Summary of Reservoir Geology

Modified form 1998, Evan K. Franseen, Timothy R. Carr, Willard J. Guy and D. Scott Beaty, Significance of Depositional and Early Diagenetic Controls on Architecture of a Karstic-Overprinted Mississippian (Osagian) Reservoir, Schaben Field, Ness County, Kansas: 1998 AAPG Meeting, Salt Lake City, Utah

A sedimentologic and diagenetic study of Mississippian (Osagian) cores from the demonstration site was undertaken to discern the interplay of original depositional facies and early to late diagenetic events in producing the complex architecture of this dominantly cherty dolomite reservoir (Summarize in Franseen and others, 1998). The strata represent deposition on a ramp. Basal strata (M0 unit) consist of normal to somewhat restricted marine strata characterized by an abundance of echinoderm-rich facies commonly containing a diverse fauna. Upper strata (M1 unit and above) are dominated by sponge spicule-rich facies and silicified original evaporite minerals indicating restricted evaporitive ramp, lagoon or supratidal environments. An internal unconformity, apparently a subaerial exposure event, separates upper and lower strata (M1 surface). The post-Mississippian unconformity caps the entire sequence.

Macroscopic, microscopic, well-log, petrophysical and minipermeameter data, and oil shows, indicate that depositional facies and early diagenetic events were dominant controls for reservoir characteristics. The best reservoir is the sponge spicule-rich wackestone/packstone facies (SWP) with replaced evaporites. Echinoderm-rich wacke-packstones (EWPG) are also locally important reservoir facies. Burrow mottling was important in creating localized networks for early diagenetic fluids. Early dissolution of grains and dolomitization created the moldic, intercrystalline and vuggy porosity important for favorable reservoir conditions. Abundant early silica cementation and replacement (chert, megaquartz, chalcedony) created complex heterogeneity. Silica replacement and cementation in EWPG facies typically results in impermeable areas whereas silica cementation and replacement in SWP and mudstone-wackestone (MW) facies results in variably tight or porous (tripolitic) areas containing vuggy, moldic, and intercrystalline porosity. Coarse calcite cementation and poikilotopic calcite replacement associated with the M1 surface significantly occluded much of the porosity in underlying M0 strata. Fracturing and brecciation from the post-Mississippian karst, burial and structural events variously enhanced or destroyed reservoir characteristics. Ramp strata were differentially eroded at the post-Mississippian unconformity resulting in complex buried paleotopography.

The most favorable areas for successful production appear to be where SWP facies containing abundant evaporites (M1 unit and above) intersect the post-Mississippian unconformity and form topographic highs. Our data suggest the EWPG facies to be locally favorable reservoirs. However EWPG facies that are dominant in the M0 unit are not likely to be favorable reservoirs because processes associated with the M1 surface significantly occluded much of the porosity. This prediction is confirmed to the northeast of our study area where M0 strata intersect the unconformity and are not productive. Our methods and results provide predictive capability, indicate the potential for deeper stratigraphic traps, and suggest alternate production strategies such as horizontal drilling or targeted infill drilling may be warranted.

The results presented here come from three recently cored wells from the Schaben field located in Ness County of west central Kansas (Figure 2.2). This area is located on the upper shelf of the Hugoton Embayment of the Anadarko Basin, on the southwest flank of the Central Kansas Uplift. This area produces oil from Mississippian (lower Meramecian Warsaw Limestone and Osagian Keokuk Limestone) dolomites and limestones. The three cores of this study are from the Osagian interval. The strata represent deposition on a ramp, the dip of which has been accentuated by post-depositional regional uplift. The ramp strata were differentially eroded at the post-Mississippian unconformity resulting in paleotopographic highs (buried hogbacks, Figures 1.3, 1.4, 1.5, 2.3). Previously, these strata have been understood using a karst reservoir model based on the truncation of these strata by the post-Mississippian subaerial unconformity (Figure 2.3). The data of this study indicate that original depositional facies and relatively early diagenetic events have a significant influence on present reservoir characteristics and that later fracturing and dissolution from karst and/or structural influences are locally important but may not be the primary control on favorable reservoir conditions. The results presented here are providing predictive capabilities to better characterize the many existing subunconformity fields in Kansas.

