W. Lynn Watney

Contrasting depositional models for Pennsylvanian black shale discerned from molybdenum abundances

Some black shales contain >1,000 ppm Mo, comparable to conventional ores. These include Mo-rich Pennsylvanian black shales that were deposited rapidly near shore as epeiric seas inundated peat swamps, while coeval beds, less enriched in Mo, accumulated slowly offshore. Concentration of Mo probably resulted from retention of the metal by organic matter in acidic pore fluids characteristic of nearshore deposits. Thus, in Pennsylvanian shales where organic productivity is significant, Mo variations may be used to distinguish nearshore from offshore conditions. Enrichments in Mo occur in black shales of other ages; therefore a high Mo content may be useful as a general criterion for nearshore environments and other conditions with strong pH gradients.

Possible Late Middle Ordovician Organic Carbon Isotope Excursion: Evidence from Ordovician Oils and Hydrocarbon Source Rocks, Mid-Continent and East-Central United States

Oils generated by Middle Ordovician rocks are found throughout the Mid-Continent and east-central regions of the United States. Gas chromatographic characteristics of these oils include a relatively high abundance of n-alkanes with carbon numbers less than 20, a strong predominance of odd-numbered n-alkanes between C10 and C20, and relatively small amounts of branched and cyclic alkanes. Saturated and aromatic hydrocarbon fractions of 43 Ordovician oils from the Anadarko, Ardmore, Forest City, Illinois, Michigan, Salina-Sedgwick, and Williston basins and the Iowa shelf demonstrate a wide range in carbon isotope composition (^dgr13Csat = -24.9 ^pmil to -33.9 ^pmil, (^dgr13Carom = -24.3 ^pmil to -33.7 ^pmil). Saturated and aromatic hydrocarbons extracted from late Middle Ordovician shales (17 core samples) show ranges in ^dgr13C similar to that of the oils. The wide ranges in ^dgr13C for oils and rock extracts reflect a major, positive excursion(s) (6-9 per ^pmil) in organic matter ^dgr13C in late Middle Ordovician rocks. This excursion has at least a regional significance in that it can be documented in sections 480 mi (770 km) apart in south-central Kansas and eastern Iowa. The distance may be as much as 930 mi (1,500 km) if the carbon isotope variations observed in Michigan basin Ordovician oils and in organic matter from late Middle Ordovician rocks in southwestern Ontario are related to the same carbon isotope excursion. Organic-matter ^dgr13C in core samples from south-central Kansas and eastern Iowa is not directly related to variations in quantity or quality of organic matter, or maceral compositi n. The positive excursion in organic matter ^dgr13C is a possible result of increased organic matter productivity and/or preservation. The parallel shifts in organic and carbonate ^dgr13C in core samples from 1 E. M. Greene well, Washington County, Iowa, imply changes in the isotope composition of the ocean-atmosphere carbon reservoir. Differences in the magnitude of the carbon isotope shifts between organic matter (8.8 ^pmil) and carbonate (4.2 ^pmil) in this core suggest a decrease, either locally or regionally, in available dissolved CO2, possibly a result of high organic-matter productivity and/or limited circulation in the late Middle Ordovician seas.

