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Little Cedar Creek Field
Country	United States of America
Region	South
Location	Conecuh County, Alabama
Offshore/onshore	Onshore
Operator	Midroc Operating Company
Field history
Discovery	1994 (Hunt Oil Company)
Start of development	1994
Production
Producing formations	Smackover

The Little Cedar Creek Field is a hydrocarbon system located in Conecuh County, Alabama in the eastern Gulf coastal plain of the United States. Little Cedar Creek Field encompasses an area of 83 km² consisting of microbial carbonate reservoirs. The field was first discovered and subsequently developed by Hunt Oil Company in 1994. Little Cedar Creek Field is now the largest oil-producing hydrocarbon field in the state of Alabama.

Geologic setting

The formation of Gulf of Mexico is the main geologic event that resulted in the creation of Little Cedar Creek Field. This process begins approximately 180 million years ago with the breakup of the super-continent Pangea. This breaking up is attributed to extensional faulting that occurred within the continent that was caused by sea-floor spreading active from around 160 – 140 Ma. This extension caused the formation of a rift basin across southern Georgia known as the South Georgia Rift Basin. A period of marine transgression followed this period of extension allowing for the South Georgia Rift Basin to begin to flood with seawater. As the basin flooded, it began separating southeast North America from the Florida Platform resulting in the formation of the Georgia Seaway (Suwanee Channel). The continuation of seafloor spreading caused the Yucatan Block and the Florida – Bahama Block, that connected to the African side of Gondwana, to the south. This marks the creation of the Gulf of Mexico Basin that extends to most of what is now Texas, Louisiana, Mississippi, Alabama, Georgia, and Florida. At this time the regional environment was arid and seawater that had previously occupied the basin evaporated resulting in halite deposits beginning to form and accumulate in massive quantities creating evaporite deposits. Overtime sediment from the surrounding continents began to deposit on top of these salt deposits known as the Louann Salt formation, and is responsible for the most of the salt deformation in the gulf allowing for oil and gas exploration due to the oil and gas traps formed from these salt diapirs.

Regional structure

The Smackover Formation characteristic of Little Cedar Creek field was deposited on a carbonate ramp that formed across the northern part of the Gulf of Mexico. The main characteristics of a ramp structure include reefs of smaller thicknesses that overlay a previously lagoonal environment that results in grainstones of the Norphle transitioning to updip lime mudstones of the Smackover that contrasts the shelf model. Due to the lateral relationship of facies as compared to the carbonate shelf model, the ramp model is a very useful concept for exploring carbonate reservoirs. The ramp model results in an up-dip, coastal environment of limestone (Smackvoer unit) deposition not associated with reefs, while the carbonate shelf model shows limestone units down-dip in a marine reef environment which consists of a distribution of facies that is uncharacteristic of the Smackover Formation in Little Cedar Creek Field.

Stratigraphy and depositional environments

Sea floor spreading and emplacement of new oceanic crust in the deep central Gulf of Mexico during the Late Jurassic resulted in a regional shift to a period of marine transgression. This shift was accompanied by crustal cooling and subsidence, the termination of the deposition of evaporites, and the transition to an open marine environment.

This image depicts the stratigraphic succession of geologic formations in the Little Cedar Creek Field Area

Norphlet Formation

During the Callovian and Oxfordian in the Late Jurassic, basin subsidence and erosion of the southern Appalachian Mountain chain resulted in the deposition of the Norphlet Formation. The Norphlet, in up-dip areas proximal to its provenance, is comprised of conglomerate, sandstones, and shale that were deposited in alluvial fan and fluvial systems. While the depositional processes of the Norphlet farther down-dip from the Appalachian Mountains are governed by aeolian processes.

Smackover Formation (Source & Reservoir)

The Smackover Formation was also deposited in the Oxfordian, and the formation is typically subdivided into three units (Lower, Middle, and Upper) based on lithofacies of the marine conditions at the time of deposition. The Lower Smackover lies above the Norphlet sandstone and is a carbonate facies with microbial algal mats, indicative of tidal flat conditions during the initial deposition of the Smackover Formation. As marine transgression continued, higher energy marine environments produced oncoloidal structures. The Middle Smackover unit is characterized by a continuation of global sea level rise separating it from the Lower Smackover due to the change in depositional facies. The Middle Smackover consists of laminated, mud-rich limestones and mudstones that are high in organic content. This is due to the continued rise in sea level creating a deeper, anoxic marine environment ideal for the preservation of organic content. The organic material preserved from this period becomes hydrocarbons which are extracted from this formation by oil companies today. The Upper Smackover, consisting of cyclic coarsening upwards sequences of peloidal, oncoidal, and oolitic packstones and grainstones, was deposited during the highstand aggradation and progradation of shallow water shoal and tidal flat complexes. The accretion of these sediments formed paleo-highs as shoals in the form of sandbanks in the Conecuh Embayment. The Smackover paleotopography consisting of various ridges and paleo-highs controlled the thickness and depositional patterns in the Smackover in southwest Alabama.

