Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 250, part 2
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Sedimentologic and Diagenetic Characteristics

Facies, Depositional Environments, and Cyclicity

Arbuckle Group and equivalent-age rocks from outside of Kansas are described as platform deposits dominated by ramp-type subtidal to peritidal carbonates, which can be subdivided into cycles based on facies type and depositional patterns. Individual cycles (from less than 0.5 m to 40 m thick) in adjacent areas (e.g. Oklahoma, Arkansas) are numbered at between 350-450 (e.g. Wilson et al., 1991). These individual cycles recently have been shown to stack into cycle sets or bundles. These cycle sets in turn make up larger-scale depositional sequences that are bounded by regional and inter-regional sequence boundaries. The major sequence boundaries usually contain evidence of subaerial exposure, indicating relative sea-level falls.

Starting in the 1980's, researchers began describing in more detail the lithologic and diagenetic features of Arbuckle strata in Kansas. The results of these studies indicate that the Arbuckle strata in Kansas are very similar in character to Arbuckle and equivalent-age strata described in other localities. Conley (1980) studied thin sections from three Arbuckle cores on the CKU. Despite almost complete dolomitization, he identified primary Arbuckle depositional textures that range from mudstone to packstone and identified oolites, pellets, intraclasts, gastropods, echinoderms, bivalves, and stromatolites. He also recognized intercrystalline, fracture, moldic, fenestral, and interparticle porosity types that range from 1 to 22%. Conley (1980) noted that porosity variability in part could be due to original depositional texture.

Dalal (1987) did a study on the characterization of the Arbuckle reservoir at the Lyons underground gas storage field in Rice County. Using insoluble residue data, he determined that the Arbuckle in the study location consists of Roubidoux Dolomite and undifferentiated Jefferson City Dolomite and Cotter Dolomite. Dalal (1987) observed that the reservoir consists of dolomitized stromatolite and silicified oolitic facies, which he used to interpret shallow subtidal (open-marine to lagoons permanently below low tides) environments for deposition. He also noted vertical variations in porosity and permeability, due to the tight silicified oolitic intervals alternating with permeable dolomitized stromatolitic intervals.

Ramondetta (1990) mentioned that Arbuckle strata in the El Dorado field (Nemaha uplift) are cyclic, upward- shoaling, and generally 10-20 ft thick. He noted that dolomudstones form the base of each cycle and grade upward to fossiliferous and oolitic grainstones up to 6 ft thick. Ramondetta (1990) indicated 15-20% vuggy and intercrystalline porosities and over 100-md permeability in grainstones and local intercrystalline porosity in the predominantly tight dolomudstones.

Franseen (1994, 2000) documented the existence of subtidal to peritidal cycles in several cores located near the Central Kansas uplift. He noted that the facies appeared, at least locally, to be arranged in cyclic succession (likely shoaling-upward cycles) on the order of a meter to 5 m or more. Depositional facies in Arbuckle strata consist predominantly of coarse-grained skeletal, intraclastic, oolitic, peloidal, dolograinstones/packstones and finer-grained dolowackestones/mudstones, thrombolites and stromatolites, intraclastic conglomerate and breccia, and minor shale and siltstone. Silicified nodules and lenses are locally abundant and some may represent replacement of original evaporite minerals. Scoured, erosional surfaces occur within the Arbuckle with mm-dm scale erosional relief. Intraclastic breccia and conglomerate commonly overlie these surfaces. Secondary features include dolomitization, breccia, fractures, and conglomerate related to intra-Arbuckle subaerial exposure, and breccia, fractures, and conglomerate related to later karst, burial or structural processes, silicification, and local mineralization (Franseen, 1994, 2000).

Steinhauff et al. (1998) documented subtidal and peritidal facies and the paragenetic sequence of events in 33 cores distributed across Kansas. They identified several scales of cycles (ranging from < 1 m to several meters) and cycle sets in cores and related cycles to electric log signatures for preliminary construction of an internal, high-resolution stratigraphic framework and correlation in the central Rice County, Kansas, study area (figs. 19, 20).

