--> ABSTRACT: Evaluation of Heterogeneous Porosity in Reservoirs on Arabian Platform: Microns to Kilometers, by R. Nurmi, M. Charara, and M. Waterhouse; #91030 (2010)

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Evaluation of Heterogeneous Porosity in Reservoirs on Arabian Platform: Microns to Kilometers

R. Nurmi, M. Charara, M. Waterhouse

A common heterogeneity is the occurrence of patchy porosity, which may occur at any scale of examination. The occurrence of patchy porosity at a scale of kilometers is a factor controlling the areal distribution of reservoirs. Although often related to depositional facies, in some areas this distribution has not been adequately explained. The three-dimensional mapping of such large-scale porosity has been carried out by well mapping and/or seismic velocity mapping.

The presence of meter-scale patchy porosity is routinely detected in Cretaceous-age reservoirs using borehole electrical imagery. Although the patterns of patchy porosity are revealed in these images, porosity variation is only shown in relative terms. It is possible, however, to use high-resolution porosity measurements to quantify these variations, which is especially important when there is a continuous network of high porosity. This type of heterogeneity sometimes erroneously causes indications of secondary porosity. Although the origin of these porosity variations is not well understood, SEM examination of some low-porosity areas indicates that they are the result of an abundance of micron-scale patchy recrystallization.

Patchy distribution of anhydrite is a common form of reservoir heterogeneity in Jurassic and Tertiary reservoirs. Electrical images clearly reveal the various fabrics of anhydrite within carbonate and sandstone reservoirs. Although this type of heterogeneity has been long recognized by geologists, it is only now being included in thorough log evaluation efforts. Unless such heterogeneously distributed anhydrite is recognized and accounted for, the evaluation of these reservoir zones can be inappropriately pessimistic or falsely indicative of hydrocarbons.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.