Haynesville Formation (Seal)

The Haynesville Formation overlies the Smackover and is made up of three units. The lower unit is composed of the Buckner Anhydrite which creates a seal for the Upper Smackover. However, in the case of the Little Cedar Creek Field, the Buckner is discontinuous and is replaced by an anhydritic shale as a seal. The middle Haynesville Formation is comprised of interbedded layers of anhydrite, shale, and sandstone which is overlain by the upper Haynesville consisting of a carbonate mudstone unit.

Trapping mechanisms

The Little Cedar Creek Field is an example of a stratigraphic trap that is formed by an abrupt change in the lithology of the rocks deposited. This compositional change is due to changes in the depositional and post depositional environments of these sediments. Stratigraphic traps can only occur if the following three elements are present: a source rock, reservoir, and a seal. In the case of this field, the microbial build-ups in the Lower Smaockover Formation are overlain by high-energy, nearshore facies that comprise the boundstone, grainstone, and packstone reservoir of the Upper Smackover Formation; and the Upper Smackover reservoir is conformably overlain by the anhydrite beds of the Haynesville/ Buckner Formation.

Reservoir characteristics

Depiction of the Little Cedar Creek Field Dual Reservoirs

Source Rock:

Lower Smackover: high concentrations of preserved organic matter, laminated microbial buildups that are rich in amorphous and microbial kerogen, prove to be a producing source rock. S1, the lower Smackover, was deposited after a pre-depositional transgression in the Conecuh Embayment that led the microbial growth that later binded with sediment to form thrombolites that makes up the laminated lime mudstone in S1. The high rate of sedimentation during this time allowed for the rapid burial and preservation of these microbial mats, producing a viable source rock.

Reservoir

Upper Smackover: high-energy, nearshore, coarse, porous boundstones, grainstones, and packstones. The porosity in these microbial reservoirs includes depositionally constructed voids as well as diagenetic solution-enhanced void and vuggy pore types, creating a pore system that provides for high permeability and connectivity in the beds. The permeability range in these dual reservoirs ranges to approximately 7953 md (milidarcys) and porosity as high as 35%. S2, the lower reservoir, is a dolomitic limestone containing the thrombolite facies that was formed in a low energy, tidal setting that is ideal for films from microbial organisms to trap and bind with the bacteria to form the thrombolite accretionary structures. S5, the upper reservoir and the youngest of the dual reservoir in the Smackover, is an oolitic grainstone that has a porosity that ranges from 0-35% due to its oolid nature and other intraclasts . The amount of clasts in the facies supports the idea that the environment of deposition was an intertidal, high energy setting. Both of these reservoirs are oriented in the SW-NE direction. However, the buildups of the thrombolite facies mainly accumulates in the southwest and northeast corner, while the oolitic grainstone facies is concentrated on the southwest corner of the field.

Seal

Haynesville/Buckner: interbedded anhydrites, shales, and tight sandstones. This formation conformably overlies the upper Smackover Formation and the microbial mounds that comprise this section. The low permeability in this formation makes it a viable caprock to the dual reservoirs of the Smackover Formation.

Drilling and production history

The first discovery well in Little Cedar Creek Field was drilled in 1994 by Hunt Oil Company. The well was drilled to a completion depth of 12,100 ft where the primary source of hydrocarbons lies in the Smackover Formation. The Smackover Formation is produced mostly in the range of 11,490 and 11,580 ft in the upper Smackover dual reservoir. Initially, the well had a flow rate of 108 barrels of oil per day and 49,000 cubic feet of gas per day. The oil produced in this field has an API of 45^o. There was only one producing well on this field until 2000, when Midroc Operating Company took over operations on the field. Since the company's takeover, there have been over 120 wells drilled on this field of which over 90 are still producing. Since 2005, Little Cedar Creek Field has been the top oil-producing field in Alabama. Production currently is focused mostly in the northeast and southeast sections, that in total spans more than 22,000 acres. In 2007, gas injections were initiated as a secondary form of recovery in the field. As of 2015, the total production of the Smackover is approximately 20 million barrels of oil (MBO) and 25 billion cubic feet of gas (BCFG).

References

An Overview of the Little Cedar Creek and Brooklyn Fields (n.d.): n. pag. Dec. 2012. Web. 1 Dec. 2015.

Benson, D. Joe. "Depositional History of the Smackover Formation in Southwest Alabama." AAPG Datapages/Archives, n.d. Web. 1 Dec. 2015.

Day, Kyle Lee. Upper Jurassic Microbolite Buildups in the Little Cedar Creek and Brooklyn Fields in SW Alabama. N.p., 2011. Web. 1 Dec. 2015.

Haddad, Sharbel Al, and Ernest A. Mancini. "Reservoir Characterization, Modeling, and Evaluation of Upper Jurassic Smackover Microbial Carbonate and Associated Facies in Little Cedar Creek Field, Southwest Alabama, Eastern Gulf Coastal Plain of the United States." AAPG Bulletin 97.11 (2013): 2059-083. Web.

Hine, Albert C., Shane C. Dunn, and Stanley D. Locker. "Geologic Beginnings of the Gulf of Mexico with Emphasis on the Formation of the De Soto Canyon." Deep-C Consortium, 14 Aug. 2013. Web. 1 Dec. 2015.

"The Smackover of Alabama and Mississippi." Vision Exploration, n.d. Web. 1 Dec. 2015.