Figure 19--Cross section of cores and corresponding gamma-ray logs in Rice County, on the Central Kansas uplift, showing shoaling-upward cycles and units defined by marker beds that provide correlation within the area. Note uppermost cycles are commonly truncated or partially truncated by the pre-Pennsylvanian unconformity surface. From Steinhauff et al. (1998).

Figure 20--#11-05 Truesdell core and corresponding gamma-ray log showing lithologic details of shoaling-upward cycles. See fig. 19 for lithologic details. From Steinhauff et al. (1998).

Development of stratigraphic and sequence-stratigraphic frameworks and correlation of Arbuckle rocks remains a primary objective in Kansas and is crucial for providing frameworks for subsequent diagenetic and structural studies. Continued documentation of cycles and sets of cycles in the Arbuckle of Kansas could aid in correlations. Studies of Arbuckle and equivalent-age strata from other areas have shown the utility of cycle-stacking patterns within the context of sequence stratigraphy as an important tool in correlating Arbuckle strata and interpreting sea-level fluctuations and orbital-forcing mechanisms that were important during Arbuckle deposition (e.g. Kerans and Lucia, 1989; Wilson et al., 1991; Kupecz, 1992; Montanez, 1992; Goldhammer et al., 1993; Overstreet et al., 2003). Incorporating the ever-increasing amount of modern well-log data that extend deeper into the Arbuckle with additional core data will be vital to establishing stratigraphic frameworks and correlation of Arbuckle strata. To date, no biostratigraphic data or chronostratigraphic correlations exist for the Arbuckle Group in Kansas. Conodonts (phosphatic microfossils that are found abundantly in marine rocks of Late Cambrian through Triassic age) have proven useful for stratigraphic subdivision and correlation of the uppermost Cambrian and throughout the entire Ordovician (Derby, Bauer, et al., 1991; Derby, Hinch, et al., 1991). Conodont studies could be useful in development of a chronostratigraphy for Arbuckle strata in Kansas, if more cores that extensively penetrate the entire Arbuckle Group become available.

Franseen (1994, 2000) and Steinhauff et al. (1998) identified facies that seem to be characteristic of Arbuckle strata in Kansas. Although Arbuckle rocks have been extensively dolomitized, much of the dolomitization is non-fabric destructive, thereby preserving original depositional facies textures. The 10 principal facies identified include (1) clotted algal boundstone, (2) laminated algal boundstone, (3) peloidal packstone-grainstone, (4) mixed packstone-grainstone, (5) ooid packstone-grainstone, (6) wackestone-mudstone, (7) intraclastic conglomerate and breccia, (8) cave fill shale, (9) depositional shale, and (10) chert. The first six lithologies listed account for more than 85% of the cored intervals described and the rest account for the remaining 15%. The facies descriptions from those studies are the most detailed published facies descriptions for Kansas Arbuckle rocks to date, and are repeated below.

Clotted Algal Boundstone

This facies consists of muddy, peloid-rich mottled (thrombolitic) to wavy-laminated clotted algal-carbonate lithology (fig. 21A). Other clotted forms (? Renalcis) are also present. Thrombolite-leopard rock texture is typically muddy with some grains. Local burrow mottling is present. Thrombolitic and clotted boundstones have a tightly bound matrix consisting of anhedral, euhedral, and polyhedral dolomite (< 0.5 mm) with peloidal cement. Thrombolitic boundstones typically have excellent sheetlike vuggy and fenestral porosity and poor intercrystalline porosity. Most thrombolitic boundstones were probably deposited in subtidal environments.

