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Margaret A. Keller

Publications and source records attributed to Margaret A. Keller.

11 recordsLinked to original sources
USGS2015

Arctic Alaska’s Lower Cretaceous (Hauterivian and Barremian) mudstone succession—Linking lithofacies, texture, and geochemistry to marine processes

We present new images and descriptions of the lithofacies and organic facies of the pebble shale unit and lower part of the Hue Shale (Lower Cretaceous) of Arctic Alaska at a high magnification that illustrates their textural characteristics. Our aims were to describe and determine the distribution of facies in these petroleum source rocks and to identify the processes that formed them. We sampled at high-resolution and applied new petrographic techniques combined with scanning electron microscopy and geochemical analyses to samples collected from three widely spaced sections—located in exposures along the Canning River and continuous core from the Mikkelsen Bay State 1 and Orion 1 wells. Results from these three locations indicate that this succession consists primarily of clay-rich mudstones that are variously silt- or sand-bearing and clay-dominated mudstones that exhibit mainly relict, 2–5 millimeter thick bedding and common but variable microbioturbation, rare macrobioturbation, and common fabrics of pelleted clay and silt. These mudstones contain rare, poorly sorted, silt-rich basal laminae that are often discontinuous and have wavy, sharp bases and crude upward fining. In addition, mud-supported, outsized clasts (dropstones) of fine sand to pebble size are present throughout the succession as isolated clasts or in clusters. We interpret these textures and much of this succession to result from intermittent deposition by suspension settling from melting seasonal sea ice—sometimes sediment-laden—and associated primary productivity. Overall, this mudstone succession fines and deepens upward from the pebble shale unit into the Hue Shale. In the Hue Shale of the Orion well, however, different processes intermittently deposited thin, discrete intervals of coarser sediment that probably represent deposition from density currents. Also in the Hue Shale of the Orion well, several thicker sandstone and silt-dominated mudstone units with discordant, scoured bases and cut and fill structures represent erosion during higher energy events such as major storms. Other lithofacies within the succession are graded tuffs/bentonites and tuffaceous/bentonitic mudstones from episodic volcanic ash falls; these are abundant in the Hue Shale, and very rare in the pebble shale unit of the two wells. Organic-carbon rich strata associated with volcanic ash intervals of the pebble shale unit and Hue Shale in the Mikkelsen 1 well have some of the best petroleum source rock potential determined for this succession. Authigenic pyrite and carbonate-cement-dominated mudstone are also present in both units of all three sections. The carbonate-cemented units indicate breaks in sedimentation and are common in the Hue Shale and in sections of the pebble shale unit interpreted to be more distal, such as along the Canning River. Our results document the variation in facies and textures of the Hauterivian and Barremian Lower Cretaceous mudstone succession of Arctic Alaska. Comparison of these characteristics to the products of modern processes on the North Slope of Alaska, in the Beaufort Sea, and elsewhere suggest that this succession formed primarily from depositional processes related to seasonal sea ice with intermittent fluvial-sourced sediment deposited by density currents and episodic erosion and reworking by storms and other currents.

Alaska↗
USGS2014

Compositional controls on early diagenetic pathways in fine-grained sedimentary rocks: Implications for predicting unconventional reservoir attributes of mudstones

Diagenesis significantly impacts mudstone lithofacies. Processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks. Selected organic-carbon-rich mudstones, from the Kimmeridge Clay and Monterey Formations, have been investigated to determine how varying starting compositions influence diagenesis. The sampled Kimmeridge Clay Formation mudstones are organized into thin homogenous beds, composed mainly of siliciclastic detritus, with some constituents derived from water-column production (e.g., coccoliths, S-depleted type-II kerogen, as much as 52.6% total organic carbon [TOC]) and others from diagenesis (e.g., pyrite, carbonate, and kaolinite). The sampled Monterey Formation mudstones are organized into thin beds that exhibit pelleted wavy lamination, and are predominantly composed of production-derived components including diatoms, coccoliths, and foraminifera, in addition to type-IIS kerogen (as much as 16.5% TOC), and apatite and silica cements. During early burial of the studied Kimmeridge Clay Formation mudstones, the availability of detrital Fe(III) and reactive clay minerals caused carbonate- and silicate-buffering reactions to operate effectively and the pore waters to be Fe(II) rich. These conditions led to pyrite, iron-poor carbonates, and kaolinite cements precipitating, preserved organic carbon being S-depleted, and sweet hydrocarbons being generated. In contrast, during the diagenesis of the sampled Monterey Formation mudstones, sulfide oxidation, coupled with opal dissolution and the reduced availability of both Fe(III) and reactive siliciclastic detritus, meant that the pore waters were poorly buffered and locally acidic. These conditions resulted in local carbonate dissolution, apatite and silica cements precipitation, natural kerogen sulfurization, and sour hydrocarbons generation. Differences in mud composition at deposition significantly influence subsequent diagenesis. These differences impact their source rock attributes and mechanical properties.

