Anadarko basin

ADVERTISEMENT
From SEG Wiki
Jump to: navigation, search
Figure 1. The Anadarko Basin occupies mostly Oklahoma with Kansas possessing the overwhelming majority of the Anadarko Shelf. The Basin also extends into Texas and even southeastern Colorado. (Courtesy Davis and Northcutt,1989)[1]

The Anadarko Basin covers western Oklahoma, western Kansas, north portion of Texas panhandle, and two counties in southeastern Colorado[2][3]. The basin is a sedimentary structure with approximately 15,000 ft deposits from late-Cambrian to Mississippian shallow-marine carbonates. It is the deepest sedimentary basin within the North American craton.[4] The basin is bordered to the south by the Southern Oklahoma Aulacogen and its development is primarily driven by the formation.

Much of the Anadarko Basin's production has been historically in the Pennsylvanian, Simpson, and Silurian-Devonian Hutton group formation.[5] It has two prominent plays the SCOOP and STACK. Since 2008, the Basin has also been a site of induced seismic activity due to waste-water disposal.

Provincial Geology

Tectonic History

Figure 2. Cross-section of the Anadarko basin showing the influence of the Wichita uplift and sedimentary deposits from Southern Oklahoma Aulacogen to the Anadarko shelf in the North. (Courtesy Debra Higley, 2010)[6]

The emergence of the Anadarko basin as an independent structure can be traced to the late Mississippian, early Pennsylvanian time. Subsiding in throughout the Paleozoic, the basin is defined by mid-continent plate collision of the Cambrian basement.[3] With the collision of the Paleozoic North American margin with the Gondwana plate, structural inversion of the Southern Oklahoma Aulacogen (SOA) resulted with thrust loading to the north of the Wichita Mountains and Amarillo uplift.[2] This event, termed the Wichita Orogeny, marked the beginning of the formation of the Anadarko basin to the north of the tectonic activity.[3]

The type of importance of the type of collision, however, is a subject of dispute. Some researchers suspect the left-lateral strike-slip is primarily minor in nature to the the orogenic plate collision.[2] Instead attributing the thrusting to the large vertical displacement in the SOA during the Pennsylvanian.[3] Regardless of the magnitude, left-lateral strike-slips are known or creating zones of compression consisting of thrusts and folds. The importance of this argument is that these folds are a found in oil producing structures of the southern Anadarko.[2]

Due to the resulting substantial vertical displacement in the Wichita Mountains, the Anadarko Basin contains more than 40,000 ft of deposited sediments. Shallow Marine Carbonates were primary deposits in the area.[3] However, erosion in the Wichita-Arbuckle uplift is another contributor to the North America's deepest sedimentary basin.[2] Erosional beheading of folds and thrust structures from the Wichita Orogeny allowed for a reconfiguration of Paleozoic quartz clastics and carbonates during renewed uplift.[3] The resulting updip after additional erosion and facies alteration created stratigraphic traps for the Anadarko basin.[2]

Development History

  1. Precambrian: crustal consolidation and metamorphism are followed by dikes emplaced in a N. 60 W. trend.
  2. Precambrian to mid-Cambrian: Development of the Southern Oklahoma Aulacogen (SOA).
  3. Cambrian to Mississippian: Subsidence of the SOA trough.
  4. Paleozoic: Tectonism between Gondwana plate and southern continental margin creates the Anadarko basin on the NW flank of SOA.[3]
Figure 3. An overview of stratigraphy for the Anadarko Basin. General stratigraphy; actual stratigraphy may deviate. (Courtesy Higley, Gianoutsos, Kruger, and Nelson. 2014)[6]

Petroleum Geology

Reservoir Rocks

The Anadarko Basin possesses an abundant of productive reserves with both quantitative proven potential in both oil and gas production. Historically exploration has been limited to exploiting the Anadarko shelf.[5] However, technological improvements such as hydraulic fracturing has allowed to tap into greater potentials of previously less important plays in oil production such as the Hunton group.

Primary oil production has been from the Simpson and the Pennsylvanian Groups, responsible for approximately 84% of Anadarko production. Lesser oil production has also been recorded in the strata groups: Hunton, pre-Chesteran Mississippian, Arbuckle, Viola, Chesteran, and Permian.[5]

While gas production is the primary concern of the Pennsylvanian, Chesteran and Hunton groups.

