediments.

The reservoir distribution is related to depositional structural highs with complex traps, commonly with strong stratigraphic control. Pinch-out of the porous zones, growth faults and high cementation levels are common features.

Basement trends (NE-SW) are the most important elements in the control high depositional energy of sediments. Expressive lows, directly associated, control the influence of clastic input and supply sufficient volume of water for the coquinas storms generation over the highs.

Regional erosional events related to the pre-Aptian unconformity stripped the top of some structural highs creating seismic truncations and canyons, however preserving the coquinas layers on the flanks. The pre-Aptian unconformity could have been the result of the superposition of many erosive events along the evolutionary history of the Coquinas Sequence, as well as tectonic uplift.

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