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The Anadarko Basin of Oklahoma represents a major Paleozoic depocenter that existed along the rifted margins of southwestern Laurentia. In its infancy it accumulated a thick series of Cambrian through Mississippian detritus while further subsidence caused by inversion of the Cambrian Southern Oklahoma Aulacogen resulted in voluminous Pennsylvanian to Permian sediment. This contribution reports new data on K-Ar ages and trace-element geochemistry of detrital illite from middle and upper Pennsylvanian shale used to reconstruct sediment origins at the peak period of subsidence of the Anadarko Basin. X-ray diffraction was used to unveil mineral compositions and abundances of illite polytypes in two size fractions of separated illite (˂1 and 2-1 μm). K-Ar isotopic analyses were completed for both fine fractions, while the laser ablation inductively coupled plasma mass spectrometry was done for the latter. All illite separates consisted of mixtures of authigenic (1Md) and detrital (2M1) illite. The Illite Age Analyses showed that the detrital age of Desmoinesian (Moscovian) shale is the late Ediacaran (584 Ma), while the age of Missourian (Kasimovian) shale is the middle Cambrian (512.5 Ma). Trace-element abundances of all analyzed illite, irrespectively of stratigraphic age, are consistent with those of mica from metamorphic rocks. Based on illite detrital age and geochemistry it was inferred that Desmoinesian (Moscovian) shale represents a mixture of Neoproterozoic and Cambrian detritus sourced locally, whereas Missourian (Kasimovian) shale records a provenance shift toward more distal easterly sources from the Ouachita-(Marathon) foreland. This study has proposed a sediment source transition between the middle and upper Pennsylvanian that likely reflected major changes in the basin paleogeography and progressive development of the east-west (transcontinental) fluvial systems.

Highlights • Coupled geomicrobiology and geomechanics to investigate alterations in shales. • Microbial process can alter the mechanics, mineralogy, and microstructure of shales. • Biogeomechanical alterations reduced permeability by 93% and porosity by 38%. • Microfractures in shales can be sealed during biogeomechanical alterations. • Biogeomechanical alterations can enhance CO2 storage security and caprock integrity. Shales have been a major focus of the energy industry over the past few decades. Recently, there is a paradigm shift in the energy industry to low-carbon solutions, such as carbon capture and storage (CCS), to mitigate global warming caused by carbon footprint. The problem of long-term safe and efficient geological CO2 storage (GCS) and caprock integrity are some of the major challenges impeding large-scale CCS application. Here, we investigated how localized and bulk biogeomechanical alterations could potentially impact caprock integrity and CO2 storage in depleted shale reservoirs. We cultivated the shale core samples (containing both artificial-induced and pre-existing natural fractures) with a cultured microbial solution at specific temperature, time, and growth conditions. Subsequently, we obtain the properties of the fractured shale rock samples impacted by this microbial process. We investigate the impact of the mechanical responses due to the microbial process, on the long-term integrity and storage potentials of CO2 in shale reservoirs. Our results suggest that in Eagle Ford, Marcellus, and Niobrara shale formations, microbially-altered local and bulk mechanical properties can enhance the long-term caprock integrity and CO2 storage security by: (1.) Increasing the localized (+19% unconfined compressive strength, −20% Poisson’s ratio, +35% fracture toughness) and bulk (+50% unconfined compressive strength, −13% Poisson’s ratio) mechanical integrity; (2.) Decreasing permeability (−93%) and porosity (−38%); (3.) Altering the clay mineral content (−56%), calcite content (+21%), and morphology; (4.) Occluding microfractures; and (5.) Mitigating any potential leakage to the atmosphere through the caprock. This study considers the heterogeneity of shales, and provide valuable insights and viable assessment in solving the long-term GCS application in depleted hydrocarbon reservoirs.