AAPG Bulletin↗
USGS2010

Algal blooms and "Marine snow": Mechanisms that enhance preservation of organic carbon in ancient fine-grained sediments

Combined petrographic and geochemical methods are used to investigate the microfabrics present in thin sections prepared from representative organic carbon-rich mudstones collected from three successions (the Kimmeridge Clay Formation, the Jet Rock Member of the Whitby Mudstone Formation, and the pebble shale and Hue Shale). This study was initiated to determine how organic carbon-rich materials were being delivered to the sediment–water interface, and what happened to them after deposition, prior to deep burial. Analyses of the fabrics present shows that they exhibit many common attributes. In particular they are all: (1) highly heterogeneous on the scale of a thin section, (2) organized into thin beds (< 10 mm thick) composed mainly of mineral mixtures of fine-grained siliciclastic detritus and carbonate materials, and (3) contain significant concentrations of organic carbon, much of which is organized into laminasets that contain abundant organomineralic aggregates and pellets. In addition, framboidal pyrite (range of sizes from < 20 μm to < 1 μm) and abundant agglutinated foraminifers are present in some units. The individual beds are commonly sharp-based and overlain by thin, silt lags. The tops of many of the beds have been homogenized and some regions of the pelleted laminasets contain small horizontal burrows. The organomineralic aggregates present in these mudstones are interpreted to be ancient examples of marine snow. This marine snow likely formed in the water column, particularly during phytoplankton blooms, and was then transported rapidly to the seafloor. The existence of the thin beds with homogenized tops and an in-situ infauna indicates that between blooms there was sufficient oxygen and time for a mixed layer to develop as a result of sediment colonization by diminutive organisms using either aerobic or dysaerobic metabolic pathways. These textures suggest that the constituents of these mudstones were delivered neither as a continuous rain of sediment nor were the bottom waters persistently anoxic. In addition, the presence of thin lags and sharp-based beds suggests that the seafloor was being episodically reworked during deposition. These fabrics indicate that conditions in the water columns and at the seafloors while these rocks were being deposited were very dynamic, and episodic fluxes of high concentrations of organic carbon to the seafloor, during phytoplankton blooms, likely enhanced preservation of organic carbon.

Journal of Sedimentary Research↗
USGS2004

Quaternary tectonic setting of South-Central coastal California

Recent geodetic, geologic, and seismologic studies show that the south-central coast of California is a region of active Quaternary deformation. Northeast-directed crustal shortening is occurring in a triangular-shaped region between the Hosgri-San Simeon fault system on the west, the Southern Coast Ranges on the northeast, and the western Transverse Ranges on the south. We informally call this region the Los Osos domain. In this study, we conducted detailed geological, seismological, and geophysical investigations to characterize the nature and rates of deformation in the domain. Locations of active and potentially active faults and folds are compiled at a scale of 1:250,000 for the entire domain based primarily on onshore geologic data and offshore geophysical data. Crustal shortening in the domain is accommodated by a series of prominent northwest-trending reverse faults and localized folding. The reverse faults separate distinct structural blocks that have little or no internal deformation. Hangingwall blocks are being uplifted at rates of up to 0.2 mm/yr. Footwall blocks are either static or slowly subsiding at rates of 0.1 mm/yr or less, except for localized areas of concentrated subsidence directly adjacent to some faults. The cumulative rate of crustal shortening is about 1 to 2 mm/yr across the northern part of the domain based on observed geologic deformation. Cumulative shortening across the central and southern parts of the domain is poorly constrained by geologic data and may approach 2 to 3 mm/yr. Historical and instrumental seismicity generally are spatially associated with the uplifted blocks and bordering reverse faults to depths of about 10 km. Together with near-surface geological data and deeper crustal geophysical imaging that show high-angle faulting, the seismicity data indicate that the reverse faults probably extend to the base of the seismogenic crust. The base of the seismogenic crust may correspond with a mid-crustal detachment or decollement surface into which the reverse faults root. We speculate that the detachment may coincide, in part, with the top of a northeast-dipping slab of oceanic crust that extends beneath the western margin of the continent or with the brittle-ductile transition above the subducted slab. The Los Osos domain of north-northeast/south-southwest crustal shortening is structurally detached from the offshore Hosgri Fault Zones. Both the pattern and regional extent of deformation in the Los Osos domain contrast sharply with that of the offshore Santa Maria Basin. The basin is undergoing minor east-northeast/west-southwest crustal shortening at rates of less than 0.1 mm/yr and is moving northwestward at a rate of about 1 to 3 mm/yr relative to the Los Osos domain along the San Simeon and Hosgri Fault Zones. Geodetic data and the kinematics of north-northeast-directed crustal shortening of the Los Osos domain east of the Hosgri Fault Zone show that the rate and cumulative amount of right-slip along the Hosgri Fault Zone progressively decrease southward. Quaternary deformation within the Los Osos domain is related to distributed dextral simple shear associated with Pacific-North American plate motion. Paleomagnetic data show that clockwise rotation of the western Transverse Ranges has occurred along the southern boundary of the domain during the past 6 m.y. During this time, the Salinian crustal block, which forms the eastern boundary of the Los Osos domain, has remained relatively stable. Internal shortening of the Los Osos domain has accommodated the relative motions of these bordering crustal blocks, particularly the rotation of the western Transverse Ranges.