Figure 21--Core photographs of major Arbuckle facies. Note the width of cores in each photo is approximately 3.5 inches. A) Clotted Algal Boundstone. This facies consists of muddy, peloid-rich mottled (thrombolitic) to wavy-laminated clotted algal carbonate lithology. Porosities are generally less than 6% and permeabilities are below 0.1 md. B) Laminated Algal Boundstone (muddy). This facies consists of wavy-laminated algal boundstones and stromatolites. Porosities are generally less than 6% and permeabilities are below 0.1 md. C) Laminated Algal Boundstone (grainy). This facies consists of wavy-laminated algal boundstones and stromatolites, and represents some of the best reservoir rock with porosity up to 32% and permeability up to 1,500 md. D) Peloidal Packstone-Grainstone. This facies is typically massive, horizontally laminated, or bedded. Porosities range from 0% to 4%, and absolute permeabilities range from 0.0003 md to 0.1 md but are generally below 0.005 md. E) Packstone-Grainstone. This facies is typically massive, horizontally bedded, or crossbedded. Porosities range from 6% to 18%, and permeabilities range from 0.1 md to 50 md. F) Ooid Packstone-Grainstone. This facies is typically massive, horizontally bedded, or crossbedded. Porosities range from 11% to 30%, and permeabilities range from 10 md to 1,500 md. G) Wackestone. This facies is typically massive to horizontally laminated. Porosities range from 2% to 11%, and permeabilities range from 0.01 md to 1 md. H) Mudstone. This facies is typically massive to horizontally laminated. Porosities range from 0% to 10%, and absolute permeabilities range from <0.0001 md to 0.1 md. I) Intra-Arbuckle Shale. Some shales are interbedded with carbonate rocks suggesting they were deposited during Arbuckle deposition. Shales are tight and represent permeability barriers. J) Breccia. Brecciation and fracturing occur with various textures. This example shows chaotically oriented clasts of various lithologies. Breccia facies typically have variable porosities and permeabilities that are primarily a function of the lithologies that were brecciated. K) Fracture-fill Shale. Much of the shale is green and clearly present as fracture or cave fill, with sediment originating from above the upper Arbuckle unconformity surface. This shale occludes original fracture porosity. L) Chert. Chert (white area) locally occurs as a replacement of carbonate facies. Chert replacement commonly results in tight and impermeable areas.

Laminated Algal Boundstone

This facies consists of wavy-laminated algal boundstones and stromatolites with muddy (fig. 21B) to grainy textures (fig. 21C). Current-modified (ripple) lamination occurs locally. Brecciated stromatolite facies typically grades upward to non-brecciated, in-place stromatolites. The stromatolites are locally tightly cemented but commonly contain abundant and distinctive differentially developed intercrystalline, fenestral, keystone-vug, and solution-enlarged porosity that closely follows laminations. This facies likely represents subtidal to peritidal (where fenestrae are present) environments. This facies is locally oil stained.

Peloidal Packstone-Grainstone

This facies is typically massive, horizontally laminated or bedded (fig. 21D), and commonly interbedded with coarser-grained lithologies. Locally, it contains wispy lenses of shale and interbedded shale layers. Burrow traces and mottling are common. Peloids are abundant and rare interclasts, lumps, and skeletal grains (gastropods) are present. Soft-sediment deformation, dewatering or tepee-like structures, mudcracks, and rip-up clasts are locally associated with this facies. The mud-rich peloidal packstone/grainstone facies represents deposition in relatively lower-energy subtidal (massive to burrowed textures) and peritidal (mudcracks, tepees, and rip-up layers and clasts) settings. This rock is tightly bound, consisting of anhedral, euhedral, and polyhedral dolomite (< 0.5 mm) and peloidal cement.

Mixed Packstone-Grainstone

This facies is typically massive, horizontally bedded or crossbedded (fig. 21E), and typically interbedded with ooid packstone-grainstone and wackestone-packstone facies. Grains include intraclasts, skeletal and algal fragments, ooids, peloids, and lumps. This facies indicates higher energy deposition in subtidal to peritidal (fenestrae and keystone vugs) settings. Locally, packstone-grainstone is tightly cemented by euhedral dolomite (< 0.5 mm). However, this facies typically has good intercrystalline porosity. In some rocks the original cement between grains has been leached, creating interparticle porosity that is open or filled with chert. The chert has a "chalky" appearance and is porous but exhibits low permeability.

Ooid Packstone-Grainstone

This facies (fig. 21F) is typically massive, horizontally bedded or crossbedded, and typically interbedded with wackestone-packstone facies. Dominant grains are ooids, but other grains including intraclasts, skeletal and algal fragments, peloids, and lumps occur in varying abundance. This facies indicates higher energy deposition in subtidal to peritidal (fenestrae and keystone vugs) settings. This facies typically has good intercrystalline porosity, but locally is tightly cemented by euhedral dolomite (< to 0.5 mm). In some rocks the original cement between grains has been leached creating interparticle porosity that is open or filled with chert. This chert has a "chalky" appearance and is porous but exhibits low permeability.

