Shale Diagenesis: A Case Study from the Albian Harmon Member (Peace River Formation), Western Canada (original) (raw)
Related papers
Sedimentary Geology, 2009
Clay mineral assemblages in alluvial mudrocks are important for paleoclimatic interpretation and for understanding burial diagenetic cementation in sandstones, but it is commonly difficult to unravel the relative importance of source weathering, pedogenesis and diagenesis in their origin. The clay mineral assemblages in fluvial overbank mudrocks from the Lower Cretaceous Chaswood Formation in central Nova Scotia, investigated by X-ray diffraction analysis of the b 2 µm fraction of 45 samples, include kaolinite, illite, vermiculite, and mixed layer kaolinite/expandable clay and mica/vermiculite. The assemblages vary with depositional facies. Wetland organic-rich mudrocks have large amounts of amorphous material and kaolinite is the dominant clay mineral. In the eastern part of the basin, where overbank mudrocks were episodically uplifted by syn-sedimentary strike-slip faulting, cumulate ultisol and alfisol paleosols are common. In the ultisols, hematite is enriched and kaolinite increases at the expense of illite in the B horizon. Alfisols contain more illite and vermiculite and the B horizon is enriched in goethite. In the western part of the basin, where thin sandstones with abundant diagenetic kaolinite cement are interbedded with the mudrocks, the distinctive clay mineral assemblage of mica/vermiculite mixed layer, vermiculite with 15.5 Å peak, and kaolinite/expandable mixed layer clay with a 17.7 Å peak is interpreted to result from bacterially-mediated oxidation of organic matter below the paleo-water table during early burial diagenesis. Deeper burial diagenesis may lead to slightly higher kaolinite crystallinity. Volcanic ash appears to alter to kaolinite/ expandable mixed layer clay with a 7.9 Å peak. Comparison with the continuously subsiding and rapidly accumulated Wessex Formation of southern England, formed at a similar paleolatitude, shows the strong role of pedogenic processes and early diagenesis by meteoric water in development of clay mineral assemblages in the locally tectonically uplifted Chaswood Formation.
Diagenesis of Clay Minerals from Lower Cretaceous Shales of North Eastern British Columbia*
Clays and Clay Minerals, 1974
Clay minerals from shale outcrops of the Lower Cretaceous Buckinghorse Formation (4250 fl thick) were investigated in order to assess their degree of diagenesis and their oil-generating potential. Crystallinity index, sharpness ratio, per cent of illite which is the 2M polymorph and presence of discrete minerals have been studied in the whole clay fraction, while the very fine clay fraction has been subjected to X-ray diffraction, differential thermal, thermogravimetric, differential thermogravimetric, i.r. spectroscopy, surface area and chemical analyses. With information derived from these studies and from published data, a classification scheme was devised which relates variation of clay mineralogy to diagenetic stages and burial depth.
2006
The mineralogy and geochemistry of shales reflect the composition of the initially deposited precursor mud, subsequently modified by diagenetic processes. To see if significant geochemical differences exist between shales that mainly owe their present-day composition to either deposition or diagenesis, we compare the published mineralogical, bulk and clay-fraction geochemical, and clay-fraction O-isotopic compositions of 2 shales. One shale is from the Western Canada Sedimentary Basin (WCSB), and its composition mainly reflects primary (depositional) chemical and mineralogical variations (smectitic to illitic illite/smectite) within this unit. The other shale is from the United States Gulf Coast (USGC), and its composition mainly reflects mixed-layer illite/smectite (US) diagenesis of deposited smectitic clay material. The chemical and mineralogical trends of WCSB and USGC shales, including one of increasing illite content in US with depth or maturity, are essentially indistinguishable, in both bulk shale and clay fraction, despite the contrasting genetic interpretations for the origin of the contained US. Thus, similar mineralogical and chemical trends with depth or temperature can result either from inherited depositional compositional heterogeneity of the sediment, from burial metamorphism of shale or a combination of both. Interestingly, the O-isotopic compositions of the clay fractions from the WCSB and USGC are significantly different, a fact that reflects original clay formation from source material and water of quite different isotopic compositions. The discrimination between depositional and diagenetic contributions to shale composition continues to pose challenges, but a combination of bentonite, illite polytype, clay isotopic and trace and rare earth elemental analyses together with illite age analysis holds promise for future work;
Clay Minerals, 2014
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 sed...
Sedimentary Geology, 2010
Lower Cretaceous deltaic sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. The influence of several factors on diagenesis has been investigated: depositional lithofacies, sea level changes, chemistry of basinal sediments and basinal fluid flux during burial. The distribution and chemistry of diagenetic minerals was determined from nine wells located along a dip section of the Sable Subbasin. Mineral type and paragenesis were characterized using a combination of optical petrography, back-scattered electron images, and electron microprobe analyses. Siderite is unusually abundant in marine sediments of the Scotian Basin and has thus been studied in detail. Siderite occurs in several generations. Early and late siderites are similar in chemical composition, suggesting buffering by pre-existing siderite, but rare low-Mg siderite is related to a greater contribution of meteoric water. Siderite has locally dissolved to create microporosity and has suppressed quartz overgrowths. Siderite is most common in those muddy prodeltaic lithofacies where there is the highest availability of detrital ilmenite. Reactive Fe released by breakdown of this ilmenite is responsible for the unusual presence of early siderite in marine sediments, with the Ca and Mg content of the siderite indicating fully marine waters. Lithofacies have a strong influence on early diagenetic mineral assemblages. Lithofacies deposited in the transgressive system tract have abundant early Fe-calcite and siderite. Early kaolinite occurs principally in proximal (fluvial and river mouth) lithofacies, where meteoric water was most likely available during the deposition. Contrary to other studies, we find little impact of sequence stratigraphy on diagenetic minerals except in the transgressive system tract. Mesogenetic minerals are related to flux of formation water and maturing hydrocarbon products, resulting first in pyrite and siderite and later in ankerite and ferroan calcite. The principal controls are interpreted to be high Fe 2+ and low Ca 2+ of formation waters and vertical movement of the waters along faults. Overall, the bulk chemical composition of terrigenous sediments and the depositional lithofacies are the most important factors controlling diagenetic minerals in the Lower Cretaceous of the Scotian Basin.
