Can seismic velocities predict sweet spots in the Woodford Shale? A case study from McNeff 2–28 Well, Grady County, Oklahoma

• Stiff sand model is tested with up-scaled dipole sonic log from Woodford Shale
• We find that the porosity and composition can be inferred using elastic velocities
• Model suggest that organic matter is a part of pore fluid rather than rock matrix
• Increase in organic matter decreases the effective (brine and gas filled) porosity

In shale, predicting sweet spots (brittle, organic-rich, and hydrocarbon-filled porous zones) ahead of the drill bit using non-intrusive methods such as seismic has been a long-standing challenge. In principle, rock properties can be inferred from P- and S-wave velocities with an appropriate rock physics model, which is a way of expressing the elastic moduli as a function of attributes such as porosity (ϕ), mineralogy and pore-fluid type and saturation. Using high-fidelity logs from McNeff 2–28 well, Grady County, Oklahoma, we demonstrate that ϕ and composition of the Woodford Shale can be inferred from dipole sonic log using the stiff-sand model. The stiff-sand model takes ϕ and composition as input and, in conjunction with Gassman's substitution, outputs elastic velocities. We find that the up-scaled McNeff 2–28 log velocities can be closely predicted by two compositional end-member input scenarios differing in location of organic matter (OM). The first scenario comprises 0–2.5% OM, 65–84% Quartz and 0% Calcite in matrix and 30–34% gas in pore-fluid. The second scenario comprises 76–20% Quartz and 1.5–3.9% Calcite in matrix and 16–35% gas and 10–40% OM in pore-fluid. In both compositional scenarios, the remainder in matrix is Illite and in pore-fluid is brine. While the input ϕ in both scenarios is close to the density-porosity (ρϕ) log, the input density (ρ) is closer to the ρ log in the second scenario. The second scenario also gives rise to the concept of effective ϕ (total ϕ − ΟΜ) which pertains to the proportion occupied by mobile components such as gas and brine, and is up to 40% lower than the total ϕ. We conclude that from a modeling perspective in the Woodford a) OM should be a part of pore fluid rather than the rock matrix, and b) realistic ϕ and composition can be inferred from the stiff-sand model. Determining a rock physics model for the Woodford enables an examination of various what-if scenarios by consistently changing the inputs and computing elastic velocities which may eventually help in creating a field guide to quantitative interpretation of the field seismic data.

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