Michie E.A.H.,University of Aberdeen |
Michie E.A.H.,Badley Geoscience Ltd
Journal of Structural Geology | Year: 2015
Relatively few studies have examined fault rock microstructures in carbonates. Understanding fault core production helps predict the hydraulic behaviour of faults and the potential for reservoir compartmentalisation. Normal faults on Malta, ranging from <1m to 90m displacement, cut two carbonate lithofacies, micrite-dominated and grain-dominated carbonates, allowing the investigation of fault rock evolution with increasing displacement in differing lithofacies. Lithological heterogeneity leads to a variety of deformation mechanisms. Nine different fault rock types have been identified, with a range of deformation microstructures along an individual slip surface. The deformation style, and hence type of fault rock produced, is a function of host rock texture, specifically grain size and sorting, porosity and uniaxial compressive strength. Homogeneously fine-grained micrtie-dominated carbonates are characterised by dispersed deformation with large fracture networks that develop into breccias. Alternatively, this lithofacies is commonly recrystallised. In contrast, in the coarse-grained, heterogeneous grain-dominated carbonates the development of faulting is characterised by localised deformation, creating protocataclasite and cataclasite fault rocks. Cementation also occurs within some grain-dominated carbonates close to and on slip surfaces. Fault rock variation is a function of displacement as well as juxtaposed lithofacies. An increase in fault rock variability is observed at higher displacements, potentially creating a more transmissible fault, which opposes what may be expected in siliciclastic and crystalline faults. Significant heterogeneity in the fault rock types formed is likely to create variable permeability along fault-strike, potentially allowing across-fault fluid flow. However, areas with homogeneous fault rocks may generate barriers to fluid flow. © 2015 Elsevier Ltd.
Freeman B.,Badley Geoscience Ltd. |
Boult P.J.,GINKGO ENPGNG |
Yielding G.,Badley Geoscience Ltd. |
Journal of Structural Geology | Year: 2010
Good seismic interpretation of faults should include a workflow that checks the interpretation against known structural properties of fault systems. Estimates of wall-rock strains provide one objective means for discriminating between correct and incorrect structural interpretations of 2D and 3D seismic data - implied wall-rock strain should be below a geologically plausible maximum. We call this the strain minimisation approach. Drawing on the large body of published data for strike dimension and maximum displacement for faults we suggest a realistic upper limit of wall-rock shear strain of 0.05, and 0.1 for maximum longitudinal strain when measured in the displacement direction. Small-scale variation of fault wall-rock strain also adheres to this rule, except in specific areas of strain localisation such as relay zones. As a case study we review an existing structural interpretation of 2D seismic surveys. Mapping of shear and longitudinal strain on the fault planes show values commonly greater than 0.05 and 0.1 respectively. Thus the model is deemed inadmissible. We then reinterpreted the area in an iterative manner using the strain minimisation approach. By using regions of implied high wall-rock strain as an indicator of high uncertainty in the interpretation, we were able to break out two self-consistent fault sets, each of which had geologically plausible wall-rock strains, where previously there had only been one fault set with highly implausible wall-rock strains. © 2009 Elsevier Ltd.
Yielding G.,Badley Geoscience Ltd |
Lykakis N.,University of Edinburgh |
Lykakis N.,Midland Valley Exploration Ltd. |
Underhill J.R.,University of Edinburgh
Petroleum Geoscience | Year: 2011
Exploration well 50/26b-6 in the UK Southern North Sea discovered a trap containing a gas-bearing Rotliegend Group (Leman Sandstone Formation) reservoir which was a major surprise at the time of drilling in that its gas composition was approximately 50% CO 2 (with 9% N2 and the remainder methane). Christened the 'Fizzy Discovery', the accumulation was appraised by well 50/26b-8. Subsequently, another CO 2-rich discovery (Oak) was made along-strike in nearby block 54/1b. Column heights at the well locations are of the order of a few tens of metres, but at the Fizzy Discovery the column height at the trap crest is estimated to be over 200 m. Interpretation of a high fidelity PSTM 3D seismic dataset has been constrained by 33 exploration wells allowing fault geometries and stratigraphic offsets to be determined with confidence. Despite late-stage (Late Cretaceous) structural inversion, the net boundary-fault offset is sufficient in both the Fizzy and Oak discoveries to almost breach the Zechstein Group evaporite super-seal, and the CO 2-bearing Rotliegend Group in the footwall is now juxtaposed against hanging wall sediments of the uppermost Zechstein Group. Hence, these Zechstein Group units evidently act as a robust long-term side-seal for the carbon dioxide column. The Fizzy and Oak accumulations are noteworthy in providing a natural demonstration of top seal and fault side-seal integrity for carbon dioxide in a subsurface reservoir, that has remained intact over a geological timescale in what is otherwise a prolific methane-rich reservoir play fairway. © 2011 EAGE/Geological Society of London.
