Accurate reserve volume determination is crucial in the early stages of a project since planned subsurface capacity is dependent on reserve expectations. The fundamental method of calculating reserves uses bulk formation resistivity and bulk porosity to determine water saturation. This approach cannot accurately quantify reserves in laminated sand-shale sequences where the sensor resolution is insufficient to characterize the fine laminae. A tensor petrophysical model can determine laminar shale volume and laminar sand-fraction conductivities reducing the problem to a single dispersed shaly sand model. Combining this with sand-fraction porosities can lead to accurate reserve quantification.

Identification and quantification of hydrocarbons within low-contrast, low-resistivity formations can be difficult when using conventional log data. This is primarily due to the presence of laminar shale and the inherent vertical resolution of measurements acquired by wireline and logging while drilling (LWD).

A Gulf of Mexico deepwater example is used to demonstrate this novel approach in quantifying hydrocarbons in laminated sand-shale sequences. Real-time shear slowness is used in conjunction with LWD triple-combo data to identify potentially productive low-contrast reservoirs. Then, advanced resistivity post processing extracts the vertical component of resistivity, enabling calculation of sand-fraction resistivity. Sand-fraction resistivity, combined with normalized sand-fraction porosity, yields sand-fraction water saturation. Shale volume, porosity and water saturation cut-offs determine the net hydrocarbon volume. The LWD-calculated hydrocarbon volumes in place are then compared to results obtained from a wireline logging suite.

This approach demonstrates that the use of conventional empirically derived bulk-volume porosity and saturation methods in laminated sand-shale sequence formations results in underestimation of the reservoir producibility and hydrocarbon reserves. Vertical resistivity, derived from LWD-acquired propagation resistivity and electrical anisotropy sensitivity, can be used to quantify reserves in these environments.

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