The following depositional facies were recognized in the cores.

1. Sponge Spicule-rich Wacke-Packstone (SWP): SWP facies are especially abundant in the upper portions of the cores and form an important reservoir facies. This facies, dark to light gray, olive green, tan, or brown in color, is characteristically mottled from burrowing, or wispy to wavy horizontal laminated. Sponge spicules (mostly monaxon) and their molds are the dominant, and commonly exclusive, grain type. Echinoderm, bryozoan, gastropod, and peloid grains occur more rarely. The sponge spicules are locally concentrated in layers or in "pockets" as a function of depositional processes (currents) and reworking by burrowing organisms. Moldic, intercrystalline, and vuggy porosity are locally abundant, and fenestral porosity occurs more rarely. Some primary porosity is solution enlarged forming vugs. The mottling texture and concentration of grains in layers locally results in variable tight and porous areas at a thin section scale. This facies has been extensively dolomitized. Dolomite occurs as very finely crystalline to aphanocrystalline (~20 µm to <100 µm), subhedral to anhedral crystals; where developed, euhedral crystals contain more intercrystalline porosity. The SWP facies commonly contains silica replaced evaporite crystals, nodules or coalesced nodules that form layers. Silica variably replaces matrix and grains. Brecciation is common, likely from dissolution and collapse of evaporites and early differential compaction between brittle chert and soft dolomitic matrix.
2. Mudstone-Wackestone (MW): MW facies occurs throughout the cores but is most abundant in upper portions of the cores. It is very similar to SWP facies except that identifiable skeletal grains or their molds are rare in MW facies and MW facies is typically tight or has minor moldic, intercrystalline, and vuggy porosity only locally developed.
3. Echinoderm Wacke-Pack-Grainstone (EWPG): EWPG facies is most abundant in the lower parts of the cores. Echinoderm fragments are typically dominant but abundant sponge spicules, bryozoan fragments, brachiopods, coral fragments, gastropods, ostracods, ooids, peloids, grapestone, calcispheres, oncolites, and other unidentifiable skeletal debris also variably occur. This facies has been extensively dolomitized. Where dolomitic, grains in this facies are typically preserved as molds. Horizontal laminations and low-angle cross laminations are locally preserved. Some intervals show sorting of grains into finer-grained layers and coarser-grained layers. Other intervals show normal grading of grains. Evidence for abundant early compaction is rare. However, locally, grains in this facies show compromise boundaries, overly-close packing, grain breakage and flat, horizontal alignment of skeletal fragments. EWPG facies is characteristically tan to dark brown in color and typically has a wispy laminated or mottled texture; locally it has a massive texture. Locally, interbedded skeletal rich layers (more porous) and skeletal poor layers (tighter) result in an alternating porous and tight layering within this facies. Moldic, intercrystalline, and vuggy porosity in this facies can exceed 35% (visual estimate). Dolomite is typically very finely crystalline (~50 µm or less) but locally exceeds 150 µm. Crystals are typically subhedral to euhedral. Some of the crystals are zoned with a clear to turbid (locally calcian) center and clear dolomite rim. Partial or pervasive replacement and cementation by chert, clear megaquartz and chalcedony is common. Where this facies has been silicified, grains have either been replaced with textures preserved or their molds are filled with silica or calcite cement; an isopachous chalcedony cement coats grains and locally lines primary pores. Abundant microcrystalline porosity occurs within tripolitic chert areas and both tripolitic and porcelaneous chert typically contains micro- and mega- fracture porosity Vugs developed within chert areas are partially or fully filled with silica cement. Fenestrae occur locally and are partially or fully filled with silica cement.

