Well logs and cores are used to define formation thickness, porosity, and water saturation. These data are required to design well completions and to prepare workover and reservoir management plans. The presence of shale (clay) in the pay zone complicates log interpretation. This paper presents a methodology utilizing a neural network to reconcile the differences between density log effective porosity and plug core total porosity values in a shaly, laminated Cretaceous sandstone.

Core plug measurements are typically taken in the better pay intervals and generally do not include the shale laminations evident in core photographs. Log measurements do include the heterogeneity that is evident in core photographs, but the log measurements represent an average of a 6-to-36-in. section that does not reflect the small scale variability. A multivariable correlating technique was used to associate gamma ray, density, and shallow and deep resistivity logs with core plug porosity measurements.

Core water saturation values were used to tune a shaly sand water saturation model. The sw model was used to choose between plug core porosity, density log porosity, or neural network porosity as the preferred measurement to use with induction log resistivity to estimate water saturation. The porosity and water saturation values were then used to construct bulk volume oil and bulk volume water logs that were combined to identify productive zones.

A case history format is used to explain the approach developed to correlate the four dependent variables measured by the logs with the core porosity and water saturation measurements. The subject field is an 80-well Gallup pool located on the southwest flank of the San Juan Basin of northwest New Mexico.

The high clay content of the Muddy Formation in the Powder River Basin is known for contributing to errors in water saturation estimates. If modern logs capable of resolving thin beds are not available, this methodology can be applied to the Muddy, the Gallup and other Cretaceous reservoirs in the Rockies that contain thin shale laminations.

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