STACK Play of the Anadarko Basin, Oklahoma
The STACK (Sooner Trend, Anadarko Basin, Canadian and Kingfisher Counties) play of northern Oklahoma has generated an amount of interest in the past few years due to its large amount of potential oil and gas reserves. The STACK was first drilled by Newfield exploration in 2011 with its first wildcat well and announced STACK play in 2013. Named for its geologic and regional location, the STACK is also so aptly named due to its multiple stacked intervals of producible formations. These formation layers allow operators to access multiple formations from a single drilling location. The STACK core area covers approximately 1,000 square miles as part of the total 70,000 square miles of the Anadarko Basin in Oklahoma (Fig. 1). With the rise of horizontal drilling, the core area of the STACK in Canadian and Kingfisher counties can be extended out to include Dewey and Blaine counties, and continues to expand while more wells are being drilling and more research is being conducted. .
It is important to understand key geologic concepts such as facies types, type of reservoir and porosity, depositional trends, and sequence stratigraphy to understand this emerging play. . Two main prolific producing reservoirs of the STACK include the Meramec and the Osage. These formations further include distinct reservoir targets that incorporate different sequence stratigraphic and lithostratigraphic intervals based on their petrophysical and lithological characteristics. Theses Mississippian units can be characterized as a mixed carbonate and siliclastic system, which includes interbedded changes in lithology.
The Meramec units are mostly composed of argillaceous and calcareous siltstones. .The Meramec has a strong shift in depositional styles across the Anadarko Basin, which creates a change from a carbonate system in the north to a siliclastic system. Understanding the sequence stratigraphy is an important part of identifying the horizontal and lateral changes from siliclastic to carbonate intervals. The Meramec unit was deposited during the Mississippian, approximately 360 to 320 million years ago, when the mid-continent region was located about 20 degrees south of the equator, on a continental shelf covered by a shallow, warm sea. This environment was optimal for the deposition of shallow marine water deposits. Figure 2 shows a reconstruction of the paleoenvironment of Anadarko Basin during the deposition of the Mississippian units.  The lateral facies changes from north to south are a result of the different depositional environments along the carbonate ramp, as well as a separation of energy environments. Clastic limestone with coarser allochems suggest a high energy environment in closer proximity to the shore, while the shaley types to the south suggest a lower energy environment. The carbonates of the Mississippian are often susceptible to diagentic alteration which makes the porosity values laterally across the area open for high variability and a difficulty in correlating throughout the subsurface.  Price identified that the primary driver for production in the Meramec is the percent of calcite cement within individual facies.  In carbonate rich intervals calcite-healed fractures are also common.  The geometry and depositional history of the Anadarko Basin is also a major influence on the lateral extent of these targets. The Anadarko Basin’s geometry is majorly influenced by the paleotectonics of the area which controlled the rate of sediment deposition, erosion, and structure accumulation that lead to the conditions for developing the successful producing sediments known generally as the Mississippian reservoirs. The geometry of the sediment accumulation was a result of the tectonic activity in the Cambrian that lead to the development of the shelf slope. Figure 3 shows a Southwest-Northeast cross section across the Basin to illustrate the overall geometry of the basin. .
Based on log and core data from the STACK, the Osage is often dominated by chert-rich grainstones and packstones.  During times of aerial exposure, the deposited sponge and spicules in the elevated silica marine waters allowed for the formation of chert in some intervals. Osage intervals are usually identified by their low clay content and aggradational and progradational stacking patterns, while Meramec strata form high frequency coarsening-upward sequences and have overall higher clay volumes. Porosity is generally higher in the Meramec than in the Osage. 
