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USGS2015

Effects of high salinity wastewater discharges on unionid mussels in the Allegheny River, Pennsylvania

We examined the effect of high salinity wastewater (brine) from oil and natural gas drilling on freshwater mussels in the Allegheny River, Pennsylvania, during 2012. Mussel cages (N = 5 per site) were deployed at two sites upstream and four sites downstream of a brine treatment facility on the Allegheny River. Each cage contained 20 juvenile northern riffleshell mussels Epioblasma torulosa rangiana). Continuous specific conductance and temperature data were recorded by water quality probes deployed at each site. To measure the amount of mixing throughout the entire study area, specific conductance surveys were completed two times during low-flow conditions along transects from bank to bank that targeted upstream (reference) reaches, a municipal wastewater treatment plant discharge upstream of the brine-facility discharge, the brine facility, and downstream reaches. Specific conductance data indicated that high specific conductance water from the brine facility (4,000–12,000 µS/cm; mean 7,846) compared to the reference reach (103–188 µS/cm; mean 151) is carried along the left descending bank of the river and that dilution of the discharge via mixing does not occur until 0.5 mi (805 m) downstream. Juvenile northern riffleshell mussel survival was severely impaired within the high specific conductance zone (2 and 34% at and downstream of the brine facility, respectively) and at the municipal wastewater treatment plant (21%) compared to background (84%). We surveyed native mussels (family Unionidae) at 10 transects: 3 upstream, 3 within, and 4 downstream of the high specific conductance zone. Unionid mussel abundance and diversity were lower for all transects within and downstream of the high conductivity zone compared to upstream. The results of this study clearly demonstrate in situ toxicity to juvenile northern riffleshell mussels, a federally endangered species, and to the native unionid mussel assemblage located downstream of a brine discharge to the Allegheny River.

Pennsylvania↗

USGS2019

An ANCOVA model for porosity and its uncertainty for oil reservoirs based on TORIS dataset

Porosity is one of the most important parameters to assess in-place oil or gas in reservoirs, and to evaluate recovery from enhanced production operations. Since it is relatively well-established to determine porosity using different laboratory and field methods, its value is usually determined at many locations across a reservoir as part of the common practice to capture reservoir heterogeneity and the variability in values. This suite of measurements and the distribution of values are most valuable for probabilistic reservoir assessments, and for spatial modeling if the exact data locations are known. Despite the importance of individual measurements to set the range of values for probabilistic studies, it is not always possible to access these data due to confidentiality. In most cases, commercial or publicly available databases that assessments may rely on usually report only mean values of porosity, like any other reservoir data, or they may not report a value at all. This makes both quantifying the mean value and the uncertainty around it difficult for probabilistic assessments. In this study, the TORIS (Tertiary Oil Recovery Information System) dataset of the National Petroleum Council and the U.S. Department of Energy was used to model porosity and the uncertainty around predicted values. TORIS is an integrated dataset of production data, reservoir properties, and project databases of crude oil reservoirs in the United States. The model presented in the paper was based on ANCOVA (Analysis of Co-Variance) of data from 1038 reservoirs from the TORIS dataset for porosity prediction, validation and testing for quantitative and qualitative parameters that may be readily available in most cases, and to estimate uncertainty around the mean values. This model also explored association of porosity values to different parameters, and to different depositional systems and diagenetic overprint conditions. Furthermore, an ANN (Artificial Neural Network) model was created to compare the predicted values of both models. Results showed that the ANN model was able to represent more of the variability, however it lacked the insights that might be gained from the ANCOVA model.

