S51B-01
Connectivity test across deformation bands: results from field pumping experiments
Sandstones of the Ilhas Group in Tucano Basin, NE Brazil, commonly present outcropping fault associated
deformation bands. In one of these outcrops, with approximately 1km length and 15m thick three wells were
drilled and in situ permeability measurements were done across the outcrop surface. The connectivity tests were
performed with one well at one side of the fault zone and the two others at the other side. Two tests were
performed and at each test whilst one well was pumped downdraws were monitored at the other two remaining
wells. The permeability profiles revealed a huge variation of permeability of up to four orders of magnitude.
Nonetheless, the well pumping tests revealed a moderate connectivity across the deformation band since the
observed stationary downdraw at monitoring well on the opposite side of the fault zone was a considerable
fraction (~1/8) of the downdraw observed in the pumped well. These results indicate that there is 3D connectivity
in the field scale across the deformation band.
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S51B-02
Effects of Pore Fluids on Off-Fault Plasticity During Earthquake Rupture Propagation
We examine factors that determine the extent and distribution of off-fault Coulomb plasticity during earthquake rupture propagations in regions where pore fluids are present at full saturation. The dynamic finite element method, in the form of ABAQUS Explicit, is used with linear slip-weakening behavior along the fault. Material surrounding the fault is described by Drucker-Prager poroelastic-plastic properties which describe the brittle behavior of rocks under compressive stress when the primary mode of inlelastic deformation is frictional sliding on fissure surfaces and microcracking. The microcracking and local uplifts at sliding asperities that occur during brittle deformation are features that lead to microscopic dilatancy. We assume that when the Drucker-Prager yield criteiron is violated, the plastic deformation either has no hardening or positive hardening and allow the possibility of dilatant plastic strains. In plane strain, the Drucker-Prager model is coincident with the Mohr-Coulomb model when the out of plane normal stress is equal to the average of the two in-plane normal stresses, but it is not precisely coincident otherwise. We incorporate pore-fluid effects into the model assuming locally drained conditions on the fault and undrained conditions off the fault. The undrained pore pressure change is calculated from the poroelastic-plastic constitutive description in terms of a Skempton coefficient, B, times the earthquake-induced change in the mean compressive normal stress, with the additional effects of dilational plastic straining included too. The few- second time scale of significant stress pulsing near the rupture front implies that poroelastic fluid diffusion effects will be active over scales of only a few mm to a few cm, much shorter than the expected multi-meter scale lengths of the slip-weakening zone (whose size sets the scale of the region of large stressing rapid rupture), allowing the approximation of undrained conditions off the fault. When the material on each side of the fault has identical poroelastic properties and permeability, the assumption of no change in pore pressure on the fault can be made, although such conditions might not generally be met [Rudnicki and Rice, JGR, 2006]. The dilatant and pressure sensitive nature of brittle rock deformation results in elastic-plastic constitutive relations in which strain localization can occur in static situations, as shown by Rudnicki and Rice [JMPS, 1975]. Our results show localizations for cases where the critical hardening required, as predicted by Rudnicki and Rice, is greater than zero. These localizations can be supressed by prescribing hardenning above the critical value in the elastic-plastic material description. We extend the studies of Templeton and Rice, [Eos. Trans. AGU, 2006] and Viesca et al. [Eos. Trans. AGU, 2006] which investigate how off-fault elastic-plastic and poroelastic-plastic response is controlled by the initial stress state, in the form of Ψ, the angle that the most compressive stress makes with the fault, and the seismic S ratio, to include the effects of dilatant plastic straining, and we investigate a range of Skempton coefficients. We show the effects of dilation and B for high and low initial angles of most compressive stress, Ψ on the extent of plastic straining and the residual stress state in the region where plastic deformation has occured. In the plastic zone, the residual fault-parallel stress is altered from its initial value close to the fault, and this change in stress can influence future rupture events.
S51B-03
Reservoir-triggered seismicity at the highest Brazillian dam
Irapé Reservoir has the highest brazillian dam, with 208 meter high, and is located at Minas Gerais State, in the Jequitinhonha river. Since September 2003 a 3-component broadband digital seismic station is in opperation at the area and more three 3-component short period digital stations were installed nine months before the impoundment of the lake. During this two years pre-impoundment monitoring no local event was detected at the area of the lake. The filling of the reservoir started in December 7, 2005, and was followed by microearthquakes just one day after. Until October 2006 more than 300 events were detected by this local network. The largest microearthquake occurred in May 14, 2006 with 3.0 magnitude and was felt by the officials of the power plant. A preliminary hypocentral determination was performed for these events showing that the seismicity is located in a small area, with hypocenters below the lake or at its margins near the dam. The clear time correlation between the start of impoundment of the lake and the occurence of seismicity shows a causative relationship for these seismicity. Also the spatial distribution of the epicenters let us to conclude that this is another case of reservoir- triggered seismicity in Brazil of the rapid or initial response type.
S51B-04
Spatio-temporal evolution of triggered seismicity at Nova Ponte Reservoir, Brazil
Nova Ponte Reservoir is located at Minas Gerais State, SE of Brazil, on the Araguari river. The lake is situated at the boundary between two geotectonic provinces: the Neoproterozoic Brasilia Mobile Belt and a Phanerozoic volcano-sedimentary basin so called Parana Basin. The impoundment started in October 1993 creating a lake with volume of 12.8 km3 and maximum depth of 132 m. The pre-impoundment monitoring was made with one station for eigth years, and no local event had been detected. Two months after the lake start to be filling, the occurrence of some events felt by the local population lead to the deployment of a five vertical analog stations network. In 1995 this network was replaced by a digital one, which is operating until now. The almost continuous monitoring shows that the seismic activiy at Nova Ponte Reservoir is a proved case of reservoir triggered seismicity. Initially the events occured 25 km south of the dam, in a cluster named Area 1, with maximum magnitude of 2.0 mD. Soon after the activity at this area decreased and a continuous seismicity appeared at another area, named Area 2, with more widespread epicentral distribution. Most of the triggered events occurs at this area. A conspicuous cluster of epicenters is located 10 km south of the dam, where the two largest events were detected, on April 1995 with 3.5 mD, and on May 1998 with 4.0 mb and VI MM maximum intensity (Assumpção et al., 2002 PAGEOPH). The last one corespond to the second largest triggered earthquake in Brazil. The seismicity at these two areas was well studied. Since July 2004 some events have been located in a small area half distance between areas 1 and 2, named Area 3. Compared to areas 1 and 2, this area show less number of events, with maximum magnitude of 2.1 mD. More recent data analysis shows another new cluster of events situated around 35 km east of the dam where occurred on July 23, 2006 an event of 2.9 mD. In this study we present preliminary results for this activity that will be investigated as a probably result of spatio-temporal evolution of reservoir-triggered seismicity.
S51B-05
Classification of Basin-Wide Fluid Flow Systems in the Barents Sea
The Barents Sea is a part of the Arctic Ocean located north of Norway and Russia. The occurrence of shallow gas, gas hydrates and seafloor expulsion features is reported from several areas of the western Barents Sea. Most of the gas is assumed to be leaking from Jurassic hydrocarbon reservoirs. Leakage of gas into the shallow sediments was probably a result of the profound Cenozoic erosion of the Barents Sea shelf. Here, we classify and compare a variety of fluid flow systems throughout the south-western (Norwegian) part of the Barents Sea in order to understand geological processes that govern fluid flow on a basin-scale. Among the various observed fluid flow feature are: buried mud volcanoes, circular vertical pipes, seafloor and buried collapse structures, giant gas chimneys with diameters up to 10 km, fluid migration along faults and bedding planes, and trapping and focussing of fluids underneath glacial sediments and gas hydrates. There are pronounced regional differences as for example the south-eastern area in the vicinity of the Nordkapp Basin and Finnmark Platform shows significantly less fluid flow features than the remainder of the investigated area. The most active fluid flow areas are located in the western part within the Soervestnaget Basin and the Veslemoey High region. The differences in fluid flow activity might result from changes in tectonic development or the Cenozoic exhumation of the Barents Sea shelf. Results of this Norwegian Research Council-funded project, entitled "Quantification of geological processes that govern basin scale fluid flow" improve our understanding of how different geological processes control fluid flow on a basin scale and its timing.