Characterization of the Mississippian chat in south-central Kansas

To understand production from low resistivity-high porosity Mississippian chat reservoirs in south-central Kansas it is necessary to understand the nature of deposition and diagenesis, how tectonics is a factor, the lithofacies controls on petrophysical properties, and log response to these properties. The initial mudstones to sponge-spicule wacke-packstones were deposited in transgressive-regressive (T-R) cycles on a shelf to shelf margin setting, resulting in a series of shallowing-upward cycles. Sponge-spicule content appears to increase upward with increasing cycle thickness. After early silicification, inter- and post-Mississippian subaerial exposure resulted in further diagenesis, including sponge-spicule dissolution, vuggy porosity development in moldic-rich rocks, and autobrecciation. Meteoric water infiltration is limited in depth below the exposure surface and in distance downdip into unaltered, cherty Cowley Formation facies. Areas of thicker preserved chat and increased diagenesis can be correlated with structural lineaments and, in some areas, with recurrent basement block movement. Combination of folding or block fault movement prior to or during development of the basal Pennsylvanian unconformity, sponge-spicule concentration, and possibly thickness of overlying bioclastic wacke-grainstones resulted in variable reservoir properties and the creation of pods of production separated by nonproductive cherty dolomite mudstones. These events also resulted in alteration of the depositional cycles to produce a series of lithofacies that exhibit unique petrophysical properties. From bottom to top in a complete cycle seven lithofacies are present: (1) argillaceous dolomite mudstone, (2) argillaceous dolomite mudstone that has chert nodules, (3) clean dolomite mudstone that has nodular chert, (4) nodular to bedded chert, (5) autoclastic chert, (6) autoclastic chert that has clay infill, and (7) bioclastic wacke-grainstone. The uppermost cycle was terminated by another lithofacies, a chert conglomerate of Mississippian and/or Pennsylvanian age. The chert facies exhibit porosities ranging from 25 to 50% and permeabilities greater than 5 md. The (Begin page 86) cherty dolomite mudstones, argillaceous dolomite mudstones, and bioclastic wacke-grainstones exhibit nonreservoir properties. Reservoir production, numerical simulation, and whole core data indicate fracturing can be present in chat reservoirs and can enhance permeability by as much as an order of magnitude. Capillary pressure data indicate the presence of microporosity and can explain high water saturations and low resistivity observed in wire-line logs. Relative permeabilities to oil decrease rapidly for saturations greater than 60% and may be influenced by dual pore systems. Archie cementation exponents increase from 1.8 for mudstones to more than 2.5 in the cherts that have increasing sponge-spicule mold and vug content. Detailed modified Pickett plot analysis of logs reveals critical aspects of chat character and can provide reliable indices of reservoir properties and pay delineation. Models developed provide additional insight into the chat of south-central Kansas and understanding of the nature of controls on shallow-shelf chert reservoir properties.

Dissolution of bedded rock salt: a seismic profile across the active eastern margin of the Hutchinson Salt Member, central Kansas

Abstract Since late Tertiary, bedded rock salt of the Permian Hutchinson Salt Member has been dissolved more-or-less continuously along its active eastern margin in central Kansas as a result of sustained contact with unconfined, undersaturated groundwater. The associated westward migration of the eastern margin has resulted in surface subsidence and the contemporaneous sedimentation of predominantly valley-filling Quarternary alluvium. In places, these alluvium deposits extend more than 25 km to the east of the present-day edge of the main body of contiguous rock salt. The margin could have receded this distance during the past several million years. From an environmental perspective, the continued leaching of the Hutchinson Salt is a major concern. This predominantly natural dissolution occurs in a broad zone across the central part of the State and adversely affects groundwater and surface-water quality as nonpoint source pollution. Significant surface subsidence occurs as well. Most of these subsidence features have formed gradually; others developed in a more catastrophic manner. The latter in particular pose real threats to roadways, railways, and buried oil and gas pipelines. In an effort to further clarify the process of natural salt dissolution in central Kansas and with the long-term goal of mitigating the adverse environmental affects of such leaching, the Kansas Geological Survey acquired a 4-km seismic profile across the eastern margin of the Hutchinson Salt in the Punkin Center area of central Kansas. The interpretation of these seismic data (and supporting surficial and borehole geologic control) is consistent with several hypotheses regarding the process and mechanisms of dissolution. More specifically these data support the theses that: 1. (1) Dissolution along the active eastern margin of the Hutchinson Salt Member was initiated during late Tertiary. Leaching has resulted in the steady westward migration of the eastern margin, surface subsidence, and the contemporaneous deposition of predominantly valley-filling Quarternary alluvium. 2. (2) Along the active eastern margin, the rock salt has been leached vertically from the top down, and horizontally along the uppermost remnant bedded soluble layer(s). As a result, the eastern margin thickens gradually (up to 90 m) and in a stepwise manner from east to west for distances on the order 5–15 km. 3. (3) In places, the Hutchinson Salt Member has been leached locally along NNE-trending paleoshear zones situated to the west of the present-day edge of the main body of contiguous rock salt. Leaching at these sites initiated when the main dissolution front impinged upon preexisting shear zones.