Journal of Petroleum Science and Engineering↗

USGS2022

Regional walrus abundance estimate in the United States Chukchi Sea in autumn

Human activities (e.g., shipping, tourism, oil, gas development) have increased in the Chukchi Sea because of declining sea ice. The declining sea ice itself and these activities may affect Pacific walrus ( Odobenus rosmarus divergens ) abundance; however, previous walrus abundance estimates have been notably imprecise. When sea ice is absent from the eastern Chukchi Sea, walruses in waters of the United States usually rest together onshore at a single Alaska coastal haulout, where they can be surveyed more easily than when they rest on dispersed offshore ice floes. We estimated the number of walruses on land (herd size) at this haulout from 13 unoccupied aircraft system (UAS) surveys flown within a 10-day period in each of 2018 and 2019. We estimated population size of walruses using the haulout over the course of the surveys by combining herd size data with data from satellite-linked transmitters that indicated whether tagged walruses were in or out of water during each survey. Our estimates of the population size of walruses using the haulout during each year's survey period were similar to each other and more precise than historical walrus abundance estimates: posterior means (95% credibility intervals) were 166,000 (133,000–201,000) for 2018 and 189,000 (135,000–251,000) for 2019. Auxiliary observations support using these estimates to represent the size of the population using the eastern Chukchi Sea in autumn during the surveyed years. Our study site was the only substantial Chukchi Sea coastal haulout in the United States during the survey periods and study-specific tracking data (consistent with known distribution and movement patterns) indicated tagged walruses remained in eastern Chukchi waters during the survey periods. In addition, the imagery, telemetry, and analytical methods developed for this study advance the prospect for precise range-wide walrus population size estimates.

Alaska↗

USGS2014

Application of near-surface geophysics as part of a hydrologic study of a subsurface drip irrigation system along the Powder River floodplain near Arvada, Wyoming

Rapid development of coalbed natural gas (CBNG) production in the Powder River Basin (PRB) of Wyoming has occurred since 1997. National attention related to CBNG development has focused on produced water management, which is the single largest cost for on-shore domestic producers. Low-cost treatment technologies allow operators to reduce their disposal costs, provide treated water for beneficial use, and stimulate oil and gas production by small operators. Subsurface drip irrigation (SDI) systems are one potential treatment option that allows for increased CBNG production by providing a beneficial use for the produced water in farmland irrigation. Water management practices in the development of CBNG in Wyoming have been aided by integrated geophysical, geochemical, and hydrologic studies of both the disposal and utilization of water. The U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) and the U.S. Geological Survey (USGS) have utilized multi-frequency airborne, ground, and borehole electromagnetic (EM) and ground resistivity methods to characterize the near-surface hydrogeology in areas of produced water disposal. These surveys provide near-surface EM data that can be compared with results of previous surveys to monitor changes in soils and local hydrology over time as the produced water is discharged through SDI. The focus of this investigation is the Headgate Draw SDI site, situated adjacent to the Powder River near the confluence of a major tributary, Crazy Woman Creek, in Johnson County, Wyoming. The SDI system was installed during the summer of 2008 and began operation in October of 2008. Ground, borehole, and helicopter electromagnetic (HEM) conductivity surveys were conducted at the site prior to the installation of the SDI system. After the installation of the subsurface drip irrigation system, ground EM surveys have been performed quarterly (weather permitting). The geophysical surveys map the heterogeneity of the near-surface geology and hydrology of the study area. The geophysical data are consistent between surveys using different techniques and between surveys carried out at different times from 2007 through 2011. This paper summarizes geophysical results from the 4-year monitoring study of the SDI system.

Wyoming↗

USGS2022

Permeability measurement and prediction with nuclear magnetic resonance analysis of gas hydrate-bearing sediments recovered from Alaska North Slope 2018 Hydrate-01 Stratigraphic Test Well

Permeability of porous media, such as oil and gas reservoirs, is the crucial material parameter for predicting their hydraulic behavior. A nuclear magnetic resonance (NMR) analyzer is widely used as a powerful tool to predict permeability of various media. NMR T 2 (transverse or spin–spin) relaxation time distribution, which is related to pore size distribution, gives the information to allow calculation of effective (initial) permeability. In this study, we investigate effective, intrinsic (absolute), and relative water and gas permeabilities of hydrate-bearing pressure core samples. These samples were recovered from the Alaska North Slope 2018 Hydrate-01 Stratigraphic Test Well by sidewall pressure coring and then analyzed in a laboratory using both fluid flow test and NMR analyzer. The peak of the NMR T 2 distribution was measured at 10–20 ms using a laboratory NMR analyzer, which compares well with in situ measurements obtained via logging while drilling NMR data for two samples with high gas hydrate saturations ( S h = 76% and 74%). Further, comparison of laboratory NMR T 2 distribution after hydrate dissociation revealed that the hydrate existed in large pore spaces. Effective permeabilities predicted by the Timur-Coates (TC) model and the Schlumberger-Doll-Research (SDR) model, with T 2 cutoff 33 ms, were about an order of magnitude less than the laboratory measured values. Alternative TC model-based calculations with the T 2 cutoff reduced to 10 ms and a newly developed hydraulic radius model better matched the laboratory data. For the analysis of the intrinsic permeabilities, the TC model with a T 2 cutoff of 33 ms and SDR model were greater than the laboratory derived values, while the hydraulic radius model more closely matched the laboratory-derived values. In addition, permeability measurements were also made relative to gas and water under constant three-phase flow (water–gas–hydrate) conditions. After hydrate dissociation, a relative permeability curve was developed for each of the analyzed core samples based on the Corey petrophysical model. The results indicate that the gas permeability changed rapidly at high water saturation around 90%. Thus, we infer that the selection of relative reservoir parameters should focus on the higher water saturation conditions.

