Abstract: Effects of Depositional Facies and Diagenesis on Calculating Petrophysical Properties from Wireline Logs in Permian Carbonate Reservoirs of West Texas

Mark H. Holtz, R. P. Major

The complex interplay between depositional facies and diagenesis in carbonate rocks presents numerous problems for calculating petrophysical properties from wireline logs. If carbonate reservoirs are divided into flow units of similar depositional and diagenetic textures, empirical equations that apply specifically to that geologically identified flow unit can be developed to accurately measure porosity and water saturation.

In Guadalupian and Leonardian reservoirs, carbonate mudstones deposited in subtidal marine settings are predominantly dolomite, although they commonly contain some shale. The presence of shale in these rocks can be detected with gamma-ray logs, and empirical equations for calculation of porosity from logs must include a gamma-ray component to compensate for the presence of shale. Because porosity in these rocks is dominantly intercrystalline, capillary pressure characteristics are predictable and saturations can be calculated with the Archie equation.

Subtidal carbonate packstones and grainstones are composed of dolomite, anhydrite, and gypsum. The matrix acoustic transit times of these three minerals are similar, and acoustic logs are the best tool for measuring porosity. Neutron logs are the least accurate porosity tools if gypsum is present. Photo-electric density logs can distinguish gypsum from anhydrite. Because porosity in these rocks is dominantly interparticle and/or moldic, dual porosity cementation exponent corrections are needed to calculate saturations with the Archie equation, and capillary pressure saturation relationships are variable.

Carbonates deposited in tidal-flat environments are generally composed of dolomite, sulfate minerals, and quartz silt. This more complex mineralogy requires a full suite of open-hole logs to make reliable porosity measurements. Porosity is commonly vuggy, and vugs are poorly connected. Thus the Archie equation may not accurately calculate saturations, and capillary pressure characteristics are unpredictable.

Diagenesis influences reservoir mineralogy and pore types. A

common style of burial diagenesis in Guadalupian and Leonardian reservoirs is hydration of anhydrite to gypsum and leaching of sulfate cement and dolomite matrix. This results in a significant amount of moldic (vuggy) porosity, although commonly these moldic pores are sufficiently well connected that saturations can be reliably calculated with the Archie equation and capillary pressure characteristics are predictable.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994