ALBERT

Description: : Mudstone-dominated shallow-marine rocks of the Kaskapau Formation were deposited on a low-gradient ramp that spanned the foredeep of the Western Canada foreland basin during the late Cenomanian to middle Turonian. Organic-, clay-rich, and silt-rich mudstone accumulated on the flank of the forebulge, 〉 200 km from the western shoreline. Within this mudstone, a large proportion of the clay minerals are organized into silt- and very fine sand-size aggregate grains. These aggregates were produced both in the water column as marine snow and phytodetritus and also through reworking of previously deposited cohesive mud to form intraclasts. The latter, along with siliceous silt grains, form wave- and combined-flow ripples, graded beds, scour-fills, and ripple-tail lamination. Where sand-size sediment (comprising detrital siliceous, calcareous bioclastic, or phosphatic grains) is present, it is molded into combined-flow ripples, HCS, gutter casts, and lags. Thus all sediment grades indicate storm- wave and current reworking of the sea floor at a distance of 〉 200 km offshore. The common occurrence of clay minerals in the form of aggregate grains, organized into combined-flow ripples and parallel lamination, implies advective transport of clay minerals as bedload, driven by combined flows across a very low-gradient ramp. On the distal part of this ramp, latest Cenomanian rocks include thin tongues of SW-prograding quartz-rich sandstone that was derived from an emergent forebulge. This source was drowned during the early Turonian eustatic rise when the sediment abruptly changed to organic-rich mudstone dominated by clay-mineral aggregates. This compositional change was mainly a response to a sudden increase in distance to detrital sources in the west, rather than a dramatic increase in water depth. Throughout much of early to middle Turonian time, the sea floor in the forebulge region lay above effective storm wave base for silt, estimated at ~ 70 m. At sea-level lowstands, wave winnowing and erosion of the sea floor concentrated bioclastic lags at the top of siltier-upward sequences; lags are interpreted to correspond to falling-stage, lowstand, and early transgressive systems tract deposits on the western margin of the basin. Previous studies may have substantially overestimated water depth for organic- and clay-rich calcareous mudstones in the Western Interior Seaway.

Description: Investigation of core and outcrop samples of the Cenomanian Natih-B Member (North Oman) indicates that the different lithofacies present experienced rather different early diagenesis shortly after deposition. Transmitted-light, cathodoluminescence and backscattered scanning-electron microscopy, as well as stable-isotopic, X-ray diffraction and total organic carbon (TOC) analyses were employed to delineate the major controls on the cyclic pattern of early diagenesis and hydrocarbon source potential.The Natih-B intrashelf basinal carbonates are composed of pelagic sediments that exhibit high-frequency cyclicity marked by decimetre-thick lithofacies alternations, mainly between: Lithofacies A compacted, partially bioturbated, skeletal, organic carbon-rich mudstone to wackestone; and Lithofacies B uncompacted, extensively bioturbated, skeletal, sparry-calcite rich wackestone to packstone. Individual units are composed variously of authigenic and biogenic calcite (58.1–97.6%, average 78.5%) and organic carbon (0.3–13.7% TOC, average 3.6%), together with minor quartz, clay, pyrite, dolomite and phosphatic material (fish debris). Lithofacies A contains relatively more organic carbon, clay, pyrite and dolomite than Lithofacies B and constitutes an excellent source rock. Diagenetic textures of Lithofacies A are dominated by compactional deformation of burrow fabrics, faecal pellets and solution seams, in addition to zoned/bright luminescent, non-ferroan sparry and isopachous calcite cement in and around uncompacted foraminifer tests, in an uncemented matrix. In contrast, Lithofacies B does not show any signs of compaction other than microstylolites and is dominated by zoned/dull luminescent, non-ferroan calcite microspar replacement, in addition to pore-filling, predominantly dull-luminescent, non-ferroan, sparry calcite cement. Moreover, Lithofacies B shows evidence of isopachous and meniscus-style cementation, together with geopetal structures and mictritic peloids. Stable-isotopic compositions of both lithofacies were determined from whole-rock samples (δ13C = −0.9 to +0.9‰, average +0.3‰; δ18O = −5.6 to −3.7‰, average −4.8‰) and sparry calcite (both cement and matrix) subsamples (δ13C = −0.6 to +1.2‰, average +0.6‰; δ18O = −5.7 to −3.7‰, average −4.3‰); all results being relative to Vienna Pee Dee Belemnite.These petrographic and isotopic characteristics suggest that the Natih-B abundant calcite cements and replacements were precipitated early, prior to compaction, mainly from ‘normal’ (open, oxic) seawater at slightly elevated depositional temperatures. Some of the slightly negative δ13C values, however, indicate an addition of isotopically light carbon, probably derived from organic-matter oxidation by organisms living in marine pore waters. Based on evidence of extensive seafloor bioturbation and cementation, and their position within the depositional succession, the tops of Lithofacies B (wackestones to packstones) are interpreted as ‘discontinuity surfaces’ that cap shallowing-upward, fifth-order cycles, formed as a function of sediment starvation and increased bottom-current activity during relative sea-level stillstand/turnaround. In contrast, Lithofacies A (mudstones to wackestones) is believed to reflect high organic production coupled with high sedimentation rate and rapid burial. These conditions limited total infaunal colonization and extensive calcite precipitation, and preserved organic matter together with some escape burrows and in-place fauna, suggesting episodic sediment influx when more accommodation was available and seafloor diagenesis was minimized during relative sea-level rises. The relatively higher amounts of pyrite and dolomite in Lithofacies A likely indicate organic-matter degradation by bacterial sulphate reduction in anoxic pore waters during shallow burial.