Alaska↗

USGS2018

Landsat time series analysis of fractional plant cover changes on abandoned energy development sites

Oil and natural gas development in the western United States has increased substantially in recent decades as technological advances like horizontal drilling and hydraulic fracturing have made extraction more commercially viable. Oil and gas pads are often developed for production, and then capped, reclaimed, and left to recover when no longer productive. Understanding the rates, controls, and degree of recovery of these reclaimed well sites to a state similar to pre-development conditions is critical for energy development and land management decision processes. Here we use a multi-decadal time series of satellite imagery (Landsat 5, 1984–2011) to assess vegetation regrowth on 365 abandoned well pads located across the Colorado Plateau in Utah, Colorado, and New Mexico. We developed high-frequency time series of the Soil-Adjusted Total Vegetation Index (SATVI) for each well pad using the Google Earth Engine cloud computing platform. BFAST time-series models were used to fit temporal trends, identifying when vegetation was cleared from the site and the magnitudes and rates of vegetation change after abandonment. The time series metrics are used to calculate the relative fractional vegetation cover (RFVC) of each pad, a measure of post-abandonment vegetation cover relative to pre-drilling condition. Mean and median RFVC were 36% (s.d. 33%) and 26%, respectively, five years after abandonment, with one third of well pads having RFVC greater than 50%. Statistical analyses suggest that much of the high vegetation cover is associated with weedy invasive annual species such as cheatgrass (Bromus tectorum) and Russian thistle (Salsola spp.). Climate conditions and the year of abandonment also play a role, with increased cover in later years associated with a wetter period. Non-linear change at many pads suggests longer recovery times than would be estimated by linear extrapolation. New techniques implemented here address a complex response of cover change to soils, management, and climate over time, and can be extended to the operational monitoring of energy development across large areas.

International Journal of Applied Earth Observation↗

USGS2023

Methane pore accessibility, densification, and accommodation by organic matter in the Niobrara Formation at wet-gas thermal maturity conditions

Petroleum within unconventional source-rock reservoirs is hosted in organic matter and mineral pore space as well as in voids and microfractures. Recent work has shown that for source-rock reservoirs in the dry gas window, significant portions of methane (CH 4 ), the main component of petroleum at elevated maturities, can be stored within fine (<5 nm) organic matter porosity. However, within reservoirs at lower thermal maturities (e.g., peak oil or wet-gas conditions), the distribution and behavior of CH 4 and the higher alkanes that comprise gas condensates across pore sizes is unclear, especially for pores with diameters <50 nm. Understanding CH 4 distribution within these settings provides insight for petroleum generation, movement, and recoverability, ultimately enabling increased accuracy of estimated ultimate recovery. Here wide Q-range total neutron scattering was used to evaluate perdeuterated methane (CD 4 ) behavior at reservoir pressures (200–750 bar) and temperature (60 °C) in a sample at the late oil/wet gas thermal maturity stage from the Late Cretaceous Niobrara Formation, an active petroleum producing formation within the Denver-Julesburg Basin, U.S. Neutron scattering data show that mesopores within the Niobrara Formation sample exhibit mass fractal scattering, similar to previously measured U.S. marine shale samples. In the presence of CD 4 , scattering intensities between Q = 0.02–0.1 Å −1 (corresponding to nominal pore diameters from 25 to 5 nm, respectively) decrease with increased pressure up to 750 bar where at least 80% of all pores with ~25 nm diameters are CD 4 accessible. In contrast, between Q = 0.1–1 Å −1 (corresponding to nominal pore diameters from 5 to 0.5 nm, respectively), scattering intensity initially increased at the lowest CD 4 pressure tested (200 bar) before decreasing with increasing pressure. These signal fluctuations with CD 4 pressure are interpreted to arise from the creation of pores with diameters <5 nm, likely through deformation of solid bitumen by supercritical CD 4 , and/or the incorporation of CD 4 within sample organic matter. This new porosity represents an increase of at least ~8% in available pore volume within the sample, although the majority of these pores do not persist following removal of CD 4 . Additionally, there is strong evidence for densification of CD 4 within the sample indicated by a shift in the CD 4 intermolecular scattering peak to higher Q-values compared to bulk CD 4 . These results provide insight into fluid properties within source-rock reservoirs at late oil/wet gas thermal maturities, especially as they relate to organic porosity interconnectivity, and are discussed with perspective toward pressure management of gas condensate wells.