Bulletin↗
USGS2003

Petroleum source potential of the Lower Cretaceous mudstone succession of the NPRA and Colville Delta area, North Slope Alaska, based on sonic and resistivity logs

Resource assessment of the North Slope of Alaska by the U. S. Geological Survey includes evaluation of the petroleum source potential of Mesozoic and Cenozoic rocks using the delta log R technique (Passey and others, 1990). Porosity and resistivity logs are used in combination with thermal maturity data to produce a continuous profile of total organic carbon content in weight % (TOC). From the pattern and amount of TOC in the profile produced, the depositional setting and thus the petroleum source-rock potential (kerogen type) of the organic matter can be inferred and compared to interpretations from other data such as Rock-Eval pyrolysis. TOC profiles determined by this technique for the contiguous interval of pebble shale unit, Hue Shale (including the Gamma Ray Zone or GRZ), and lower part of the Torok Formation indicate important potential for petroleum generation in the Tunalik 1, Inigok 1, N. Inigok 1, Kuyanak 1, Texaco Colville Delta 1, Nechelik 1, and Bergschrund 1 wells of the western North Slope region. TOC profiles suggest that this interval contains both type II and III kerogens – consistent with proposed depositional models -- and is predominantly greater than 2 wt. % TOC (cut-off used for effective source potential). Average TOC for the total effective section of the pebble shale unit + Hue Shale ranges from 2.6 to 4.1 wt % TOC (values predominantly 2-8% TOC) over 192-352 ft. Source potential for the lower Torok Formation, which also has interbedded sandstone and lean mudstone, is good to negligible in these 7 wells.

Alaska↗
USGS2001

High resolution study of petroleum source rock variation, Lower Cretaceous (Hauterivian and Barremian) of Mikkelsen Bay, North Slope, Alaska

Open File Report 01-480 was designed as a large format poster for the Annual Meeting of the American Association of Petroleum Geologists and the Society for Sedimentary Geology in Denver Colorado in June 2001. It is reproduced here in digital format to make widely available some unique images of mudstones. The images include description, interpretation, and Rock-Eval data that resulted from a high-resolution study of petroleum source rock variation of the Lower Cretaceous succession of the Mobil-Phillips Mikkelsen Bay State #1 well on the North Slope of Alaska. Our mudstone samples with Rock-Eval data plus color images are significant because they come from one of the few continuously cored and complete intervals of the Lower Cretaceous succession on the North Slope. This succession, which is rarely preserved in outcrop and very rarely cored in the subsurface, is considered to include important petroleum source rocks that have not previously been described nor explained Another reason these images are unique is that the lithofacies variability within mudstone dominated successions is relatively poorly known in comparison with that observed in coarser clastic and carbonate successions. They are also among the first published scans of thin sections of mudstone, and are of excellent quality because the sections are well made, cut perpendicular to bedding, and unusually thin, 20 microns. For each of 15 samples, we show a thin section scan (cm scale) and an optical photomicrograph (mm scale) that illustrates the variability present. Several backscattered SEM images are also shown. Rock-Eval data for the samples can be compared with the textures and mineralogy present by correlating sample numbers and core depth.

Open-File Report↗
USGS1990

Introduction to stratigraphy and hydrocarbon occurrence in Oligocene and Miocene rocks of the Santa Barbara-Ventura and Santa Maria Basins of California

The Santa Barbara-Ventura and Santa Maria basins of southern California have been of great interest to the oil industry because oil or gas has been produced from nearly all the sedimentary units ranging in age from the Cretaceous to the Pleistocene. In particular, the Neogene section of these basins is oil-rich, containing eight giant oil fields (greater than 100 million barrels) in the onshore and at least five in the offshore. In the Santa Barbara-Ventura basin the sedimentary sequence includes marine beds ranging in age from Cretaceous to Pleistocene and two nonmarine sequences of Eocene-Oligocene and Pleistocene ages. The main reservoir for hydrocarbons in this basin is the thick deep-sea sand sequence of the Pliocene and Pleistocene Pico Formation. The other important production in this basin is from reservoirs in the Miocene and, to a lesser extent, the Oligocene sequences. These reservoirs, including the nonmarine Sespe Formation and the marine Alegria, Vaqueros, Monterey, and Sisquoc Formations, are the emphasis of this core workshop. In the Santa Maria basins to the north and northwest of the Santa Barbara-Ventura basin, the Paleogene sequence is absent for the most part, and the upper Neogene sequence does not include the thick Pliocene and Pleistocene Pico Formation. In both the onshore and offshore Santa Maria basins the fractured rocks of the deep-marine Miocene Monterey Formation are the main reservoir for hydrocarbons. Another important reservoir is the upper Miocene and Pliocene marine Sisquoc Formation; both Monterey and Sisquoc cores from the Santa Maria basin are shown in this workshop. As in the Santa Barbara-Ventura basin, the pre-Monterey sedimentary sequence of the Santa Maria basins also contains minor petroleum accumulations.

California↗