Evidence For Fluid Migration Research Articles

Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

Abstract The Shenhu area is an important gas hydrate exploration and production test zone in the northern slope of South China Sea. A sediment layer with high gas hydrate saturations and abnormally high pore-water chlorinity concentrations has been identified from recently acquired logging-while-drilling (LWD) data and core samples. Data from four sites (W19, W18, SC-01 and SC-02) indicates that most gas hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log inferred coarser-grained units, except at SC-02. At this site, a striking increase in pore-water chlorinity values, with a maximum value exceeding 816 mM at a depth of 158 mbsf, indicates an active or recently-active system. A one-dimensional (1D) diffusion model is used to estimate the time when the gas hydrate formed based on the saturation, thickness, and porosity of gas hydrate-bearing units at each site. The results show that gas hydrates at sites SC-02 and W18 respectively formed 19–29 thousand and 26–28 thousand years ago (assuming a closed system). To further investigate the apparently young age of these highly saturated gas hydrate accumulations, three-dimensional (3D) seismic variance and frequency attributes extracted along different sedimentary layers are shown to provide evidence for vertical and lateral gas migration along normal faults and gas chimneys. The high pore-water chlorinities and evidence for fluid migration from deeper sedimentary sections suggest that upward migration of methane gas into the hydrate stability zone may have contributed to the recent formation of the highly concentrated gas hydrate occurrences identified in the Shenhu area.

Variable spatio-temporal clustering of microseismicity in the Hellenic Subduction Zone as possible indicator for fluid migration

This study investigates the variability of spatio-temporal microseismicity clustering and the occurrence of mutual triggering of events along the subduction interface in south-eastern Aegean as indication for fluid flow along and above the plate interface. We quantify spatial, temporal and spatio-temporal microseismicity clustering from the outer to the inner forearc and at intermediate depths. Waveform similarity indicates a decreasing of spatially clustered events from the outer and central towards the inner forearc and at intermediate depths. Highly similar events (cross-correlation >0.9), used as proxy for spatial clustering, decrease from the outer (30.2%) and central forearc (34.9%), towards the inner forearc (20.5%) and at intermediate depth (6.9%). Such highly similar events show increasing median inter-event times from the outer and central towards the inner forearc and at intermediate depth: 0.35, 0.34, 16.45, and 70 days, respectively. The Epidemic-Type-Aftershock-Sequences (ETAS) model, employed to investigate microseismicity temporal clustering, indicates an increase of the percentage of independent events from the outer (32%) and central (46%) forearc, to the inner forearc (93%) and at intermediate depth (93%). Hence, ETAS results suggest that mutual triggering of events is significant in the outer and central forearc, and it is almost absent in the inner forearc and at intermediate depths. Autocorrelation analysis, investigating spatio-temporal clustering, shows the tendency of earthquakes to occur close in space and time in the outer and central forearc, while in the inner forearc, and especially at intermediate depth, earthquakes are more homogeneously and randomly distributed. Combining the results from spatial, temporal and spatio-temporal analysis, we suggest that the different spatio-temporal patterns hint at systematic variations in the presence of migrating fluids on active faults close to failure. Triggering of seismicity is significant in the outer and central forearc, indicating fluid flow from the subduction interface, and it is diminishing towards the inner forearc. At intermediate depths, the nearly complete absence of mutual triggering of earthquakes indicates that there is little evidence for migration of fluids on active faults close to failure. Because intermediate depth seismicity in the Hellenic subduction zone occurs at P-T conditions where dehydration reactions are expected, fluids released by dehydration reactions within the slab are very likely migrating directly into the overlying mantle without triggering earthquakes.