Description: A bstract :  An integrated nomenclature scheme is proposed to capture the inherent heterogeneity of fine-grained sedimentary rocks at the 10 2 to 10 –3  mm scale and to assist the evaluation of these rocks as sinks of organic carbon, barriers to fluid flows, and reservoirs of oil and gas. This scheme incorporates previous knowledge and the latest field, petrographic, and laboratory observations. We propose to name fine-grained sedimentary rocks using a root term based on their texture (grain size), which is modified by description of bedding, composition, and grain origin. Regarding texture, we suggest the use of "mudstone" as a class name for the entire spectrum of fine-grained sedimentary rocks. We define mudstone as a rock in which more than fifty percent of its grains are mud (clay and silt) size (〈 62.5 µm). Similar to the approach used for the description of sandstone texture, mudstone texture can be refined by a "coarse," "medium," or "fine" size-range term. Regarding bedding, we follow Campbell's (1967) genetic approach to define laminae, laminasets, and beds, and describe lamina geometry, continuity, and shape. Regarding composition, we propose terms such as "siliceous," "calcareous," "argillaceous," and "carbonaceous" to capture differences in rock composition. The name of a mudstone can be further modified by additional attributes that detail the form and origin of the rock components. Application of this approach to the Cretaceous Eagle Ford Shale illustrates the variability typically present in mudstone successions and demonstrates how our detailed characterization can be used to decipher and predict rock properties of economic interest.

Description: : Few studies describe and compare the transport mechanisms operating to disperse mud in different parts of basins. Instead, the physical processes operating to disperse mud in offshore environments, where storm and tidal processes are interpreted to dominate, are generally considered in isolation from those occurring in basinal settings where changes in bottom-water anoxia and suspension settling from buoyant plumes are mostly interpreted to dominate. Using microtextural, mineralogical and geochemical data derived from the analyses of 151 thin sections obtained from the Lower Jurassic mudstone-dominated succession exposed on the coast of NE England, we investigate how varying sediment dispersal mechanisms, bioturbation and early diagenesis operated to produce the lithofacies variability observed. In particular, we consider the processes of sediment delivery while bottom waters were interpreted to be euxinic. Analyses of these samples reveal that the succession is highly variable at millimetre to centimetre scales. Six main lithofacies were observed: (1) sand- and clay-bearing, silt-rich mudstones; (2) silt-bearing, clay-rich mudstones; (3) clay-rich mudstones; (4) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; (5) fine-grained muddy sandstones; (6) cement-rich mudstones. These units are organized typically into stacked successions of sharp-based, normally graded, thin (〈10 mm) beds. Single beds exhibit a variety of sedimentary structures. Specifically tempestites, wave-enhanced sediment gravity flows of fluid mud, ripples and gutter casts are common in the coarser-grained mudstone facies. In contrast, thin siltstone lags, compacted ripples and organo-mineralic aggregates are common in the finer grained mudstone facies and those with significant primary production-derived components. Bioturbation is common throughout. These data indicate that sediment was transported by density flows and traction currents operating at the sediment–water interface in all parts of the studied succession. Bioturbation has overprinted depositional fabrics in the majority of samples. The extent to which persistent bottom-water anoxia and low-energy suspension settling influenced lithofacies variability in the basinal parts of the succession has been overstated; these environments were more dynamic than most researchers have previously concluded.

Description: 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.

Description: An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as for elucidating chemical evolution pathways within mudstones. Here we integrate field observations with thin section descriptions (optical and electron optical techniques) and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah, to determine the impacts of early and burial diagenesis on this mudstone succession. The detrital assemblage in the Mancos Shale comprises quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km scale), ferroan dolomite cemented units up to 0.3 m thick, are present. These cemented units cap both coarsening-upward units (1 to 3 m thick), and stacked successions of coarsening-upward units (5 to 15 m thick). These upward-coarsening sediment packages, capped by dolomite cemented strata, correlate to bedsets and parasequences in updip settings. Pervasive cementation in these dolomite-cemented units is likely to have occurred prior to compaction as a result of bacterially mediated respiratory processes. Cementation at these levels is particularly evident because cement precipitation occurred during breaks in sediment accumulation below marine flooding surfaces. The abundance of dolomite cements highlights the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones. In addition to carbonate-cements, diagenetic alteration and precipitation of quartz and aluminasilicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions. Kaolinite precipitation is interpreted to have occurred prior to significant compaction and indicates that both silicon and aluminium were mobile during early diagenesis. We interpret the abundance of early diagenetic kaolinite cement to be the result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. There is also indirect evidence for burial diagenetic kaolinitization of feldspar grains. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Textures and CL spectra for the quartz microcrystalline cement suggests that recrystallization of biogenic silica (opal-A) was likely to have been an important source for quartz cements, although smectite-to-illite transformation may have contributed some. These mineral phases highlight that microscopic-scale diagenetic mobility of silica is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.