Diatomaceous origin of siliceous shale in Eocene lake beds of central British Columbia
Canadian Journal of Earth Sciences, 2005
The abundance of diatoms in Neogene lacustrine sediments and their apparent scarcity in Paleogene deposits have long perplexed geoscientists, but siliceous shales from Eocene lake beds of central British Columbia provide new insights. Major element geochemistry, X-ray diffraction patterns, and relict diatom frustules suggest that Eocene beds at Horsefly, McAbee, and Princeton originated as lacustrine diatomite that underwent diagenetic alteration to produce siliceous shale. The combination of high SiO2 and low Al2O3 values and the presence of opal-CT X-ray diffraction peaks provide a distinctive geochemical fingerprint for biogenic silica deposits that have been remineralized. The discovery of diatomaceous geochemical signatures in siliceous shales may prove to be a useful tool for extending the geologic record of diatoms, perhaps helping to reduce the apparent discrepancy between fossil evidence and evolutionary interpretations based extant species.
2019
In the Cenomanian-Turonian Dunvegan and Kaskapau formations in Alberta and British Columbia, terrestrial, and nearshore to offshore environments can be related temporally and spatially using a well-established allostratigraphic framework. In these varied environments, ten mudstone microfacies were identified, using optical and scanning electron microscopy. Microfacies 1 through 3 were deposited in a freshwater lacustrine environment by hyperpycnal flows, and consist of rippled, structureless, and inverse-to-normally graded thin beds that record turbulent, laminar, and waxing and waning flow. Microfacies 4 and 5 were deposited from turbulent flow in a fluvial environment. Microfacies 6 represents a paleosol that underlies a maximum flooding surface above the base of Dunvegan allomember E. Marine prodelta mudstones contain microfacies 7 to 9 that comprise bioturbated mudstone characterised by a randomly oriented clay fabric, and storm-generated interlaminated siltstone and mudstone. Microfacies 10 is the most distal offshore facies and includes organo-mineralic aggregates and reworked intraclastic aggregates.
Geologos, 2009
The study of upper Cretaceous -lower Tertiary fluvial deposits of the Coalspur Formation in the Foothills region of west-central Alberta reveals that the distribution of early authigenic kaolinite has a well-defined relation to the sequence stratigraphic framework. In this context, it has been observed that the kaolin mineral content increases in sandstones lying below subaerial unconformities, which mark the most significant stratigraphic hiatuses and hence the sequence boundaries in fully fluvial successions. The increased abundance of authigenic kaolinite immediately below sequence boundaries may have been caused by the infiltration of meteoric water during times of subaerial erosion, resulting in the dissolution of unstable minerals (e.g., micas and feldspar) and the formation of kaolinite and secondary porosity. It is therefore suggested that the change in clay mineral assemblages in the stratigraphic section depends in part on the position of the analyzed sandstone samples relative to the sequence boundaries. In a larger context, the method of using authigenic clays to delineate depositional sequences in non-marine successions needs to be evaluated on a caseby-case basis, as the diagnostic early diagenetic minerals underlying the sequence boundary may change as a function of palaeoclimate and also as a function of late diagenetic processes.
Marine and Petroleum Geology, 2012
Diagenesis in the uppermost Jurassic to Lower Cretaceous deltaic sandstones and shales of the Scotian Basin is an important control on reservoir quality. Ferruginous zone (sub-oxic) marine pore-water diagenesis controls the initial formation of Fe 2þ-silicates that are the precursors of grain-rimming chlorite that preserves porosity. This study assesses the regional controls on the type of marine pore-water diagenesis by studying the sedimentology, mineralogy, and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin. Coated grains preserve a record of whether marine pore-water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone. Four types of coated grain were distinguished, each with a different mineral paragenesis. Mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite, and the correlation of P with Fe or Ti. Pyrite and Fe-calcite are found where the sulphidic zone is more significant than the ferruginous zone. Ferruginous zone diagenesis was common in low-sedimentation rate retrogradational sediments with low organic carbon, and in delta-front turbidites and river-mouth sandstones. Estuarine, tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower Fe:Ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products. The relative contribution of colloidal iron (hydr)-oxides appears greater in distal low-sedimentation rate environments. Where large changes in sedimentation rate occurred at ravinement surfaces, the underlying progradational rocks have evidence of ferruginous zone diagenesis, whatever their facies. Rapid upward migration of the pore-water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore-water zones. This study has shown that the availability of Fe and organic carbon varying in a complex manner in marine deltaic sediments, but that the resulting diagenesis by marine pore-water can be predicted from facies and paleogeographic setting.