S51B-06
Seismic Imaging of Focused Fluid-Flow-Related Pipe Structures; a Case Study at the Mid- Norwegian Margin
The project entitled "Quantification of geological processes that govern basin-scale fluid flow" within the PETROMAKS-project financed by the Norwegian Research Council aims to understand the complexity of fluid- flow processes. Focussed fluid flow processes are expressed in a multitude of seismic signatures and structures observed on the Mid-Norwegian margin. Here, we study the Nyegga-area located close to the Storegga slide and at the border of two large oil- and gas-prone sedimentary basins, the Møre- and Vøring Basin. Multibeam bathymetry data and conventional 3D-seismic data demonstrate hundreds of pockmarks at the seabed and corresponding acoustic pipe structures in the subsurface. These features are undoubtedly caused by focusing of fluids, but what are the geological processes and what contributes to their appearance on seismic data? Increased seismic energy response from the pockmarks may stem from precipitated authigenic carbonates and/or gas hydrates within the near-surface sediments. Seismic data indicate semi-circular near- vertical geometry of the pipe structures, contained by low amplitude pull-up reflections towards the central zone. The pipe structures varies in size, but commonly the larger the pockmark the wider and deeper the pipe structure. The pipe structures have an apparent depth of 300-600 meters, but additional pipe structures may be rooted as deep as 1000 mbsl, and hence, pierce the whole Miocene-Pleistocene succession. We compare conventional 2D- and 3D-seismic data with medium and high resolution single-channel data to investigate internal acoustic characteristics of individual pipe structures. Single-channel data and near-offset data reveal an acoustic turbidity within pipe structures that apparently are related to scattering of seismic energy from the pockmarks. To investigate the deeper internal pipe characteristics we conduct the undershooting technique to improve the seismic imaging of the pipes below the complex pockmark sites. This technique involves shot-domain elimination of all traces with ray-paths travelling through the pockmark, followed by a standard processing work flow. The finite undershot stack-section improves seismic imaging of focussed fluid flow pathways due to eliminating surface-generated acoustic signatures at the pipe structures.
S51B-07
An example of extruded sandstones resulting from hydrothermal venting associated with igneous intrusions
The presence of anomalous sands within deep water Palaeogene sediments offshore south east India can be explained by the effects of hydrothermal venting. The sands are found within a deep marine sequence of pelagic shales, with no obvious lateral sedimentary provenance, they are believed to have been extruded at the sea bed via hydrothermal venting. Seismic examples along with well data of these unusual phenomena are presented which support this origin for their presence within this pelagic section. The Mannar sub basin, which lies off the south east coast of India, has its origin in Mid Cretaceous rifting. It is one of a series of SW-NE trending rift basins which together are collectively referred to as the Cauvery Basin. Sediments in the syn-rift section are in excess of 3000m are comprised of a series of both continental and marine clastics. Late Cretaceous and early Palaeogene drift sediments are comprised of deep marine pelagic shales. The Period from the Maastrichtian to the Early Eocene was one of limited sediment input into the Mannar basin, resulting from a westward tilt of the Indian plate, a time of widespread deposition on the western margin of the sub continent. Within this predominantly shale prone interval there are encountered isolated sandstone intervals. These sands exhibit unusual petrographic and sedimentary characteristics which indicate that they have been transported relatively short distances and not long distance from the shelf edge which in this basin may have been several hundred kilometres to the North and West. Along the entire margin of western and south eastern India there is widespread evidence of igneous activity, the most notable being the Deccan Traps of Central and Western India. This igneous activity has been dated from Late Cretaceous up to Early Eocene. The igneous activity in the Mannar basin is postulated to be Eocene in age. In the Mannar Sub basin the igneous activity can be recognised on 3D seismic data and exhibits the increasingly familiar geometries seen in other similar settings such as the NW margin of the European plate. Within the area being presented one particularly well imaged igneous complex is observed beneath an anomalous high amplitude set of seismic reflections which are interpreted to be extruded clastic sediments, brought to the sea bed be as a result of hydrothermal venting, triggered by the igneous intrusions. Two wells have been drilled which penetrated intervals with similar seismic responses to those illustrated. These wells penetrated sands with very high GR readings and cores taken in the wells indicate that they are debris flows with many typical debris flow characteristics such as fluid escape features. The lateral extent and thickness of the extruded section can cover several square kilometres and range in thickness up to 50m. These dimensions indicate that these "extrudites" could well be of economic importance as hydrocarbon bearing reservoirs