Fluid flow in accretionary prisms
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Fluid expulsion from actively deforming accretionary prisms occurs primarily along faults because matrix permeability is low, tectonic consolidation is rapid, and fluid pressures are high. Field observational, geophysical, and geochemical data from modern (Barbados, Oregon/Washington, and S. Mexico) and ancient (Kodiak, Alaska) setting show: 1) that fault zones (=melanges) are primary conduits of fluid flow and preserve a unique history of that fluid evolution; 2) that fluid flow is rapid and episodic; 3) and that with increasing deformation chemical systems in active faults become more rock dominated and contain more homogeneous fluids. 1) In modern accretionary environments near lithostatic fluid pressures and temperature anomalies suggest fault zones are preferential conduits of fluid flow. 2) Temperature discontinuities measured along modern faults (..delta..T approx. 12/sup 0/C) and vein/wall rock temperature contrasts in ancient rocks (..delta..T approx. 25-30/sup 0/C, from fluid inclusions) both indicate fluids move significant distances before heat dissipates). 3) During initial deformation, substantial movement of fluids along faults assures complete mixing. Sr/Ca measurements indicate waters evolve from seawater, and delta 18-0 analyses of calcite cements and veins (18-23 percent per thousand SMOW) suggest that for T < approx. 125/sup 0/C calcite crystallizes from unaltered seawater (..delta.. 18-0 approx. 0 percent more » per thousand). As melanges become increasingly pulverized and as temperatures rise, the calculated delta 18-0 value of water increases to approx. 12-14 percent per thousand, reflecting a decrease in apparent water/rock ratio and significant exchange with the host rock. The increased fault density serves to obliterate differences in delta 13-C (-10-0 percent per thousand) and homogenize fluids. « lessKeywords:
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Volume‐loss strain accompanying pressure solution of calcite occurred within both the Umbria‐Marches Apennines of Italy and the Appalachian Mountains of western New York. Data from strain markers show that volume‐loss strain was greater within the shallow portions of the Apennines than within the Appalachians. Within the deeper portions of both fold and thrust belts, strain was nearly volume‐constant. Calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35% volume‐loss strain of the limestones within the Apennines. Strain at a depth of about 1 km was volume‐constant and is interpretated as indicative of restricted pore fluid circulation. In the Appalachians, calcite comprises less than 1% of the clastic rocks, and a 10% volume‐loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water. Here, the volume of calcite removed (0.1% of the total rock) is so small that circulation of meteoric water is not necessary for strain by pressure solution.
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Accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence. Compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration. These latter processes, in conjunction with fermentation or thermal maturation of entrained organic matter, yield hydrocarbon‐bearing pore fluids that are expelled from the prism. Regional fluxes of heat and a number of dissolved chemical species, most notably carbon, are controlled by the advective expulsion of the pore waters. Numerical modeling, observation and monitoring of flow patterns and rates, and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow. Dispersed, intergranular flow (10 −8 to 10 −11 m/s), controlled by the vertical permeability of the prism (10 −14 to 10 −20 m²), is limited by low‐permeability lithologies and seems not to vary much from margin to margin. Focused flow (10 −1 to 10 −8 m/s) above the décollement is controlled by fault zones or sedimentary intrusions (diapiric structures). At low‐fluid pressures, fault zone permeability may be similar to that of adjacent wall rock, but as fluid pressure increases from hydrostatic (λ* = 0) to near lithostatic levels (λ* ≈ 1.0), fault zones dilate, and (fracture) permeability increases by 2–4 orders of magnitude (10 −10 to 10 −16 m²). Similarly, mud volcanoes and diapirs provide high‐permeability fluid conduits to the sediment‐water interface. As a result, faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits (carbonate and gas hydrates) accumulate and chemosynthetic organisms cluster. Models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable. That conclusion has been quantified by extended (1–10 months) seafloor and borehole experiments that measured temperature anomalies associated with flow events. On the Cascadia prism, flow (estimated velocity ∼3 × 10 −5 m/s; 950 m/yr), confined to a thrust fault that cuts upward through the prism, has brought thermogenic hydrocarbons from depths >1.5 km to the surface within the last 400 years.
