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Kansas Geological Survey, Open-file Report 1999-49


A Review of Arbuckle Group Strata in Kansas from a Sedimentologic Perspective

by
Evan K. Franseen

KGS Open-file Report 1999-49

Abstract

Arbuckle Group and equivalent-age rocks (Cambrian and Lower Ordovician) represent an important record of sediment deposition in the history of the North American continent and they contain important accumulations of hydrocarbons (oil and gas) and base metal deposits. This is true for Kansas as well where Arbuckle strata account for approximately 40% of the volume of produced petroleum and known reserves. However, in comparison to their counterparts in other areas, such as the Ellenburger and Knox, Arbuckle rocks in Kansas remain relatively understudied, especially with respect to sedimentology and diagenesis.

The Arbuckle is present in the subsurface in most of Kansas and is absent only in areas of northeastern and northwestern Kansas, and over ancient uplifts and buried Precambrian highs. Arbuckle rocks thicken from north to south and are up to 1390 feet in the southeastern corner of Kansas. Arbuckle Group and equivalent-age rocks from Kansas and surrounding areas are similar, consisting of platform deposits dominated by ramp-type subtidal to peritidal carbonates (mostly dolomitized) which can be subdivided into cycles, less than 0.5 m to 40 m thick, based on facies type and depositional patterns. Recent studies from central Kansas show that major depositional facies consist of coarse-grained packstones/grainstones, fine-grained packstones/wackestones/mudstones, stromatolites-thrombolites, intraclastic conglomerate and breccia, and shale. In addition, secondary features include dolomitization, breccia, fracture and conglomerate related to early subaerial exposure and later karst, burial or structural processes, silicification, and local mineralization.

Arbuckle and equivalent strata in the Midcontinent were affected by prolonged subaerial exposure that began immediately after Arbuckle deposition, forming the sub-Tippecanoe to sub-Absaroka unconformity. Favorable reservoir qualities are generally thought to be directly related to basement structural elements and karstic features from the post-Arbuckle subaerial exposure event. Although most production in Kansas is from the top of the Arbuckle, some early and recent studies indicate that the Arbuckle is not a simple homogeneous reservoir, that complex vertical and lateral heterogeneities exist including both non-porous and porous horizons in the formation, and that high probability exists of finding additional oil with improved reservoir characterization. Although fracture and vuggy porosity contribute importantly to the production of Arbuckle strata, recent observations indicate a significant amount of porosity (over 50%) in many cores is controlled by depositional facies and dolon-titization.

Studies of Arbuckle and equivalent-age strata from other areas indicate that Arbuckle strata and diagenetic processes are complex and that porosity/permeability patterns are related to a number of processes. These studies underscore the importance of continued study of Arbuckle rocks in Kansas for improved reservoir characterization. Ongoing and future geologic studies of Arbuckle rocks in Kansas are being directed toward: 1) continued sedimentologic, stratigraphic and sequence stratigraphic analyses incorporating core, well log and seismic data; 2) petrophysical studies. Initial studies indicate that core plug petrophysical properties are controlled by matrix grain size and that upscaling from plug to whole-core and drill stem test data can identify and quantify the relative contribution of karstic, fracture and matrix porosity and permeability; 3) regional and local structural analyses and mapping of the upper Arbuckle surface to provide more details on the contribution of structural features and karst paleogeomorphology to reservoir character; 4) diagenetic and geochemical studies focussing especially on the timing of, and processes associated with, dolormitization and karstification events and their contributions to creating or occluding porosity.

Introduction

Arbuckle Group rocks (Cambrian and Lower Ordovician) represent an important record of sediment deposition in the history of the North American continent and they contain important accumulations of hydrocarbons (oil and gas) and base metal deposits. Arbuckle strata account for about 40% of the volume of produced oil and known reserves in Kansas (Newell et al., 1987). This represents a significant amount of revenue for the over 2 billion-dollar Kansas oil and gas industry (Petroleum Independent, 1993). Despite their economic importance, the geologic history of these rocks in Kansas remains poorly understood, both on a local and regional scale.

The Arbuckle and equivalent reservoirs in the Midcontinent are generally considered to have favorable reservoir qualities (i.e. porosity and permeability) that are directly related to basement structural elements (e.g. fractures, regional uplifts, minor horst and graben features) and enhanced by karstic features developed by prolonged subaerial exposure that began immediately after Arbuckle deposition and likely continued in some areas for over 200 million years. The overwhelming acceptance of karst control on reservoir development in the Arbuckle has led to drilling practices that have focused on the upper Arbuckle surface and upper 30 feet or so of Arbuckle strata, virtually ignoring lower Arbuckle strata and any potential additional petroleum reservoirs within the Arbuckle. In Kansas, where the Arbuckle is contained entirely in the subsurface, this has resulted in a relative paucity of sedimentologic and stratigraphic data extending through the entire Arbuckle section.

