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The AAPG/Datapages Combined Publications Database

West Texas Geological Society

Abstract

Resource Plays in the Permian Basin: Resources to Reserves, 2006
Pages 1-8

Modeling the DEW Plot [Rt/Rw versus Rxo/Rmf] in Vuggy Reservoirs using the Maxwell-Garnett Equation and the Lucia (1995) Petrophysical Classes

George B. Asquith

Abstract

DEW Plots (Rt/Rw versus Rxo/Rmf) for vuggy carbonates were modeled by calculating Rt using the Maxwell-Garnett Equation and matrix water saturations either fixed (Sw = 0.25) or based on the Lucia (1995) petrophysical classes with Rw = 0.03, Sw(vug) = 0.0 and Rxo/Rmf = (Rt/Rw)/10, and where Φ = porosity. The following examples were run:

  1. Φ(matrix) = 0.12, Sw(matrix) = 0.25, Φ(vug) = 0.0 - 0.20, Sw(vug) = 0.0

  2. Φ(matrix) = 0.02 - 0.12, Sw(matrix) = 0.25, Φ(vug) = 0.0 - 0.20, Sw(vug) = 0.0

  3. Φ(matrix) = 0.12, Sw(matrix) = Lucia (1995) CLASS 1, 2 and 3, Φ(vug) = 0.0 - 0.20, Sw(vug) = 0.0

  4. Φ(matrix) = 0.02 - 0.12, Sw(matrix) = Lucia (1995) CLASS 1, 2 and 3, Φ(vug) = 0.0 - 0.20, Sw(vug) = 0.0

The DEW Plots from these four examples revealed the following:

  1. Resistivity logs are relatively insensitive to the presence of vuggy porosity.

  2. Rt/Rw values ranged from 450 to 55,000.

  3. The magnitude of Rt/Rw is dependent on the amount of matrix porosity relative to the amount of vuggy porosity and the grain or crystal size of the matrix porosity (decrease in matrix porosity greater Rt/Rw and increase in grain or crystal size the greater Rt/Rw).

  4. Rt/Rw greater than 4,444 (BVW < 0.015) indicates vugs are the dominant pore type with a degree of porosity isolation [Separate Vug Porosity (SVP)].

A DEW Plot from the Siluro-Devonian Hunton vuggy dolostone in the Texas Panhandle with a 82 foot thick (16,304’ – 16,386’) porosity zone, exhibited Rt/Rw trends (600 – 29,000) similar to the examples indicating a complex vuggy reservoir with a mixture of vuggy and matrix porosity. The upper 21 feet of the Hunton porosity zone exhibited Rt/Rw values greater than 10,000 indicating a predominantly vuggy reservoir with some separate vug porosity (SVP). Below the upper 21 feet the resistivity decreased from 400 ohm-m to 30 ohm-m. Due to the decrease in resistivity the operator only perforated in the top 21 feet which had a CAOF of 67 MMCFGPD and no water. However, the DEW Plot revealed moveable hydrocarbons (Sw/Sxo < 0.6) over the entire 82 feet. Thus the decrease in resistivity (i.e. lower Rt/Rw) is due to a textural change not potential water production (i.e. increase in matrix porosity and/or decrease in crystal size).

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