Prominent Reservoir Rocks

The type of reservoirs are varied with typical marine sandstones in the Simpson Group, Dolomite in the Arbuckle Group, and even dolomitized limestone in the Hunton Group. Four prominent production fields are highlighted as follows:

  1. Arbuckle - Cambrian and early Ordovician age. Primary production has been located in the bald-headed anticlines along Nemaha Ridge and structural traps on the Shelf in northern Oklahoma. The Group is primarily a carbonate structure with high porosity near the eroded surface of the strata due to fracturing.[5]
  2. Chesteran - Mississippian age. Typically made of of oolitic and fossilferous limestone inter-bedded with shale. The formation is prominent for its gas production found in its erratically porous limestone.[5]
  3. Hunton - Silurian and Devonian age.[4] Essentially a carbonate strata, the Hunton is primarily made up of the Chimney Hill Limestone and the Henryhouse Shale. The geology of the Hunton Group is primarily dolomitized creating stratigraphic traps with unusually high production considering the geology.[4]
  4. Simpson - Pennsylvanian age. The primary oil producing reservoir of the Anadarko consists of marine sandstones, shale and carbonate rocks with primary production from its sandstones with green shales offering good source rock.[5]

Primary Geological Risks

The Anadarko basin has a few geological risks and issues that have affected the production and profitability of the basin. One of the issues is the issue of some reservoirs in the basin being under pressured. This can be an issue due to initially high GORs being recorded with the hydrocarbons not being pressured into oil instead of natural gas. This problem is escalated in Oklahoma due to large costs associated with moving and selling natural gas, and low gas prices. This leads to wells being shut in at very early stages in the well’s life.[4]

There are also some petrophysical issues that exist in the basin. There are areas where low matrix porosity and permeability have been recorded. The producible levels of porosity and permeability have been found near naturally occurring fractures in the reservoir rock. These can be hard to identify leading to dry wells being drilled away from these fractures. This basin has also had large amounts of water saturation. This is another issue that can lead to uneconomic wells producing large amounts of water.[4]

Traps and Seals

The Anadarko basin has a few different stratigraphic and structural traps to hold the hydrocarbons in the reservoir. The major type of trap of the basin is the pinch out that exists in the northeast section of the basin. Here the Hunton formation becomes thinner and thinner until the formation is pinched off causing a reservoir trap. There is also tectonic faulting that has caused the faults to form structural traps. Another aspect of the stratigraphic traps that exist is the large amounts of dolomitization that occurred in the limestones of the basin. The dolomitization of the limestones caused the rocks to have lower porosity and permeability which allowed for the formation to become a good reservoir trap.

The Anadarko basin has a few layers of shale in its stratigraphy that provide good seals for the reservoirs. The major seal for the basin is the Woodford Shale that overlies the Hunton group. Other shale and non-porous limestones would be the Pennsylvanian sediments as well as the Ochleta formation.

Source Rock

The Anadarko basin has had many decades of high levels of production, this is due to its abundance of source rocks located in the stratigraphy of the basin. The major source rock for the basin is the Woodford shale, which provides the hydrocarbons to the Hunton group, Viola group, Simpson group and the Arbuckle group. There are other stratigraphic layers that can act as source rocks including, the Viola group, Sylvan Shale, Lower Mississippian limestone, Chester group, and the Morrow group.

One key indicator of the quality of the potential source rock is the Total Organic Carbon (TOC) weight percentage. Data from a study published by the USGS, different potential source rocks had cores tested to obtain the TOC levels in the rock. Some of the better results of potential source rocks were:

· Ordovician: .02 wt% to 18.2wt% (higher values were found mostly in Kansas)

· Woodford Shale: .1 wt% to 26 wt%

· Mississippian Limestone: 0.5 wt% to 3.4 wt%

· Morrowan Shale: 0.48 wt% to 10.71 wt%

· Thirteen Fingers Limestone: 4 wt% to 6 wt%

· Desmonian Shale: 0.08 wt% to 11.2 wt%

When discussing the potential of the source rocks using TOC a good benchmark for a source rock is 1 wt%. The previously mentioned rock formations all pass this benchmark in varying degrees.

Hydrocarbon and maturation type

Hydrocarbon generation started around the same time as the Pennsylvanian, about 300 million years ago. The majority of hydrocarbons migrated from the deepest source rocks in the Basin.[2] This is due to the increasing temperatures from the geothermal gradient, which is the primary influence on maturation.[7] The vast majority of hydrocarbons produced in the Anadarko basin will be in the form of gas, however there are a few plays with some lower GOR such as the Springer-Morrow Sandstone Play and the Post-Morrow Pennsylvanian Sandstone Play.[2]

Main source rocks:

Shale of middle Ordovician Simpson Group

Simpson Group is around 50% shale. Organic material is primarily algal which means the kerogen is a great source of oil. Source rock quality is moderate deep in the basin where maturity is reached.[2]

Woodford and Pennsylvania shales

Nearly the entire Woodford section is a great source rock. Maximum thickness was found to be 900 feet towards the bottom of the basin. Kerogen here is made up of marine planktonic material and plant debris, which provides an excellent source for both oil and gas.[2]

The upper Pennsylvanian is composed of 70% shale while the lower Pennsylvanian is made up of 80% shale. The upper shale contains kerogen that produces good oil and gas and the lower shale consists of kerogen that is gas prone.[2]

Geological Uncertainties

The Anadarko basin has a few geological risks and issues that have affected the production and profitability of the basin. One of the issues is the issue of some reservoirs in the basin being under pressured. This can be an issue due to initially high GORs being recorded with the hydrocarbons not being pressured into oil instead of natural gas. This problem is escalated in Oklahoma due to large costs associated with moving and selling natural gas, and low gas prices. This leads to wells being shut in at very early stages in the well’s life.[4]