The shallow drilling philosophy based on the fracture-controlled karst reservoir model has worked well in Kansas, but may be only scratching the surface of an important resource. Despite the prolific production history, Arbuckle strata in Kansas have remained virtually unstudied with respect to sedimentology and diagenesis, apart from earlier general descriptive studies (e.g. Wallace, 1943; McCracken, 1955; Jewett, 1951, 1954; Walters, 1946, 1958; Merriam, 1963; Zeller, 1968; Adler, 1971; Cole, 1975), and some more recent studies detailing sedimentology and stratigraphy (Franseen, 1994; Steinhauff et al., 1998). Thus, the level of detail known about Arbuckle rocks in Kansas is much less than is known about age-equivalent strata, such as the Ellenburger and Knox, that are producing significant petroleum in surrounding states.

This paper provides a synopsis of our understanding of the geology of Arbuckle strata in Kansas at this time, especially pertaining to sedimentologic and diagenetic features from past, recent and ongoing studies. A rigorous treatment of Arbuckle reservoir studies and historical aspects of Arbuckle petroleum geology of Kansas are beyond the scope of this paper. Instead, as highlighted in this review, the recent and ongoing geologic studies being undertaken, predominantly at the University of Kansas, and lessons learned from detailed studies of Arbuckle and equivalent-age strata from surrounding areas, are providing critical information and opening new avenues of applied research that should lead to improved understanding and economic development of the Arbuckle in Kansas.

Regional Geology and Petroleum Aspects of Arbuckle and Arbuckle-equivalent Geology

Arbuckle Group rocks are part of the cratonic-wide Sauk Sequence which is bounded at its base and top by major interregional unconformities (Sloss, 1963). These interregional unconformities represent major regressions of the sea and erosion and subaerial exposure of vast areas of the craton. Sauk Sequence strata represent a major transgression of the sea onto the craton. Arbuckle shallow shelf carbonate strata in Kansas and age equivalent strata elsewhere are part of the Cambro-Ordovician "Great American Bank" that stretched along the present southern and eastern flanks of the North American craton (Ross, 1976). The Arbuckle Group in Kansas is stratigraphically equivalent to the Arbuckle Group of Oklahoma, the Ellenburger Group of Texas, and the Upper Knox Group of the Appalachian fold-thrust belt and Black Warrior Basin, all of which are prolific hydrocarbon reservoirs in the subsurface (Bartram et al., 1950; Loucks and Anderson, 1985; Kerans, 1988; Mazzullo, 1989).

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 other areas are numbered at between 350-450 (e.g. Wilson et al., 1991). These individual cycles have recently 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. Recent studies have focused on 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 time (e.g. Kerans and Lucia, 1989; Wilson et al., 1991; Kupecz, 1992; Montanez, 1992; Montanez and Read, 1992; Goldhammer et al., 1992, 1993).

In much of North America the Arbuckle and equivalent units experienced a prolonged period of subaerial exposure that coincided with a Middle Ordovician sea-level lowstand. In some areas (such as the Central Kansas Uplift) there has been extensive erosion and numerous additional prolonged periods of subaerial exposure up through the Early Pennsylvanian. During these episodes of widespread exposure, a vast system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed, and is represented throughout the Midcontinent by extensive development of karst features in the Arbuckle, Knox, Beekmantown, Prairie du Chien, St. George and Ellenburger groups (Gore, 1954; Merriam and Atkinson, 1956; Fisher and Barrett, 1985; Kerans, 1990; Hammes, 1997). However, the nature of the post-Sauk unconformity surface is quite variable. Sloss (1988) notes that only locally does the surface involve angular discordance or truncation observable at outcrop scale.

One of the most significant oil and gas producing regions in the U.S. is the Arbuckle-Ellenburger trend of the southern Midcontinent stretching nearly a thousand miles from the Central Kansas Uplift across Oklahoma and Texas to the Delaware basin of West Texas (Bartram et al., 1950; Gatewood and Fay, 1992). In much of North America reservoir development in the Arbuckle and equivalent units is attributed to the prolonged period of subaerial exposure that coincided with the Middle Ordovician sea-level lowstand. The El Paso, Ellenburger, Arbuckle, Knox, Beekmantown, and St. George groups all contain karstic solution-collapse breccias induced by cavern collapse below this unconformity and superimposed younger unconformities. It appears that most of the production regionally occurs at the top of the Lower Ordovician strata near the post-Sauk unconformity and is related to karst and fracturing associated with the unconformity. Also, the breccia zones, locally forming significant reservoirs, may be associated with tectonic fracturing which may play an important role in localizing solution collapse, and an accessory role in reservoir development. In addition to the Ellenburger, recently there has been a resurgence in exploration interest of the Upper Knox and Arbuckle carbonates as a result of significant oil and gas discoveries in Oklahoma, Alabama, and Mississippi (Montanez, 1992).

