Reports: UNI856080-UNI8: Mixed Carbonate-Siliclastic Sediment Gravity Flows: Depositional Processes in the Mississippian Fort Payne Formation, Tennessee

Jeannette M. Wolak, PhD, Tennessee Technological University

Importance of the Work

Located in central Tennessee and southern Kentucky, oil and gas production in the Mississippian Fort Payne Formation began in the middle 1800s and continues to present day. According to the Tennessee Department of Environment and Conservation (TDEC), more than 2,440 wells have targeted the Fort Payne interval since 1969. The general play style is stratigraphic and characterized by facies changes between carbonate bioherms and siliciclastic siltstones and shales. The primary purpose of this project is to: (a) define sedimentological processes operating during deposition of the Fort Payne Formation, and (b) characterize resulting geometries. Application of these results will impact current exploration in mixed carbonate-siliciclastic systems, particularly in calciclastic submarine fans (CSFs).

Research Hypotheses

We posit that deposition in the study area (TN and KY) was dominated by carbonate-laden sediment gravity flows (SGFs), similar to those classically defined for siliciclastic deepwater systems, e.g. turbidite sequences and debris flow deposits. If so, it follows that carbonate allochems observed in the Fort Payne are detritus transported down the slope in submarine channels, and alternating layers of siliciclastic and carbonate mudstones record periods of increased SGF activity and quiescence, respectively. At the stratigraphic scale, we expect that architectures in the Fort Payne should be predictably organized, both in a vertical stratigraphic order and from axis to margin within individual channels. Results from sedimentological inquiry and stratigraphic inquiry may be combined to demonstrate that the Fort Payne is a Mississippian-aged calciclastic submarine fan (CSF) system, a conclusion that will drive future exploration strategies.

Methods and Initial Results

Our work is divided into two concurrent study objectives: (1) sedimentological characterization of mixed carbonate-clastic architectures in the Fort Payne Formation; and (2) prediction of geometries in the Fort Payne using wireline log signatures. To accomplish both goals, our research team identified a series of outcrops in northern Tennessee and southern Kentucky where thick intervals of the Fort Payne are exposed and stratigraphic associations are clear, i.e. contacts with the underlying Maury Shale and Chattanooga Shale are mappable. Initial field work on these outcrops focused on measuring stratigraphic sections and identifying sedimentological facies in the mixed lithology system. One significant result of this effort is the description of seven lithofacies in the Fort Payne and linkage of these facies to sedimentological processes operating during deposition (part of study objective 1, above). In general, we are able to conclude that deposition in the mixed carbonate-siliciclastic system records both turbulent and laminar flow styles, and carbonate fragments such as crinoid stalks and skeletal debris were transported some distance down the continental slope.

The second project objective focuses on characterizing geometries of sedimentary bodies in the Fort Payne and linking those shapes to wireline log signatures to aid in subsurface interpretation. With respect to the former, our team has identified archetypal sedimentary body styles in outcrop: submarine channels, overbanks (levees), lobes and carbonate mounds. While most of these are fairly common in siliciclastic systems, the bioherms of the Fort Payne are particularly interesting because they record a mix of skeletal debris in a clastic mudstone matrix, a result that will impact porosity and permeability in reservoirs adjacent to these features. To characterize what each geometry may look like in the subsurface, we have acquired a handheld gamma ray spectrometer/scintillometer, the Gamma Surveyor II (GMS II). This instrument measures total gamma dose rate as well as assays of potassium (%), uranium (ppm) and thorium (ppm). Our most recent field work incorporates the GMS II, and we are presently determining how gamma radiation values change within submarine channels and carbonate mounds.

Continuing Work

With respect to sedimentological study, the next step in this research is petrographic analysis of the Fort Payne lithofacies. Recrystallization is evident in outcrop and hand sample, which suggests diagenetic effects have overprinted at least some primary sedimentary structures. Current field work is focused on collecting an appropriate suite of facies samples for thin section analysis and petrophotography in 2017-2018.

Finally, field investigations of Fort Payne geometries using the handheld spectrometer/scintillometer will be extended to include overbank deposits and lobes, thus encompassing each type of geometry observed in the study area. We anticipate that these bodies will prove more challenging to characterize because silicilastic and carbonate beds are thinner (cm-scale) and likely below instrument resolution. With the completion of outcrop gamma ray analysis, we will extend our results to signatures observed in subsurface wireline logs in Tennessee and Kentucky.

Research Team and Dissemination of Research

To date, this funding has supported six research projects for undergraduate geoscience students at Tennessee Tech University (TTU): H. Blaylock, C. Clay, K. Hillis, S. Huskey, D. Koehl and G. Winkle. Students Huskey and Koehl assisted with characterizing lithofacies of the Fort Payne Formation and presented their results at the Southeastern Geological Society of America (SE GSA) conference in 2017. Following graduation in May of 2017, Huskey and Koehl are graduate students (MS) at Bowling Green State University and Wright State University, respectively. Students Clay and Hillis used a 10 ft. recirculating flume to investigate alignment and deposition of crinoid skeletal fragments in the Fort Payne. Both students attended the 2017 SE GSA meeting, and they are currently applying for graduate programs. Most recently (summer 2017), undergraduates Blaylock and Winkle completed field work using the GMS II to characterize gamma ray signatures in the Fort Payne. An abstract summarizing results has been accepted for presentation at the 2017 National Geological Society of America meeting in Seattle, Washington. In addition to regional and national meetings, Clay, Hillis, Huskey and Koehl presented their research at the 2017 TTU Student Research Day; students Clay and Hillis received a Best Student Poster Award. Finally, four of the six students have received senior thesis credit for their research, a required component of the geoscience curriculum at TTU.

Additional Support

To aid with conference travel costs, all six undergraduate students were awarded travel grants from the TTU Undergraduate Research and Creative Activity (UReCA!) program. Additionally, Blaylock received an On to the Future Award from GSA to attend the upcoming conference in Seattle.