Cenozoic Structural Framework and Tectono-Sedimentary Evolution of the Southern Nenana Basin, Alaska: Implications for Conventional and Unconventional Hydrocarbon Exploration

Abstract

The Tertiary Nenana basin of Interior Alaska is a petroleum frontier basin, and is currently the focus of both new oil exploration and coalbed methane exploitation. The basin holds large volumes of subsurface, oil and gas-prone Late Paleocene sub-bituminous coals that have an excellent potential for the geologic sequestration of CO₂. The distribution of sediments and structural features of the southern Nenana basin record four distinct episodes of regional deformation. The coeval normal-slip motion across the northeast-southwest trending master faults caused by northwest-southeast extension suggests a pull-apart type opening of the basin in the Late Paleocene. The Late Paleocene extensional rift episode resulted in the deposition of coal-bearing non-marine sediments with excellent source potential (kerogen II and III). Basin inversion from the Early Eocene to the Early Oligocene resulted in reverse faulting and folding of higher stratigraphic levels, marked by pinch-out structures and stratigraphic unconformities. During the later episode of active rifting from the Middle Oligocene to Late Miocene, a progressive migration of active faulting towards the western margin of the basin caused a widening of the area of extension, as well as the widespread deposition of reservoir and seal rocks of the Usibelli Group. Onset of rapid tectonic subsidence in the Pliocene that continues to the present-day has resulted in the development of the principle half-graben that is accommodated by oblique-slip motion along the northeast-southwest striking faults to the east. This episode of rapid tectonic subsidence has provided significant pressure and temperature gradients for the source rocks. Apatite fission-track and vitrinite reflectance data suggest that the basin has experienced higher paleotemperatures associated with two major paleo-thermal episodes: Late Paleocene to Early Eocene (60 Ma to 54.8 Ma) and Late Miocene to present-day (7 Ma to present). During these episodes of maximum paleotemperatures, petroleum source rocks of the Late Paleocene were exposed to peak temperatures greater than 90°C, corresponding to the oil and gas windows. In this study, we are also investigating the effect of CO₂ injection in the Late Paleocene coals for the recovery and production of coal bed methane. Our preliminary analyses demonstrate that 150 MMSCF of methane could be produced, while 33000 tonnes of CO₂ per injection well (base case of ~9 years) can be sequestered in the vicinity of the existing infrastructure. However, these volumes of sequestered CO₂ and coal bed methane recovery are estimates, and they are sensitive to the reservoir's geomechanical and flow properties.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015