Wackestone-Mudstone

This facies is typically massive to horizontally laminated (figs. 21G, 21H). Burrow mottling is typically present in most intervals. This facies typically is composed of euhedral dolomite (< 0.05 mm) with little or no porosity. Replacement of evaporite nodules with chert is observed locally. This facies is interpreted as being deposited in shallow-water, low-energy restricted environments.

Intra-Arbuckle Shale

Some shales are interbedded with carbonate rocks suggesting they were deposited during Arbuckle deposition (fig. 21I). In addition, horizons with wavy-horizontal to horizontally interbedded shale and carbonate mudstone-wackestone are present. Several shale layers contain silicified nodules and lenses that may have replaced evaporites. These shale layers likely represent relatively low-energy subtidal to peritidal conditions. Supratidal conditions may be indicated for some horizons where silicified nodules apparently represent replacement of original evaporite minerals.

Conglomerate and Breccia

Many conglomerates or breccias consist of rip-up clasts derived from underlying lithologies. Textures range from clast- to matrix-supported. Conglomerates and breccias are commonly associated with desiccation and mud cracks, dewatering structures, and tepees. Local autoclastic breccia textures indicate subaerial exposure of some Arbuckle horizons. Some collapse breccia may have resulted from the dissolution of evaporites. These conglomerates evidence intra-Arbuckle high-energy erosional and subaerial exposure events in subtidal to peritidal settings. The conglomerate and breccia facies typically have variable porosities and permeabilities that are primarily a function of the lithologies that were brecciated. Later brecciation and fracturing occurs with various textures ranging from incipient fracturing and brecciation with a fitted clast texture to extreme brecciation with chaotically oriented clasts of various lithologies (fig. 21J). The features are consistent with a karst origin from exposure at the post-Sauk unconformity. These late-stage breccias and fractures are variably open to tight.

Fracture-fill Shale

Much of the shale is green and clearly present as fracture (fig. 21K) or cave fill, with sediment originating from above the upper Arbuckle unconformity surface. Locally, fracture fills contain fragments of dolomite rhombs and subangular to rounded silt-size to coarse-grained detrital quartz grains.

Chert

Chert commonly occurs as a replacement of carbonate facies (typically preserving original textures) and, locally, original evaporite minerals. Chert replacement commonly results in tight and impermeable areas (fig. 21L). Locally, where only partial replacement occurs, some vuggy and intercrystalline porosity is developed.

Diagenetic Features and Porosity Types

Franseen (1994, 2000) and Steinhauff et al. (1998) noted the following general paragenetic sequence of events listed from oldest to youngest: (1) deposition of original facies including intraformational erosion and subaerial exposure events as indicated by autobrecciation, mudcracks, fenestrae, and rip-up conglomerates; (2) early dolomitization evidenced by silicified dolomite rhombs in chert clasts that occur in later fractures; (3) silicification, as indicated by displacive growth textures of silica areas in dolomite, differential compaction between silica (brittle fracturing) and surrounding dolomite matrix (soft-sediment deformation), and silicified carbonate grains and facies occurring as clasts (with truncated grains at clast boundaries) occurring in later fractured and brecciated areas; (4) subaerial exposure and karstification producing the post-Sauk unconformity; open, partially occluded, or fully occluded fractures and breccia fabrics; and dissolution that cut across all previous events; and (5) burial. Associated features include later dolomitization(s), including some baroque dolomite, mineralization, compaction, and fracturing that cuts across all previous features.

