Petrophysical discrimination of Seven Rivers Formation reservoir zones using natural (spectral) gamma ray logs, Grayburg Jackson Pool, Eddy County, New Mexico

By

Brian S. Brister, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM

The shallow Seven Rivers Formation has been a volumetrically minor contributor to Permian oil and gas production from southeastern New Mexico. It tends to contain thin reservoir zones that are difficult to evaluate petrophysically. In fact, most discoveries date to the era of cable-tool drilling when subtle shows were more apparent. It is rarely evaluated today in wells drilled for deeper targets. Thus, it has the potential to be an overlooked, behind-pipe pay zone in thousands of existing wells.

At Grayburg Jackson Pool (formerly Fren Pool), the upper part of the formation consists primarily of anhydrite with thin beds (< 4 feet) of dolomitized packstone to grainstone oil reservoir, non-reservoir dolomitized algal boundstone-mudstone, and red mudstone (shale), all deposited in a sabkha environment. Porosity of the grainstone facies ranges up to 28.5% and it is uranium enriched with corresponding high gamma ray activity. Due to common drilling and logging problems in the field related to water flows and poor borehole stability, it is often difficult to successfully acquire open-hole logs. Common cased-hole gamma ray-neutron logs do not allow discrimination between the dolomite reservoir and shale, which have nearly identical log characteristics.

A core-based petrologic study of the upper Seven Rivers Formation was used to calibrate log curves in a modern open-hole log suite that included the natural (spectral) gamma ray log. Simple methods of comparing the relative concentrations of U, K, and Th from the gamma ray log, combined with a porosity log, discriminates reservoir zones. The combination is also effective in the cased-hole environment and is a low-cost tool for finding otherwise overlooked, subtle, behind-pipe pay at Grayburg Jackson, and presumably, other fields in the Seven Rivers play. Such logs should also be useful in evaluating other radioactive reservoirs zones in the region such as in the Grayburg and Yates Formations.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003