ALBERT

Description: Large tracts of the NW European continental shelf and Atlantic margin have experienced kilometre-scale exhumation during the Cenozoic, the timing and causes of which are debated. There is particular uncertainty about the exhumation history of the Irish Sea basin system, Western UK, which has been suggested to be a focal point of Cenozoic exhumation across the NW European continental shelf. Many studies have attributed the exhumation of this region to processes associated with the early Palaeogene initiation of the Iceland Plume, whilst the magnitude and causes of Neogene exhumation have attracted little attention. However, the sedimentary basins of the southern Irish Sea contain a mid-late Cenozoic sedimentary succession up to 1.5 km in thickness, the analysis of which should permit the contributions of Palaeogene and Neogene events to the Cenozoic exhumation of this region to be separated. In this paper, an analysis of the palaeothermal, mechanical and structural properties of the Cenozoic succession is presented with the aim of quantifying the timing and magnitude of Neogene exhumation, and identifying its ultimate causes. Synthesis of an extensive apatite fission-track analysis (AFTA), vitrinite reflectance (VR) and compaction (sonic velocity and density log-derived porosities) database shows that the preserved Cenozoic sediments in the southern Irish Sea were more deeply buried by up to 1.5 km of additional section prior to exhumation which began between 20 and 15 Ma. Maximum burial depths of the preserved sedimentary succession in the St George's Channel Basin were reached during mid-late Cenozoic times meaning that no evidence for early Palaeogene exhumation is preserved whereas AFTA data from the Mochras borehole (onshore NW Wales) show that early Palaeogene cooling (i.e. exhumation) at this location was not significant. Seismic reflection data indicate that compressional shortening was the principal driving mechanism for the Neogene exhumation of the southern Irish Sea. Coeval Neogene shortening and exhumation is observed in several sedimentary basins around the British Isles, including those along the UK Atlantic margin. This suggests that the forces responsible for the deformation and exhumation of the margin may also be responsible for the generation of kilometre-scale exhumation in an intraplate sedimentary basin system located 〉1000 km from the most proximal plate boundary. The results presented here show that compressional deformation has made an important contribution to the Neogene exhumation of the NW European continental shelf.

Description: Neogene-to-Recent deformation is widespread on and adjacent to Australia's passive' margins. Elevated historical seismic activity and relatively high levels of Neogene-to-Recent tectonic activity are recognized in the Flinders and Mount Lofty Ranges, the SE Australian Passive Margin, SW Western Australia and the North West Shelf. In all cases the orientation of palaeostresses inferred from Neogene-to-Recent structures is consistent with independent determinations of the orientation of the present-day stress field. Present-day stress orientations (and neotectonic palaeostress trends) vary across the Australian continent. Plate-scale stress modelling that incorporates the complex nature of the convergent plate boundary of the Indo-Australian Plate (with segments of continent-continent collision, continent-arc collision and subduction) indicates that present-day stress orientations in the Australian continent are consistent with a first-order control by plate-boundary forces. The consistency between the present-day, plate-boundary-sourced stress orientations and the record of deformation deduced from neotectonic structures implicates plate boundary forces in the ongoing intraplate deformation of the Australian continent. Deformation rates inferred from seismicity and neotectonics (as high as 10-16 s-1) are faster than seismic strain rates in many other stable' intraplate regions, suggestive of unusually high stress levels imposed on the Australian intraplate environment from plate boundary interactions many thousands of kilometres distant. The spatial overlap of neotectonic structures and zones of concentrated historical seismicity with ancient fault zones and/or regions of enhanced crustal heat flow indicates that patterns of active deformation in Australia are in part, governed, by prior tectonic structuring and are also related to structural and thermal weakening of continental crust. Neogene-to-Recent intraplate deformation within the Australian continent has had profound and under-recognized effects on hydrocarbon occurrence, both by amplifying some hydrocarbon-hosting structures and by inducing leakage from pre-existing traps due to fault reactivation or tilting.

