SPECTRAL GAMMA RAY RESPONSE IN A MIXED CARBONATE-SILICICLASTIC CHANNEL SYSTEM, FORT PAYNE (MIDDLE MISSISSIPPIAN), TENNESSEE

Oil and gas exploration in north-central Tennessee and south-central Kentucky is primarily based on stratigraphic plays including mixed carbonate-siliciclastic submarine channels and Waulsortian-type mounds. Identifying these features in downhole logs is challenging due to similar responses in gamma ray, density, and resistivity measurements. To better constrain the response of different lithologies in gamma ray logs, this study focused on using a hand-held gamma ray scintillometer to characterize rocks types in a siliciclastic and carbonate-filled submarine channel in the Fort Payne Formation.

The research was conducted on an outcrop on TN Highway 52 (36°30’11.97”N, 85°26’49.62”W), three miles east of Celina, Tennessee. Here, the Fort Payne Formation is characterized by interbedded crinoidal packstones, grainstones, and siliciclastic shales. These lithologies fill a submarine channel incised into micritic and siliciclastic mudstone.

Scintillometer measurements were collected on a 10x10m grid at the base of the channel. A total of 100 data points were collected at 20 cm increments, comparable to downhole wireline log spacing but including a lateral as well as vertical (grid) component. A Gamma Surveyor II was used to measure potassium (%), thorium (ppm), uranium (ppm) and a general measurement of gamma radiation dose rate. Preliminary results suggest that the margins of submarine channels in the Fort Payne Formation are only resolvable where channel fill includes a siliciclastic shale component. In cases where channels incise similar lithologies it is not possible to use wireline logs to resolve submarine architectural elements.