Colorado, Kansas, Nebraska, Wyoming↗

USGS2022

Planning and operations of the Hydrate 01 Stratigraphic Test Well, Prudhoe Bay Unit, Alaska North Slope

The National Energy Technology Laboratory, the Japan Oil, Gas and Metals National Corporation, and the U.S. Geological Survey are leading an effort to conduct an extended gas hydrate production test in northern Alaska. The proposed production test required the drilling of an initial stratigraphic test well (STW) to confirm the geologic conditions of the proposed test site. This well was completed in January 2019 in cooperation with the Prudhoe Bay Unit Working Interest Owners. The Prudhoe Bay Unit Hydrate-01 STW was spudded on 10-December-2018. Downhole data acquisition was completed on 25-December-2018, and the rig was released on 01-January-2019. The Hydrate-01 STW was drilled in two sections, including the surface hole that was drilled to a depth of 2248 ft measured depth (MD) (685 m MD) and cased, and the production hole section that was drilled to a depth of 3558 ft MD (1084 m MD) and also cased. A thermally chilled mineral-oil-based mud was used in the main (production) hole section of the well to maintain wellbore stability and quality of the wellbore acquired data. The primary wellbore data were acquired using logging-while-drilling tools. A sidewall pressure core system was also deployed to gather grain size and other data needed for the design of the future production test wells. In addition to confirming the geologic conditions at the test site, the Hydrate-01 STW was designed to serve as a monitoring well during future field operations. Therefore, two sets of fiber-optic cables, each including a bundled distributed acoustic sensor (DAS) and a distributed temperature sensor (DTS), were clamped to the outside of the production casing and cemented in place. In March 2019, the project team acquired three-dimensional (3D) DAS vertical seismic profiling data in the Hydrate-01 STW. Temperature surveys were also acquired with the DTS as deployed in the Hydrate-01 STW during the completion of the well and nearly continuously since March-2019.

Alaska↗

USGS2016

Hydrogeochemistry and coal-associated bacterial populations from a methanogenic coal bed

Biogenic coalbed methane (CBM), a microbially-generated source of natural gas trapped within coal beds, is an important energy resource in many countries. Specific bacterial populations and enzymes involved in coal degradation, the potential rate-limiting step of CBM formation, are relatively unknown. The U.S. Geological Survey (USGS) has established a field site, (Birney test site), in an undeveloped area of the Powder River Basin (PRB), with four wells completed in the Flowers-Goodale coal bed, one in the overlying sandstone formation, and four in overlying and underlying coal beds (Knoblach, Nance, and Terret). The nine wells were positioned to characterize the hydraulic conductivity of the Flowers-Goodale coal bed and were selectively cored to investigate the hydrogeochemistry and microbiology associated with CBM production at the Birney test site. Aquifer-test results indicated the Flowers-Goodale coal bed, in a zone from about 112 to 120 m below land surface at the test site, had very low hydraulic conductivity (0.005 m/d) compared to other PRB coal beds examined. Consistent with microbial methanogenesis, groundwater in the coal bed and overlying sandstone contain dissolved methane (46 mg/L average) with low δ 13 C values (−67&permil; average), high alkalinity values (22 meq/kg average), relatively positive δ 13 C-DIC values (4&permil; average), and no detectable higher chain hydrocarbons, NO 3 − , or SO 4 2− . Bioassay methane production was greatest at the upper interface of the Flowers-Goodale coal bed near the overlying sandstone. Pyrotag analysis identified Aeribacillus as a dominant in situ bacterial community member in the coal near the sandstone and statistical analysis indicated Actinobacteria predominated coal core samples compared to claystone or sandstone cores. These bacteria, which previously have been correlated with hydrocarbon-containing environments such as oil reservoirs, have demonstrated the ability to produce biosurfactants to break down hydrocarbons. Identifying microorganisms involved in coal degradation and the hydrogeochemical conditions that promote their activity is crucial to understanding and improving in situ CBM production.

