Domestic shale plays, whether oil or natural gas, have become so numerous it’s difficult to continue to think of them as unconventional.
Shale formations such as the Bakken, Fayetteville, Marcellus, Barnett and others all have attained household name status in the industry.
The Woodford Shale in Oklahoma’s western Arkoma Basin is another active yet until recently, somewhat less publicized shale play.
In fact, activity in the play has skyrocketed since 2004. Twenty-five rigs currently are active there, and over 350 wells – horizontal holes for the most part – already have been drilled.
The most active operators are Devon, Newfield, Chesapeake and Antero Resources.
“The Devonian-Mississippian age Woodford Shale is an important source rock in central and southern Oklahoma, where it’s unconformably underlain by carbonates of the early Devonian Hunton group and overlain by the Mississippian Sycamore and Mayes shales and limestones,” said AAPG member Bob Tucker, a development geologist at Denver-based Antero.
“It’s both an oil and gas source, depending on the thermal maturity and kerogen type,” he continued. “It’s a black siliceous shale, with abundant chert beds in the upper portion.
“The lower portion is organic rich, and pyrite is abundant throughout, along with minor phosphate and dolomite beds.”
Dealing With Complexities
Compared to relatively structure-free resource plays like the Bakken and the popular Barnett, the Woodford is unusually complicated.
The Arkoma Basin itself is structurally complex, with multiple generations and styles of faulting. Faults and unexpected structural geometries can have adverse effects on both the length of the laterals and placement of the laterals in the section.
Antero decided to attack these challenges head-on with 3-D seismic data. The company participated in a spec data shoot that CGGVeritas implemented to acquire 284 square miles of 3-D data in Oklahoma’s Atoka and southern Coal counties.
Prior to this Atoka 3-D survey in 2007, much of the area had only a limited grid of 2-D data.
Tucker gave a presentation at the RMAG/Denver Geophysical Society 3-D symposium earlier this year discussing the use of these data to reduce drilling risk in Woodford Shale horizontal wells.
The seismic data are used principally to map faults and dip orientation to reduce drilling and completion risk, Tucker said, while noting that the data facilitate:
- Characterization of steep dip panels, which enables better well planning and geosteering.
- Identification of faults that can cause problems, e.g., laterals drilling out of zone, conduits for water production, sinks for frac energy.
- Recognition of significant unconformities that would yield lower quality wells.
Preferred Targets
Unfaulted dip panels are the preferred sites for horizontal well placement, according to Tucker.
“The western Arkoma is unique in that the shallow Atoka and younger section are structurally detached from the deeper section,” he said. “Prior to acquisition of the Atoka 3-D survey, well control and 2-D data allowed recognition of 10-degree-northeast true dip at the Woodford level.
“Dip panels generally behave as rigid blocks,” Tucker noted. “Minor folding does exist as drag along faults, and the Woodford level structure locally has an undulose character. This may be due to subseismic faults.
“Knowledge of dip magnitude facilitates well planning and efficient drilling.”
In such a structurally complicated area, it comes as no surprise that the 3-D survey was particularly valuable to identify faults.
“The greatest benefit from the Atoka survey is the recognition of faults – intersecting a fault can cause significant problems for a horizontal well,” Tucker said.
“Faults can either be avoided entirely by stopping the lateral short of the trace of the fault,” he said, “or by anticipating the sense and magnitude of the throw and drilling through the fault.”
A ‘Significant’ Unconformity
There are three generations of faults present in the Atoka area:
- Pre-Ouachita normal faults.
- Ouachita thrust belt-related thrust faults.
- Tishomingo uplift-related reverse faults.
Tucker noted the pre-Ouachita normal faults are the most important with respect to drilling Woodford horizontal wells.
“They generally have a north-70-degree-east orientation, although east-west faults are locally significant,” he said. “Separation is from a few tens of feet to over 1,500 feet, and throw is generally down to the south over most of the Atoka 3-D survey area.”
He added that down-to-the-south normal faults are present throughout the Arkoma Basin.
A locally significant unconformity that removed the Wapanucka to upper Hunton interval – including the Woodford – was recognized via the Atoka 3-D survey. The area where the Woodford is absent is localized to the crest of a rotated fault block.
“Wells with laterals beneath the unconformity are poor producers for the most part,” Tucker said, “with lower gas rates and higher water production.
“Understanding the extent of the unconformity resulted in structurally favorable locations being bypassed because of potentially poor well performance or missing Woodford.”
Problems and Possibilities
Even though the Atoka 3-D data are high quality overall, Tucker noted a few problems have been identified:
- Geosteering analysis doesn’t yield results comparable to what is expected from seismic.
- Some areas have velocity anomalies related to overlying structural complexities, yielding time structure that differs from depth structure.
- Structurally complex areas are difficult to image.
“The Woodford shale play is complex, and the geology, drilling and completion design, and production practices all have major impacts on well productivity,” Tucker said. “The application of 3-D seismic data is one component of risk management.”
The Atoka 3-D survey clearly was a vital component in that it allowed recognition of structural and stratigraphic features that impact whether a horizontal well is successful.
Tucker summarized the advantages afforded by the survey:
- An accurate estimate of dip magnitude enables correct placement of the lateral in the stratigraphic interval of interest.
- Dip changes or small-scale faults may cause wells to drill out of zone but can be dealt with given sufficient planning.
- Large faults that compartmentalize the area should be avoided.
- Wells drilled in areas with truncated Woodford may perform poorly.
- Velocity anomalies can cause problems with time-based structural geometries.