The Geology of the Arbuckle Group in the Midcontinent<subtitle>Sequence Stratigraphy, Reservoir Development, and the Potential for Hydrocarbon Exploration</subtitle>

Abstract

The Arbuckle Group of the midcontinent comprises the mid-southern part of the great American carbonate bank (GACB) and consists mostly of carbonates with a few laterally consistent sandstones. The Arbuckle Group is found in the Anadarko, Ardmore, and Arkoma Basins and surrounding environs in the Texas panhandle, Oklahoma, and Arkansas. These basins represented a significant downwarp associated with early rifting in the area now located in the southern one half of both the states of Oklahoma and Arkansas. Similar to other parts of the GACB, the thick widespread Cambrian–Ordovician Arbuckle Group was deposited as mostly restricted shallow-water marine carbonates. The Arbuckle is a cyclic carbonate dominated by intertidal and shallow subtidal facies. In some areas, supratidal or deeper subtidal facies are observed. The depositional model is represented by an extensive, dominantly regressive, tidal flat with persistent peritidal facies across much of the GACB. These peritidal cycles shallow upward with significant variation in thickness from as thin as 4 ft (1.2 m) to more than 110 ft (33.5 m) thick. Large-scale regional changes in relative sea level may have had a large influence on the type of cycles and sequences that formed during Arbuckle deposition. Arbuckle strata, especially within third-order sequence boundaries, are correlatable across the basin. Within the sequence boundaries, cycles can be further grouped into packages of sequences that are composed mostly of either intertidally or subtidally dominated cycles. Detailed local to regional correlation of the facies bundles can be made with gamma-ray and resistivity logs; however, facies are commonly obscured by a strong diagenetic overprint that makes detailed correlation difficult. Reservoirs in the Arbuckle are complex, and porosity is controlled by original depositional fabric, diagenesis, paleokarst, and fracture overprint. Upper subtidal and lower intertidal facies typically have the depositional fabric most conducive to reservoir development. Diagenetic changes are a continuum that begins with early diagenesis, including hypersaline or evaporative conditions as well as vadose and phreatic conditions, and followed by deep phreatic to late thermal diagenesis. Evidence that porosity formed during multiple diagenetic phases exists. Dolomitization and precipitation events are also evidenced at various levels of the profile. Dolomite is the most abundant mineral and can be subdivided into early (syngenetic to penecontemporaneous) hypersaline dolomite, shallow burial mixed-water (phreatic) dolomite, and deeper burial to thermal (baroque and xenotopic) dolomite. The super-Sauk unconformity is recognized as evidence of a eustatic sea level drop and has been used to mark the boundary between the Sauk and Tippecanoe depositional megasequences. The Arbuckle Group contains multiple unconformities at major sequence boundaries. Paleokarst is especially prevalent beneath the super-Sauk unconformity, especially along major sequence boundaries with related unconformity surfaces. Paleokarstic features in the Arbuckle Group have been identified in outcrop in the Arbuckle Mountains of southern Oklahoma and in the southern Ozark uplift in northeastern Oklahoma. Numerous cores and logs indicate collapse breccias that are interpreted to have formed in response to karst conditions. The Arbuckle Group is an important petroleum reservoir in the midcontinent, and has great potential especially for natural gas. Exploration is enhanced by understanding the complex relationships of depositional processes, stratigraphic relationships, paragenesis, and structural overprints. Reservoir development is typically along sequence boundaries, especially where facies have strong diagenetic overprints from dolomitization and dissolution associated with paleokarstic events. No major source rocks exist below or within the Arbuckle Group, so the best reservoirs are structurally related with strong fracture overprints and juxtaposed with source rocks or are along migration pathways.