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Abstract Many faults in active and exhumed hydrocarbon‐generating basins are characterized by thick deposits of carbonate fault cement of limited vertical and horizontal extent. Based on fluid inclusion and stable isotope characteristics, these deposits have been attributed to upward flow of formation water and hydrocarbons. The present study sought to test this hypothesis by using numerical reactive transport modeling to investigate the origin of calcite cements in the Refugio‐Carneros fault located on the northern flank of the Santa Barbara Basin of southern California. Previous research has shown this calcite to have low δ 13 C values of about −40 to −30‰PDB, suggesting that methane‐rich fluids ascended the fault and contributed carbon for the mineralization. Fluid inclusion homogenization temperatures of 80–125°C in the calcite indicate that the fluids also transported significant quantities of heat. Fluid inclusion salinities ranging from fresh water to seawater values and the proximity of the Refugio‐Carneros fault to a zone of groundwater recharge in the Santa Ynez Mountains suggest that calcite precipitation in the fault may have been induced by the oxidation of methane‐rich basinal fluids by infiltrating meteoric fluids descending steeply dipping sedimentary layers on the northern basin flank. This oxidation could have occurred via at least two different mixing scenarios. In the first, overpressures in the central part of the basin may have driven methane‐rich formation waters derived from the Monterey Formation northward toward the basin flanks where they mixed with meteoric water descending from the Santa Ynez Mountains and diverted upward through the Refugio‐Carneros fault. In the second scenario, methane‐rich fluids sourced from deeper Paleogene sediments would have been driven upward by overpressures generated in the fault zones because of deformation, pressure solution, and flow, and released during fault rupture, ultimately mixing with meteoric water at shallow depth. The models in the present study were designed to test this second scenario, and show that in order for the observed fluid inclusion temperatures to be reached within 200 m of the surface, moderate overpressures and high permeabilities were required in the fault zone. Sudden release of overpressure may have been triggered by earthquakes and led to transient pulses of accelerated fluid flow and heat transport along faults, most likely on the order of tens to hundreds of years in duration. While the models also showed that methane‐rich fluids ascending the Refugio‐Carneros fault could be oxidized by meteoric water traversing the Vaqueros Sandstone to form calcite, they raised doubts about whether the length of time and the number of fault pulses needed for mineralization by the fault overpressuring mechanism were too high given existing geologic constraints.
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Fluid pressures affect migration of oil, gas, and water in continental margins. Burial and thermal history models describe the degree to which indercompaction or thermal expansion of fluids contribute to fluid pressure histories, but it is more difficult to evaluate how source-terms, such as oil yield or mineral dehydration reactions, impact paleo-fluid pressures. In this study, we document how a thick, maturing source rock helped create near-lithostatic fluid pressures that generated overpressures in reservoir rocks. We analyzed abundant oil-filled and rare aqueous fluid inclusions in calcite-filled fractures in the La Luna Fm. source rock and in the underlying Cogollo Gp. carbonate reservoir in the W. Maracaibo Basin, Venezuela. Homogenization temperatures (Th) of oil-filled inclusions range from 25-42[degrees]C in the La Luna Fm. and from 25-105[degrees]C in the Cogollo Gp., and associated gravities (determined from fluorescence properties) range from 28-43[degrees]API and 17-45[degrees]API, respectively. Integration of Th with the burial and thermal history of the sampled horizons leads to the conclusion that fractures in the La Luna Fm. formed under near-lithostatic fluid pressure conditions in the presence of a gas-charged oil. The values from fractures in the Cogollo Gp. are higher than in the La Luna Fm and become more variable withmore » increasing depth below La Luna. We interpret those fractures to have formed under lower fluid pressure conditions and/or with a less gas-charged oil than for La Luna. This interpretation of the distribution of paleo-fluid pressures is supported by the observation of modern inverted fluid pressure gradients between upper and lower Cogollo Gp. reservoirs. Thus late expulsion of a gas-charged oil created near-lithostatic fluid pressures in the La Luna Fm. source rock, and those fluid pressures bled downward through fractures into the adjoining reservoir rocks, contributing to the overpressures we observe today.« less
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