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The producing sandstone of Riacho da Barra field represents the middle and distal portions of uplap sublacustrine gravitational fans, deposited in a northeast-southwest elongate graben developed during the Early Cretaceous on the northeastern part of the Reconcavo rift basin, Brazil.
Since the earliest stages of exploitation of the field, geologists and engineers have worked together to describe the reservoirs. A geologic and hydrologic model for the Riacho da Barra field was proposed, with emphasis on the lateral continuity of the reservoirs, which was mainly controlled by pressure-gradient correlations. This model was created to guide not only the development of the field, but also to define the possible use of waterflooding as a secondary recovery method.
Two main reservoir sets were identified. The first group corresponds to medium-grained, well-sorted, massive sandstones, with centimetric conglomerate levels, deposited in channels in the middle of gravitational fans. This group represents the best reservoirs, with an average porosity of 16% and average permeability of 100 md, but restricted lateral continuity. Major trends of channel deposits are the most favorable directions for waterflooding.
The second group includes a cyclic sequence of coarse-grained massive sandstones and medium-grained parallel-stratified sandstones, deposited as lobes of middle and distal fans. These sandstones have a wider distribution and contain 85% of the original oil in place of the field (50 million stock tank bbl). However, they have poorer reservoir quality, with average porosity of 12% and average permeability of 20 md. These characteristics are due to the significantly large thickness of poorly sorted parallel-stratified sandstones and also to the thin sandstone beds interlayered with shales, which show high contents of calcite cement.
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