Kansas Arbuckle Geology

Kansas is characterized by broad basins and domes covering thousands of square kilometers. Prominent uplift and subsidence occurred episodically throughout the Phanerozoic, separated by periods of gradual deformation (Newell et al., 1989). Two prominent structural uplifts that affect the Paleozoic rocks in Kansas, the Nemaha uplift and Central Kansas uplift (Fig. 1), represent significant Early Pennsylvanian deformation likely associated with similarly aged plate convergence along the Ouachita Mountains orogenic belt in Arkansas (Newell et al., 1989). This uplift and erosion locally affected Arbuckle strata, especially on the Central Kansas uplift where Pennsylvanian strata directly overlie Arbuckle strata or basement rocks where Arbuckle strata are absent (Fig. 2).

Figure 1--Map showing Arbuckle production in relation to major structural features in Kansas. Structural features modified from Merriam (1963). Data on oil and gas fields from Kansas Geological Survey.

Most Arbuckle production in Central Kansas Uplift (Pratt-Stafford north through Barton, Ellis, Russell toward Norton and Decatur; also in southern Nemaha Uplift (Butler , Cowley).

The Arbuckle is present in most of Kansas and is absent only in areas of northeastern and northwestern Kansas, and over ancient uplifts and buried Precambrian highs (Cole, 1962, 1975; Denison, 1966; Walters, 1946). The Arbuckle is thin to absent in parts of northeastern Kansas due to pre-Simpson uplift and erosion (Newell et al., 1987). The Arbuckle is locally absent on the Nemaha uplift, Cambridge arch and Central Kansas uplift due to pre-Pennsylvanian erosion (Walters, 1946; Jewett, 1951, 1954; Merriam, 1963).

The basal contact of the Arbuckle Group is an unconformity (sub-Sauk surface of Sloss, 1963) with basement rocks which in Kansas can range in age from Precambrian to Lower Ordovician (Cole, 1975). As throughout the craton of North America, the upper contact of the Arbuckle Group is also an unconformity (sub-Tippecanoe surface to sub-Absaroka surface of Sloss, 1963). Middle Ordovician Simpson sandstone and shale typically unconformably overlie the Arbuckle, although Viola Shale or Mississippian strata locally directly overlie Arbuckle rocks (Fig. 2). In areas of greatest uplift since Ordovician time, such as the Central Kansas uplift, the Arbuckle Group may be unconformably overlain by beds as young as Pennsylvanian (Merriam, 1963; Cole, 1975) (Fig. 2).

Figure 2--Diagrammatic cross section and map showing location of cross section in Kansas. Note Arbuckle relationships to basement rocks and overlying strata, especially across Central Kansas Uplift (from Adler, 1971). A larger version of this figure is available as an Acrobat PDF (512 kB).

Thickness of Pennsylvanian and above are not as affected by Central Kansas Uplift; Mississippian and Arbuckle are very much thinned by uplift.

Kansas Production History and Reservoir Aspects of the Arbuckle

Since the 1920's, several billion barrels of oil have been produced from the Central Kansas uplift, primarily from carbonate reservoirs within the Arbuckle and Lansing-Kansas City groups. The majority of Arbuckle reservoirs of central Kansas were drilled from 1923 through the 1940's and constitute a series of giant and near giant oil fields.

These Arbuckle fields were characterized by prolific oil production (averaging 50,000 to 100,000 BO per well) from several hundred to over a thousand relatively shallow wells (depths of 3,000 to 3,400 ft) distributed normally on 10 acre spacing (Walters, 1991). Previous to discovery of these prolific carbonate reservoirs, production was dominated by sandstone reservoirs. The Arbuckle fields of central Kansas followed closely on the heels of the Arbuckle discovery in 1917 at El Dorado field in Butler County, Kansas, and represent the first oil production in the prolific western ranges of Kansas. The significance of the Arbuckle to Kansas and U.S. production and reserves is highlighted by the estimate that Arbuckle reservoirs account for over 47% of the 16.3 billion barrels of original oil in place in Kansas, and 43% of the cumulative 5 to 6 billion barrels of the total oil produced from the state (Watney and Paul, 1983; Newell et al., 1987). According to Newell et al. (1987), the Arbuckle Group is the most significant pay zone on the Central Kansas uplift (Fig. 1). From 1929-1968, over 1.4 billion barrels had been produced from the Arbuckle rocks in this area (Adler, 1971). Updated estimates indicate that Arbuckle Group strata have produced a total of 2.95 billion barrels of oil from the Central Kansas uplift up to 1991 (K.D. Newell pers. comm., 1992).

