Arbuckle Reservoirs in Central Kansas: Relative Importance of Depositional Facies, Early Diagenesis and Unconformity Karst Processes on Reservoir Properties

Kansas Geological Survey Open-file Report 98-55

Reservoir and Non-reservoir Facies

Ten principal facies are evident in the cores studied. Listed in approximate order of relative abundance, with the most abundant facies first, these include: (1) Clotted algal boundstone, (2) laminated algal boundstones, (3) peloidal packstone-grainstone, (4) 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%.

Clotted Algal Boundstone: This facies consists of muddy, peloid-rich mottled (thrombolitic) to wavy laminated clotted algal carbonate lithology. Other clotted forms (Renalcis?) are also present. Thrombolite-leopard rock texture is typically muddy with some grains. 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; fenestral porosity evidences at least local subaerial exposure.

Laminated Algal Boundstone: This facies consists of wavy laminated algal boundstones and stromatolites with muddy to grainy textures. Current-modified (ripple) lamination occurs locally. Brecciated stromatolite facies typically grades upwards to non-brecciated, in-place stromatolites. The stromatolites are locally tightly cemented but commonly contain abundant and distinctive differentially developed intercyrstalline, 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 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 teepee-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.
Packstone-grainstone: This facies is typically massive, horizontally bedded or crossbedded 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 pertidal settings (fenestrae and keystone vugs). Locally packstone-grainstone is tightly cemented by euhedral dolomite (< to 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. This chert has a "chalky" appearance and is porous but exhibits low permeability. This facies typically has good intercrystalline porosity.

Ooid packstone-grainstone: This facies is typically massive, horizontally bedded or crossbedded and typically interbedded with undifferentiated packstone-grainstone and wackestone-packstone facies. Ooids compose more than 80% of the grains but other coated grains, peloids, and lumps are also present. This facies indicates higher energy deposition in subtidal to pertidal settings. This facies typically has good oomoldic and intercrystalline porosity.

Wackestone-mudstone: This facies is typically massive to horizontally laminated. 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 in some intervals. 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. 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 teepees. 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.

Fracture-fill Shale: Much of the shale is green and clearly present as fracture or cave fill, with sediment originating from above the unconformity. Locally, fracture fills contains 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. Locally, where only partial replacement occurs, some vuggy and intercrystalline porosity is developed.

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