Jackfork play grows but economics tough

G. Alan Petzet
Exploration Editor

An exploration play for gas in the deep Mississippian-Pennsylvanian Jackfork Group of southeastern Oklahoma the past 4 years seems poised for a large expansion.

However, economics is the big question concerning this play along the frontal portion of the Ouachita thrust belt, wrote Scott Montgomery in the latest issue of Petroleum Frontiers.

Only about 15 Jackfork wells have been drilled, of which 13 were successful completions. Most drilling occurred in 1995-96.

The range of estimated ultimate recovery of selected wells is 1.9-7.6 bcf/well with calculated drainage areas significantly less than 160 acres at 7,500-12,000 ft.

"Drilling currently is concentrated along the back limb of a surface-mapped anticline that extends over 3-4 townships. The structurally highest and lowest productive wells on this fold are separated by 6,000 vertical ft, suggesting that a total gas column approaching this figure exists," Montgomery wrote.

The zone extends more than 150 miles along the entire length of the Ouachitas in Arkansas and Oklahoma.

The play, centered in the south half of Latimer County, Okla., seems poised to expand to the southwest, east, and possibly to the south, he wrote. The current play lies within 6-8 miles north and northeast of the Potato Hills uplift.

Key operators are Chesapeake Operating Co. and GHK Petroleum Co., both of Oklahoma City, Ward Petroleum Corp., Enid, Okla., Vastar Resources Inc., Texaco, and Amoco. Marathon and Chevron are assembling lease blocks in the area.

Jackfork gas play

Exploratory and development drilling are occurring in the frontal Ouachitas between the Ti Valley and Windingstair thrust faults (Fig. 1 [93875 bytes]).

H&H Star Energy Inc., Traverse City, Mich., doing business as Petrostar Energy, initiated the play in 1992.

Its 1 Hope, in 4-3n-20e, targeted Pennsylvanian Spiro but encountered gas bearing sands in thrusted lower Jackfork at 17,957 ft. Now operated by Ward, it has produced more than 1 bcf of gas. Ward's EUR is 2.1 bcf.

A second discovery, H&H 1 Lady Luck, 10 miles east-northeast of the 1 Hope, encountered two productive sand zones above 9,900 ft in 1993. It is also expected to recover 2 bcf. Five of six offsets have produced. They have shown two or more zones with 8-17% porosity and IPs above 1.5 MMcfd of gas, Montgomery wrote.

Jackfork geology

Jackfork hydrocarbon potential until recently was believed to be "nonexistent, minor in magnitude, or else overly elusive to useful determination," Montgomery wrote.

Existence of several shallow, intermittent producing oil fields in Atoka and Pittsburg counties, Okla., combined with new geologic interpretations, "strongly suggest that a detailed reevaluation of Jackfork potential is now required throughout much of eastern Oklahoma."

Most workers in eastern Oklahoma divide Jackfork into four or five formations. From base to top, they are the Wildhorse Mountain formation, Prairie Mountain formation, Markham Mill formation, Wesley formation, and Game Refuge sandstone.

Jackfork depositional models will remain speculative, at least until stratigraphic and sedimentological analysis of Jackfork outcrops in eastern Oklahoma and western Arkansas can be carried out and integrated with well data from the current play. Nevertheless, recent work has established the presence of important channel facies with possible reservoir quality.

Such channel sequences show up on high resolution seismic lines, the 59 page Petroleum Frontiers publication points out, and the presence of gas, condensate, and oil in the Jackfork is unlikely to be localized in a few, scattered locations.

Interpretations of cross sections such as Fig. 2 [110188 bytes] indicate the possibility that wells drilled south of the surface trace of the Teechem anticline to 15,000 ft or deeper have the best chance of encountering multiple repeats of thicker sands in the lower Wildhorse Mountain formation. Wells drilled closer to the anticlinal crest are capable of penetrating at least two full Jackfork sections. This is also the case for wells drilled along the back limb of the more northern fold.

Jackfork sediments are also roughly comparable to other occurrences of productive sandstone-bearing foreland basin "flysch" of Pennsylvanian age associated with the Marathon-Ouachita structural front in southwestern Texas.

Operating factors

Drilling costs are commonly $1.2-2.0 million for an 8,000 ft hole. The cost of air drilled holes is at the low end of the range, and operators think further reduction is possible, Montgomery wrote.

Air drilling can minimize a wellbore's tendency to "walk" downdip but produces no cuttings. Other pitfalls can include sloughing of Johns Valley shales, wet channel sands in Johns Valley and Jackfork, and the presence of up to 13% clay, most of which is illite/smectite, in Jackfork reservoirs.

Operators indicate that every well is "exploratory" with regard to completion practices, Montgomery wrote. Stimulating Jackfork reservoirs usually results in only a 100-200 ft half fracture length, even though wells are then capable of producing 1-2 bcf within 2 years.

Only one well, the 1-4 Hope, producing from 18,800 ft, has been completed without successful stimulation. Two frac jobs failed, probably due to the depths involved.

"This is presumed due to a significantly higher degree of fracturing. It suggests, however, that deeper wells may be able to offset higher drilling costs with much reduced completion costs," Mongtomery wrote.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.