Abstract We present a systematic model that links the generation, migration, phase partitioning, and accumulation of methane into a closed loop as the sediment is deposited from the seafloor and buried through the base of hydrate stability zone (BHSZ). In our model, methane is generated by biodegradation of organic carbon in muds. Hydrate does not form and methane is not trapped until a coarse‐grained layer is deposited, because the small pores prevent hydrate formation in muds. Instead, methane diffuses into sands/silts where methane solidifies into hydrate. As hydrate‐bearing sands/silts pass through the BHSZ during sediment burial, methane hydrate dissociates, and releases free gas. The released and the newly generated free gas below the BHSZ concentrates into a vertical/dipping zone with low capillary entry pressure and high permeability and flows upward driven its buoyancy. When free gas reaches the hydrate stability zone (HSZ), capillary forces drive free gas to flow laterally, preferentially enter sands/silts, feed hydrate growth, and elevate hydrate saturation. With three‐dimensional focused free gas flow, microbial methane that is generated from a much larger fetch area of the entire basin, both above and below the BHSZ, is concentrated into coarse‐grained layers at structural closures for hydrate formation. Our model illustrates how geological evolution, microbial methane generation, and gas flow by buoyancy couple to generate concentrated hydrate deposits in geological system. These insights can be used to explore for high‐concentration methane hydrate and are important for understanding the methane budget and carbon cycle under the seafloor.
Abstract The Fengjia barite–fluorite deposit in southeast Sichuan is a stratabound ore deposit which occurs mainly in Lower Ordovician carbonate rocks. Here we present results from fluid inclusion and oxygen and hydrogen isotope studies to determine the nature and origin of the hydrothermal fluids that generated the deposit. The temperature of the ore‐forming fluid shows a range of 86 to 302 °C. Our detailed microthermometric data show that the temperature during mineralization of the fluorite and barite in the early ore‐forming stage was higher than that during the formation of the calcite in the late ore‐forming stage. The salinity varied substantially from 0.18% to 21.19% NaCl eqv., whereas the density was around 1.00 g/cm 3 . The fluid composition was mainly H 2 O (>91.33%), followed by CO 2 , CH 4 and traces of C 2 H 6 , CO, Ar, and H 2 S. The dominant cation was Na + and the dominant anion Cl ‐ , followed by Ca 2+ , SO 4 2‐ , K + , and Mg 2+ , indicating a mid–low‐temperature, mid‐low‐salinity, low‐density NaCl–H 2 O system. Our results demonstrate that the temperature decreased during the ore‐forming process and the fluid system changed from a closed reducing environment to an open oxidizing environment. The hydrogen and oxygen isotope data demonstrate that the hydrothermal fluids in the study area had multiple sources, primarily formation water, as well as meteoric water and metamorphic water. Combined with the geological setting and mineralization features we infer that the stratabound barite–fluorite deposits originated from mid–low‐temperature hydrothermal fluids and formed vein filling in the fault zone.
Subsurface fluid injections can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures. Laboratory studies indicate that fracture rheology and permeability in such reactivation events are linked to the roughness of the fracture surfaces. In this study, we construct numerical models using discrete element method (DEM) to explore the influence of fracture surface roughness on the shear strength, slip stability, and permeability evolution during such slip events. For each simulation, a pair of analog rock coupons (three-dimensional bonded quartz particle analogs) representing a mated fracture is sheared under a velocity-stepping scheme. The roughness of the fracture is defined in terms of asperity height and asperity wavelength. Results show that (1) Samples with larger asperity heights (rougher), when sheared, exhibit a higher peak strength which quickly devolves to a residual strength after reaching a threshold shear displacement; (2) These rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products; (3) Long-term suppression of permeability is observed with rougher fractures, possibly due to the removal of asperities and redistribution of wear products, which locally reduces porosity in the dilating fracture; and (4) Increasing shear-parallel asperity wavelength reduces magnitudes of stress drops after peak strength and enhances fracture permeability, while increasing shear-perpendicular asperity wavelength results in sequential stress drops and a delay in permeability enhancement. This study provides insights into understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.
