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Extensional Systems

29 Apr

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Bulletin Article

Experimental models of transfer zones in rift systems

Transfer zones in rift basins are classified into convergent, divergent, and synthetic, based on the relative dip directions of adjacent faults within the transfer zone. Experimental models were constructed to determine the geometry, evolution, and fault patterns associated with each of these transfer zones. In addition, basement faults with initially approaching, laterally offset, and overlapping geometries were modeled. The models consisted of two layers, with stiff clay representing basement and soft clay representing the sedimentary cover. Laser scanning and three-dimensional surface modeling were used to determine the map geometry to compare the models with examples of natural structures. The experimental models showed many similarities with conceptual models but also showed more details and a few significant differences. Typically, divergent transfer zones are narrower than convergent transfer zones, for the same initial spacing between basement faults. The differences between the different initial fault configurations (approaching, laterally offset, or overlapping) are the degree of interaction of the secondary faults, the amount of overlap between the fault zones, and in some cases, the width of the transfer zone. The main faults propagate laterally and upward and curve in the direction of dip of the faults, so that the faults curve toward each other in convergent transfer zones, away from each other in divergent transfer zones, and in the same direction in synthetic transfer zones. A primary difference with schematic models is the significant component of extensional fault propagation folding (drape folding), accompanied by secondary faulting within the sedimentary cover, especially in the early stages of fault propagation. Therefore, all three types of transfer zones are characterized by significant folding and related variations in the shapes of structures. The transfer zones are marked by a progressive change in relief from the footwall to the hanging wall, resulting in a saddle-shaped geometry. The hanging walls of the faults are marked by a gentle flexure or rollover into the fault, with the amount of flexure increasing with fault throw away from the fault tip. The geometries and fault patterns of the experimental structures match some of the observations in natural structures and also provide predictive analogs for interpretation of surface and subsurface structures and the delineation of structural traps in rift basins.

Added on 30 April, 2013

Bulletin Article

Three-dimensional structure of experimentally produced clay smears: Implications for fault seal analysis

The geometries of clay smears produced in a series of direct shear experiments on composite blocks containing a clay-rich seal layer sandwiched between sandstone reservoir layers have been analyzed in detail. The geometries of the evolving shear zones and volume clay distributions are related back to the monitored hydraulic response, the deformation conditions, and the clay content and strength of the seal rock. The laboratory experiments were conducted under 4 to 24 MPa (580–3481 psi) fault normal effective stress, equivalent to burial depths spanning from less than approximately 0.8 to 4.2 km (0.5 to 2.6 mi) in a sedimentary basin. The sheared blocks were imaged using medical-type x-ray computed tomography (CT) imaging validated with optical photography of sawn blocks. The interpretation of CT scans was used to construct digital geomodels of clay smears and surrounding volumes from which quantitative information was obtained. The distribution patterns and thickness variations of the clay smears were found to vary considerably according to the level of stress applied during shear and to the brittleness of the seal layer. The stiffest seal layers with the lowest clay percentage formed the most segmented clay smears. Segmentation does not necessarily indicate that the fault seal was breached because wear products may maintain the seal between the individual smear segments as they form. In experiments with the seal layer formed of softer clays, a more uniform smear thickness is observed, but the average thickness of the clay smear tends to be lower than in stiffer clays. Fault drag and tapering of the seal layer are limited to a region close to the fault cutoffs. Therefore, the comparative decrease of sealing potential away from the cutoff zones differs from predictions of clay smear potential type models. Instead of showing a power-law decrease away from the cutoffs toward the midpoint of the shear zone, the clay smear thickness is either uniform, segmented, or undulating, reflecting the accumulated effects of kinematic processes other than drag. Increased normal stress improved fault sealing in the experiments mainly by increasing fault zone thickness, which led to more clay involvement in the fault zone per unit of source layer thickness. The average clay fraction of the fault zone conforms to the prediction of the shale gouge ratio (SGR) model because clay volume is essentially preserved during the deformation process. However, the hydraulic seal performance does not correlate to the clay fraction or SGR but does increase as the net clay volume in the fault zone increases. We introduce a scaled form of SGR called SSGR to account for increased clay involvement in the fault zone caused by higher stress and variable obliquity of the seal layer to the fault zone. The scaled SGR gives an improved correlation to seal performance in our samples compared to the other algorithms.

