Federico F. Krause

Genesis and geometry of the Meiklejohn Peak lime mud-mound, Bare Mountain Quadrangle, Nevada, USA: Ordovician limestone with submarine frost heave structures—a possible response to gas clathrate hydrate evolution

Abstract During the Early Middle Ordovician (Early Whiterockian) the Meiklejohn Peak lime mud-mound, a large whaleback or dolphin back dome, grew on a carbonate ramp tens to hundreds of kilometres offshore. This ramp extended from the northwest margin of Laurentia into the open waters of the ancestral Pacific Ocean to the north. The mound developed in an outer ramp environment, in relatively deep and cold water. A steep northern margin with a slope that exceeds 55° characterizes the mound. This margin is split by a 14-m long vertical fracture that separates a zone of slumped, drag-folded and brecciated rocks from the main mass of the mound. Failure along this fracture occurred subcutaneously, as highlighted by covering beds that are folded next to the mound. Brecciated blocks and clasts contain zebra and stromatactis structures indicating that these rocks and structures were lithified early in the history of the mound. The southern end of the mound is less steep and is characterized by large, echinodermal grainstone cross-beds. These deposits are part of a large, subaqueous dune that grew northwards and preceded the main development of the mound. Southward dipping and downlapping layers of mud-mound mudstone and wackestone overlie the dune. These muddy limestone layers are cut in several places by injection dykes and are pierced, near the contact with the underlying dune, by a 25-m long pipe filled with rotated nodular and brecciated mud-mound clasts. This long pipe extends to the edge of the mound and appears to have been a conduit where fluidized materials that came from the mounds interior were vented. The interior of the mound is typified by light grey limestone with zebra bands and stromatactis structures. Both structures represent former cavity systems that are filled with fibrous and bladed calcite and pelleted and laminated geopetal mudstone. Spar bands of zebra limestone often extend for several metres and appear to have been unsupported over these distances. Zebra banded rocks are also accompanied by snout and socket structures and, in some instances, are folded and sheared by curving kink bands. Zebra and stromatactis limestone structures found throughout the mud-mound resemble frost heave and cryoturbation structures identified in both Holocene and Pleistocene cryosols, and in laboratory experiments with advancing freezing fronts in clay-size sediment. Significantly, modern occurrences of methane clathrate hydrate (methane-charged ice) display parallel and digitate layering similar in depositional appearance to that of zebra and stromatactis limestone from Meiklejohn Peak. Early carbonate cements are also commonly associated with these modern clathrate hydrate deposits. Consequently, gas clathrate hydrates may have been the propping agent for zebra and stromatactis structures observed in the mud-mound. In this scenario, carbonate cements would have precipitated and stabilized these structures, both with the consolidation and dissociation of gas clathrate hydrates, and with the oxidation and reduction of associated gases. Stable δ 13 C and δ 18 O isotope ratios collected from mudstone and spar of zebra and stromatactis structures indicate that they were lithified in equilibrium with Ordovician seawater. The δ 13 C isotope ratios recorded at Meiklejohn Peak are similar to δ 13 C isotopic ratios obtained from ∑CO 2 evolving from modern seafloor. These isotopic ratios may indicate that frost heave structures in the Meiklejohn Peak mud-mound are the result of consolidation and dissociation of carbon dioxide clathrate hydrates. Even though the bulk of gas clathrate hydrates identified to date in modern ocean floors are composed of methane, carbon dioxide clathrate hydrates are known from the modern seafloor of the Okinawa Trough. They may also be common in areas of abundant carbonate sediment accumulation, as suggested by recent observations from the Great Australian Bight.

Paleozoic stromatactis and zebra carbonate mud-mounds: Global abundance and paleogeographic distribution

Carbonate mud-mounds with zebra and stromatactis structures are present in every Paleozoic system and series, but are more common in Devonian and Carboniferous deposits, reaching their acme in Mississippian System (lower Carboniferous) rocks. Global distributions illustrate that mud-mounds spanned the planet ranging from tropical to polar circles. Such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments, but also in settings where oceanic waters were cold and seasonally light limited. Moreover, their proliferation during the Devonian and Carboniferous was at a time when planet-wide climatic ice-house conditions are thought to have prevailed. Mud-mounds, therefore, may also be products of cool and cold-water carbonate sedimentation.

