Net pay identification is of critical importance in evaluating the potential of an exploration or development prospect. In the carbonate formations of Permian Basin, traps are formed due to combination of structural and stratigraphic features which can exhibit differing fluid contacts. Further hindering development and economic evaluation in such areas are the presence of long transition zones and residual oil zones.

Identification of oil water contact and free water level from conventional log/core derived saturation is unreliable due to the heterogenous nature of the rock, mixed wettability, and hydraulic forces. Modeling of fluid profiles by static pressure gradient and saturation height models are made difficult by the lack of RFT data and imbibition capillary pressure curves.

This paper introduces a sonic porosity and resistivity derived bulk volume water overlay for the identification of top of residual oil zones and producing oil water contact. This estimate is verified by gas chromatograph data from mud logs and core-tube gas analysis. Comparison of gas data and shows in cutting and core clearly indicate a depleted/residual oil zone in close agreement with the overlay technique. Gas analysis also indicates the change in quality of the oil in the residual column and its producibility.

The analytical techniques presented are resilient to uncertainties in data such as water resistivity, porosity and mineralogy. The techniques can be applied on a well to well without the need to pool data across the field or engaging in characterization techniques like rock typing.

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