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Abstract: Stochastic Modeling of Calcite Concretions in Cretaceous Turbiditic Sandstones of the Albacora Oil-field, Campos Basin, Brazil

Sombra, C. L.; A. B. Barreto Jr.; R. K. Romeu and W. L. Lanzarini - Petrobras/ Cenpes; Lopes, H.; S. Pesco and G. Tavares - Puc-Rio / MatComp

Vertical cores from the Namorado Sandstone display a great number of elongate calcite concretions, tabular to sub-tabular, 10 to 15 cm thick, that parallel stratification. The vertical frequency of occurrence is one concretion per meter, arranged in a cyclic pattern governed by framework composition (bioclast-rich levels). The Namorado Sandstone is about 100 m thick in this area, composed by 85% fine to very fine grained massive sand (turbidites), 15% lime mudstone, marlstone, diamictite and shale.

Horizontal cores also cut concretions, and aggregates of concretions are up to 6 m long, but it was not observed periodicity. Lengths are the greatest uncertainty in concretion geometry: horizontal well cores are sparse and are in many cases tangential to concretion borders. The sandstone grain-size is very uniform (fine to very fine sand), and the main control on permeability is the amount of calcite.

Besides vertical and horizontal well core data, published outcrop descriptions from other reservoirs were used to construct a conceptual model for the concretions, since there are no outcrops from the Namorado Sandstone.

After having built a conceptual model, small scale 3-D models (25 m x 25 m x 2 m) were generated using a Boolean simulation technique. Horizontally elongate ellipsoids were simulated without any repulsion criteria, and simulated concretions were allowed to interpenetrate. Simulations honored global proportions of calcite, as determined from core data, and vertical periodicity was imposed on vertical proportion curves. Concretion thicknesses were set to 10 cm, and length histograms varied at each model. Figure 1 is one image generated. These models are representative of portions of the reservoir with abundant calcite concretions and where muddy interbeddings are absent.

Flow properties of these small scale 3-D models - average porosity, average vertical and horizontal permeability, capillary pressure and relative permeability curves - were determined using upscaling techniques.

Vertical/horizontal permeability anisotropy (Kv/Kh) results from the small scale models were consistent with anisotropy interpreted from horizontal well tests. The models that best fit the Kv/Kh ratio from well tests were the ones constructed with small concretions, one to two meters long.

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