Structure has played a major role in the development of Arbuckle reservoirs on several different scales. On a state-wide scale, most major hydrocarbon production within the Arbuckle in Kansas occurs on two major uplift features, the Central Kansas uplift and the Nemaha Ridge (Fig. 1). On a more local scale, major fields within the El Dorado field in Butler County are associated with several large-scale fault-bounded domes. As an example of structural influence on a still smaller scale, production within the West Shumway dome in the El Dorado field follow what appear to be original tectonic fracture systems with north-south and east-west orientations (Ramondetta, 1990). Most productive fields on the Central Kansas uplift are structural and structural-stratigraphic traps that produce oil or gas from the top of the Arbuckle section (Walters, 1958). The weathering and secondary solution of the upper Arbuckle beds, due to subaerial exposure, significantly enhanced porosity and permeability and created petroleum reservoirs in these strata (Walters, 1958: Merriam, 1963; Adler, 1971). Most of the oil and gas zones in the Arbuckle are contained in the top 25 ft, some are 25-50 ft within the Arbuckle (Adler, 1971), and thus, Arbuckle reservoirs typically have been visualized as an oil column on top of a strong aquifer. This conceptual model of the Arbuckle reservoir resulted in drilling and completion practices in which wells were drilled into the top of the Arbuckle with relatively short penetration (under 10 ft. or 3 m) and completed open hole. Production from these Arbuckle wells is characterized by high initial potential, steep decline rates and production of large quantities of oil at high water/oil ratios (500,000 to a 1,000,000 BO per well is not uncommon).

Although most production is from the top of the Arbuckle, Bloesch (1964) concluded that the Arbuckle couldn't be considered fully tested until Precambrian rocks are reached. Bloesch (1964) reported that oil and gas shows existed well below the top of the Arbuckle in some areas of Oklahoma. Recent drilling in Kansas has revealed that the Arbuckle is not a simple homogeneous reservoir, that complex vertical and lateral heterogeneities exist including both non-porous and porous horizons in the formation, and that high probability exists of finding additional oil with improved reservoir characterization (e.g. Ramondetta, 1990). Observations by Franseen (1994) in two Arbuckle cores from the Central Kansas Uplift area (extending up to 133 feet into the Arbuckle from the upper contact) indicate a significant amount of porosity (over 50%) is controlled by depositional facies and dolomitization with relatively minor porosity related to late stage brecciation and fracturing that resulted from structural controls or karst (discussed further in a later section).

Stratigraphy

Definitions of the Arbuckle Group in Kansas differ somewhat, but generally these rocks are thought to consist of Cambrian and Lower Ordovician rock units between the underlying Reagan or Lamotte Sandstone and overlying Simpson Group (Merriam, 1963; Zeller, 1968). Some authors include the Lamotte Sandstone, Bonneterre Formation, and Reagan Sandstone in the Arbuckle Group (e.g. Cole, 1975) (Fig. 3).

Figure 3--Stratigraphic column of Cambro-Ordovician units in Kansas showing two different (but similar) usages for Arbuckle Group strata in Kansas (from Cole, 1975).

Maquoketa, Viola, and Simpson are same in formal and petroleum nomenclature; Arbuckle of petroleum usage is split into  Arbuckle, Bonneterre, and Lamotte under formal usage.

Arbuckle Group rocks thicken from north to south and are thickest, up to 1390 feet, in the southeastern corner of Kansas (Cole, 1975). In Kansas the Arbuckle Group consists mainly of dolomite with scattered beds containing chert and sand (Wallace, 1943; Merriam, 1963). Minor amounts of glauconite and pyrite occur throughout the Arbuckle (Merriam, 1963). Despite the dolomitization, original depositional textures are still easily identifiable in many cores (Franseen, 1994) (discussed further in a later section).

In earlier studies the Arbuckle had been subdivided and correlated with equivalent exposed strata in adjacent states by study of insoluble residues (McCracken, 1955) and various local subdivisions were proposed (e.g., Walters, 1946, 1991). Where the Arbuckle was eroded along the flanks of uplifted areas, the dolomites and limestones were deeply weathered (Walters, 1958; Merriam, 1963). Some early stratigraphic studies using lithologic data attempted to subdivide and correlate Arbuckle strata using chert horizons and insoluble residues to correlate from the subsurface of Kansas to outcrops in Missouri (e.g. Ockerman, 1935; McCracken, 1955).

