G. Alan Petzet
Exploration Editor
A third East Texas Jurassic Cotton Valley lime pinnacle reef discovery in two years has jump started a play that began in the early 1980s.
Driving the play is Marathon Oil Co., which has been keying off two early 1980s discoveries and extensively employing 3D seismic technology to evaluate Cotton Valley reefs at 14,000-15,000 ft. TXO Production Corp., which Marathon later acquired, made the earlier discoveries.
Marathon said it has secured more than 40,000 net acres containing a large inventory of reef prospects. It is running one rig full time, and plans to add one or two others periodically.
The most recent discovery, Marathon's 1 Riley Trust in Robertson County, flowed 21.9 MMcfd of gas (82419 bytes) without stimulation through a 3464 in. adjustable choke at 3,100 psi flowing tubing pressure from perforations at 14,827-15,007 ft opposite a Cotton Valley reef. Working interests are Marathon 75% and Eastern Exploration (Broughton & Associates) 25%.
Marathon said it has 3D seismic data over about 200 sq miles in the play and is acquiring more. The company has identified 35 leads and prospects. It previously disclosed plans to drill five wells this year.
Big flows, small reefs
High pressures and flow rates indicate that the small reefs, drainable by one or two wells each, have good to excellent porosities and permeabilities.
TXO opened one well McSwane field in Freestone County in 1980 and noticed that production characteristics differed from wells in updip Cotton Valley fields. It discovered a productive reef in Branton field in Leon County in 1981.
The Branton pinnacle reef is an estimated 250 ft vertically by about 1 mile long. Petroleum Information cited production reports that show the well averaged 25.5 MMcfd of gas during 1984.
Marathon-TXO's two Branton field pinnacle reef wells have produced a combined total of more than 40 bcf of gas. Cumulative production from five identified productive reefs totals about 60 bcf.
PI characterized McSwane and Branton as "very high pressured reservoirs, defined by local carbonate buildups encased in (Jurassic) Lower Bossier shale." Logs indicate the reefs have 25-27% porosity.
"The individual pinnacle reservoirs at McSwane and Branton are small, on the order of a square kilometer or so. Their relief is substantial, being up to 250 ft, with flanks dipping at 30-40 as measured by dipmeter. Productive intervals are much thicker than in other Cotton Valley lime reservoirs, involving nearly the total thickness of individual pinnacles."
Drilling trends
The frontier nature of the play is evident in the fact that the entire northern extension of the shelf edge, all the way around to the Sabine uplift, a distance of about 150 miles, remains to be explored, PI noted.
Marathon plans to spud the 2 Poth confirmation well in September. It is to start the 1 Peiskee wildcat late this year or in early 1996. It is attempting to complete the 1 Mischer in a shallower horizon and has drilled the 1 Rachui in Robertson County. And it spudded the 1 Brounkowski, permitted to 20,000 ft, on July 26.
American Cometra Inc., Fort Worth, is well along at the 1 Frost between Branton and McSwane fields. Projected to 16,000 ft, it was spudded in mid-June.
The previous four Mara- thon reef discovery wells are:
- 1 McSwane, May 1980, 6.2 MMcfd with 1,200 psi FTP. McSwane field discovery well.
- 1A Marshall, June 1982, 7 MMcfd with 8,350 psi FTP. Branton field well.
- 1 Poth, November 1993, 20.4 MMcfd with 4,650 psi FTP. The Pearwood field discovery well, it started production at 10 MMcfd with 8,800 psi FTP.
- 1 Nash, March 1995, 4.15 MMcfd with 2,000 psi FTP. Kenwood field discovery well.
Marathon also opened Jamucker field in 1995 at the 1 Beddingfield, 1 mile northeast of the 1 Poth. The Beddingfield produces from Cotton Valley lime below the reef interval, Marathon said.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