A general upward facies pattern from bottom to tops of cores is discernible in all three cores. Echinoderm facies predominates in the bases of all the cores (M0 unit, Figure 2.3). Although a variety of facies occur, importantly, evidence of evaporites is generally lacking. The upper portions of all cores (M1 unit and above) are dominated by sponge spicule-rich and mudstone/wackestone facies and contain abundant evidence of evaporites. Correlations of facies in cores and well-logs indicate deposition on a ramp. The abundance of echinoderm-rich facies with other diverse fauna, abundance of burrowing organisms and only rare occurrence of evaporites in M0 suggest deposition in relatively normal to somewhat restricted marine environments. In contrast, the abundance of mudstone, wackestone and spicule-rich facies, relative rarity of echinoderm-rich facies, and abundance of evaporites in M1 and above suggest deposition in restricted, evaporitive ramp, lagoon or supratidal environments. However, the abundance of burrow mottling indicates conditions sufficient to support organisms that reworked the sediment. Evaporite characteristics (radiating blades, isolated to coalesced nodules, and coalesced vertically elongate nodule layers) indicate formation in a supratidal to shallow subaqueous setting at or just below the sediment/water interface. Local fenestral fabric indicates at least intermittent subaerial exposure.

EWPG facies are mostly characteristic of a shallow subtidal ramp setting. Horizontal lamination, cross-lamination, normal grading, packing of grains, grain breakage, and scoured contacts evidence at least intermittent high energy. In the M0 unit, this facies likely represents deposition from storm and turbidity currents or from migration of subtidal shoals or banks. Local fenestral fabrics in the echinoderm facies, especially in the M1 unit, indicate at least local subaerial exposure. Local echinoderm-rich layers in the M1 unit may represent shelfward migration of subtidal shoals or shelfward deposition from storm currents.

SWP and MW facies are more indicative of a low energy and restricted setting. Although sponge spicule facies are commonly thought to represent deeper water basinal sections, their association with evaporites and local evidence of subaerial exposure evidence shallow water environments. As demonstrated by studies in other areas, sponges thrived and likely formed sponge mats, gardens or mounds in this environment. Local wispy and wavy horizontal lamination, horizontal planar lamination and interbeds of grainstone in mudstone to packstone indicate transport and reworking of sediment by currents likely generated from tides or storms. The change between the normal to restricted marine ramp (M0 unit) and the evaporitic ramp to lagoon (M1 unit and above) is marked by a sharp contact (termed the M1 surface). This surface and the strata immediately below for several meters show significant alteration. A coarse calcite cement and replacive poikilotopic calcite is associated with the M1 surface and occurs variably throughout strata below this surface to the bottom of the cores. This cement is very important for occluding porosity in strata below the surface. A similar calcite cement and calcite replacement in the upper five feet of the cores is presumably a separate event related to post-Mississippian subaerial exposure.

The interpretation of two separate events is supported by:

1. the absence of calcite in strata lying between the M1 surface and the occurrence of calcite in the upper five feet,
2. fractures that were filled with coarse calcite cement below the M1 surface are cross cut by fractures filled with sediment from facies overlying the M1 surface, and
3. brecciated areas just below the M1 surface that contain clasts of the poikilotopic calcite-replaced facies in dolomitic matrix.

These features are not observed in strata above the M1 surface. The strata immediately below the M1 surface contain local fenestral fabric, fractures, breccia, autobreccia, clay-rich horizons with abundant horizontal fenestrae interlaminated with fine- to coarse-grained detrital quartz, locally abundant glauconite, oblong altered areas with abundant fenestrae and local branching, downward tapering microfractures (roots?). In combination, these features and crosscutting relationships strongly support a subaerial exposure event at the M1 surface.

Petrographic, minipermeameter, and nuclear magnetic remanence (NMR) data indicate SWP facies with abundant original evaporites are the most favorable reservoirs. EWPG facies are also locally favorable reservoir facies. Burrow mottling was very important in creating networks for later diagenetic fluids that resulted in variable porous and tight areas on macroscopic and microscopic scales. Very finely crystalline (<10-50 µm) dolomite is characteristic of early reflux or mixing zone dolomitization. The predominance of original evaporites in M1 strata is supportive of a reflux mechanism. Early dissolution of grains and dolomitization created moldic, intercrystalline and vuggy porosity important for favorable reservoir conditions.

