Discrimination of effective from ineffective porosity in heterogeneous Cretaceous carbonates, Al Ghubar field, Oman

Langhorne B. Smith, Gregor P. Eberli, Jose Luis Masaferro, Salah Al-Dhahab

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

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 skeletalpelletal 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.

Original languageEnglish (US)
Pages (from-to)1509-1529
Number of pages21
JournalAmerican Association of Petroleum Geologists Bulletin
Volume87
Issue number9
DOIs
StatePublished - Sep 2003

ASJC Scopus subject areas

  • Fuel Technology
  • Energy Engineering and Power Technology
  • Geology
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)

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