Spatiotemporal distribution of low-frequency earthquakes in Southwest Japan: Evidence for fluid migration and magmatic activity

Low-frequency earthquakes (LFEs) in the lower crust and uppermost mantle are widely observed in Southwest Japan, and they occur not only along the subducting Philippine Sea (PHS) slab interface but also beneath active arc volcanoes. The volcanic LFEs are still not well understood because of their limited quantities and less reliable hypocenter locations. In this work, seismic tomography is used to determine detailed three-dimensional (3-D) P- and S-wave velocity (Vp and Vs) models of the crust and upper mantle beneath Southwest Japan, and then the obtained 3-D Vp and Vs models are used to relocate the volcanic LFEs precisely. The results show that the volcanic LFEs can be classified into two types: pipe-like and swarm-like LFEs, and both of them are located in or around zones of low-velocity and high-Poisson’s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes. The pipe-like LFEs may be related to the fluid migration from the lower crust or the uppermost mantle, whereas the swarm-like LFEs may be related to local magmatic activities or small magma chambers. The number of LFEs sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration. The spatiotemporal distribution of the LFEs may indicate the track of migrating fluids. As compared with the tectonic LFEs along the PHS slab interface, the volcanic LFEs are more sensitive to fluid migration and local magmatic activities. High pore pressures play an important role in triggering both types of LFEs in Southwest Japan.

Evidence for fluid migration following pockmark formation: Examples from the Nile Deep Sea Fan

A large number of pockmarks have been identified in the Rosetta Region of the Western Nile Deep Sea Fan. Co-located high resolution 2D and Chirp datasets have been used to identify seabed and buried pockmarks which are interpreted to span the Holocene to Pleistocene period. The pockmarks range in size and stratigraphic position from large buried pockmarks to small unit seabed pockmarks. Clusters of small unit pockmarks are stratigraphically positioned above the centre of large buried Pleistocene pockmarks and are used to infer: 1) rejuvenation of the larger pockmark conduit once burial has occurred, and 2) a change in the frequency and magnitude of pockmark formation events.

Evidence for fluid migration as the source of deformation at Campi Flegrei caldera (Italy)

We model the location, geometry and density of the source of the recent geological unrest at Campi Flegrei caldera (Italy) by inverting levelling, trilateration and gravity measurements collected between 1980 and 1995. The best fitting source for the 1980–84 inflation is a horizontal penny‐shaped crack with a density 142 to 1115 kg/m3. The source best fitting the deflation period (1990–95) is a vertical spheroid with density between 902 and 1015 kg/m3. These results exclude the intrusion of magma, and indicate the migration of fluid to and from the caldera hydrothermal system as the cause of ground deformation and consequent unrest.

Fate of ‘warm’ migrating fluids in the central Appalachians during the Late Paleozoic Alleghanian orogeny

Middle to Upper Devonian elastic rocks throughout the Valley and Ridge province in the central Appalachians contain evidence for fluid migration during the Late Paleozoic Alleghanian orogeny. Fluid inclusion microthermometry of vein minerals indicate that the migrating fluid was a ‘warm’ (160 to >220 °C) CH 4-saturated NaCl-CaCl 2 brine. However, rocks in the same stratigraphic interval immediately forelandward of the Valley and Ridge contain veins that have significantly different fluid inclusions, indicating that the ‘warm’ fluids did not significantly migrate into the rocks of the Appalachian Plateau province. The ‘warm’ fluids are interpreted to have migrated to the paleosurface as hot springs during the Late Paleozoic. Stable and strontium isotope chemistry of the vein minerals suggests that some infiltration of the 'warm' fluids into the easternmost Plateau province did occur. This indicates that wholesale flushing of a foreland during an orogeny may not be as common as originally thought.

Seismic reflectivity anomalies in sediments at the eastern flank of the Juan de Fuca Ridge: Evidence for fluid migration?

A high‐resolution seismic survey was carried out in the vicinity of Ocean Drilling Program (ODP) Leg 168 drill sites at the eastern flank of the Juan de Fuca Ridge. Three seismic systems with different source frequencies up to 4000 Hz were used simultaneously along each seismic line. The data sets were combined to provide the best possible resolution at any given depth level. An integrated interpretation of narrow zones of low reflection amplitudes is presented, which are related to basement highs but are independent of seafloor topography. The effect is most pronounced above a buried basement ridge in the vicinity of ODP Sites 1030 and 1031, where higher porosity values are present. Higher porosities can reduce seismic impedance contrasts and may therefore cause the observed low reflection amplitudes. In addition, Biot‐Stoll's [Biot, 1956a, b; Stoll, 1989] model suggests that attenuation coefficients are porosity dependent and reflection amplitudes are further decreased at higher seismic frequencies when porosity increases. Since zones of higher porosities are potential pathways for fluids, they may be associated with hydrothermally driven fluid discharge, which was previously predicted above the buried basement ridge. It is therefore proposed that the observed seismic amplitude reduction in the vicinity of ODP Sites 1030 and 1031 indicates zones of upward fluid migration.

