Kansas Geological Survey, Open-file Report 91-52, p. 98-107
Tulsa, Oklahoma 74170
Geological applications for horizontal drilling have proved most effective in tight fractured reservoirs, heterogeneous paleokarst reservoirs, and porous and permeable reservoirs with coning problems. A modified version of standard fracture classification can be used to define most potential horizontal drilling targets in the midcontinent into four types (fig. 1). Carbonate reservoirs with potential horizontal drilling (HD-reservoir) such as the Cambro-Ordovician Arbuckle Group can be further categorized by facies-related reservoir characteristics.
Figure 1--Geological classification model for the four basic types of HD-reservoirs (MASERA, 1991).
Tight fractured carbonate reservoirs, with similar features to the Bakken Shale, are often developed in low-energy basinal settings. These reservoirs are characterized by little or no effective matrix porosity with the only porosity and permeability provided by fractures. The Mississippian Sycamore Limestone and Devono-Mississippian Woodford Shale currently produce from vertical wells in southern Oklahoma and are indicative of this first type of potential HD-reservoir (Type A); they are currently key horizontal drilling targets in the midcontinent (fig. 1).
The Austin Chalk is a basinal pelagic carbonate and typifies the second type of HD-reservoir(Type B), which has fair to good matrix porosity with very low permeability (fig. 1). The only producing chalk reservoirs near the midcontinent region are in the northern and northwestern parts of the Denver basin, as exemplified by the Upper Cretaceous Niobrara Formation.
Although in the northern part of the midcontinent area, Ordovician Viola carbonates and the "Mississippi lime" contain high-energy deposits, in Oklahoma they were deposited in relatively low-energy subtidal conditions on shallow marine ramps and can have porosity and permeability profiles similar to chalks. Both reservoirs require fracturing for good production and are actively being pursued as horizontal drilling candidates.
The limestones and dolomites of the Siluro-Devonian Hunton Group are similar to those of the Viola and "Mississippi lime" in that they were deposited on a carbonate ramp. Except for the Frisco and Sallisaw formations, reservoirs in the Hunton Group are often composed of interfingering upper subtidal, intertidal, and supratidal facies. Although these carbonates represent different facies, their reservoir characteristics are somewhat similar to those found in the Permian-age Abo "Reef" trend reservoirs in southeastern New Mexico which define the third type of HD-reservoir (Type C). These carbonates are characterized by their heterogeneity and good porosity with fair permeability, which is often complemented by fractures (fig. 1). Reservoir heterogeneity is further complicated by a long and varied diagenetic overprint.
Platform margin carbonates, in particular reefs, may also represent good horizontal drilling targets; however, there are few significant reefal buildups in the southern midcontinent region. Mud mounds, such as those found in the Devonian Frisco Limestone, are a type of reef and can be quite heterogeneous.
Although the fourth type of HD-reservoir (Type D) may be found in carbonates, the analogue is the Permo-Triassic Sadlerochit Sandstone in Prudhoe Bay field (fig. 1). This fourth type of HD-reservoir may or may not be fractured and is represented by zones with good porosity and permeability. with either gas- or water-coning problems. These zones are usually homogeneous and in this case an abundance of fractures may actually be detrimental to the reservoir and increase coning problems.
Some carbonates, such as the platform carbonates of the Arbuckle Group, may contain examples of more than one or even all four types of HD-reservoirs (fig. 2). Type A reservoirs may be exploited from fractured tidal flat deposits such as those found in core from the Pure Pruitt No. 1 well in sec. 26, T. 3 S., R. 1 E. in Carter County, Oklahoma (pl. I). This core is mostly limestone with minor amounts of dolomite. It is extensively fractured with very little matrix porosity.
Figure 2--Arbuckle HD-reservoir model (MASERA, 1990).
Most Arbuckle production is from rather heterogeneous fractured and karstic dolomites which represent both Type B and Type C HD-reservoirs. Core from the Shell Wesley Unit No. 1A-3 in sec. 3, T. 4 S., R. 3 W. in Carter County, Oklahoma, has the qualities of Type B HD-reservoir in that it has fair porosity defined by intercrystalline pores with fractures providing much of the permeability (pl. II). The rock is composed mostly of thermal dolomite.
The E. L. Cox Wesley No. A-1 well in sec. 3, T. 4 S., R. 3 W. also in Carter County, Oklahoma, contains core which represents a Type C HD-reservoir even though it is from the same stratigraphic interval as in the Shell Wesley Unit No. 1A-3. Much of the rock has been altered by karstification and thermal dolomitization (pl. III). The rock is very heterogeneous with fair to good matrix porosity and permeability.
Although most Type D reservoirs in the midcontinent are sandstones, extremely porous reservoirs within the Arbuckle, such as shown in core from the Texaco Osage No. C-1 well in sec. 24, T. 20 N., R. 11 E. in Osage County, Oklahoma, are also considered as Type D reservoirs, especially those zones with coning problems (pl. IV).
The midcontinent has an abundance of potential HD-reservoirs due in part to its complex structural history. Fractures and heterogeneity are the most important factors in developing good HD-reservoirs. Heterogeneity is often caused by secondary processes such as karstification, which is pervasive in midcontinent carbonates due to the presence of numerous disconformities and several unconformities. Fractured and karstic carbonates such as these of the Arbuckle Group provide excellent targets for future horizontal drilling.
[Note: An Acrobat PDF file containing all of the plates is available.]
Plate I--Pure Oil Pruitt Unit No. 1 (8,573-8,589 ft).
Plate II--Shell Oil Wesley Unit No. A-1 (3,584-3,598 ft).
Plate III--Edwin L. Cox Wesley A-1 (3,806-3,822 ft).
Plate IV--Texaco Osage No. C-1 (3,397-3,415 ft).
Kansas Geological Survey
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Web version created July 3, 2012. Original publication date 1991.