Flow unit modeling and fine-scale predicted permeability validation in Atokan sandstones: Norcan East field, Kansas

Characterizing the reservoir interval into flow units is an effective way to subdivide the net-pay zone into layers for reservoir simulation. Commonly used flow unit identification techniques require a reliable estimate of permeability in the net pay on a foot-by-foot basis. Most of the wells do not have cores, and the literature is replete with different kinds of correlations, transforms, and prediction methods for profiling permeability in pay. However, for robust flow unit determination, predicted permeability at noncored wells requires validation and, if necessary, refinement. This study outlines the use of a spreadsheet-based permeability validation technique to characterize flow units in wells from the Norcan East field, Clark County, Kansas, that produce from Atokan aged fine- to very fine-grained quartzarenite sandstones interpreted to have been deposited in brackish-water, tidally dominated restricted tidal-flat, tidal-channel, tidal-bar, and estuary bay environments within a small incised-valley-fill system. The methodology outlined enables the identification of fieldwide free-water level and validates and refines predicted permeability at 0.5-ft (0.15-m) intervals by iteratively reconciling differences in water saturation calculated from wire-line log and a capillary-pressure formulation that models fine- to very fine-grained sandstone with diagenetic clay and silt or shale laminae. The effectiveness of this methodology was confirmed by successfully matching primary and secondary production histories using a flow unit-based reservoir model of the Norcan East field without permeability modifications. The methodologies discussed should prove useful for robust flow unit characterization of different kinds of reservoirs.

What controls porosity in cherty fine-grained carbonate reservoir rocks? Impact of stratigraphy, unconformities, structural setting and hydrothermal fluid flow: Mississippian, SE Kansas

Abstract The localization and heterogeneity of carbonate oil and gas reservoirs are commonly controlled by extensive diagenetic alteration. Mississippian (Osagean–Meramecian) strata in SE Kansas are investigated to determine structural, relative sea-level, diagenetic and depositional controls on stratigraphy, lithofacies distribution and reservoir character. This project shows how karst horizons and fractured zones can provide preferred conduits for hydrothermal porosity enhancement. Thus, enhanced porosity in karst horizons may have a late origin, with chemically aggressive hydrothermal fluids following preferred pathways of fluid flow. Lithofacies include echinoderm-rich bioclastic wacke–packstone, sponge-spicule-rich packstone, dolomitic bioclastic wackestone, argillaceous dolomite, tripolitic chert and chert breccia. Four cores are used to construct a 10 mile-long SW–NE-trending cross-section, showing three genetic units deposited on a mostly south-facing distally steepened ramp, with periods of upwelling. Paragenesis reveals that early and late dissolution enhances porosity in chert and carbonate facies. Fluid inclusion microthermometry from megaquartz and baroque dolomite reveals variable but increasing homogenization temperatures (70–160 °C) and increasing salinity through time. The best reservoirs may be controlled by depositional setting that led to large amounts of chert, alteration associated with subaerial exposure, and a hydrological and structural setting that led to enhanced hydrothermal fluid flow for later dissolution.

Shear-wave anisotropy reveals pore fluid pressure–induced seismicity in the U.S. midcontinent

S-wave anisotropy observations in the U.S. midcontinent provide direct evidence of pore fluid pressure–induced earthquakes. Seismicity in the U.S. midcontinent has increased by orders of magnitude over the past decade. Spatiotemporal correlations of seismicity to wastewater injection operations have suggested that injection-related pore fluid pressure increases are inducing the earthquakes. We present direct evidence linking earthquake occurrence to pore pressure increase in the U.S. midcontinent through time-lapse shear-wave (S-wave) anisotropy analysis. Since the onset of the observation period in 2010, the orientation of the fast S-wave polarization has flipped from inline with the maximum horizontal stress to inline with the minimum horizontal stress, a change known to be associated with critical pore pressure buildup. The time delay between fast and slow S-wave arrivals exhibits increased variance through time, which is common in critical pore fluid settings. Near-basement borehole fluid pressure measurements indicate pore pressure increase in the region over the earthquake monitoring period.

