Langhorne B. Smith¹, Gregor P. Eberli¹, Jose Luis Massaferro², Jan-Henk van Konijnenburg³, Hildegard Westphal⁴
(1) University of Miami, Miami, FL
(2) Shell RTS, Rijswijk, Netherlands
(3) Shell RTS, Rijsvijk, Netherlands
(4) Universität Hannover, Hannover, Germany

Abstract: Not all porosity is good porosity: evaluation of heterogeneous Upper Cretaceous carbonates, Al Ghubar Field, Oman

Neutron, density and sonic porosity logs show porosity values of more than 30% in most of the Cretaceous (Cenomanian) Natih E heavy oil reservoir at Al-Ghubar Field, Oman. These high porosity values led to initial reserves estimates that are higher than current production trends suggest will ultimately be recovered. This discrepancy is likely due to the occurrence of abundant ineffective porosity that can only be discriminated from effective porosity using resistivity logs.

The reservoir is composed of a hierarchy of cycles that typically have burrowed wackestone bases and skeletal-pelletal grainstone caps. Four important types of porosity were identified: interparticle porosity in the grainstones and within burrow fills, and microporosity, intra-skeletal, and moldic porosity in the wackestone facies. Of these, only the interparticle porosity within the grainstone is likely to be effective. The pore throats between the micropores, intra-skeletal pores and isolated molds are too small to produce heavy oil rendering this porosity ineffective. The porous burrow fills do not appear to be connected well enough to make an effective reservoir rock.

Almost all of the high-permeability grainstone units had resistivity over 100 ohm/m and separation of more than 10 ohm/m between the medium and deep induction curves. There was little to no separation between the induction curves over the burrowed wackestone facies, suggesting that this facies has poor bulk permeability.

In carbonate reservoirs with abundant microporosity and multiple porosity types, resistivity logs may be a much better tool for assessing effective porosity than any combination of porosity logs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana