Pore Fluid Migration Research Articles

Poroelastic stress-triggering of the 2005 M8.7 Nias earthquake by the 2004 M9.2 Sumatra–Andaman earthquake

The M9.2 Sumatra–Andaman earthquake (SAE) occurred three months prior to the M8.7 Nias earthquake (NE). We propose that the NE was mechanically triggered by the SAE, and that poroelastic effects were a major component of this triggering. This study uses 3D finite element models (FEMs) of the Sumatra–Andaman subduction zone (SASZ) to predict the deformation, stress, and pore pressure fields of the SAE. The coseismic slip distribution for the SAE is calibrated to near-field GPS data using FEM-generated Green's Functions and linear inverse methods. The calibrated FEM is then used to predict the postseismic poroelastic contribution to stress-triggering along the rupture surface of the NE, which is adjacent to the southern margin of the SAE. The coseismic deformation of the SAE, combined with the rheologic configuration of the SASZ produces two transient fluid flow regimes having separate time constants. SAE coseismic pore pressures in the relatively shallow forearc and volcanic arc regions (within a few km depth) dissipate within one month after the SAE. However, pore pressures in the oceanic crust of the down-going slab persist several months after the SAE. Predictions suggest that the SAE initially induced MPa-scale negative pore pressure near the hypocenter of the NE. This pore pressure slowly recovered (increased) during the three-month interval separating the SAE and NE due to lateral migration of pore fluids, driven by coseismic pressure gradients, within the subducting oceanic crust. Because pore pressure is a fundamental component of Coulomb stress, the MPa-scale increase in pore pressure significantly decreased stability of the NE fault during the three-month interval after the SAE and prior to rupture of the NE. A complete analysis of stress-triggering due to the SAE must include a poroelastic component. Failure to include poroelastic mechanics will lead to an incomplete model that cannot account for the time interval between the SAE and NE. Our transient poroelastic model explains both the spatial and temporal characteristics of triggering of the NE by the SAE.

Geothermal heat flow in the northeast margin of the Gulf of Mexico

Eighty-two seafloor heat-flow measurements were recently obtained across the Mississippi Fan region in the deepwater northeastern Gulf of Mexico. These data display an abrupt transition in heat flow between an area near the center of Pleistocene deposition (∼20 mW/m 2 ) and the eastern margin of the fan (∼40 mW/m 2 ). Although deposition of fan sediments has very likely suppressed the shallow subseafloor thermal regime, causing lower seafloor heat-flow values near the center, the magnitude and abruptness of the heat-flow contrast cannot be fully accounted for by the mechanisms related to sedimentary deposition, which include radiogenic heat production in sediments, pore-fluid migration, and presence of salt structures. The most plausible explanation for the sharp heat-flow contrast is that the heat released from the igneous basement is significantly greater in the eastern margin of the fan. The zone of contrasting heat flow lies along a previously suggested boundary between the oceanic crust and the thin transitional crust in the northeastern Gulf of Mexico. The area of higher heat flow coincides with the suggested zone of transitional crust, which, because of its granitic origin, generates greater amounts of radiogenic heat than oceanic crust. This finding opens up the possibility that heat-flow data may be used in delineating crustal lithologic boundaries along continental margins.

Subsurface gas offshore of northern California and its link to submarine geomorphology

The northern California continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology. Analyses of seismic reflection, sidescan sonar, and high-resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope. Subsurface gas, inferred from enhanced reflectors and other geophysical indicators, is spatially variable and related more to total depth and stratigraphy than to underlying structure, with the exception of one band of gas that follows the regional structural trend. Shallow depressions on the seafloor (pockmarks) are used to infer expulsion sites. The largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m, where expulsion features are rare. The upper slope (water depths 400–600 m) has a high concentration of pockmarks (diameter 10–20 m), in contrast to a near-absence of pockmarks at water depths shallower than 400 m. Of nearly 4000 pockmarks observed on sidescan sonar records, more than 95% are located in water depths deeper than 400 m. Bottom simulating reflectors (BSRs) on some seismic reflection profiles indicate the possible presence of gas hydrate. We find that gas and pore-fluid migration in the offshore Eel River Basin is: (1) correlated to surface morphology; (2) a contributor to seabed roughness; (3) a significant mode of sediment redistribution on the upper slope; and (4) potentially a factor in large slope failures.

Sediment dewatering and pore fluid migration along thrust faults in a foreland basin inferred from isotopic and elemental geochemical analyses (Eocene southern Pyrenees, Spain)

The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migration of imbricated thrust-folds. Isotopic and elemental geochemistry of syn-kinematic veins (calcite and celestite) and their marly host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of their evolution and circulation during the early deformation of the basin-fill. The isotopic composition of sulfur and the 87Sr 86Sr ratios of calcites and celestites from the veins in the footwall syncline show that the original fluid had the isotopic composition of Eocene seawater. The different 87Sr 86Sr ratio in veins from the thrust-fault zones compared with the same ratio in the marly host-rock of the footwall syncline indicates that the thrust-fault zones acted as conduits for advective fluids. The relatively high 87Sr 86Sr ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from deeper sources or from the meteoric weathering of the emerged part of the belt. δ 18O and δ 13C values of calcites show that the isotopic composition of the calcite-cements in veins was controlled by the isotopic composition of the marly host sediment. Depletion of both δ 18O and δ 13C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a formation water composition. The distribution of δ 18O and δ 13C values of the marly host-rock and calcite cements in veins of the four outcrops probably resulted from differences in the meteoric water influences. The hydrogeological regime at the toe of the submarine thrust system was dominated by tectonically-induced dewatering of the foreland basin sediments. The thrust-fault zones were the channelizing paths for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.

Diminished amplitude reflections (fadeout) in Bermuda Rise stratification

Single and multichannel deep-tow seismic records collected on the southwestern Bermuda Rise reveal areas within the acoustic layering where the intensity of reflectors fade or even disappear over lateral distances of 20–60 m. The fadeouts are conspicuously present in a highly stratified interval consisting of lower to middle Eocene siliceous turbidites and thin, cherty layers interbedded with hemipelagic/pelagic clays and oozes. Acoustic scattering at rough interfaces, gas pockets, and changes in geoacoustic properties do not readily account for the diminished amplitudes of the reflections. Detailed examination of the wave coda from selected fadeout areas reveal changes in reflector geometry due to faulting of the sediments. Strong reflections within the stratified sequence are thought to result from the thin, high impedance chert beds that occur within the turbidite layers. Secondary reflections (multiples) generated by the hard, chert interfaces are thought to interact constructively with primary reflections to generate strong interference reflections, but also destructively to cause fadeouts where the faulting has changed the layer configuration. Alternatively, the fadeouts may result from the dissolution of chert beds due to the upward migration of silica-deficient pore fluids along faults and fractures in response to excess pore pressures.

High heat flow anomalies over salt structures on the Texas Continental Slope, Gulf of Mexico

We made 74 closely spaced (< 2 km apart) heat flow measurements around and over two salt structures on the Texas continental slope, Gulf of Mexico. The values outlined the shape of the heat flow anomalies over both structures. Based on a preceding high resolution seismic survey, we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall‐shaped feeder. The heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation. The values over the salt plug are nearly all greater than 70 mW/m². The measurements over the salt tongue have a sharp heat flow peak of 90 mW/m² associated with the presumed feeder and rather uniform values around 60 mW/m² over the remainder. The variation of heat flow over both structures is smooth and shows no apparent scatter. Heat flow values off these features are uniformly low, around 30 mW/m². Thermal effects from bottom water temperature fluctuation, slope sedimentation, diapiric movement of the salt body, and pore fluid migration appear unable to provide a satisfactory explanation for the observations. However, thickness variations of a highly conductive salt body can easily account for the heat flow anomalies. We suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt.

Thermal anomalies in the central Indian Ocean: Evidence for dewatering of the Bengal Fan

Seafloor heat flow measurements in the central Indian Ocean are locally higher than predicted by lithospheric cooling models. These anomalously high heat flow values occur in a region affected by folding and faulting of oceanic crust and overlying distal Bengal Fan sediments. Three closely spaced sites drilled during Ocean Drilling Program Leg 116 dated the onset of deformation in this area at about 7–8 Ma. Here apatite fission track thermochronometric data from sites 717 and 718 and numerical simulations are used to (1) quantify the effects of Bengal Fan sedimentation on seafloor heat flow, (2) evaluate the thermal history of sediments penetrated at these two sites and (3) constrain mechanisms responsible for anomalous seafloor heat flow in this area. At the rates sediments were deposited along the distal fan (≤ 350 m/m.y.). numerical simulations predict a 6% to 9% reduction in present‐day heat flow at the seafloor relative to heat flow through the base of the lithosphere. At site 717, thermal history simulations assuming heat transfer dominated by conduction are consistent with available down‐hole temperature measurements and apatite fission track data. Site 718 is located 8 km south of Site 717 and 2 km south of a high‐angle fault that offsets oceanic crust by about 300 m. A previously published detailed heat flow survey conducted around site 718 documented seafloor heat flow ranging from 44 to 166 mW/m2 over kilometer‐scale distances. In addition, a monotonic 10 to 20% reduction in the mean length of fission tracks in apatite from 560 to 960 m below the seafloor at site 718 implies bottom‐hole temperatures of the order of 50°–60°C for a time period greater than a few million years. Both the seafloor heat flow and apatite fission track data argue for localized, possibly intermittent, advection of heat by lateral, updip migration of pore fluids in the vicinity of site 718. Taken together, the heat flow data and temperature range inferred from the apatite fission track data at site 718 imply a fluid source in the upper crust. Upward movement of pore fluids at velocities of less than 1 cm/yr through a basement‐sediment boundary maintained at temperatures ranging from 65° to 80°C is capable of explaining both sets of observations. As a hypothesis to reconcile existing observations, I suggest that the anomalous thermal structure in the central Indian Ocean represents a joint response to compaction‐driven dewatering of the Bengal Fan and local enhancement of the vertical permeability in the upper crust and sediment pile due to intraplate deformation.

Active Fault Participation in the Diagenetic Modification of Sandstone Reservoir Properties

In sedimentary basins undergoing regional strain, faults have a potential for influencing subsurface fluid flow by providing some of the driving energy for fluid movement. Variable displacement on faults in the slip direction results in systematic volume changes in the surrounding sedimentary rocks. Compressed and dilated volumes are distributed according to position relative to the fault. Intermittent seismic slip produces rapid pressure changes and high hydraulic gradients capable of causing movement of large fluid volumes or of maintaining pressure differentials if pore fluid migration is obstructed. As a result of the hydraulic gradients generated by individual faults, subsurface fluids may either be transferred between formations juxtaposed across the fault or vertically transported along the fault. Faults may thus provide the means of mixing of subsurface fluids, and are potentially zones of intense diagenetic modification. An appreciation of fault-influenced diagenetic modification is particularly pertinent to an understanding of the heterogeneity of sandstone reservoirs where pore water and hydrocarbon migration from source to reservoir rocks are an integral part of the hydrocarbon accumulation process. Mineralogical and fabric modifications to rock components may result in either significant enhancement of porosity or extensive cementation and compaction that overprints regional burial diagenetic assemblages. Inactive ormore » cemented faults may behave as hydrocarbon seals. The potential contribution of faults to sandstone reservoir heterogeneity should always be considered in models of burial diagenesis and hydrocarbon migration.« less