There are also some petrophysical issues that exist in the basin. There are areas where low matrix porosity and permeability have been recorded. There are a lot of thinning formations in the area as well, which causes certain target formations to be more difficult to hit on the first try. The producible levels of porosity and permeability have been found near naturally occurring fractures in the reservoir rock. These can be hard to identify leading to dry wells being drilled away from these fractures.[4]

Another issue that exists in the Anadarko basin is well interference from well spacing not being properly managed. The basin has been in production for a few decades and has had a lot of wells placed nearby each other. When a new well is placed too close to an existing well it can cause the existing well to see a sharp drop in production. The drop in production is also seen in the new well as it will have a much shorter economic life due to lower amounts of producible hydrocarbons in the area. There have also been large amounts of water produced in the area due to the overproduction of certain areas of the basin.[4]

The high amounts of produced water may be due to the effects of water coning that exist in the Anadarko basin. The basin has had issues with high water oil ratios early in its production life due to the issue of water coning. With the basin having generally high amounts of water in the basin, when a well is brought online it starts to pull the fluids upwards causing a water cone to be made. This cone will cause large amounts of water production, even though there may still be large amounts of hydrocarbons nearby. Having high WOR’s early on in the production life of a well could cause some operators to shut down the production from this well, leading to uneconomic wells throughout the basin.[6]

Future Assessment

Figure 4. Illustration of a directional drilling (horizontal drilling) operation such as that used in the Anadarko Basin. (Courtesy of Trevor Cone, 2014)[8]

Future development in the Anadarko basin is limited due to the long life of the basin as an energy producer; now entering its mature phase. Nevertheless, additional shale plays are still being evaluated, as of 2018, despite the industry downturn.[9] Anadarko has been a key player in the hydraulic fracturing boom. For instance, in 2010, the USGS estimated that the basin still showed promise with 495 million bbl of oil and 27.5 Tcf gas.[10] Hydraulic fracturing and horizontal wells have become imperative for the competitive production. Recently, longer laterals in wells have allowed for increased recovery. The advances in these new methods extend the limits of the Anadarko’s prospect as an energy provider. However, these new techniques are expensive and require higher oil and gas prices in order to remain cost effective.

Further Reading

Woodford Shale

Hydraulic Fracturing in Shale

USGS Assessment of the Anadarko Basin

References

  1. Davis, H. G., & Northcutt, R. A. The greater anadarko basin; an overview of petroleum exploration and development. Circular - Oklahoma Geological Survey, 90, 13-24. 1989. Print
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Ball, M. M., Henry, Mitchell E, Frezon, Sherwood E., and Geological Survey. Petroleum Geology of the Anadarko Basin Region, Province (115), Kansas, Oklahoma, and Texas Mahlon M. Ball, Mitchell E. Henry, and Sherwood E. Frezon. (1991). Print
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Perry, William J. Jr. “ Tectonic Evolution of the Anadarko Basin region, Oklahoma.” USGS Bulletin, 1866, 1989
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Gaswirth, Stephanie B. and Higley, Debra K. " Petroleum System Analysis of the Hunton Group in West Edmond Field, Oklahoma." AAPG Bulletin Vol. 97, No. 7 2013. DOI: 10.1306/12031212075
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Adler, F. J. (1971). Anadarko basin and central oklahoma area. Memoir - American Association of Petroleum Geologists, 15, 1061-1070.Retrieved from https://search-proquest-com.ezproxy.lib.ou.edu/docview/52588585?accountid=12964
  6. 6.0 6.1 6.2 Higley, D., Gianoutsos, N. J.,Kruger J. D. ,Nelson, P.H. Petroleum Systems and Assessment of Undiscovered Oil and Gas in the Anadarko Basin Province, Colorado, Kansas, Oklahoma, Texas - Mississippian Through Permian Assessment Units. United States Geological Survey Digital Data Series DDS-69-EE, US Dept. of the Interior. 2010
  7. Pawlewicz, J. M. Thermal Maturation of the Eastern Anadarko Basin, Oklahoma. USGS bulletin, 1866, 1989.
  8. Cone, T. Horizontal Drilling: How Do They Get It To Go Sideways? Rigzone. February, 25 2015. https://rigzonenews.wordpress.com/2015/02/25/horizontal-drilling-how-do-they-get-it-to-go-sideways/ (retrieved Nov. 23, 2019).
  9. Hintzman, T., Walls, J. D., & Morcote, A. Applying Digital Rock Characterization to a New Anadarko Basin Shale Play. Unconventional Resources Technology Conference. 2016, August 1. doi: 10.15530/URTEC-2016-2459710
  10. Greenberg, J. Regional report: Anadarko/Permian - high oil prices, horizontal drilling, multistage completions increasing anadarko, permian basin activity. World Oil, September 2012.