Transformation of iron sulfide to greigite by nitrite produced by oil field bacteria.

Nitrate, injected into oil fields, can oxidize sulfide formed by sulfate-reducing bacteria (SRB) through the action of nitrate-reducing sulfide-oxidizing bacteria (NR-SOB). When reservoir rock contains siderite (FeCO3), the sulfide formed is immobilized as iron sulfide minerals, e.g. mackinawite (FeS). The aim of our study was to determine the extent to which oil field NR-SOB can oxidize or transform FeS. Because no NR-SOB capable of growth with FeS were isolated, the well-characterized oil field isolate Sulfurimonas sp. strain CVO was used. When strain CVO was presented with a mixture of chemically formed FeS and dissolved sulfide (HS−), it only oxidized the HS−. The FeS remained acid soluble and non-magnetic indicating that it was not transformed. In contrast, when the FeS was formed by adding FeCl2 to a culture of SRB which gradually produced sulfide, precipitating FeS, and to which strain CVO and nitrate were subsequently added, transformation of the FeS to a magnetic, less acid-soluble form was observed. X-ray diffraction and energy-dispersive spectrometry indicated the transformed mineral to be greigite (Fe3S4). Addition of nitrite to cultures of SRB, containing microbially formed FeS, was similarly effective. Nitrite reacts chemically with HS− to form polysulfide and sulfur (S0), which then transforms SRB-formed FeS to greigite, possibly via a sulfur addition pathway (3FeS + S0 → Fe3S4). Further chemical transformation to pyrite (FeS2) is expected at higher temperatures (>60°C). Hence, nitrate injection into oil fields may lead to NR-SOB-mediated and chemical mineral transformations, increasing the sulfide-binding capacity of reservoir rock. Because of mineral volume decreases, these transformations may also increase reservoir injectivity.

Implied basement-tectonic control on deposition of Lower . Carboniferous carbonate ramp, southern Cordillera, Canada

cambrian (Kanasevich et al., 1969) and preThe Mount Head embayment is a regional downwarp along the west coast of Lower Middle Devonian (Norris and Price, 1966; Carboniferous western Canada that developed by differential subsidence, which we recog- Benvenuto and Price, 1979). nize from lithostratigraphic and biostratigraphic patterns. Palinspastic restoration of the Mount Head embayment illustrates that subsidence domains are geometrically coincident with previously identified tectonic elements of the autochthonous basement. Thus, it ap- METHODS pears that basement-tectonic elements controlled sediment accommodation and accumu- Geologic observations were recorded on lation within the Mount Head embayment. The overall shape of this embayment is probably palinspastically restored maps and cross setinherited from the arcuate shape of the Late Proterozoic cratonic rift margin. Within the tions and to basement-tectonic Mount Head embayment, several northeast-southwest-trending Archean and Proterozoic domains defined on aeromagnetic and gravbasement-tectonic elements intersect the autochthonous cratonic margin at high angles. ity maps (Brandley, 1993). The palinspastic Carboniferous piano-key-like structural reactivation of these tectonic elements produced base map is modified from Gibson,s (1985) oriented trends of differential subsidence that partitioned the Mount Head embayment into map of displacement vectors for Jurassic two subbasins (Crowsnest and Kananaskis depocenters) separated by a more positive area rocks. The magnitudes of the vectors were (Highwood high). Moreover, our data imply that Precambrian basement structure noted by others under the Plains and Rocky Mountain fold and thrust belt continued across the altered to reflect displacements at the MisRocky Mountain trench to the west, and that tectonic control on sedimentation noted by sissippian level documented by Bally et al. others for Precambrian and pre-Middle Devonian rocks extends clearly into overlying (1966), Price (1981), and Price and Fermor Carboniferous deposits. (1985), university theses, and Geological Survey of Canada maps and sections across

A comparison of plug-derived and probe-derived permeability in cross-bedded sandstones of the Virgelle Member, Alberta, Canada: The influence of flow directions on probe permeametry

