James D. Henry
Old Aulacogen LP
Frisco, Tex.
The early Pennsylvanian Marble Falls formation has recently been determined to be a fractured carbonate reservoir much like the famous Barnett shale.
The Marble Falls lies a few hundred feet shallower than the Barnett and is sourced by it. In May 2010, Halek Energy of Southlake, Tex., drilled the 8 Johnson well as a horizontal borehole just north of Bartons Chapel in rural southwestern Jack County. This was the breakthrough well in the entire Fort Worth basin for the concept of directionally drilling the Marble Falls limestone.
The Marble Falls formation has been a ubiquitous, albeit modest, reservoir in the Fort Worth basin for decades, and until the Halek Energy well, it had only been drilled and tested in vertical boreholes. At this relatively early point, we have just seen the tip of the Marble Falls iceberg, but it appears that this reservoir will likely contain substantially more crude oil than natural gas.
Jack County occupies the west-central part of the Fort Worth basin. The north-south axis of the Bend arch, which nominally separates the Fort Worth basin on the east from the Eastern Shelf of the Midland basin to the west, lies beneath central and western Young County, which is immediately adjacent to Jack County on the west.
Structure plays a very small part in the petroleum geology of this area and neither contributes to nor detracts from this play. Faults are rare and of small magnitude. Folding is very subtle and largely monoclinal in nature. The fractured Marble Falls reservoir is strictly stratigraphic in nature.
At the moment, this play is centered in Jack County and the northern half of Palo Pinto County immediately to the south, but the pay zone extends under adjacent counties including Young, Wise, Parker, Stephens, and probably several more to the south.
Over the decades, the Marble Falls has been characterized by good shows of oil and gas, but it also had been stigmatized by weak production histories until Halek Energy drilled the 8 Johnson as a directional hole in the spring of 2010. As a relatively new player in the Fort Worth basin, Halek did not know that it was impossible to drill a big well in the Marble Falls, so he just spudded and drilled one.
When hole and rig problems arose, the planned 1,000-ft lateral in the Marble Falls was terminated short of 600 ft and a slotted liner was run. After only a small acid job, the well was placed on gas lift in early June and immediately began flowing more than 700 b/d of oil and 1,800 Mcfd of gas. A few weeks later it was IPd for 481 b/d and 1,814 Mcfd.
By the end of the year, the 8 Johnson (RRC lease 694208) had sold 54,571 bbl of oil and more than 267,000 Mcf and was still averaging 218 b/d and 1,427 Mcfd. From the start this well had also been making large volumes of water. The salinity of the water has been analyzed at between 88,000 and 92,000 ppm, so it is undoubtedly coming from the producing formation, not the deeper (and saltier) Ellenburger or the shallower (and less saline) Atoka conglomerates. According to data at the Railroad Commission, such water production is normal and to be expected from the Marble Falls.
The Halek Energy 8 Johnson has been the one single-most productive well in Jack County—and probably the entire Fort Worth basin—since the day it first came on line. So much for conventional wisdom about the marginally economic Marble Falls.
In 2010, the year Halek's discovery well was drilled, the Railroad Commission issued 20 drilling permits targeting the Marble Falls formation. It issued 53 in 2011 and has issued 150 so far in 2012.
The commission's laissez faire attitude toward required drilling and production paperwork makes it impossible to determine the number of actual completions in the Marble Falls or the reservoir's current or cumulative output. To date, the major players in the Marble Falls trend have been DTE Gas Resources, Swan Production Co., Best Petroleum, and Cobra Oil & Gas.
Horizontal drilling revolution
The advent of horizontal drilling technology in the past dozen years has literally changed the rules for domestic onshore exploration.
More than half of all new well permits in Texas and Oklahoma are for directionally drilled boreholes and have been for several years now. Completion techniques for the Marble Falls are evolving quickly and usually—but not always—include multistage frac jobs.
Most operators begin each new location with a vertical pilot hole drilled through the Marble Falls and the underlying Jermyn in order to obtain a simple suite of wireline logs for the purpose of stratigraphic orientation. Those logs will furnish information for the decision as to exactly where to drill the directional hole.
The lateral in Halek Energy's big well was only somewhere between 550 and 600 ft in length, but a horizontal hole of 1,500-2,500 ft would probably be more cost-effective in this part of the Fort Worth basin. As a general rule of thumb, 160-acre spacing will suffice to keep adjacent wellbores from communicating with each other after frac jobs.
The play is too new for us to have a good handle on ultimate per-well reserves. Accordingly, some operators opt for vertical wells in this play, reasoning that the anticipated additional reserves in a horizontal well do not justify the extra drilling and completion costs. At this time, a conventional vertical Marble Falls well can be completed for about $850,000, and a horizontal well will cost almost exactly twice that figure.
There is a temptation to compare the Marble Falls play to the state of the Barnett shale play along about 1989, but the Marble Falls is substantially different from that of the Barnett. It is starting out to be more about oil than gas, and since it is somewhat shallower, it will be less expensive to develop.
A decade or 2 from now, we will undoubtedly understand the Marble Falls far better than we do today. In the meantime, a lucrative period of development awaits those who dare.