Montana, Wyoming↗

USGS2019

Caribou use of habitat near energy development in Arctic Alaska

Increasing demands for energy have generated interest in expanding oil and gas production on the North Slope of Alaska, raising questions about the resilience of barren-ground caribou populations to new development. Although the amount of habitat lost directly to energy development in the Arctic will likely be relatively small, there are significant concerns about habitat that may be indirectly impacted due to caribou avoidance behaviors. Behavioral responses to energy development for wildlife have been well-documented, but such responses are often assumed to dissipate over time, despite scant information on the ability of animals to habituate. To understand the long-term effects of energy development on barren-ground caribou we investigated the behavior of the Central Arctic Herd in northern Alaska, which has been exposed to oil development on its summer range for approximately 40 years. Using recent (2015-2017) location data from GPS collared females, we conducted a zone of influence analysis to assess whether caribou reduced their use of habitat near energy development, and if so, the distance the effects attenuated. We conducted this analysis for the calving, post-calving and mosquito harassment periods when caribou exhibit distinct resource selection patterns, and contrasted our results to past research that investigated the responses of the Central Arctic Caribou Herd immediately following the construction of the oil fields. Despite the long-term presence of energy development within the Central Arctic Herd summer range, we found that female caribou exhibited avoidance responses to infrastructure during all time periods, although the effects waned across the summer. Caribou reduced their use of habitat within 5 km of development during the calving period, within 2 km during the post-calving period, and within 1 km during the mosquito harassment period, areas which were predicted to overlap 12%, 15% and 17% of important calving, post-calving, and mosquito habitat areas, respectively. During the calving period, the indirect effects we observed were similar to those observed in past research, whereas during the post-calving and mosquito periods, we detected avoidance responses which had not been previously reported. These findings corroborate a growing body of evidence suggesting that habituation to industrial development in Arctic caribou is likely to be weak or absent, and emphasizes the value of minimizing the footprint of infrastructure within important seasonal habitat areas to reduce behavioral impacts to barren-ground caribou.

Alaska↗

USGS2017

Effects of industrial and investigator disturbance on Arctic-nesting geese

Oil and gas development on the Arctic Coastal Plain (ACP) of Alaska, USA may have effects on Arctic-nesting birds. To estimate effects of industrial activity and investigator disturbance on avian productivity, we monitored nests of greater white-fronted geese ( Anser albifrons ) with digital cameras and periodic nest visits during 2013–2014 at 2 sites on the ACP. A disturbed site was adjacent to human-made infrastructure and industrial clean-up activities initiated at the onset of the study and a control site was >2 km from sources of industrial disturbance. We assessed variation in estimates of incubation constancy, nest survival, and predator behavior relative to site, year, and distance from industrial activity using nest photographs obtained at 1-minute intervals. We compared analysis of hourly nest survival informed by intensive monitoring with cameras to analysis of daily nest survival informed by traditional nest visit data obtained at intervals of 5–7 days to assess how method and time scale of sampling affect ecological inference. Geese in both sites exhibited high levels of nest attendance and initiated incubation breaks less than once per day. Observer-caused incubation breaks associated with nest visits ( = 37.8 min) were longer than other types of incubation breaks ( = 8.7 min), demonstrating a differential response by nesting geese to direct human encroachment versus indirect vehicular and aircraft traffic. During both years, geese were absent from nests more frequently in the disturbed ( = 0.9 breaks/day) than control ( = 0.6 breaks/day) site, and this break frequency was slightly higher for nests closer to industrial activity. In the year with high rates of depredation, nest survival was positively related to distance from industrial activity and abandoned infrastructure, consistent with predictions of industry-caused effects. This relationship, however, was not evident in the year with reduced predation pressure, likely because of annual variation in arctic fox ( Vulpes lagopus ) behavior. Analysis of nest survival probability informed by camera data allowed for detection of detailed patterns of variation that were not supported when using only visit data for the same nests. Observer visits were responsible for reductions of 7–35% in nest survival probability, highlighting the importance of minimizing, and controlling for, observer effects in studies of avian productivity. Indirect vehicular and aircraft disturbance posed less risk to nest survival than direct encroachment by observers at nest sites. Therefore, effects of industrial activities on avian productivity in the Arctic can be minimized through practices that limit direct encounters with nests.

