Integrated Approach to the Identification of Middle Atoka Sediment Gravity Flows in Red Oak Field, Arkoma Basin (SE Oklahoma): Methods to Aid in Exploitation of Similar Siliciclastic Reservoirs
Middle Atokan sandstone reservoirs, deposited within a deepwater foreland basin, are key targets and form the largest reserve base of the giant Red Oak gas field in the Arkoma Basin of SE Oklahoma. One of these, the Fanshawe reservoir “cluster,” is a genetically-related group of sandstones interpreted as a complex sediment gravity flow/canyon fill depositional sequence. This interpretation evolved over several years, and is supported by high-quality image logs (FMI & OBMI), standard openhole wireline logs, detailed mud log data, log correlations and the resultant mapping of individual sandstone bodies.
Some of the early field development targeted Fanshawe sandstones, but in recent years it has become a secondary objective, after the more obvious early prospects were drilled. A detailed re-interpretation began with simple sandstone mapping, which showed a wide range of thicknesses both regionally and locally. Exhaustive log correlations demonstrated that the interval consisted of vertically and laterally heterogeneous sandstone packages.
Careful scrutiny of mud log data identified quartz-rich intervals which had previously been interpreted from wireline logs to be finer-grained siltstones or even shales. This was an important breakthrough. Incorporating image log data proved that many intervals were actually thin-bedded, steeply-dipping sandstones and shales, commonly underlain by scour/slump surfaces, and overlain by flooding surfaces. Re-integrating standard wireline logs, particularly gamma ray and resistivity curves, helped to delineate the basal Fanshawe canyon.
The Fanshawe depositional system is now recognized as a dynamic gravity flow/canyon fill environment in which local paleobathymetry influenced location of submarine channels, overbank deposits, sheet sandstones and debris flows. These sandstone-bearing intervals are typified by high dips, bypass zones and complex stacking architecture. Mapping fine-scaled thicknesses has proven useful in targeting areas with remaining potential. It also allows for better prediction of pressure regimes. Thus, a more systematic and predictive method to target well placement has resulted.
The techniques used to develop the understanding of the Fanshawe reservoir “cluster” are based on commonly acquired data types and stress the importance of data integration and circling back through the interpretation to re-visit where appropriate.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009