Fault-zone samples recovered from 159 to 174
m below the seafloor (mbsf) in Hole 1309D from the Integrated Ocean Drilling Program (IODP) at the Atlantis Massif, Mid-Atlantic Ridge, 30°N, include both ...cataclastic and mylonitic fault rocks, and their gabbroic and ultramafic host rocks. Laboratory experiments determined the strength and permeability of nine fault-zone and host-rock samples with a triaxial apparatus under conditions that simulate present-day,
in situ pressure conditions. The permeabilities of cataclasites are ∼10
−18
m
2, while host-rock and mylonite permeabilities are <2
×
10
−19
m
2. When subjected to increasing differential stress, visible fractures increased the permeability of most rock types to >10
−17
m
2. The strength of cataclasites is 260–380
MPa, weaker than that of mylonites of ∼600
MPa. High resolution X-ray computed tomography (HRXCT) and optical microscopy shows that experimentally produced fractures preferentially form interconnected networks within cataclastic matrices. Thus, permeability and strength are a function of the fault-zone microstructure, which evolved during exhumation from upper mantle and lower crustal depths. Localization of cataclastic zones adjacent to altered ultramafic and mylonitic gabbroic rocks likely make the cataclastic portion of this fault a long-lived fluid conduit within the Atlantis Massif Oceanic core complex.
•Arbuckle Group in Osage County, Oklahoma, is rigorously characterized for a potential deployment CCS.•Various fields and experimental data were used to characterize Arbuckle and its caprocks.•CO2 ...sequestration model was built to estimate CO2 storage capacity and caprock properties.•Two sweet spots were identified with higher potential for CO2-storage in North Burbank field.•A new perspective on using active CO2-EOR for CO2-storage in the Arbuckle Group is presented.
This paper examines a rigorous site characterization and analysis of geological storage capacity of CO2 in Arbuckle Group in the north part of Oklahoma to accelerate Carbon Capture and Storage (CCS) and Utilization (CCUS) technology deployment. Data obtained from the core, logs, and historic wastewater injection and production data were used to build and validate a geological model. Subsurface structure, depth required to attain supercritical CO2, rock properties and required caprock criteria were applied to Arbuckle geological model to identify suitable region for geological storage of CO2. The model estimates that the western Osage County has a storage capacity of > 50 million metric tons of CO2. More specifically, two sweet spots with a higher potential for CO2 storage were identified. The presence of several anthropogenic CO2 sources in the vicinity of site, existing pipelines, and compression infrastructure are the significant elements of a techno-economic analysis of the prospect storage project(s). This study demonstrates that the carbonate Arbuckle Group could be a strategic geological unit for CO2 sequestration, thus contributing toward emissions reduction from nearby industrial complex.
The escarpments that bound the Pito Deep Rift (northeastern Easter microplate) expose in situ upper oceanic crust that was accreted ∼3 Ma ago at the superfast spreading (∼142 mm/a, full rate) ...southeast Pacific Rise (SEPR). Samples and images of these escarpments were taken during transects utilizing the human‐occupied vehicle Alvin and remotely operated vehicle Jason II. The dive areas were mapped with a “deformation intensity scale” revealing that the sheeted dike complex and the base of the lavas contain approximately meter‐wide fault zones surrounded by fractured “damage zones.” Fault zones are spaced several hundred meters apart, in places offset the base of the lavas, separate areas with differently oriented dikes, and are locally crosscut by (younger) dikes. Fault rocks are rich in interstitial amphibole, matrix and vein chlorite, prominent veins of quartz, and accessory grains of sulfides, oxides, and sphene. These phases form the fine‐grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt. Bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins. When compositions are normalized to compensate for the excess silica, the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration, including the loss of mobile elements and gain of some trace metals and magnesium. Microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the SEPR; faults related to the opening of the Pito Deep Rift can be distinguished by their orientation and fault rock microstructure. Some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones. Other increments were linked with the influx of relatively fresh seawater. The spacing of the faults is consistent with fault localization occurring every 7000 to 14,000 years, with long‐term slip rates of <3 mm/a. Once spread from the ridge axis, the faults were probably not active, and damage zones likely played a more significant role in axial flank and off‐axis crustal permeability.