In studying thin sections of three cores from the Central Kansas Uplift, Conley (1980) noted 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 1-22%. In a study of the Lyons underground gas storage field in Rice County, Dalal (1987) observed that the reservoir consists of dolomitized stromatolite and silicified oolitic facies. Ramondetta (1990) briefly 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 occasional intercrystalline porosity in the predominantly tight dolomudstones. Franseen (1994) documented the existence of subtidal to peritidal cycles in reconnaissance and detailed studies of Kansas Arbuckle cores housed at the Kansas Geological Survey and was the first to describe facies and paragenetic sequence in detail for several cores located near the Central Kansas Uplift. Franseen (1994) 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 meters or more. Steinhauff et al. (1998) also documented in detail the existence of similar subtidal and peritidal facies and paragenetic sequence of events in six cores from central Kansas. In addition, Steinhauff et al. (1998) identified several scales of cycles (ranging from < 1 m to several m's) in cores and related cycles to electric log signatures for preliminary construction of a stratigraphic framework and correlation in the central Kansas study area. As of this writing, the sedimentologic and stratigraphic work is continuing by that team and extending to other areas in Kansas outside the initial central Kansas study area.

Depositional Facies And Environments

As reported in Franseen (1994), original depositional facies in Arbuckle strata consist predominantly of coarse-grained skeletal, intraclastic, oolitic, peloidal, dolograinstones/packstones and finer-grained dolowackestones/mudstones, thrombolites and digitate to LLH stromatolites, intraclastic conglomerate and breccia, and minor shale and siltstone. Silicified nodules and lenses are locally abundant and at least some likely represent replacement of original evaporite minerals. Scoured, erosional surfaces occur within the Arbuckle with medium scale erosional relief. These surfaces are commonly overlain by intraclastic breccia and conglomerate.

Based on study of cores from central Kansas, Franseen (1994) defined the following broad depositional facies categories: 1) coarse-grained packstones/grainstones, 2) fine-grained packstones/wackestones/mudstones, 3) stromatolites-thrombolites, 4) intraclastic conglomerate and breccia, and 5) shale. In addition, Franseen (1994) recognized the presence of secondary features including, dolomitization, breccia, fractures and conglomerate related to intra-Arbuckle subaerial exposure, breccia, fractures and conglomerate related to later karst, burial or structural processes, silicification, and local mineralization. Steinhauff et al. (1998) split facies further into: (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 depositional facies descriptions below follow those from Franseen (1994) and are thought to generally be representative of Arbuckle depositional facies throughout Kansas.

Coarse-Grained Packstones/Grainstones: This common facies is typically massive, or horizontally bedded to crossbedded and commonly interbedded with fine-grained wackestone-packstone facies. Locally, burrow traces are distinguishable or the facies has a mottled texture that may due to extensive burrowing. Grains include intraclasts, skeletal fragments, ooids, and peloids. This facies indicates high-energy deposition in subtidal to peritidal settings. Fenestrae, keystone vugs, and vugs occur locally indicating exposure. Locally, the coarse-grained packstone/grainstone facies is tightly cemented but more commonly contains significant intercrystalline, vuggy, local channel, local fenestral, moldic, and fracture porosity. This facies is also locally oil-stained.

Fine-Grained Packstones/Wackestones/Mudstones: This facies is typically massive to horizontally laminated and bedded, is commonly interbedded with coarser-grained carbonates, or locally contains wispy lenses of shale and interbedded shale layers. Burrow traces and mottling are common. Peloids are common and rare intraclasts and skeletal grains (e.g. gastropods) occur locally. Soft sediment deformation, dewatering or incipient teepee-like structures, mudcrack and desiccation cracks, and rip-up clasts are locally associated with this facies. The fine-grained packstone/mudstone facies represents deposition in relatively lower energy conditions in subtidal (massive and burrowed textures) to peritidal (mudcrack and desiccation cracks, incipient teepee-like structures, and rip-up layers and clasts) settings. This facies is typically tightly cemented, but locally contains some vuggy, intercrystalline, fracture and localized fenestral porosity.