Production in the STACK
Production in the STACK is due to multiple stacked target intervals of the Meramec, Osage, and up to as many as 8 other formation targets including the Morrow, Oswego, and Red Fork. Figure 4 highlights the stratigraphic intervals of the main producers in the STACK. The producing interval of Mississippian reservoirs can be as much as 400 ft for oil and 225 feet for gas. With the rise of horizontal drilling, a targeting specific benches with ideal reservoir quality has become an important concept when effectively and economically producing from these intervals.  Meramec wells in Kingfisher county have ranged anywhere between 7,000 to 8,500 feet. To the west of Kingfisher County, the play deepens below 10,000 feet and liquids content falls—the western area wells produce dry gas or wet gas, with very few oil-prone wells. According to a report on the STACK play by Droege and Vick in 2018, the highest amounts of estimated oil-in-place occur in the chert-rich Lower Osage in the western part of the play, while the Meramec is the most targeted bench and is the highest producer overall.  Calcite poor siltstone facies within the Meramec and vuggy chert facies in the Osage have been referenced as the best reservoir qualities in the STACK. Areas of overpressure in the Meramec have also been researched in southeastern Kingfisher and have been recent areas of great interest for operators. Rig count has doubled from the beginning of 2016 to May 2018. In July 2017, Devon Energy announced a “record setting” Meramec well in Kingfisher county that achieved a peak rate of 6,000 BOE per day.
Technology the STACK
The STACK has been yielding impressive results since its discovery and has only continued to evolve with the implementation of more research and technological advances. Seismic data has been used to aid oil and gas exploration in Oklahoma since 1921. 3-D seismic surveys have been extremely valuable helped to pinpoint areas of predicted well success and drill with less risk. 3D seismic surveys have been a key asset in the past few years to finding optimal drilling locations, and understanding the subsurface within the STACK. This data has been extremely valuable in helping producers reduce risk while saving time and money. It also has refined the overall understanding of the STACK while also provided insight into target formations, as well as systems of natural fractures. More advanced geoscience work is being done to help illuminate the driving forces behind this play, such as reservoir modeling, fracture modeling, seismic inversion, petrophysical characterization, fluid analysis, etc. Seismic processing and further inversion work can give geoscientists more information about the rock properties of the area, while fracture modeling can help evaluate risk while it can also provide information about the stimulated rock volume of the reservoir. In 2018, TGS began conducting the survey for the Canton 3-D seismic survey (figure 5) in Blaine and Major Counties at a size of about 452 square miles and will be a major development for data across the area.
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- Blakey, R.C., 2017, Paleogeography: Colorado Plateau Geosystems, Phoenix, AZ
- Higley, D.K. and Gaswirth, S.B., 2014, Petroleum systems and assessment of undiscovered oil and gas in the Anadarko Basin province, Colorado, Kansas, Oklahoma, and Texas – USGS Province 58, USGS Digital Data Series DDS-69-EE.
- “BACK TO Stack & Scoop.” STACK & SCOOP Overview - Maps - Geology - Counties, www.shaleexperts.com/plays/stack-scoop/Overview?menu
- Price, B., Haustveit, K., & Lamb, A. (2017, July 24). Influence of Stratigraphy on Barriers to Fracture Growth and Completion Optimization in the Meramec Stack Play, Anadarko Basin, Oklahoma.
- Droege,L. & Vick, H.,2018. Redefining the STACK Play from Subsurface to Commercialization: Identifying Stacked Pay Sweet Spots in the Northern Anadarko Basin. Search and Discovery 11104.
- Perry, W. J. Jr., 1989, Tectonic evolution of the Anadarko basin region, Oklahoma. In: U.S. Geological Survey bulletin 1866-A, A multidisciplinary approach to research studies of sedimentary rocks and their constituents and the evolution of sedimentary basins, both ancient and modern.
- Shelley,S.,Grammer, G.M.,& Pranter,M.,2017. Reservoir Characterization and Modeling of a Subsurface Meramec Analog from a Quarry in Northeastern Oklahoma, Oklahoma City Geological Society – The Shale Shaker, September-October 2017, p. 224-243.
- Ball, M., Henry, M., and Frezon, S., 1991.Petroleum Geology of the Anadarko Basin Region, Province (115), Kansas, Oklahoma, and Texas. USGS Open File report 88- 450W.
- [null Droege,L. & Vick, H.,2018.] Redefining the STACK Play from Subsurface to Commercialization: Identifying Stacked Pay Sweet Spots in the Northern Anadarko Basin. Search and Discovery 11104.