Small fractures in deep sea sediments: indicators of pore fluid migration along compaction faults : Buckley, D E Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20–21 September 1988P115–135. Publ London: Graham and Trotman, 1989 (Advances in Underwater Technology, Ocean Science and Offshore Engineering, V18)

Small fractures in deep sea sediments: indicators of pore fluid migration along compaction faults : Buckley, D E Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20–21 September 1988P115–135. Publ London: Graham and Trotman, 1989 (Advances in Underwater Technology, Ocean Science and Offshore Engineering, V18)

Fluid Flow and Mass Transfer in Central San Joaquin Basin, California: Strontium Isotope Study of Carbonate Cements: ABSTRACT

Ratios of /sup 87/Sr//sup 86/Sr can be used as tracers in sedimentary basins for studying the evolution and migration of pore fluids. The central San Joaquin basin is well suited for such a study, because potential diagenetic sources and sinks for calcium (and thus strontium) are constrained in both space and time. Analyses of carbonate cements from the marine Stevens sandstone (upper Miocene) at North Coles Levee reveal a trend of decreasing /sup 87/Sr//sup 86/Sr with precipitation temperature. Calculated isotopic temperatures for the calcite cements range from 50/sup 0/ to 80/sup 0/C, and strontium ratios vary systematically from 0.7083 to 0.7074. Further evidence of progressive cementation events is the systematic decrease in interstitial cement volume with decreasing strontium ratio. The /sup 87/Sr//sup 86/Sr values of the cements at North Coles Levee lie between the Miocene seawater ratio (0.7090) and present-day pore waters (105/sup 0/C, /sup 87/Sr//sup 86/Sr = 0.7072), and thus define a strontium evolutionary trend in the basin. Potential sources of strontium in the central San Joaquin basin have been identified and analyzed for their strontium content and /sup 87/Sr//sup 86/Sr ratios. Preliminary analyses indicate that dissolution and/or albitization of plagioclase is the most likely source of lower ratiomore » strontium in the carbonate cements. Because the timing of strontium availability from these reactions is known, potential fluid or oil-migration pathways and mass-transfer rates can be modeled. At North Coles Levee, mass-balance calculations indicate that a dissolution of approximately 10% of the detrital plagioclase would account for the strontium content and ratio of the present-day pore water.« less

Clastic diagenesis

Summary Diagenesis is the sum of those processes by which originally sedimentary clastic assemblages attempt to reach equilibrium with their environments. The subject has rapidly evolved, over the last 20 years, after several decades of routine petrographical analysis, to become a discipline of sophisticated analytical geochemistry. This progression may be traced through the evolution of depositional facies models, the growth of theoretical geochemistry, and the development and application of quantitative analytical techniques. Consequently, the study of diagenesis today involves the integration of data gathered from a range of interrelated disciplines. The publication in recent years of such integrated studies, particularly from the Texas Gulf Coast of the U.S.A., has led to a number of wide ranging models being established. These models allow an interpretation of the sequence of authigenic minerals in terms of their relationship to the depositional environment or surface chemistry (eogenesis), burial or subsurface conditions (mesogenesis) and weathering or reexposure to surface conditions (telogenesis). More specifically, once the pre-depositional controls on diagenesis have been established, it is possible to relate the inferred eogenetic mineral assemblages to exact geochemical sedimentary environments. Sedimentary mineral assemblages chemically are characterized by relative instability and so tend to interact with interstitial pore waters. Thus, in non-marine environments, mineral authigenesis may reflect arid-oxidizing, or wet-reducing pore-water conditions, and in the marine environment, either oxidizing or reducing pore waters. During mesogenesis different processes become important. Elevated temperatures add energy to the reacting system, lowering reaction barriers and increasing reaction rates. Furthermore, widespread pore fluid migration at depth, transporting large quantities of solute, is likely to impart major, regional diagenetic changes to sediments. That there is a remarkable degree of consistency in deep burial settings suggests that depth-related processes conform to a predictable pattern. Additional geochemical instability is introduced into the diagenetic system during uplift and exposure to the telogenetic realm. Minerals formed during burial, at elevated temperatures and pressures, and from concentrated formation waters, may become unstable in oxidizing, meteoric waters. Diagenetic research requires the complete dissection of sedimentary rocks and, subsequently, the quantitative chemical and mineralogical analysis of their individual components. Such an approach to diagenetic studies, when related to an assessment of their paragenesis, may eventually lead to a predictive, integrated model for the evolution of sedimentary clastic rocks.