Stromatolites-Thrombolites: This facies consists of muddy, mottled to wavy laminated lithology interpreted to represent thrombolites (locally leopard rock texture), wavy algal-laminites, brecciated stromatolitic horizons, and digitate to LLH stromatolites; locally these individual facies occur together in a vertical gradational sequence. Thrombolite-leopard rock texture is typically developed in mudstone-wackestone facies and locally contains some possible burrow mottling. It is usually tightly cemented but locally contains some vuggy and intercrystalline porosity, and more rarely some fenestral porosity. The thrombolite-leopard rock texture likely represents deposition in mostly a subtidal setting, but locally may reflect peritidal environments where fenestrae are developed. The wavy algal laminites occur in mudstone to packstone lithology. Locally, the laminations show evidence of current (ripple) modification. Typically the laminites are tightly cemented. The algal laminite facies most likely represents a peritidal depositional setting. The brecciated stromatolite and stromatolite facies occurs in mudstone to packstone lithology. Typically brecciated stromatolite texture grades upward to non-brecciated, inplace stromatolites. The stromatolites locally are tightly cemented but more commonly contain abundant and distinctive differentially developed intercrystalline, fenestral, keystone vug and solution enlarged vuggy porosity that closely follow laminations. The stromatolite to brecciated stromatolite facies likely represents local subtidal (where subaerial exposure features are absent) to mostly peritidal environments. This facies is locally oil-stained.

Intraclastic Conglomerate and Breccia: Many of the conglomerates or breccias consist of rip-up clasts derived from underlying lithologies whereas other horizons show a mixture of clast lithologies. Textures range from clast- to matrix-support. Conglomerates and breccias are commonly associated with desiccation and mud cracks, dewatering structures, and incipient teepee structures. Although the intraclastic conglomerates and breccias commonly overlie a sharp erosional surface, some conglomerates and breccias are associated with an upwards gradation from undisrupted beds to deformed and fractured beds, to ripped-up conglomerate and brecciated textures. Local autoclastic breccia textures indicate subaerial exposure of some Arbuckle horizons. A variety of mechanisms (including storms, tides, and relative sea level fluctuations) are plausible for conglomerate and breccia development in subtidal to peritidal settings. A peritidal setting is indicated where the conglomerates and breccias are closely associated with desiccation and mud cracks, incipient teepee structure development, and autoclastic breccia textures. Some collapse breccia textures may have resulted from dissolution of evaporites. The conglomerate and breccia facies commonly has differentially developed porosity. The matrix is typically grainy with abundant intercrystalline, and vuggy (locally fracture) porosity whereas the clasts (e.g. mudstones to wackestones) are relatively tightly cemented. Locally, where there is a mixture of packstone-grainstone clasts and mudstone-wackestone clasts, the grainstone-packstone clasts are locally porous as well.

Shale: Green shale occurs throughout the core as thin depositional layers and as later fracture filling sediment. Shale is commonly associated with stylolite development. Locally, as shown in thin section, fracture fill contains fragments of dolomite rhombs and subangular to rounded silt-size to coarse-grained detrital quartz grains. Several horizons consist of wavy horizontal to horizontally interbedded shale and carbonate mudstonewackestone. Several shale layers contain silicified nodules and lenses that replaced primary evaporites. The shale layers likely represent relatively low energy subtidal to peritidal conditions. Shallow lagoonal to supratidal conditions may be indicated for some horizons where silicified nodules apparently represent replacement of original evaporite minerals.

Diagenetic Features and Porosity Types

Franseen (1994) 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. The small crystal size of some dolomite (<0.01 mm), evidence of silica replaced evaporite minerals (anhydrite/gypsum?), and local occurrence of spheroidal or polyhedral dolomite are supportive of early reflux or mixing zone dolomitization; (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), 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; (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 occurs 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 is also 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 of this study 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 a preferred orientation 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.

A 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 that is related to depositional facies, early diagenesis, and dolomitization and not necessarily related to the upper post-Sauk subaerial exposure surface. Initial observations indicate that more than 50% of the preserved porosity are these types of "matrix" porosity. shows various types of Arbuckle dolomite textures and variations in 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). Several of the coarse-grained and stromatolite intervals in the cores are heavily stained with oil. It is apparent that these intervals are significant in their potential for storage of fluids and deserve increased attention in exploration/production strategies. Initial results of ongoing petrophysical studies were reported in Steinhauff et al. (1998). They indicate that petrophysical properties of the facies at the core plug scale can be tied directly into depositional facies and are generally controlled by matrix grain size. Stromatolite-thrombolite facies and coarse-grained packstone-grainstone facies may have porosities from 10-25% and permeabilities from 10 to 1500 millidarcies. Other facies exhibit non-reservoir properties.

Just as striking in many of the cores studied are the subtleties and relative lack of macroscopic exposure features and porosity associated with the upper subaerial exposure event, especially near the upper contact. These observations indicate that vuggy, fracture and breccia porosity associated with karst and structure, thought to be the dominant controlling factors in Arbuckle reservoir character, is highly variable in occurrence. Where present these porosity types serves to create more effective porosity and may affect the producibility of the matrix-dominated porosity zones.

