Abstract

Reservoir flow units are typically defined by combining core descriptions with sequence stratigraphic concepts to understand depositional cycles. This framework can then be incorporated into an engineering model in order to design, implement, or monitor an advanced recovery program. As most fields do not have extensive core coverage, some other method must be used to estimate reservoir properties in uncored wells. Log data is a popular and fairly reliable method by which to define those properties. However, log data is not without pitfalls. Like all geophysical methods, log data must be interpreted. It represents a range of possibilities rather than a single unique solution.

We have examined 1500 feet of core in the Grayburg and San Andres formations of Vacuum Field in Lea County, New Mexico, and attempted to correlate core features with log responses. We have noted some high gamma ray zones that would typically be interpreted as depositional sandstones, but are NOT. Instead, the gamma ray response is caused by sand-filled fractures or by slightly sandy/silty tidal flat caps. It is important to be able to identify these karst and tidal flat zones during geologic model construction. In Vacuum field, these zones represent vertical flow barriers, with very different lateral distribution than bedded sands. It is possible to use modern log suites to separate depositional sands from other features, an important step in defining the geometry and flow properties of a geologic model.