Lithofacies

Detailed study of 12 cores from four counties in central Kansas and reconnaissance of about 20 other Arbuckle cores throughout Kansas (Figure 7a) suggest that the facies described in this paper are 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. Ten principal facies are evident. Listed in approximate order of relative abundance, with the most abundant facies first, these include:

The first six lithologies listed account for more than 85% of the cored intervals described and the rest account for the remaining 15%.

	Clotted Algal Boundstone: This facies consists of muddy, peloid-rich mottled (thrombolitic) to wavy laminated clotted algal carbonate lithology (Figure 8A). 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 sheet-like vuggy and fenestral porosity and poor intercrystalline porosity. Most thrombolitic boundstones were probably deposited in subtidal environments.
Laminated Algal Boundstone: This facies consists of wavy laminated algal boundstones and stromatolites with muddy (Figure 8B) to grainy textures (Figure 8C). 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 (Figure 8D), 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 (Figure 8E), 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 (Figure 8F) 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 inter-crystalline 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 (Figures 8G, 8H). 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 (Figure 8I). 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 has 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 (Figure 8J). 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 (Figure 8K) 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 (Figure 8L). Locally, where only partial replacement occurs, some vuggy and intercrystalline porosity is developed.

Dominance of Early Matrix Porosity: 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. The relative lack of karst associated fracture, breccia, and dissolution porosity in most cores is especially surprising considering that the cores came from flanks or tops of structural highs where karst processes would likely have been most extensive

Observations indicate that more than 50% of the preserved porosity is “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).

Figure 8: 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 0.1md. 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 0.1md. 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 1,500md. D) Peloidal Packstone-Grainstone. This facies is typically massive, horizontally laminated or bedded. Porosities range from 0% to 4% and absolute permeabilities range from 0.0003md to 0.1md but are generally below 0.005md. E) Packstone-Grainstone. This facies is typically massive, horizontally bedded or crossbedded. Porosities range from 6%, to 18%, and permeabilities range from 0.1md to 50md. F) Ooid Packstone-Grainstone. This facies is typically massive, horizontally bedded or crossbedded. Porosities range from 11% to 30%, and permeabilities range from 10md to 1,500md. G) Wackestone: This facies is typically massive to horizontally laminated. Porosities range from 2% to 11% and permeabilities range from 0.01md to 1md. H) Mudstone: This facies is typically massive to horizontally laminated. Porosities range from zero to 10% and absolute permeabilities range from <0.0001md to 0.1md. 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 occurs 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.