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.
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.
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.
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.
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.