Unique near-surface seismic-reflection characteristics within an abandoned salt-mine well field, Hutchinson, Kansas

Summary High-resolution seismic reflections have been used effectively to investigate sinkholes resulting from the dissolution of a bedded salt unit found throughout most of central Kansas. A seismic reflection survey was conducted to investigate the shallow subsurface between a sinkhole that formed catastrophically within a few tens of meters of a main east/west rail line. Data quality was significantly below expectations and not equivalent to other seismic data from this area where acquisition parameters, equipment, and target intervals were similar. Near-surface tomographic and MASW analyses revealed a highly irregular bedrock surface characterized by what appear to be a high concentration of short wavelength dissolution features. These bedrock features are below about 20 m of unconsolidated sediments with physical dimensions several meters deep and several meters wide. Data quality is quite good on other seismic reflection surveys from this general area where these bedrock features are not present. Pronounced static shifts and degradation in spectral characteristics of reflections where these bedrock features are present seems to be isolated to an area suspected to be the crest of a relatively broad anticlinal structure where surface fractures could have provided a conduit for fresh water to access shallow, thin evaporite layers within the thick shale sequence in the upper 200 m. Broadband high-resolution compressional-wave energy suffered significantly from this highly irregular bedrock topography.

Improving Geologic and Engineering Models of Midcontinent Fracture and Karst-Modified Reservoirs Using New 3-D Seismic Attributes

Our project goal was to develop innovative seismic-based workflows for the incremental recovery of oil from karst-modified reservoirs within the onshore continental United States. Specific project objectives were: (1) to calibrate new multi-trace seismic attributes (volumetric curvature, in particular) for improved imaging of karst-modified reservoirs, (2) to develop attribute-based, cost-effective workflows to better characterize karst-modified carbonate reservoirs and fracture systems, and (3) to improve accuracy and predictiveness of resulting geomodels and reservoir simulations. In order to develop our workflows and validate our techniques, we conducted integrated studies of five karst-modified reservoirs in west Texas, Colorado, and Kansas. Our studies show that 3-D seismic volumetric curvature attributes have the ability to re-veal previously unknown features or provide enhanced visibility of karst and fracture features compared with other seismic analysis methods. Using these attributes, we recognize collapse features, solution-enlarged fractures, and geomorphologies that appear to be related to mature, cockpit landscapes. In four of our reservoir studies, volumetric curvature attributes appear to delineate reservoir compartment boundaries that impact production. The presence of these compartment boundaries was corroborated by reservoir simulations in two of the study areas. Based on our study results, we conclude that volumetric curvature attributes are valuable tools for mapping compartment boundaries in fracture- and karst-modified reservoirs, and we propose a best practices workflow for incorporating these attributes into reservoir characterization. When properly calibrated with geological and production data, these attributes can be used to predict the locations and sizes of undrained reservoir compartments. Technology transfer of our project work has been accomplished through presentations at professional society meetings, peer-reviewed publications, Kansas Geological Survey Open-file reports, Masters theses, and postings on the project website: http://www.kgs.ku.edu/SEISKARST.

Seismic detection of shallow natural gas beneath Hutchinson, Kansas

A high-resolution seismic reflection survey was conducted in order to identify shallow natural gas that had leaked from a gas storage facility near Hutchinson, Kansas. Gas presence produced both bright spots and dim spots on the seismic reflection profiles. Core and well log data from wells drilled to vent the gas indicate that the gas-bearing interval corresponds to thin dolomite layers, which have higher P-wave velocities than the surrounding shales. Gas within fractures in these dolomites appears to reduce the velocity of the dolomite interval down to or below that of the shales. Depending upon the magnitude of the gas effect, a dim out or bright spot is produced. As gas dissipates from a given location, the associated seismic anomaly is reduced.