In this article we compare permeability measured on radially confined cylindrical plugs to permeabilities measured using a millimeter-scale probe tip on unconfined end-faces of the same plugs. Our investigation focuses on directional attributes of probe permeametry data as a means to understand the differences between plug-scale and probe-scale permeability magnitudes. Horizontal and vertical plug and probe flow tests were carried out on fine- to medium-grained, three-dimensional (3-D) cross-bedded sandstones collected from estuarine channel successions within the Cretaceous-age Virgelle Member at Writing-on-Stone Provincial Park, southern Alberta, Canada. The sandstones are heterogeneous in that they contain thin, discontinuous micaceous/carbonaceous laminae and apparent bedding-parallel grain-scale fabric. The resulting probe-derived permeabilities (0.4-3.5 d) are mostly higher than corresponding plug permeabilities (0.5-1.6 d), and mean probe-derived permeability anisotropy is significantly lower than that derived from plug-scale measurements, that is, plug k V / k H is less than probe-derived k V / k H . The results can be understood as a consequence of the model flow geometry of probe flow tests (Goggin et al., 1988), namely, that flow is preferentially directed parallel to the sample surface, radially outward from the probe tip. On end-faces of vertical plugs the bedding-parallel fabric further enhances flow parallel to the sample surface, and probe measurements yield reliable estimates of horizontal permeability, having magnitudes comparable to the permeability of equivalent horizontal plugs. For probe measurements on end-faces of horizontal plugs, flow paths appear to be variably confined parallel to bedding as a function of grain-scale and laminae-scale fabric, systematically yielding an overestimate of horizontal permeability ( k PRH > k PLH ). Interpretation of probe data in this orientation, however, is very uncertain, due to partitioning of flow paths parallel to and at angle to bedding. Significantly, almost all probe permeameter core and outcrop studies are based on measurements of this type.

Tube worm fossils or relic methane expulsing conduits

Abstract Chemosynthetic ecosystems teeming with tubeworm colonies were discovered at hydrothermal vents in the Galapagos Ridge in 1977 and at cold seeps at the base of the Florida Escarpment in 1984. As a result of these reports a number of fossil examples were identified in the rock record. One such assemblage was recognized in the Western Interior Seaway, in the Middle Campanian Pierre Shale Formation, where previous researchers noted siboglinid (formerly vestimentiferan and pogonophoran) tubeworms in methane-derived nodular limestones with tubules. On the inside these tubules have an outer ring of micrite with microparticulate siliciclastic materials and a core of calcite cement. Alternatively, they have an outer annulus of calcite cement and a core of microparticulate siliciclastic materials with calcite cements. Interestingly, the cemented cores can contain meniscate and vesiculate fabrics in association with the microparticulate linings. With this evidence we infer that the tubules preserve fabrics of former gas bubbles; the microparticulate linings are deposits that accumulated on the walls of the tubules as fluids streamed through them. Methane bubbles would have carried adhered siliciclastic microparticles and bubble wakes would have held entrained microparticles. We, thus, interpret the tubules to be former, small, subseafloor conduits along which fluid and particulate transport occurred. Particle transport by gas bubbles is a well-known process in chemical and mineral industries. Our observations highlight this process for the first time in an ancient geologic conduit system and provide a mechanism for maintaining particulate plumes that accompany effusing methane streams at modern seeps and vents.

Preserved organic matter and miospores in buried Middle Devonian (Givetian) paleosols: indicators of weathering, oxidation and maturity

Abstract Preserved organic matter and miospores from buried Middle Devonian (Givetian), cumulate, hydromorphic, floodplain paleosols in a fluvio-deltaic setting reflect the intensity of pedogenic processes as indicated by pedologic evidence. Amorphous organic material, fungal clusters, cuticle and miospores were examined in the paleosols and compared with organic matter in unweathered parent material. The paleosols include Protosols, Gleysols and a Calcisol and range from mature to immature, strongly oxidized to strongly gleyed, and are with and without calcareous nodules. The indicators of increasing soil maturity are: (1) increased abundance of amorphous organic residue and fungal clusters relative to miospores and cuticle; and (2) increased ripping, edge degradation and fungal, bacterial and mineral alteration of miospores.

