--> Abstract: Stratigraphy and Reservoir Characteristics of the Albian Limestones in Santos Basin, Brazil, by M. F. B. Moraes, M. R. Rodriguez, J. J. G. Rodrigues, and E. Duraes; #90933 (1998).

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Abstract: Stratigraphy and Reservoir Characteristics of the Albian Limestones in Santos Basin, Brazil

Moraes, M.F.B.; Rodriguez, M.R.; Rodrigues, J.J.G.; Duraes, E. - Petrobras/E&P

The Albian carbonates of Santos basin exceed 1,000m in thickness. Three tectono-sedimentary episodes were recognized in this carbonate package: a) the lowermost one overlies conformably the Aptian evaporites, defining a very gentle ramp; b) the intermediate one is strongly influenced by the halokinesis; c) in the last episode, less affected by salt movement, a broad shallow carbonate shelf was established. The first episode is seismically characterized by parallel reflectors, as a result of a weak salt movement and strong thermal subsidence. The depositional morphology is interpreted as a low energy gentle ramp, represented dominantly by fine-grained limestones. The intermediate episode, is characterized by truncations and onlaps related to the morphology generated by halokinesis. A progressive shallowing is marked by the passage from deep-water facies at the base to oncolitic/oolitic facies at the top. In the last episode, an agradational pattern was established, and a broad shallow carbonate shelf was installed. Seismically, this episode is characterized by more continuous and high amplitudes reflections. The depositional pattern is the typical shoaling-upward cycles, composed at the base by peloidal-oncolitic wackestones/packstones and at the top by high energy grainstones, formed by ooids and associated oncolites and peloids. These cycles reflect sea-level oscillations driven by climatic changes.

As a result of a regional deep burial (more than 4,500m), the fine-grained and the mud-bearing rocks were strongly compacted (physical and chemical compaction), resulting in very tight rocks. On the other hand, the grainstones/packstones, because of their strength, were less compacted with part of their original porosity preserved; modification on their original permo-porosity features were dictated mostly by the meteoric diagenesis.

At present, four Albian carbonate zones were recognized in the Santos basin. Stratigraphically these zones show lateral continuity and a high vertical compartmentalization. The reservoirs are formed by the higher energy facies, at the upper part of shoaling-upward cycles. The reservoirs are formed by oolitic/oncolitic grainstones and associated packstones, with a minor amount of peloids. Their quality depends largely on the diagenetic history. The upper zone was submitted to an intense meteoric activity, which obliterated the original intergranular porosity through grain dissolution and generation of micropores, resulting in a porous but poorly permeable reservoir (less than 1mD). In the other zones, the meteoric diagenesis are much less dramatic, which allowed the preservation of intergranular porosity, producing a domain of macropores, with high permeability values (up to 1,000mD). The general seismic attributes of the Albian carbonates allow the recognition and mapping of the porous reservoirs through seismic velocity and amplitude maps.

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