John C. McCaslin
Exploration Editor
Exploratory interest in the Arkoma basin of eastern Oklahoma and western Arkansas has never been as high as in these days of deeper wildcatting into Paleozoic rocks all the way down to the Cambro-Ordovician Arbuckle.
Recent gas discoveries in the Oklahoma portion of the basin have set off one of the most active leasing and drilling forays in many years in the Mid Continent province. Each new well completion lends more information to the geologists who are interested in the future of this vast gas bonanza.
STRUCTURAL STYLES
The latest edition of Petroleum Frontiers by Petroleum Information in Denver, Drilling the Deep Arkoma, points out that major structures in the gas producing portions of the basin can be divided into three general categories:
- Basement-involved high angle faults (normal and reverse)
- Foreland-style thrusts and imbricates.
- And, folds, (mostly open synclines separated by narrow anticlines).
Cross (strike-slip) faults also exist within the frontal Ouachita Thrust province, as in the Klippe area northeast of Black Knob Ridge, but are generally of small importance to exploration. In addition, PI says that some component of lateral motion may have occurred along certain basement structures in the Arkoma basin.
INTO DIFFERENT LEVELS
Petroleum Frontiers notes that to some degree, the above three types of structures can be divided into different levels.
Folds are most often the shallow expression of shortening related to underlying thrusts, which, in places, cut to the surface along anticlinal axes. These thrusts have overriden basement fault zones where the maximum period of displacement (early-mid Atoka, as evidenced by syntectonic depositional thickening) pre-dated the time when thrusting reached the shelf by only a few million years.
These three structural levels have separate trapping possibilities. This has been revealed by the history of gas finding in many Arkoma basin fields, particularly Wilburton where much of the action is today, and Red Oak.
At Wilburton, for example, middle Atoka sandstones were found productive at 2,500 ft on a surface anticline (Wilburton) in the last 1920's. Then, three decades later, drilling at about 7,000 ft found Spiro reserves where simple folding had given way to thrust-rollover.
The recent ARCO Oil & Gas Co. strikes at 13,000 ft and deeper have found gas trapped in the Arbuckle on a reverse-faulted basement block beneath the basal thrust.
PI adds that a very similar history characterizes Red Oak and Kinta, except that, being higher on the shelf, surface folding involves the Hartshorne, underlying thrusts relate to the middle Atoka sands, and basement faulting involves the Spiro and Cromwell sands. Arbuckle gas production this far north has so far been confined to the Roy Reed well, located on a basement horst in Red Oak field, roughly 2,500 ft deeper than Spiro.
Continued exploratory work in this fascinating province will uncover more and more geologic secrets.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.