Subsurface Sequence Stratigraphy and Characterization of Mississippian (Kinderhookian to Meramecian) Carbonate Reservoirs of the Anadarko Shelf and Implications on Reservoir Quality, North-Central Oklahoma
Wittman, Brett; Cahill, Thomas; Xie, Xiangyang
Lower Mississippian carbonate reservoirs of the midcontinent are prolific oil and gas producers. Production from these carbonates has been primarily from two reservoir intervals, the Mississippi "chat" and recently denser tripolitic mudstone intervals. The chat interval is a high porosity chert residuum at the Mississippian, Pennsylvanian Kaskaskian unconformity. The distribution of the chat reservoir is very discontinuous and heterogeneous. Recent horizontal drilling successes have reinvigorated academic and industry interest in the Lower Mississippian strata. Much of the activity is now targeting lower porosity cherty mudstone intervals of the Reeds Springs and Cowley formation, which was previously considered to be a non-economic interval.
The study area lies along the shelf edge and slope margin near the Kansas-Oklahoma border in all or parts of Alfalfa, Grant, Kay, Woods, and Garfield counties, Oklahoma. The dataset is made up of 150 wells with raster image logs. Gamma ray, resistivity, microlog, photoelectric effect and density logs are all used to make stratigraphic correlations. These reservoir intervals are found near sequence bounding unconformities that were exposed at periodic lowstands at the top of transgressive regressive sequences, however, their spatial position and quality has not been well understood in north-central Oklahoma. The purpose of this study is to document a high resolution sequence stratigraphic study of the Lower Mississippian section from the shelf edge near the state line to the south toward the distally starved basin.
Lower Mississippian subcrops are a succession of prograding clinoforms. The compartments are interformational units within the Reeds Spring and Cowley formations that are individually correlatable. The best quality reservoir is found at the tops of shallowing upwards cycles below fourth order unconformities. Episodic subaerial exposure provided numerous opportunities for the formation of tripolitic chert reservoirs. There are multiple, individual and distinct clinoforms with reservoir potential in the study area which differ from conventional chat reservoirs. Reservoir quality clinoforms are usually found near the paleo shelf edge as this was an ideal environment for secondary porosity development during lowstand. Porosity decreases basinward within the wedges and with depth beneath sequence boundaries. Documentation of the spatial extent and quality of reservoir clinoforms within the Lower Mississippian adds value to the exploration and production potential of north-central Oklahoma.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013