The northern escarpments of the Hess Deep Rift provide cross‐sectional views of in situ, ∼1‐Ma‐old, upper oceanic crust that underwent extensive, spreading‐related brittle deformation. Most of the ...deformation and associated alteration occurred within the locus of magmatic construction of the East Pacific Rise, in the presence of high‐temperature hydrothermal fluids. Passing laterally from undeformed host rocks, brittle deformation zones are classified as (1) damage zones where densely spaced fractures overprint the primary structure of dikes and lavas, (2) cataclastic zones where interconnected fractures, comminuted grains, and matrix minerals define deformational fabrics, and (3) very fine‐grained, gouge‐filled fault cores. Relative to the host rock, damage and cataclastic zones are rich in veins of chlorite and/or actinolite, and lesser amounts of titanite, epidote, and quartz. These phases mark relict hydrothermal fluid pathways. Trace and major element compositions of representative samples also indicate fault‐localized hydrothermal alteration, including an increase in MgO by several weight percent within cataclastic and damage zones. In contrast, the fault cores are composed of very finely comminuted basaltic material and have MgO concentrations similar to the damage zones. Integrated compositional, textural, and outcrop‐scale structural data inform an evolutionary model for fault growth from the early, widespread dilational phases of damage‐zone development to more restricted noncoaxial strain in the cataclastic zones. With continued fault development, gouge develops and seals the fault cores. While the fault cores are sealed by gouge, surrounding zones remain conduits to hydrothermal fluid flow, except where sealed by secondary minerals. Sealed faults can later be reactivated as conduits with additional increments of fault slip. The dual behavior of faults as conduits and seals inevitably leads to compartmentalization of the flow regime in subaxial and ridge‐flank areas.
IODP Hole 1256D penetrates a complete section of upper oceanic crust spread from the East Pacific Rise at a full rate of up to 220 mm/yr. Here we present U-Pb zircon data from Hole 1256D gabbroic ...rocks using isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS). Three gabbro samples yielded Th-corrected weighted mean 206Pb/238U SIMS dates of 15.03 ± 0.14 Ma, 15.13 ± 0.35 Ma, and 15.23 ± 0.12 Ma. An ID-TIMS Th-corrected weighted mean 206Pb/238U date of 15.191 ± 0.040 Ma from the first of the three samples provides verification and refinement of the timing of magmatism. The U-Pb dates agree with the predicted age of the crust based on marine magnetic anomalies, suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis. Zircons from two of the studied samples have anomalously high Th/U ratios, likely reflecting oxidation of U prior to or during zircon crystallization. The elevated ƒO2 of late stage melts in the axial melt lens may be related to progressive crystallization, assimilation of hydrothermally altered sheeted dikes and gabbros, and/or contamination of the melt lens by seawater derived saline brines.
•TIMS and SIMS zircon U-Pb dates from in situ Miocene-age East Pacific Rise spread gabbros indicate axial melt emplacement.•In the context of global data, elevated Th/U values are consistent with oxidation of the axial melt lens.•The analyses support previous conclusions from petrologic and magnetic analyses of the Hole 1256D core.
We performed a series of laboratory and image analysis on organic shale samples before and confined compressive strength tests. Following failure, we often observe an increase in pore volume in the ...sub-micron range, which appears to be related to the formation of microcracks that in some cases cross or terminate in organic matter, intersecting the organic-hosted pores. Samples with higher clay content tended not to display this behavior. The microcrack networks allow the hydrocarbons to migrate to the main induced tensile fractures. The disconnected nature of the microcracks causes only a slight increase in permeability, consistent with other observations.
The rock product of shallow‐crustal faulting includes fine‐grained breccia and clay‐rich gouge. Many gouges and breccias have a fabric produced by distributed deformation. The orientation of fabric ...elements provides constraints on the kinematics of fault slip and is the structural record of intrafault strain not accommodated by planar and penetrative surfaces. However, it can be difficult to quantify the deformational fabric of fault rocks, especially the preferred orientations of fine‐grained minerals, or to uniquely determine the relationship between fabric geometry and finite strain. Here, we present the results of a fabric study of gouge and breccia sampled from low‐angle normal (detachment) faults in the Black Mountains, Death Valley, CA. We measured a preferred orientation of the long axes of the clasts inherited from the crystalline footwall of the fault and compared the shape preferred orientation to the anisotropy of magnetic susceptibility of the fault rocks. The two measurements of fabric exhibit systematic similarities and differences in orientation and anisotropy that are compatible with the large‐scale kinematics of fault slip. The dominant carriers of the magnetic susceptibility are micron‐ and sub‐micron scale iron oxides and clay minerals. Therefore even the finest grains in the fault rock were sensitive to the distributed deformation and the micro‐mechanics of particle interaction must have departed from those assumed by the passive‐marker kinematic model that best explains the fabric.
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