Discussion--Ongoing and Future Studies

Our current understanding of depositional facies, stratigraphy and diagenetic features indicates that the Arbuckle strata in Kansas are similar to Arbuckle and equivalent strata in areas surrounding Kansas that have received much more detailed study over the years. Therefore, studies from these other areas provide insight and direction for further study of Arbuckle strata in Kansas which are much needed, especially in light of the economic importance of Arbuckle strata to Kansas and their declining production. Studies from these other areas indicate that present-day petrophysical characteristics and lateral and vertical heterogeneity of reservoirs in the Arbuckle, Ellenburger and Knox Groups can be attributed to: 1) karstification associated with low frequency (third- and second-order) sea level falls and subaerial exposure (Loucks and Anderson, 1985; Kerans, 1988; Fritz, 1991; Bliefnick and Belfield, 1992; Kupecz, 1992); 2) diagenetic modification of karst features during burial and tectonism (Kupecz and Land, 1991; Lynch and Al-Shaieb, 1991; Kupecz, 1992); 3) tectonically induced fracture and breccia porosity (Ijirigho and Schreiber, 1986); and 4) matrix (intercrystalline and vuggy) porosity developed in replacive dolomites that results from rapid lateral and vertical changes in the distribution of depositional and diagenetic facies, and from the process of dolomitization itself (Mazzullo, 1989; Kerans and Lucia, 1989; Gosh and Friedman, 1989; Montanez, 1992).

Mazzullo (1990) points out that most Ellenburger fields are composite types of traps resulting from the complex diagenetic history, lithofacies patterns, paleogeomorphic, and tectonic history which has resulted in a variety of structural, subunconformity, and stratigraphic trap possibilities within the section, in addition to traps and fields associated with paleocaverns. Some authors have documented several karstic events within Lower Ordovician strata, interpreted as forming from eustatic sea level excursions during Lower Ordovician deposition (e.g. Mazzullo, 1990; Holtz and Kerans, 1992: Lynch and Al-Shaieb, 1991). Several recent studies suggest that dolomitization, in combination with depositional lithofacies distribution, was an important influence on the aerial and stratigraphic distribution of economically important porous and permeable zones within Lower Ordovician strata that are not related to karstification processes (e.g. Amthor and Friedman, 1991; Montanez, 1992).

Therefore, despite the prevalence in thinking of fracture and karst-modified reservoirs for the Arbuckle and equivalent strata, these studies described above demonstrate that porosity/permeability patterns are related to a number of processes and underscore the importance of a much more detailed understanding of the relatively understudied Arbuckle strata in Kansas for improved reservoir characterization. This is especially true as alternative infill and horizontal drilling strategies are considered and as the Vbuckle is evaluated for disposal of water and CO2. An example of the necessity of more detailed understanding of reservoir controls in Kansas is provided by a recent short radius horizontal well drilled in Arbuckle strata in Rooks County, Kansas, by Helmerich & Payne, Inc. (Tulsa, Oklahoma). Although the well, drilled as a short radius horizontal well in a strong water drive, heavy Arbuckle reservoir, was successful from an engineering standpoint, it failed to meet its geological objective as a result of the poorly understood complexities of facies, fracture and diagenetic patterns in the Arbuckle.

The sedimentologic and stratigraphic studies, as described earlier and initially reported by Franseen (1994) and Steinhauff et al. (1998), are ongoing and continue to incorporate Arbuckle data from areas throughout Kansas. These studies are utilizing numerous well logs and core data already available and will benefit from the incorporation of additional data, including 2-d and 3-d seismic, from other sources. Detailed stratigraphic and sequence-stratigraphic studies are crucial for providing frameworks for subsequent diagenetic and structural studies.

As mentioned above, petrophysical studies of Arbuckle strata in Kansas are currently underway and initial results were reported in Steinhauff et al. (1998). Additional studies reported in Byrnes and Franseen (1999) confirmed that matrix properties are an important control on flow in some Arbuckle rocks. In addition, they showed that using an upscaling approach between core-plug and whole-core porosity and permeability measurements and drill stem tests can identify and quantify the relative contributions of karstic, fracture and matrix porosity and permeability. Continued petrophysical studies are critical for constructing numerical flow models, understanding hydrocarbon recovery efficiencies and determining where hydrocarbons remain in Arbuckle reservoirs in Kansas.

As indicated by (Newell et al., 1989), prominent uplift and subsidence occurred episodically in Kansas throughout the Phanerozoic, separated by periods of gradual deformation. The Midcontinent rift system and related Humboldt fault zone constitute a north-northeast by south-southwest-trending fault swarm that was reactivated several times during the Paleozoic. Local uplift and interstratal truncation suggest rejuvenation of structural blocks during the Late Ordovician, before the better known Pennsylvanian uplift and erosion of the major structural features (Baars and Watney, 1991). Reactivation of these structural features throughout time created unconformity-bounded trapping conditions at several stratigraphic intervals (Baars and Watney, 1991). Pre-existing regional structure also influenced initial depositional lithofacies and the development of later regional dolomitization. The regional and stratigraphic extent of subaerial exposure and karstification in the Arbuckle was largely controlled by the regional patterns of uplift and subsidence. Most importantly, karstification of the Arbuckle surface was likely influenced strongly by pre-existing tectonically-induced fracture systems.