The Gabon coastal region located along the western African margin hosts several sedimentary basins that developed prior, during and after the opening of the South Atlantic Ocean. A range of diagenetic processes controlled the distribution of clay paragenesis shown to be of high importance for the understanding of the basin's burial history and geotectonic development. Materials acquired for this study stem from the siliciclastic fluvio-lacustrine-deltaic, petroleum bearing, Early Cretaceous Dentale and Gamba formations cored by two respective wells at depths of ~ 1500 m. Sampled materials were analysed by X-ray diffraction, automated electron microscopy, and inductively coupled plasma mass spectrometry in order to reconstruct eogenetic and mesogenetic variations directing the formation of clay assemblages in the basin. The clay contents in both cores consist of authigenic mixed-layer minerals like illite-smectite, chlorite-smectite and berthierine-chlorite, and some minor detrital illite/mica and chlorite. I-Sm and C-Sm phase chemistry implied that the original dioctahedral (montmorillonite to beidellite) and trioctahedral (saponite) smectite precursors formed out of acid volcanic feedstock during eogenesis. Different magmatic fractionation degrees, from rhyodacite to trachyandesite, reflected in the uniform REE curves of volcanic glass conform to an active geotectonic development of the Cretaceous margins of Africa. Mesogenesis led to the decrease of smectite and formation of mixed-layered phases; I-Sm composition showed maximal burial depths of sediments to be ~ 1000 m and ~ 500 m deeper than today for Gamba and Dentale sediments, respectively. Besides, temperature, a major role in the formation of mixed-layer minerals had the porosity of sediments and geochemistry of smectitic precursors. Thus, Fe-Mg smectite showed higher thermal stability and lower rates of transformations in non-expanding clays during burial compared to Al-rich smectite. Different burial histories of sediments from the two cores can be attributed to rift-related normal faulting and subsequent differential denudation.

The Avalon Zone hosts well-preserved examples of high- and low-sulfidation epithermal Au-Ag mineralization and associated phyllosilicate alteration. Using examples from the Hope Brook and Hickey's Pond high-sulfidation systems (HSS) and the Heritage low-sulfidation system (LSS), this study presents new data on the mineralogical and chemical characterization of phyllosilicates associated with epithermal Au-Ag deposits. Electron probe microanalysis and X-ray diffraction studies revealed that the HSS examples contain Fe muscovite ((K0.62–0.79Na0.10–0.37Ca0.00–0.01)Al1.31–1.72(Mg0.01–0.08Fe0.05–0.19)(Al1.00–1.24Si2.76–3.00O10)(OH)2), paragonite ((K0.05–0.30Na0.78–1.02Ca0.00–0.01)Al1.66–1.84(Mg0.00–0.01Fe0.02–0.04)(Al1.01–1.11Si2.89–2.99O10)(OH)2), Al clinochlore ((Mg2.42–2.65Fe1.62–1.78)(Si2.57–2.69Al1.31–1.43O10)(OH)8), pyrophyllite, kaolin minerals, illite-smectite (I-Sme) and chlorite-smectite (C-Sme). Conversely, the LSS examples contain Fe-Mg muscovite ((K0.44–0.76Na0.00–0.02Ca0.00–0.01)Al1.55–1.82(Mg0.17–0.31Fe0.05–0.12)(Al0.55–0.71Si3.29–3.45O10)(OH)2), Al clinochlore ((Mg2.17–2.56Fe1.59–2.01)(Si2.71–2.92Al1.08–1.29O10)(OH)8), and I-Sme and C-Sme intermediates. Chlorite geothermometry indicated that HSS and LSS assemblages crystallized at average temperatures of 261 °C and 145 °C, respectively. Mica and chlorite from HSS and LSS are defined by Tschermak substitution and high-temperature polytypism (2M1 and IIb, respectively). In the HSS examples, one or more pulses of hot, acidic fluid reacted with host rocks generating an envelope of advanced argillic alteration (Fe-Ms, Kln, Pg, Prl) around a vuggy silica core. This is bounded by a zone of argillic alteration (Fe-Ms, Kln), with sericitic/chloritic alterations (Fe-Ms, Al-Clc) developed in distal parts of the systems. In the LSS, near-neutral hydrothermal fluids led to the formation of broad phyllic/chloritic zones (Fe-Mg Ms and Al-Clc). A low temperature overprint with 1M mica, Ib clinochlore ((Mg2.48–2.74Fe1.58–1.73)(Si3.05–3.15Al0.85–0.95O10)(OH)8), kaolinite and Sme-poor I-Sme and C-Sme is attributed to late waning stage hydrothermal activity and/or weathering. A close paragenetic link was established between Fe-Mg muscovite and Ag mineralization at Heritage, as well as between Fe-Mg muscovite and bladed calcite, suggesting boiling as the precipitation mechanism. The present study provides guidance on how phyllosilicate assemblages have the potential to discern HSS versus LSS mineralization during early prospection in terranes with limited exposure, and to contribute to the reconstruction of their hydrothermal record.