Morley C.K.,Chiang Mai University |
Alvey A.,Badley Geoscience Ltd
Journal of Asian Earth Sciences | Year: 2016
We consider the presence of thick section in the axial trough indicates that the Pliocene-Recent continuous spreading model proposed by Curray (2015) is not appropriate for the Andaman Sea. The high frequency of eustatic sea level changes during the Plio-Pleistocene is incompatible with large, rapid influxes of sediment, widely separated in time but related to eustacy as proposed by Curray (2015). Instead spreading is probably of Miocene age, and was only renewed recently, if at all. The nature of the crust adjacent to the spreading centre remains uncertain. However, industry seismic data from the East Andaman Basin strongly suggests if the western part of the basin is underlain by oceanic crust it must be Oligocene or older, it is not Early Miocene. This in turn suggests the volcanics dredged from the Alcock rise were extruded onto older crust (whether it is oceanic, island arc or continental), and do not represent the age of oceanic crust formation. Gravity and seismic reflection data point to the East Andaman Basin being dominantly underlain by extended, to hyper extended continental crust and transitional crust. The Alcock and Sewell rises are interpreted as representing the western side of the strongly necked zone of continental crust. © 2015 Elsevier Ltd.
Bretan P.G.,Badley Geoscience Ltd.
Fault and Top Seals: From Characterization to Modelling | Year: 2012
Evaluating the structural integrity of fault-bounded traps for CO2 storage requires a thorough assessment of the likely sealing or non-sealing behavior of faults, in particular, i) will the increase in pressure generated by CO2 injection (or by a CO2 column) trigger fault instability and reactivation, thus leading to loss of CO2 from the trap, and ii) will the fault act as a capillary barrier, thus permitting CO2 to accumulate, and if so what might the likely height of the trapped column be before the fault leaks? The structural integrity of fault-bounded CO2 traps can be evaluated using workflows and predictive algorithms originally developed for the prediction of capillary seal of hydrocarbon, using appropriate CO2 fluid densities. Three-dimensional faulted-framework models are an essential first step in assessing the integrity of a fault-bounded CO2 trap. Fault-plane diagrams are used to investigate the juxtaposition geometry of CO2 bearing reservoir/non-reservoir intervals at the fault plane. Predictive algorithms for fault-sealing, such as Shale Gouge Ratio, and for stress-driven leakage enable a better understanding of the possible fault behavior to be derived.
Bretan P.G.,Badley Geoscience Ltd
4th International Conference on Fault and Top Seals 2015: Art or Science? | Year: 2015
The deterministic method for predicting column heights in traps involves constructing a fault framework model and populating the model with attributes. Shale Gouge Ratio (SGR) is calculated at sand-on-sand juxtapositions and transformed to hydrocarbon column height. The application of the deterministic method is straightforward for traps defined by few faults. Fault-plane sections are inspected visually to identify the column height that could be supported at the fault. However, for traps bounded by multiple intersecting faults identifying column heights through the visual inspection of fault-plane sections is practically impossible. A new automated approach is described that enables leak points and column heights to be quickly derived and evaluated for traps bounded by multiple intersecting faults. Fault 'side walls' defined by branch lines are simultaneously interrogated to derive a unique location of the leak point. The leak point is that point on a fault side wall which, when trappable column heights are calculated, implies the shallowest hydrocarbon contact in the trap. The new approach has shown that the location of a leak point in a trap can depend upon the transformation used to convert SGR to capillary pressure and has important implications for migration studies in complex fault-bounded traps.
Yielding G.,Badley Geoscience Ltd
Petroleum Geoscience | Year: 2012
Fault-seal analysis in hydrocarbon exploration often involves prediction of the sealing capacity of fault rock at reservoir-reservoir juxtapositions on subsurface faults. A proxy property, such as Shale Gouge Ratio (SGR), is mapped on to the fault surface, and then SGR is either (a) calibrated by observations of known sealing faults, to define its sealing capacity (empirical approach), or (b) assumed to be equal to the composition of the fault rock, for which a database of capillary threshold pressures is available from cores (deterministic approach). The deterministic approach implicitly assumes that capillary pressures measured on centimetre-scale samples are representative of seismically mappable faults, for example that faults of intermediate SGR are equivalent to phyllosilicate framework fault rocks.This contribution builds on earlier outcrop and modelling work to suggest an alternative explanation for the observed progressive increase in sealing capacity on faults of increasing SGR. Stochastic models of disrupted shale smears display the same pattern of increasing sealing capacity as SGR increases. These models have a bimodal 'fault rock' composed only of sealing shale smears and non-sealing matrix and, yet, at intermediate SGR the predicted column heights are similar to those normally ascribed to intermediate composition fault rocks. The resulting 'fault-seal envelope' in the models is a statistical estimate of the maximum trappable column height, dependent on the random occurrence of a gap in the smeared fault surface. © 2012 EAGE/Geological Society of London.
Roberts A.M.,Badley Geoscience Ltd |
Kusznir N.J.,University of Liverpool |
Corfield R.I.,BP Exploration Operating Co. |
Thompson M.,BP Exploration Operating Co. |
Woodfine R.,BP Exploration Operating Co.
Petroleum Geoscience | Year: 2013
An integrated workflow has been devised for the investigation of deep-water rifted continental margins. At a margin this allows us to predict the crustal structure, the distribution of continental-lithosphere thinning and the location of the ocean-continent transition with a new degree of confidence. The workflow combines the analytical techniques of 2D or 3D gravity inversion, 2D or 3D flexural backstripping with reverse thermal subsidence modelling, upper-crustal fault analysis and rifted margin forward modelling. No one technique on its own can provide all of the required answers, nor can it provide answers without some degree of uncertainty. The use of a combination of techniques, however, provides answers to several different problems and, crucially, more confidence in these answers. The workflow provides direct information on the present-day geometry of rifted margins and leads towards a better understanding of the geodynamic evolution of these margins. It also provides information which can inform the exploration process by making predictions about crustal structure at the ocean-continent transition, the location of the continent-ocean boundary, stretching-factor, heat-flow magnitude and history, palaeobathymetric history and subsurface palaeostructure. Application of the workflow is illustrated here with reference to the continental margins of West India, Brazil, West Australia, Norway and Newfoundland-Iberia. © 2013 EAGE/Geological Society of London.
Morley C.K.,Chiang Mai University |
Alvey A.,Badley Geoscience Ltd.
Journal of Asian Earth Sciences | Year: 2015
The Central Andaman Basin (CAB) is generally accepted to be a site of continuous sea floor spreading since the Early Pliocene (~4.0. Ma). The adjacent Alcock and Sewell Rises, and part of the East Andaman basin have been interpreted as probable Miocene oceanic crust. Published seismic lines across the eastern half of the spreading centre show that 100's. m thickness of sediment are present right up to the central trough. The central trough margins are faulted, uplifted and tilted away from the central trough. The youngest sediment is ponded and onlaps the tilted central trough margin, while older faulted sediment lies within the trough. Such a configuration is incompatible with continuous spreading. Instead, either spreading in the central basin was episodic, probably comprising a Late Miocene-Early Pliocene phase of spreading, followed by extension accommodated in the Alcock and Sewell rise area (by faulting and dike intrusion), and then a recent (Quaternary) return to spreading in the central trough; or the central trough marks an incipient spreading centre in hyper-thinned continental (or possibly island arc) crust. To resolve these possibilities regional satellite gravity data was inverted to determine crustal type and thickness. The results indicate the CAB is oceanic crust, however the adjacent regions of the Alcock and Sewell Rises and the East Andaman Basin are extended continental crust. These regions were able to undergo extension before seafloor spreading, and when seafloor spreading ceased. Unpublished seismic reflection data across the East Andaman Basin supports the presence of continental crust under the basin that thins drastically westwards towards the spreading centre. Episodic seafloor spreading fits with GPS data onshore that indicate the differential motion of India with respect to SE Asia is accommodated on widely distributed structures that lie between the trench and the Sagaing Fault. © 2014 Elsevier Ltd.
Yielding G.,Badley Geoscience Ltd |
Bretan P.,Badley Geoscience Ltd |
Freeman B.,Badley Geoscience Ltd
Geological Society Special Publication | Year: 2010
Calibration is a necessary step in the workflow for prediction of fault seal because there is no direct way to detect the hydraulic behaviour of a fault at the scale of a hydrocarbon trap. Over the last 20 years two general approaches have been developed: (i) Measurement of hydraulic properties of fault-zone samples (lab calibration), then mapping these results onto the appropriate parts of trap-bounding faults. (ii) Design of simple algorithms which attempt to capture a salient feature of the fault zone (e.g. CSP, SSF, SGR), then looking at known trap-bounding faults to find a relationship between the algorithm and the presence or capacity of a seal (sub-surface calibration). Seal capacity is typically y Hg-air threshold pressure in the lab or static prdescribed bessure differences in the subsurface (e.g. hydrocarbon buoyancy pressure). In addition to likely interpretation and geometry errors in approaches (i) and (ii), further uncertainty is introduced when converting the calibrated seal strength to potential hydrocarbon column height, because of the variability of subsurface hydrocarbon fluids (interfacial tension). Despite these potential problems, the different methodologies typically agree reasonably well in their predictions for fault-seal capacity. However, this agreement may be largely coincidental and is likely to be a response to the heterogeneity of fault-zone structure (especially at intermediate 'compositions' or SGR). © The Geological Society of London 2010.