Understanding of the structural setting and potential structural elements that could have affected Arbuckle strata in any given location is aided by regional maps of basement rocks (e.g. Baars and Watney, 1991; Berendsen and Blair, 1992), Arbuckle rocks (Merriam and Smith, 1961) and gravity and magnetic maps (Kruger, 1996) that exist for Kansas. Recent and ongoing studies are focussing on detailed local (field scale) structural mapping of the upper Arbuckle surface. Carr, et al. (1994, 1995) integrated seismic data with detailed structural mapping of the upper Arbuckle surface on several fields along the western flank of the Central Kansas Uplift to determine locations and causes of Arbuckle structural elements. They demonstrated reservoir development and performance was related to the development of mature karst features associated with the post-Arbuckle subaerial exposure event. An ongoing study by J. Cansler (KU MS candidate) of the upper Arbuckle surface in central Kansas is further detailing structural and karst paleogeomorphology controls on Arbuckle production.

As summarized earlier in this paper, Franseen (1994) and Steinhauff et al. (1998) documented the wide range of diagenetic events affecting the Arbuckle rocks in Kansas. Much more diagenetic study, especially on dolomitization and karst processes, is needed to understand their environments, relative timing and effects on reservoir character. Dolomite is important in both the development and destruction of porosity within the Arbuckle Group and may either predate or postdate various events of fracture formation. In many areas, intercrystalline and moldic porosity is dominant within reservoir lithologies. Its distribution appears to be limited to certain stratigraphic horizons and certain lithofacies, some resulting in nonporous dolomite and others resulting in porous and permeable dolomites. Furthermore, one or more later phases of dolomitization appear to have significantly reduced porosity and permeability through dolomite cementation. This later dolomite may have been important in recrystallizing and coarsening early dolomite and may have had a marked effect on porosity structure. The occlusion of porosity associated with this late dolomitization may be related to migration of post-Arbuckle fluids, including hydrocarbons, that seek appropriate conduits for flow. Thus, distribution of this dolomite may be intimately associated with structural setting.

Although porosity potential is usually high in paleokarst facies, it has been emphasized recently that during periods of subaerial exposure, carbonate sediments or rocks are susceptible to diagenetic changes which can both create or occlude porosity (e.g. Budd et al., 1995). In addition to studies mentioned above on the upper Arbuckle karst surface, a focus for additional Arbuckle karst studies will be to determine porosity creation versus occlusion associated with the post-Sauk exposure events and overprinting of these events on earlier fabrics (including earlier exposure events).

Summary

Arbuckle Group and equivalent-age rocks from Kansas and surrounding areas are similar, consisting of platform deposits dominated by ramp-type subtidal to peritidal carbonates (mostly dolomitized) which can be subdivided into cycles, less than 0.5 m to 40 m thick, based on facies type and depositional patterns. In Kansas, the Arbuckle thickens from north to south (up to 1390 feet thick) and is absent only northeastern and northwestern portions of Kansas, and over ancient uplifts and buried Precambrian highs.

Arbuckle strata account for about 40% of the volume of produced oil and known reserves in Kansas and represents a significant amount of revenue the Kansas oil and gas industry. Most of the present oil and gas zones in the Arbuckle are contained in the top 25 ft, some are 25-50 ft within the Arbuckle and favorable reservoir qualities (i.e. porosity and permeability) that are generally considered to be directly related to basement structural elements and enhanced by karstic features.

Although most production is from the top of the Arbuckle, some earlier and recent studies have documented complex vertical and lateral heterogeneities including both nonporous and porous horizons, deeper in the formation, and that the deeper productive zones may be controlled by several factors such as depositional facies, dolomitization, silicification, or intra-formational exposure events.

To date, current understanding of depositional facies, stratigraphy, diagenetic features and structural controls of Arbuckle strata in Kansas lags behind that in areas surrounding Kansas that have received much more detailed study over the years. However, the level of detail is increasing from recent and ongoing studies on sedimentology, stratigraphy, petrophysics, structure, paleogeomorphology and diagenesis. These studies are rapidly adding to the understanding of porosity/permeability patterns relating to structural setting, fracture patterns, karst and other diagenetic controlling factors on porosity/permeability development that should result in better exploration and production strategies in the Arbuckle.

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