Waterflood response of reservoirs in an estuarine valley fill; upper Mannville G, U, and W pools, Little Bow Field, Alberta, Canada

Lower Cretaceous upper Mannville G, U, and W pools in Little Bow field are hosted by separate, parallel, elongate estuarine sandstone bodies within an incised valley fill. Each sandstone body is 3-4 km long, 300-500 m wide, and up to 22 m thick. G pool was discovered in 1972 and placed on primary production; oil production declined gradually and was accompanied by modestly increasing gas-to-oil ratios (GOR) and water-to-oil ratios (WOR). U and W pools were discovered in 1982 and 1983, respectively, and were produced by primary methods until initiation of waterflooding in 1985. Response to waterflooding these two pools has been a rise, then decline, in the GOR, followed by rapidly rising WOR, which is currently up to 10:1 in wells adjacent to water injectors. Production re ponse indicates control by mesoscale and microscale reservoir heterogeneities. Mesoscale heterogeneities include permeable sandstone beds several meters thick, that are continuous between adjacent wells, and stochastic shale beds up to 80 cm thick, which are not yet correlatable between wells. Rapid breakthrough of water occurred in producing wells adjacent to injectors due to channeling in thick permeable sandstone beds between shale beds. Microscale heterogeneities are principally mineralogically segregated laminae of cross-stratified sandstones. Permeability values from cores indicate variations of one order of magnitude among laminae. It appears that water passes preferentially through low-permeability sandstone laminae due to the higher mobility of water compared to oil.

Application of micro-FTIR mapping and SEM to study compositional heterogeneity of siltstones: Example from the Late Devonian–Early Mississippian Middle Bakken Member

This paper explores the applicability of micro‐FTIR mapping to study heterogeneity of organic matter‐lean siltstones. Closely spaced samples of Late Devonian dolomitic siltstones of the Middle Bakken Member were analysed with micro‐FTIR, powder X‐ray diffraction, and scanning electron microscopy (SEM) to explore the distribution and chemical properties of organic matter (OM), muscovite/feldspar/clay group, carbonates, and quartz, and their influence on porosity and permeability of these rocks. Our results show that quartz is the dominant component of the samples, and the main mineralogical differences between the samples are reflected in the abundance of carbonate minerals. Organic matter content is usually far below 1 wt. % and dominantly represented by terrestrially derived vitrinite and inertinite. Micro‐FTIR mapping demonstrates that the more spatially connected quartz and muscovite/feldspar/clays become, the larger permeability in the rock develops, and these correlations are especially strong for planes parallel to bedding. In contrast, carbonate connectivity shows a strong negative correlation with permeability. No correlations between connectivity of components and porosity have been detected. These observations suggest that micro‐FTIR not only can document compositional heterogeneity of siltstones, but also has potential to help understanding their permeability systems.

Modeling core-scale permeability anisotropy in highly bioturbated “tight oil” reservoir rocks

A high-resolution simulation model of a heterogeneous low-permeability rock sample is used to investigate the effects of physical and biogenic sedimentary structures on scaling and anisotropy of absolute permeability at the core scale. Several simulation sub-samples with random locations and volumes were also selected for evaluation of the effects of scale and lithological composition on the calculated permeability. Vertical and horizontal permeability values (from whole core simulation) are in good agreement with routine core analysis (RCA) measurements from offsetting cores. Despite relatively good reservoir quality associated with geobodies of biogenic and relic bedding structures, results from the full diameter core simulation demonstrate that their limited volumetric abundance and restricted connectivity prevent these features from controlling fluid flow in these rocks. In fact, permeability seems to be dominated by the tighter encasing matrix, which exhibits average permeability values very close to those reported from RCA. Geometric averaging offers a better representation for the upscaling of horizontal permeability datasets; whereas, both geometric and harmonic averaging work similarly well for the vertical measurements. The methodology used in this work is particularly applicable to the detailed characterization of reservoir rocks with a high degree of heterogeneity caused by biological reworking and diagenesis.