Abstract

The Natih E heavy-oil reservoir (21° API) at Al Ghubar field, Oman has produced less than 5% of the calculated oil in place. Porosity logs used to calculate reserves show high porosity throughout the reservoir, but further analysis of the only continuous core taken from the field indicates that much of the porosity is ineffective.

There are four heavily oil-stained, high-permeability skeletal-pelletal grainstone units with interparticle porosity in the core that probably contributed most of the production. The four permeable grainstone units occur at the top of small-scale accommodation cycles that have wackestone and packstone bases. These grainstones make up about 20% of the total thickness of the porous Natih E reservoir. The other 80% is composed of packstone and wackestone with ineffective microporosity, interparticle porosity in burrows, and isolated moldic and intraskeletal porosity. The small-scale reservoir-bearing cycles can be correlated across the field using the separation between the medium and deep induction curves as a guide.

Resistivity logs are the most reliable tool to distinguish effective from ineffective porosity. Most high-permeability grainstone units have deep induction values more than 100 ohm m and separation of more than 10 ohm m between the medium and deep induction curves. The ineffective intervals with microporosity, burrow porosity, and moldic porosity have lower resistivity and little separation between the medium and deep induction curves.

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