FACIES AND POROSITY CONTROL IN A DEEPWATER RESERVOIR OUTCROP ANALOG, JACKFORK GROUP, OKLAHOMA

The importance of understanding depositional and post-depositional geologic controls on porosity and permeability is very critical for predicting and understanding the production performance of reservoirs. Thus, characterizing outcrops of deepwater sandstones in the Pennsylvanian Jackfork Group in Southeast Oklahoma may be useful to understanding production performance in deepwater reservoirs

The Pennsylvanian Jackfork Group provides classic outcrop exposures of deepwater turbidites deposited in an elongate east-west trending Pennsylvanian basin. The sandstones in the study area are predominantly well-cemented, fractured, tabular-bedded, quartz arenites. However, extremely friable sandstones occur stratigraphically between well-cemented sandstone beds in certain outcrops.

Three stratigraphic sections have been measured with corresponding outcrop gamma-ray logs. The stratigraphy consists of multiple Depositional Sequences and Systems Tract deposits, which aid in characterizing the distribution of facies in the sections. The different sandstones are representative of differing porosity and facies types. In a reservoir, porosity in similar cemented sandstones is due to fracture, while in similar friable sandstones, porosity is matrix controlled.

Petrographic studies of the sandstones show differing diagenetic features of quartz overgrowth, pressure solution and clay content between the two sandstone types. Though distinguishable in outcrop, results from the outcrop gamma ray log show that it is difficult to impossible to distinguish the zones containing matrix porosity from those containing fracture porosity. This could be the case in other deepwater reservoirs and may have direct implications for on-going and future exploration in the Ouachita Mountains of Southeast Oklahoma.