Added on 30 April, 2013

Bulletin Article

Insight into petrophysical properties of deformed sandstone reservoirs

We use samples from undeformed and deformed sandstones (single deformation band, deformation band cluster, slip-surface cataclasite, and fault core slip zone) to characterize their petrophysical properties (porosity, permeability, and capillary pressure). Relationships between permeability and porosity are described by power-law regressions where the power-law exponent (D) decreases with the increasing degree of deformation (strain) experienced by the sample from host rock (D, 9) to fault core (D, 5). The approaches introduced in this work will allow geologists to use permeability and/or porosity measurements to estimate the capillary pressures and sealing capacity of different fault-related rocks without requiring direct laboratory measurements of capillary pressure. Results show that fault core slip zones have the highest theoretical sealing capacity (140-m [459-ft] oil column in extreme cases), although our calculations suggest that deformation bands can locally act as efficiently as fault core slip zones in sealing nonwetting fluids (in this study, oil and CO2). Higher interfacial tension between brine and CO2 (because of the sensitivity of CO2 to temperature and pressure) results in higher capillary pressure and sealing capacity in a brine and CO2 system than a brine and oil system for the same samples.

Added on 31 March, 2013

Bulletin Article

Measuring and modeling fault density for CO2 storage plume-fault encounter probability estimation

Emission of carbon dioxide (CO2) from fossil-fueled power generation stations contributes to global climate change. Capture of CO2 from such stationary sources and storage within the pores of geologic strata (geologic carbon storage) is one approach to mitigating anthropogenic climate change. The large storage volume needed for this approach to be effective requires injection into pore space saturated with saline water in reservoir strata overlain by cap rocks. One of the main concerns regarding storage in such rocks is leakage via faults. Such leakage requires, first, that the CO2 plume encounter a fault and, second, that the properties of the fault allow CO2 to flow upward. Considering only the first step of encounter, fault population statistics suggest an approach to calculate the probability of a plume encountering a fault, particularly in the early site-selection stage when site-specific characterization data may be lacking. The resulting fault encounter probability approach is applied to a case study in the southern part of the San Joaquin Basin, California. The CO2 plume from a previously planned injection was calculated to have a 4.1% chance of encountering a fully seal offsetting fault and a 9% chance of encountering a fault with a throw half the seal thickness. Subsequently available information indicated the presence of a half-seal offsetting fault at a location 2.8 km (1.7 mi) northeast of the injection site. The encounter probability for a plume large enough to encounter a fault with this throw at this distance from the injection site is 25%, providing a single before and after test of the encounter probability estimation method.

Added on 31 March, 2013

Bulletin GeoHorizon

Summary of the AAPG–SPE–SEG Hedberg Research Conference on “Fundamental Controls on Flow in Carbonates”

A joint AAPG–Society of Petroleum Engineers–Society of Exploration Geophysicists Hedberg Research Conference was held in Saint-Cyr sur Mer, France, on July 8 to 13, 2012, to review current research and explore future research directions related to improved production from carbonate reservoirs. Eighty-seven scientists from academia and industry (split roughly equally) attended for five days. A primary objective for the conference was to explore novel connections among different disciplines (primarily within geoscience and reservoir engineering) as a way to define new research opportunities. Research areas represented included carbonate sedimentology and stratigraphy, structural geology, geomechanics, hydrology, reactive transport modeling, seismic imaging (including four-dimensional seismic, tomography, and seismic forward modeling), geologic modeling and forward modeling of geologic processes, petrophysics, statistical methods, numerical methods for simulation, reservoir engineering, pore-scale processes, in-situ flow experiments (e.g., x-ray computed tomography), visualization, and methods for data interaction.

Added on 31 March, 2013

Explorer Emphasis Article By David Brown

Looking Deeper Into Fracturing's Impacts

What happens in the reservoir following hydrofracturing? Microseismic monitoring is providing some important answers.