Controls on mineral authigenesis in Coal Measures sandstones of the East Midlands, UK

AbstractAuthigenic clay mineral formation in fluvial sandstones and one brackish sandstone from the lower and middle Coal Measures of the East Midlands has been studied using XRD, optical microscopy and a variety of electron optical techniques. The sandstones are quartz-rich with minor feldspar and mica. The clay minerals present are kaolinite, chlorite and illite; they form 10–15% by volume of most of the sandstones. Both detrital and authigenic illite and chlorite are present but the kaolinite is entirely authigenic. Kaolinite and illite authi-genesis has been important in all the sandstones but authigenic chlorite is only abundant in the phyllosilicate-rich Silkstone sandstone. The controlling factors in the diagenesis of the sandstones were primary composition, environment of deposition, bed thickness, pore-fluid migration and composition, pressure and temperature. These interacting factors have been of varying importance in time and space in the diagenesis of the sandstones. Authigenic clay morphology appears to have been controlled by time and environment of formation, pore-fluid chemistry and, possibly, parent minerals.

Early diagenetic carbonate precipitation and pore fluid migration in the Kimmeridge Clay of Dorset, England

ABSTRACTIn the argillaceous sequence of Kimmeridge Clay a carbonate rich bed is composed of ferroan dolomite cement with varying amounts of excess CaCO3, and Fe2+ substitution in the Mg2+ sites. The isotopic and chemical compositions change symmetrically about the centre of the band proving that it grew by vertical accretion during diagenesis. Textural and isotopic evidence shows that growth centred on a horizon rich in primary carbonate which became dolomitized and assimilated during production of diagenetic carbonate. This accounts for the lateral extent of the concretion. Early central diagenetic carbonate was produced from organic matter by bacterial fermentation (δ13C =+0.59‰) and later marginal carbonate by abiotic breakdown, (δ13C tending towards — 2.73‰). δ18O values range from — 1.56 to — 4.46‰ because the dolomite precipitated during progressive burial. As burial increased, magnesium, whose dominant source was trapped seawater, became depleted while the relative availability of Fe2+, whose source was dominantly reduced detrital oxides, increased. Dolomitization and the source of diagenetic components for dolomite formation are discussed. Diffusion and pore fluid migration transported ions to the site of precipitation. Early cementation of the band served to influence pore fluid migration, but thereafter pore fluid migration controlled carbonate precipitation.

Dilatancy-induced fluid migration and the velocity anomaly

This article describes a numerical model capable of simulating the fluid-rock processes that appear to precede some earthquakes. More precisely, the two-dimensional finite element-finite difference model can be used for investigating dilatancy and the associated migration of the pore fluids in a water-saturated rock. The numerical model is applied to a tectonically simple region containing a thrust fault. Fault creep is used to produce local stress concentrations (at fault ends) and dilatancy. Numerical results illustrate the somewhat conflicting requirements on rock permeability in the dilatancy-fluid diffusion model. During the first stage of the process the permeability must be low enough to allow a drop in fluid pressure sufficient to cause vaporization; at a later stage the permeability needs to be large enough to enable the fluid to move sufficiently rapidly into the dilated region. The numerical solution also yields such quantities as surface displacements which can be used for constraining the choice of model parameters for a particular earthquake region.