Silica cementation and replacement of original lithologies is abundant throughout all three cores and replaces all facies types. The abundance of silica replacement is at least partially due to the abundance of sponge spicules. Early silica cementation and replacement is evidenced by silicified areas closely following burrow networks, displacive growth of silica nodules, brittle fracturing of silica areas and soft-sediment deformation of surrounding dolomite sediment and fractures in silica filled with dolomitic sediment. In addition, preservation of radiating bladed evaporite crystal and nodule textures without much breakage or compaction is supportive of early silica replacement. Silica replacement and cementation tend to result in relatively tight and pervasive replacement in EWPG facies whereas in SWP and MW facies, silica replacement is variable, especially where evaporites were replaced and contain more moldic and vuggy porosity. Brittle fracturing results in local micro- and macro-fracture porosity The silicified areas in EWPG are typically tight and form impermeable layers. However, some silicified areas contain abundant microcrystalline, vuggy and moldic porosity (tripolitic chert). In SWP and MW facies, silica replacement may partially or totally replace matrix and grains or replace the dolomite matrix and leave spicules as molds.

Silicified areas, MW facies and local shale layers that tend to be tight impart a complex vertical heterogeneity in the cores. Minipermeameter data collected every 0.25 feet (0.08 meters) support macroscopic and petrographic observations and document this vertical heterogeneity at a detailed scale not revealed by whole-core and standard plug data.

The coarse calcite cement and replacement associated with the M1 surface was extremely important in occluding porosity in underlying strata. This is confirmed by low minipermeameter readings, NMR data, the relative lack of oil staining, and the lack of production even where the M0 interval intersects the post-Mississippian unconformity in favorable structurally high areas to the northeast of Schaben Field (Figures 2.2, 2.3).

Brecciation and fracturing are ubiquitous throughout the three cores on macroscopic and microscopic scales. Fracture fill and breccia matrix includes shale, subangular to rounded, silt- to coarse-grained size detrital quartz, chert, megaquartz, chalcedony grains, carbonate micrite, carbonate grains, and skeletal grains. Clasts (ranging from rounded to angular) include chert/chalcedony/megaquartz fragments, clasts of original carbonate facies, replacive poikilotopic calcite clasts, coarse calcite cement fragments, and rubble of red and greenish limy clay.

Brecciation and fracturing were caused by a variety of processes interacting at different times. Some areas reveal several generations of fracturing, brecciation, cementation and sediment fills resulting in complex fabrics. As discussed earlier, early differential compaction between silicified areas and surrounding matrix resulted in brittle fracturing and soft-sediment deformation of surrounding matrix which imparted a fracture and breccia fabric. Local fracturing and brecciation were caused by early subaerial exposure (M1 surface). Post-Mississippian subaerial exposure, burial compaction and structural uplift resulted in brittle fracturing and brecciation of all facies and previous diagenetic events. Early and late fracturing and brecciation variably enhanced or destroyed reservoir characteristics as indicated by minipermeameter and NMR data and oil stain patterns. Fracture and breccia matrix porosity that remained open results in vertical communication among the thin intervals with favorable reservoir characteristics controlled by depositional facies and early diagenesis.

Historically, topographic highs just underlying the post-Mississippian unconformity are viewed as the most favorable locations for petroleum exploration and production. This study indicates that structurally uplifted and tilted Mississippian ramp strata were differentially eroded at the post-Mississippian unconformity resulting in a complex buried paleotopography. Sedimentologic, stratigraphic, diagenetic, petrophysical and well log data indicate that the most favorable areas for successful production may be where SWP facies originally containing abundant evaporites intersect the post-Mississippian unconformity and form the topographic highs. In contrast, areas where EWPG facies intersect the unconformity and form topographic highs are more variable as to reservoir characteristics. Our results indicate that M0 strata are relatively tight due to calcite cementation and replacement associated with subaerial exposure at the M1 surface and, therefore, are not favorable reservoir intervals. The fact that M0 strata are not productive where the M0 interval intersects the post-Mississippian unconformity in structurally favorable positions to the NE of our study area confirms the above prediction and highlights the value of detailed sedimentologic and diagenetic studies for understanding the relative importance of controls on reservoirs and providing predictive capabilities.

Although drilling strategies have been based on a karst-controlled model, the complex vertical heterogeneity and significance of depositional facies and early diagenesis in controlling reservoir architecture demonstrated in this study indicate that other production strategies such as horizontal drilling or targeted infill drilling may be viable alternative strategies. Future studies should address whether or not fracture porosity is necessary for making the depositional and early diagenetic controlled reservoir facies productive intervals.