End Member Arbuckle Reservoir Types - An Emerging Picture

The various aspects of research on Arbuckle strata in Kansas are providing insight on Arbuckle reservoir character and directions for future research to improve the emerging picture of Arbuckle reservoir types. Arbuckle strata consist of complex stacks of reservoir and non-reservoir lithologies at various frequency and thickness scales. Knowing this, the Arbuckle in Kansas can be viewed as having three end-member reservoir architectures, representing fracture-, karst-, and matrix-dominated architectural systems (Figure 17), with numerous possible variations between these end-member architectures. Notably, in all three architectures, lithofacies and stratal packaging of reservoir and non-reservoir strata exert a control on reservoir architecture, despite other processes that affect the rocks.

The first end-member type is dominated by fractures, possibly related to faults in structurally complex areas (Figure 17A). In this type of reservoir scenario, lithofacies control porosity and permeability within individual layers (beds), but fractures dominate overall permeability.

The second end-member type of reservoir is the classic conceptual reservoir model for Arbuckle strata in Kansas. In this scenario processes associated with karst overprint lithofacies and fractures, resulting in complex porosity and permeability (Figure 17B). However, karst overprinting can vary laterally from being extensive to insignificant. Also, even where extensively developed, karst processes can both enhance or destroy reservoir properties. For example, open fractures, caves, and vugs in reservoir facies near the unconformity may enhance permeability. However, open fractures and vugs in non-reservoir facies are likely to be insignificant in enhancing overall reservoir character. This emphasizes the importance of understanding which lithologies intersect the unconformity. Additionally, infilling of fractures and vugs with karst sediment and cement associated with karst can completely destroy reservoir properties, even in favorable reservoir facies.

The third end-member type of reservoir shows relatively little fracturing and karst overprinting (Figure 17C). In this reservoir type, lithofacies control porosity and permeability, and laterally persistent stratal packages may exert significant control on flow properties. These types of reservoirs may be significant, especially where karst overprinting is not extensive, in areas away from faults, in areas off structural highs, and stratigraphically deeper within the Arbuckle.

Figure 17: End-member Arbuckle reservoir types. A) Abundant fracturing of strata. Lithofacies control porosity and permeability within individual layers (beds), but fractures dominate overall permeability. B) Karst overprinting on lithofacies and fractures, resulting in complex porosity and permeability. Karst overprinting can vary laterally from insignificant to extensive. Even where extensively developed, karst processes can both enhance and destroy reservoir properties. C) Lithofacies control porosity and permeability, and laterally persistent stratal packages may exert significant control on flow properties.