Interpretation Of Depositional Facies And Reservoir Rock Quality From Borehole, Logs, San Andres Dolomite, New Mexico

Abstract

The Permian San Andres Formation in three cored wells in the Twin Lakes and Cato fields, New Mexico is composed of five vertically stacked tidal flat sequences. An individual tidal fiat sequence consists of evaporite (anhydrite) at the top, followed by supratidal (dolomite, anhydrite and mixtures of anhydrite and dolomite) and subtidal (dolomite or limestone) at the bottom. The core study shows that dolomite and anhydritic dolomite are the reservoir rocks. Marine and some intertidal dolomites are the most porous and permeable. Porosity is intercrystalline with or without vugs. Comparison of lithology, porosity, Archie porosity types in the cores with sonic, density, neutron and gamma ray borehole logs shows that lithology, vuggy porosity and tidal flat sequence may be determined from log response alone in the San Andres rocks. Vuggy porosity can be estimated from the reponses of the sonic log compared with that of the density or neutron log. Vuggy dolomites will have an Archie m (cementation exponent) value higher than 2. Calculation of water saturation will be too low and oil and gas calculations too high if the standard value of m = 2 is used in calculations in vuggy intervals.