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AGS / Energy / Natural Gas / Major Producing Formations in North Arkansas


Upper Cambrian to Ordovician (Arbuckle, St. Peter, Fernvale)

Arbuckle Group

The term Arbuckle Group was first defined for the Arbuckle and Wichita Mountains of southern Oklahoma. The correlative Arbuckle Group in Arkansas is divided into seven formations in ascending order: Potosi, Eminence, Gasconade, Roubidoux, Jefferson City, Cotter, and Powell. The Arbuckle consists primarily of dolostone with minor amounts of limestone, sandstone and shale.

St. Peter Formation

The St. Peter sandstone, Middle Ordovician in age, is generally exposed throughout the northern counties in Arkansas. Its distribution in the subsurface is relatively unknown due to the lack of well control. The St. Peter is composed primarily of massive bedded, medium to fine grained, well rounded, friable, white sandstone. A few minor beds of shale, limestone, and/or dolostones have been noted in some sections. In the subsurface, the porosity of the sandstone varies from 9.8 to 13.5% in accordance with the relative cementation or secondary silification. No body fossils are known from the St. Peter in Arkansas, but a few trace fossils have been reported. The formation ranges from a feather edge to as much as 175 feet thick.

Fernvale Formation

The Upper Ordovician Fernvale limestone is widely exposed in the Salem Plateau of northern Arkansas in Izard, Stone, and Independence Counties. It consists generally of a massive, sometimes cross-bedded, coarsely crystalline, light gray to pink, crinoidal limestone. Various types of fossils are found throughout the formation, including orthoconic nautiloids, brachiopods, bryozoans, and crinoids. The thickness of the Fernvale ranges from 0 to over 100 feet. The porosity is reported 10% in the Cecil field of Crawford County. The Fernvale Formation in the Arkansas Valley area is stratigraphically equivalent to the Viola Formation in Oklahoma.



Silurian to Devonian (St. Clair, Penters, Sylamore)

St. Clair Formation

The Silurian St. Clair Formation is exposed in portions of Izard, Stone, Searcy and Independence Counties. The St. Clair is composed of a coarse-grained, highly fossiliferous, generally light gray to pinkish-gray limestone. The formation contains well preserved whole shells of brachiopods, ostracods, and trilobite pygidia and cephala. Fragmentary remains of crinoids, corals, bryozoa, bivalves, gastropods, and cephalopods are also found. The thickness of the St. Clair ranges from 0 to 100 feet. The St. Clair is stratigraphically equivalent to the Hunton Formation in Oklahoma.

Penters Formation

The Devonian Penters Formation is exposed at Penters Bluff in Independence County, which is its type locality, and in part of eastern Stone County along the White River. The Penters is composed principally of a fine-grained, fossiliferous, dolomitic, limestone with some chert and siliceous beds. The reservoir porosity ranges from 5 to 18%.

Sylamore Formation

The Sylamore Sandstone is a lower member of the Chattanooga Formation. The Sylamore is a gray, brown or white, fine to coarse-grained, slightly shaly, pyritic sandstone with angular to sub-rounded quartz grains. It is generally characterized by the presence of dark-colored phosphate pebbles. The Sylamore ranges from 0 to about 75 feet in outcrop thickness and the reservoir porosity ranges from 7 to 14%.



Mississippian (Boone, Fayetteville, Pitkin)

Boone Formation

The Boone Formation consists of gray, fine to coarse-grained fossiliferous limestone interbedded with brown, brownish-gray to dark gray dense chert. The quantity of chert varies considerably both vertically and horizontally. The Boone is well known for dissolutional features such as sinkholes, caves, and enlarged fissures. The thickness of the Boone is 300 to 350 feet in most of northern Arkansas but as much as 390 feet has been reported. The reservoir porosity ranges from 7 to 17%.

Fayetteville Formation

The Fayetteville Formation is composed mainly of a black, fissile, concretionary, clay shale. Dark gray, fine-grained limestones are commonly interbedded with the shales in north central Arkansas. The Wedington Sandstone Member consists of gray to brown, fine-grained, sometimes calcareous sandstone. As of March 2008, approximately 2.5 million acres in north central Arkansas have been leased in the Fayetteville Shale gas play with a cumulative production of approximately 107 BCF. The thickness of the producing zone (Lower Fayetteville Shale) ranges from 50 to 550 feet and wells range in depth from 1,500 to 6,500 feet.

Pitkin Formation

The Pitkin Formation is composed of a fine to coarse-grained, oolitic, bioclastic limestone. The Pitkin rests conformably on the underlying Fayetteville Shale. The thickness of the Pitkin ranges from a thin edge to over 400 feet. The reservoir porosity ranges from 7.5 to 12%.



Pennsylvanian (Morrow, Atoka, Hartshorne Coal)

Morrow Group

The Pennsylvanian Morrow Group is locally exposed in northwestern Arkansas where it is subdivided into the Bloyd Shale and the Hale Sandstone in descending stratigraphic order. Locally, the Morrow rests unconformably on the underlying Pitkin Formation, which makes a distinctive stratigraphic break between the Mississippian and Pennsylvanian systems in this area.

The Bloyd Formation consists of (in ascending stratigraphic order) the Brentwood Limestone Member, the Woolsey Member, the Dye Shale Member, the Kessler Limestone Member and the Trace Creek Shale Member. The typical thickness of the Bloyd ranges between 175 and 200 feet.

The Hale Formation consists mainly of sandstone and shale, but includes some beds of limestone and conglomerate. The formation is made up of two members: a lower Cane Hill Member and an upper Prairie Grove Member. The thickness of the Hale ranges from a few feet to more than 300 feet. The reservoir porosity of the Morrow Group ranges from 6 to 22%.

Atoka Group

The Pennsylvanian Atoka Formation is a sequence of marine, mostly tan to gray, silty sandstones and grayish-black shales. Some rare calcareous beds and siliceous shales are known. This unit has the largest areal extent of any of the Paleozoic formations in the state. In the Arkansas River Valley and the Frontal Ouachita Mountains the Atoka has been subdivided into upper, middle, and lower lithic members based on regionally mappable shale or sandstone sequences. This unit may reach up to 25,000 feet thick in the Ouachita Mountains region.

The principal producing and correlative sands of the Atoka Formation are the Carpenter "A", Upper Alma, Lower Alma, Carpenter "B", Morris, Areci, Freiberg, Casey, Dunn "A", Dunn "C", Paul Barton, Cecil Spiro, and Orr. The reservoir porosity of the Atoka ranges from 6 to 27%.

Lower Hartshorne Coal

The Lower Hartshorne coalbed lies in the lower portion of the McAlester Formation. It is the most extensive and productive coalbed in Arkansas. The Lower Hartshorne Coal is more than 14 inches thick over an area of about 740 square miles and is less than 14 inches thick in 580 additional square miles. West of Huntington in Sebastian County, it is approximately 8 feet thick. Coal rank ranges from low-volatile bituminous in the western part of the Arkansas Valley coalfield to semianthracite in the eastern part.

The development of Arkansas’ coalbed natural gas resources began in 2001 and has yielded an approximate cumulative production of 12 BCF from the Lower Hartshorne coalbed. In the gas producing area, the thickness of the coalbed ranges from 1 to 5 feet.


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Arkansas Geological Survey
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Little Rock, AR 72204
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