Mantle Amphiboles and Mantle Plumes

Metasomatic minerals such as clinopyroxene, amphibole, phlogopite or apatite found in many mantle xenoliths provide evidence for fluid migration through mantle rocks and reaction with mantle minerals. The chemical composition of these minerals give us insight into the nature and composition of the percolating fluids, the processes of fluid-melt interactions as well as the fractionation of incompatible elements in mantle minerals. This is particularly true above mantle plumes where fluids are thought to be released during ascent of hot material and interact with the lithospheric mantle. The chemical study of mantle xenoliths in such a context should give us valuable information about plume lithosphere interactions with implications for magma generation within mantle plumes. We report on the chemical composition of mantle minerals from two different localities: alnphibolebearing mantle xenoliths are found in two recent volcanic fields in Yemen, in relation to the uprising of the Afar mantle plume. These xenoliths and the processes they recorded are compared with mantle peridotites sampled in Tertiary and Quaternary volcanoes of the French Massif Central where the presence of a mantle plume is still highly questioned. Laboratoire de G6ologie, Universit6 Blaise Pascal, UMR 6524, 5 rue Kessler, 63038 Clermont-Ferrand, France

Petroleum Field Evidence for Fluid Migration across Faults in Abnormally Pressured Siliciclastic Systems: ABSTRACT

In the deep portions of sedimentary basins, faults commonly form portions of boundaries between domains of contrasting datum fluid pressure. Fluid escape from high-pressure domains may be achieved by either hydraulic fracturing of an essentially impermeable seal (fault or top) or fluid-flow along or across a semi-permeable fault zone. In this study, hydrocarbon distribution patterns in petroleum fields within Tertiary siliciclastic strata of the offshore Gulf of Mexico and Trinidad areas are used to distinguish between these two possibilities. Hydrocarbon columns in Vermilion 14 field (Louisiana) and Flambouyant and Poui fields (Trinidad) are trapped in partial sand-against-sand juxtaposition in the footwall of normal faults. Fluid pressure differences of 700-5000 psi are observed across some portions of these faults. Column heights are several times longer in reservoirs that are underpressured relative to juxtaposed strata than along fault segments that juxtapose reservoirs with equivalent datum fluid pressures (either normal or abnormal). Sandstones that are juxtaposed against lower pressure strata contain anomalously short oil or gas columns. In these cases, buoyancy pressures generated by the hydrocarbon columns are insufficient to elevate the reservoir pressures to those necessary for hydraulic fracturing of the seal. The hydrocarbon distribution in these fields is most consistent withmore » a hydrodynamic model in which fluid flow occurs across semi-permeable fault zones from high to low datum fluid-pressure domains. Superposition of hydrodynamic effects on the capillary properties of the fault zones creates the observed enhancement or degradation of seal capacity (inferred from column heights) relative to the hydrostatic case. This model provides a simple mechanism for fluid migration out of the abnormally pressured regime, without invoking hydraulic fracturing of the seal.« less

Genetic link between Ouachita foldbelt tectonism and the Mississippi Valley–type lead-zinc deposits of the Ozarks

Fluids expelled from the Arkoma basin in response to Late Pennsylvanian–Early Permian orogenesis in the Ouachita foldbelt may have been ultimately responsible for formation of the Mississippi Valley–type lead-zinc deposits in the Ozark region of Missouri, Arkansas, Kansas, and Oklahoma. Fluid inclusions in both mineralized and barren rock record the passage of hot, saline fluids regionally throughout most of the Paleozoic section in the Ozarks. Dating studies in addition to structural and geologic observations provide further evidence for fluid migration and mineralization approximately coincident with the Ouachita orogeny. Formation of foreland basins, such as the Arkoma, during convergent tectonism creates conditions exceptionally favorable to the migration of fluids from deep sedimentary basins. Proximity to a basin whose margin has undergone some form of tectonic deformation or uplift may be the unifying factor in the genesis of Mississippi Valley–type deposits in geologically diverse settings.