Date of Graduation

5-2013

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Xiangyang Xie

Committee Member

Ralph K. Davis

Second Committee Member

Doy L. Zachry

Keywords

Earth sciences, Anadarko shelf, Carbonate reservoirs, Mississppi lime, Mississippian, Sequence stratigraphy

Abstract

Mississippian carbonate strata of the midcontinent contain prolific oil and gas reservoirs. Production from these carbonates has been primarily from two reservoir types, the Mississippi "chat" and recently denser chert-rich mudstone intervals. The"chat" interval is a high porosity chert residuum associated with the both the Osagean and basal Pennsylvanian unconformity. The distribution of the "chat" reservoir is discontinuous and heterogeneous. Recent horizontal drilling successes have reinvigorated academic and industry interest in the Lower Mississippian. Much of the activity is now targeting lower porosity, cherty, mudstone intervals of the Reeds Spring and Cowley Formations, which were previously considered to be non-economic.

The study area lies along the shelf edge and slope margin near the Kansas-Oklahoma border comprising all or parts of Alfalfa, Grant, Kay, Woods, and Garfield Counties, Oklahoma. The dataset totals 150 wells with raster image logs. Gamma-ray, resistivity, microlog, photoelectric effect and density logs were all used to make stratigraphic correlations. Reservoir intervals occur near sequence-bounding unconformities that were exposed at periodic lowstands at the top of transgressive-regressive sequences, however, the spatial position and quality has not been well understood in north-central Oklahoma. The purpose of this study is to compile a high resolution sequence stratigraphic study of the Mississippian section from the shelf edge near the state line and south toward the distally starved basin.

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 at the tops of shallowing upwards cycles below third and fourth order unconformities. Episodic subaerial exposure provided numerous opportunities for the formation of tripolitic chert reservoirs. There are multiple, distinct clinoforms with reservoir potential in the study area, which differ from conventional chat reservoirs. Reservoir quality clinoforms usually occur near the paleo shelf edge as this was an ideal environment for secondary porosity development during lowstands. 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.

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