Added on 01 March, 2013

Bulletin Article

New evidences for the formation of and for petroleum exploration in the fold-thrust zones of the central Black Sea Basin of Turkey

The central Black Sea Basin of Turkey is filled by more than 9 km (6 mi) of Upper Triassic to Holocene sedimentary and volcanic rocks. The basin has a complex history, having evolved from a rift basin to an arc basin and finally having become a retroarc foreland basin. The Upper Triassic–Lower Jurassic Akgol and Lower Cretaceous Cağlayan Formations have a poor to good hydrocarbon source rock potential, and the middle Eocene Kusuri Formation has a limited hydrocarbon source rock potential. The basin has oil and gas seeps. Many large structures associated with extensional and compressional tectonics, which could be traps for hydrocarbon accumulations, exist. Fifteen onshore and three offshore exploration wells were drilled in the central Black Sea Basin, but none of them had commercial quantities of hydrocarbons. The assessment of these drilling results suggests that many wells were drilled near the Ekinveren, Erikli, and Ballıfakı thrusts, where structures are complex and oil and gas seeps are common. Many wells were not drilled deep enough to test the potential carbonate and clastic reservoirs of the İnaltı and Cağlayan Formations because these intervals are locally buried by as much as 5 km (3 mi) of sedimentary and volcanic rocks. No wells have tested prospective structures in the north and east where the prospective İnalti and Cağlayan Formations are not as deeply buried. Untested hydrocarbons may exist in this area.

Added on 28 February, 2013

Bulletin Article

Permeability and flow impact of faults and deformation bands in high-porosity sand reservoirs: Southeast Basin, France, analog

Outcrops of the Cretaceous high-porosity sandstone of the Southeast Basin, France, show two main types of deformation structures: a large number of small-offset, shear-enhanced cataclastic deformation bands (DBs); and a small number of large (meters to decameters)-offset ultracataclastic fault zones. Microstructural analyses of the cataclastic DBs show that fragmentation produces strands of cataclastic fragment-supported matrix, separated by weakly fractured host rock, which cluster to form the DBs. The ultracataclastic fault zones, however, are composed of a matrix-supported ultracataclasite material. Permeability data show that the DBs reduce host-rock permeability by 0.5 to 2 orders of magnitude, whereas the ultracataclasites reduce permeability by approximately 4 orders. Simple calculations considering the structural frequency, thickness, and permeability of these faults suggest that, although the DBs may have an impact on single-phase flow, it is most likely to be less than a 50% reduction in flow rate in extensional contexts, but it may be more severe in the most extreme cases of structural density in tectonic shortening contexts. The larger ultracataclastic faults, however, despite their much lower frequency, will have a more significant reduction in flow rate, probably of approximately 90 to 95%. Hence, although they are commonly at or below the limit of seismic resolution, the detection and/or prediction of such ultracataclastic faults is likely to be more important for single-phase flow problems than DBs (although important two-phase questions remain). The study also suggests that it is inappropriate to use the petrophysical properties of core-scale DB structures as analogs to larger seismic-scale faults.

Added on 28 February, 2013

Bulletin GeoHorizon

2011 Napa Hedberg Research Conference report on enhanced geothermal systems

The American Association of Petroleum Geologists sponsored a Hedberg Research Conference on Enhanced Geothermal Systems in Napa, California, March 18 to 23, 2011. The workshop was attended by 67 participants from 10 different countries: United States, Australia, Austria, Canada, Colombia, Germany, Malaysia, Netherlands, New Zealand, and Norway.

Added on 28 February, 2013

Explorer Geophysical Corner By Carlos Cabarcas,Oswaldo Davogustto,Satinder Chopra

Micro MDPs Can Be a BIG Tool in Fault Finding

Microseismic technology is crucial these days for understanding reservoirs and planning development programs.

Added on 01 February, 2013

VG Abstract By Juan I. Soto

How Orogens Could Collapse Forming Sedimentary Basins: The Case of the Betic-Rif Orogen

The Betic hinterland, in the westernmost Mediterranean, constitutes a unique example of a stack of metamorphic units. Using a three-dimensional model for the crustal structure of the Betics-Rif area this talk will address the role of crustal flow simultaneously to upper-crustal low-angle faulting in the origin and evolution of the topography.

Request a visit from Juan I. Soto!

Added on 26 June, 2019

VG Abstract By Juan I. Soto

Salt versus Shales in Continental Margins and Orogens

In comparison with the known boundary conditions that promote salt deformation and flow in sedimentary basins, the processes involved with the mobilization of clay-rich detrital sediments are far less well established. This talk will use seismic examples in different tectonic settings to document the variety of shale geometries that can be formed under brittle and ductile deformations.

Request a visit from Juan I. Soto!

Added on 26 June, 2019

Extensional Systems

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