Abstract Massive fluid injection into the subsurface can induce microearthquakes by reactivating preexisting faults or fractures as seismic or aseismic slip. Such seismic or aseismic shear deformations may result in different modes of permeability evolution. Previous experimental studies have explored frictional stability‐permeability relationships of carbonate‐rich and phyllosilicate‐rich samples under shear, suggesting that friction‐permeability relationship may be primarily controlled by fracture minerals. We examine this relationship and identify the role of mineralogy (i.e., tectosilicate, carbonate, and phyllosilicate content) using direct‐shear experiments on smooth saw‐cut fractures of natural rocks and sintered fractures with distinct mineralogical compositions. These results indicate that the friction‐permeability relationship is controlled by mineralogy. Frictional strength and permeability change upon reactivation decrease with phyllosilicate content but increase with tectosilicate content. In contrast, the reverse trend is observed for frictional stability ( a ‐ b ). However, the permeability change decreases with carbonate content while both frictional strength and stability increase. The permeability change always decreases with an increase in frictional stability. This relationship implies a new mechanical‐hydro‐chemical coupling loop via a linkage of frictional properties, mineralogy, and permeability.
Abstract We conduct numerical shear experiments on mixtures of quartz and talc gouge using a three‐dimensional (3D) distinct element model. A modified slip‐weakening constitutive law is applied at contacts. We perform velocity‐stepping experiments on both uniform and layered mixtures of quartz and talc analogs. We separately vary the proportion of talc in the uniform mixtures and talc layer thickness in the layered mixtures. Shear displacements are cycled through velocities of 1 and 10 μm/s. We follow the resulting evolution of ensemble shear strength, slip stability, and permeability of the gouge mixture and explore the mesoscopic mechanisms. Simulation results show that talc has a strong weakening effect on shear strength—a thin shear‐parallel layer of talc (three particles wide) can induce significant weakening. However, the model offsets laboratory‐derived strong weakening effects of talc observed in uniform mixtures, implying the governing mechanisms may be the shear localization effect of talc, which is enhanced by its natural platy shape or preimposed layered structure. Ensemble stability ( a − b ) can be enhanced by increasing talc content in uniform talc‐quartz mixtures. Reactivation‐induced permeability increase is amplified with increased quartz content before the maturation of shear localization. Postmaturation permeability enhances on velocity upsteps and diminishes on velocity downsteps. Talc enhances compaction at velocity downsteps, potentially reducing fault permeability. Evolution trends of stability relating to the composition and structure of the fault gouge are straightforwardly obtained from the 3D simulation. Local friction evolution indicates that talc preferentially organizes and localizes in the shear zone, dominating the shear strength and frictional stability of faults.
We explore the petrophysical behavior of the two interbedded lithofacies (sandy silt and clayey silt) that constitute the Green Canyon Block 955 hydrate reservoir in the deep-water Gulf of Mexico by performing experiments on reconstituted samples of the reservoir material. Sandy silts reconstituted to the in situ porosity have a permeability of 11.8 md (1.18 × 10−14 m2), which is similar to the intrinsic permeabilities measured in intact cores from hydrate reservoirs of similar grain size offshore Japan (Nankai Trough) and offshore India. Reconstituted clayey silts have a much lower intrinsic permeability of 3.84 × 10−4 md (3.84 × 10−19 m2) at the in situ stress. The reconstituted sandy silt is less compressible than the clayey silt. Mercury injection capillary pressure measurements demonstrate that the largest pores with the clayey silt are still smaller than the pores remaining after 90% hydrate saturation in sandy silt. We interpret that the methane solubility in pores of clayey silt is always less than that necessary to form hydrate, which explains why no hydrate is present in the clayey silt. We upscale the reservoir properties to estimate the behavior of interbedded sandy silt and clayey silt. We find the upscaled intrinsic horizontal and vertical permeabilities for the entire reservoir interval are 8.6 md (8.6 × 10−15 m2) and 1.4 × 10−3 md (1.4 × 10−18 m2). We estimate that during reservoir production, a maximum vertical strain of approximately 12% will result. Ultimately, this study will inform reservoir simulation models with petrophysical properties at scales of both individual lithofacies and reservoir formation.
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