Alaska↗

USGS2019

India National Gas Hydrate Program Expedition-02: Operational and technical summary

The India National Gas Hydrate Program is being steered by the government of India's Ministry of Petroleum and Natural Gas (MoPNG) with participation of Directorate General of Hydrocarbons (DGH), Oil and Natural Gas Corporation Limited (ONGC), and the National Oil Companies and Research Institutes of India. The India National Gas Hydrate Program Expedition 01 (NGHP-01) established the presence of gas hydrate in the Krishna Godavari (KG) and Mahanadi Basins and in the offshore area of the Andaman Sea Basin. However, the gas hydrates discovered during NGHP-01 were mainly distributed as fracture-filling material in fine-grained clay-rich sediments. The India National Gas Hydrate Program Expedition 02 (NGHP-02) was carried out with an objective to discover gas hydrate in sand-rich sediment along the eastern offshore margin of India. ONGC planned and executed NGHP-02 on the behalf of the MoPNG. NGHP-02 started on March 3, 2015 and was completed on July 28, 2015 (total 147 days) using the Japanese scientific Drilling Vessel Chikyu (D/V Chikyu). During NGHP-02, 42 holes at 25 sites were drilled, cored, and/or surveyed with downhole logging tools. These sites were located in four areas along the eastern margin of India and formally named Area A (Mahanadi Basin, three sites), Area B (northern part of the KG-Basin, twelve sites), Area C (central part of the KG-Basin, six sites), and Area E (southern part to the KG-Basin, four sites). All 25 sites established during NGHP-02 were first drilled and logged with logging-while-drilling (LWD) tools and an additional 17 holes were then drilled and/or cored with conventional coring tools (HPCS/ESCS) or pressure coring tools (PCTB). Wireline logging was conducted in 10 holes and formation tests using a dual packer Modular Formation Dynamics Tester (MDT) tool were carried out in two holes. The onboard science team used the laboratory facilities on the D/V Chikyu to examine and analyse the physical properties, geochemistry, and sedimentology of all the cores collected during the expedition. Core samples were also analysed in additional post-expedition shore-based studies conducted in numerous domestic and international gas hydrate research laboratories. The NGHP-02 sediment cores were archived at the National Gas Hydrate Core Repository in Mumbai (India), which is associated with the ONGC Gas Hydrate Research and Technology Centre (GHRTC). The necessary data for characterizing the occurrence of gas hydrate, such as interstitial water chlorinities, core-derived gas chemistry, physical and sedimentological properties, thermal images of the recovered cores, pressure core and downhole measured logging data (LWD and/or conventional wireline log data), were obtained from most of the drill sites established during NGHP-02. Almost all the drill sites yielded evidence for the occurrence of gas hydrate; however, the inferred in situ concentration of gas hydrate varied substantially from site to site. For the most part, the interpretation of downhole logging data, core thermal images, interstitial water analyses, and pressure core images from the sites established during NGHP-02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained (sand-rich) sediments. This paper presents the operational and technical summary of NGHP-02. NGHP-02 started on March 3, 2015 and was completed on July 28, 2015 (total 147 days) using the Japanese scientific Drilling Vessel Chikyu (D/V Chikyu). During NGHP-02, 42 holes at 25 sites were drilled, cored, and/or surveyed with downhole logging tools. These sites were located in four areas along the eastern margin of India and formally named Area A (Mahanadi Basin, three sites), Area B (northern part of the KG-Basin, twelve sites), Area C (central part of the KG-Basin, six sites), and Area E (southern part to the KG-Basin, four sites). All 25 sites established during NGHP-02 were first drilled and logged with logging-while-drilling (LWD) to

Journal of Marine and Petroleum Geology↗

USGS2000

A review of Arbuckle Group strata in Kansas from a sedimentologic perspective: Insights for future research from past and recent studies

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 1,390 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 generally are thought to be related directly 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 nonporous and porous horizons in the formation, and that high probability exist of locating 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 (about 50%) in many cores is controlled by depositional facies and dolomitization. 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 upscalling 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; and (4) diagenetic and geochemical studies focusing especially on the timing of, and processes associated with, dolomitization and karstification events and their contributions to creating or occluding porosity.

The Compass: Earth Science Journal of Sigma Gamma ↗