These secondary features significantly affected the original depositional facies. However, although the dolomitization is pervasive, in cores studied to date it is mostly non-fabric destructive. Later brecciation and fracturing occur with various textures, ranging from incipient fracturing and brecciation with a "fitted" clast texture and little clast rotation to extreme fracturing and brecciation with chaotically oriented clasts of various lithologies. This later-stage fracturing and brecciation also is characterized by various types of sediment fill and locally by cements. Many of these late-stage fractures are only partially filled, or in some cases completely open, and represent significant effective secondary porosity. Some of these features are similar to those described by Kerans (1988) in the Ellenburger that he attributed to karsting associated with the pre-Simpson Group (Middle Ordovician) subaerial exposure event. Several horizons in the cores studied by Franseen (1994, 2000) and Steinhauff et al. (1998) show an upward transition of chaotic clast-supported or matrix-supported breccia texture to a fitted, fracture breccia texture with little to no clast rotation, which may represent a transition from cave-collapse zone upward to cave-roof facies possibly developed during the post-Sauk subaerial exposure event. Elsewhere, fractures appear to have preferred orientations that likely reflect a tectonic influence. In addition to the open fractures, other late-stage porosity development includes vuggy, channel, and interparticle porosity, much of which is likely related to the post-Arbuckle exposure event.

Franseen (1994, 2000) and Steinhauff et al. (1998) noted that the striking feature in many cores is the abundance and apparent importance of "matrix" porosity (intercrystalline, moldic, fenestral, vuggy) throughout the entire lengths of the cores, which is related to depositional facies, early diagenesis, and dolomitization and not necessarily related to the upper post-Sauk subaerial exposure surface. Their initial observations indicate that more than 50% of the preserved porosity are these types of "matrix" porosity. Much of the matrix porosity-rich intervals are associated with coarse-grained, laminated to bedded facies that are differentially cemented or with stromatolitic intervals that show differential porosity development likely due to differences in original texture (e.g. mud content) and early diagenesis (e.g. development of fenestral and vuggy porosity during early subaerial-exposure events).

Core-Scale Petrophysical Trends

Byrnes et al. (1999) collected petrophysical data from a number of core-plug samples of the different Arbuckle facies described above. That study emphasized the important contribution of original facies character and matrix properties to reservoir character (fig. 22). The following is a summary of petrophysical properties from Byrnes et al. (1999).

Figure 22--Core-plug petrophysics. Petrophysical properties of the facies at the core-plug scale are generally controlled by matrix grain size. Each lithology exhibits a generally unique range of petrophysical properties. All lithologies exhibit increasing permeability with increasing porosity and can be characterized as lying along the same general porosity permeability trend. Note that fracturing of lithologies enhances permeability but does not add significantly to porosity. Vuggy pores can be well connected where vuggy porosity is extensive near the unconformity surface.

Peloidal Packstone-Grainstone: Cementation of matrix has resulted in nearly total occlusion of porosity. Porosities range from 0% to 4% and absolute permeabilities range from 0.0003 md to 0.1 md but are generally below 0.005 md. Fenestrae within this facies may range up to several centimeters in length and may enhance porosity by several percent. Fenestrae in this lithology are not interconnected but are isolated by low-permeability matrix.

Mudstone: Without fractures or fenestrae, these exhibit porosities ranging from zero to 10% and absolute permeabilities ranging from < 0.0001 md to 0.1 md. Where fenestrae are present, porosity may be enhanced up to values as high as 17%; however, the fenestrae are primarily isolated and permeabilities are not increased significantly.

Wackestone: Without vugs these exhibit porosities ranging from 2% to 11% and permeabilities ranging from 0.01 md to 1 md. Where vugs are present, porosities can range from 9% to 17%, and permeabilities can range from 1 md to 1,000 md.

Packstone: Porosities range from 6%, for finer-grained rock and where packstone is mottled with wackestone, to 18% for cleaner more coarse-grained rock. Permeabilities in the packstone lithology range from 0.1 md to 50 md.

Ooid Packstone-Grainstone: Generally these contain little to no vuggy porosity but exhibit intercrystalline and moldic porosities ranging from 11% to 30%; associated permeabilities range from 10 md to 1,500 md. The highest porosity and permeability values are exhibited by clean, homogeneous, medium-grained moldic packstones.

Muddy Clotted and Laminated Algal Boundstones: Laminated muddy algal boundstones exhibit porosities generally less than 6% and permeabilities below 0.1 md. Where fenestral or vuggy porosity is developed, these may exhibit high permeability at the core-scale, but it is unlikely that these high permeabilities are laterally pervasive at the interwell scale.

Laminated Grainy Algal Boundstones: Laminated grainy algal boundstones represent some of the best reservoir rock ranging in porosity up to 32% and permeability up to 1,500 md.

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