Description: The Naylor structure in the Port Campbell Embayment, Otway Basin, South Australia is proposed as a demonstration site for the subsurface geological storage of carbon dioxide (CO2). The Naylor structure is a fault-bounded high with normal faults to the north and west to SW. Seismic interpretation shows evidence of recent fault reactivation in the Otway Basin. It is postulated that residual hydrocarbon columns (accumulated and leaked prior to present day) in the Otway Basin leaked due to fault reactivation. Thus, a critical issue in the geological storage of CO2 in the Port Campbell Embayment is the potential for the reactivation of faults bounding the Naylor structure. The propensity of faults to be reactivated is assessed by determining the in-situ stress field, the mechanical properties of the fault rock and the orientations of the existing faults. The in-situ stress field lies on the boundary of a strike-slip and reverse faulting regime in the Port Campbell Embayment. The vertical, minimum horizontal and maximum horizontal stress gradients are 21 MPa km-1, 19 MPa km-1 and 38 MPa km-1 respectively and the pore pressure gradient is hydrostatic. The maximum horizontal stress in the Port Campbell Embayment is oriented at 150{degrees}N. One planar and two curviplanar faults were identified within the Naylor structure. Two fault segments act to trap accumulations at the crest of the structure. These fault segments have relatively low propensities to reactivate near the crest of the structure. The intended migration pathway of the CO2 plume does not intersect the identified faults until it reaches the crest of the Naylor structure. However, reservoir heterogeneities such as sub-seismic faults may cause the migrating CO2 plume to move towards identified fault segments which are not intended to trap the injected CO2 and have a relatively high propensity to reactivate.

Description: Parts of the Australian continent, including the Otway Basin of the southern Australian margin, exhibit unusually high levels of neotectonic deformation for a so-called stable continental region. The onset of deformation in the Otway Basin is marked by a regional Miocene–Pliocene unconformity and inversion and exhumation of the Cretaceous–Cenozoic basin fill by up to c. 1 km. While it is generally agreed that this deformation is controlled by a mildly compressional intraplate stress field generated by the interaction of distant plate-boundary forces, it is less clear whether the present-day record of deformation manifested by seismicity is representative of the longer-term geological record of deformation. We present estimates of strain rates in the eastern Otway Basin since 10 Ma based on seismic moment release, geological observations, exhumation measurements and structural restorations. Our results demonstrate significant temporal variation in bulk crustal strain rates, from a peak of c. 2×10−16 s−1 in the Miocene–Pliocene to c. 1.09×10−17 s−1 at the present day, and indicate that the observed exhumation can be accounted for solely by crustal shortening. The Miocene–Pliocene peak in tectonic activity, along with the orthogonal alignment of inverted post-Miocene structures to measured and predicted maximum horizontal stress orientations, validates the notion that plate-boundary forces are capable of generating mild but appreciable deformation and uplift within continental interiors.

Description: Shale dykes, diapirs and mud volcanoes are common in the onshore and offshore regions of Brunei Darussalam. Outcrop examples show that shale has intruded along both faults and tensile fractures. Conventional models of overpressure-induced brittle failure assume that pore pressure and total stresses are independent of one another. However, data worldwide and from Brunei show that changes in pore pressure are coupled with changes in total minimum horizontal stress. The pore pressure/stress-coupling ratio ({Delta}{sigma}h/{Delta}Pp) describes the rate of change of minimum horizontal stress magnitude with changing pore pressure. Minimum horizontal stress measurements for a major offshore field where undepleted pore pressures range from normal to highly overpressured show a pore pressure/stress-coupling ratio of 0.59. As a consequence of pore pressure/stress coupling, rocks can sustain a greater increase in pore pressure prior to failure than predicted by the prevailing values of pore pressure and stress. Pore pressure/stress-coupling may favour the formation of tensile fractures with increasing pore pressure rather than reactivation of pre-existing faults. Anthropogenically-induced tensile fracturing in offshore Brunei supports this hypothesis.

Description: Subsurface sediment mobilization (SSM) -- which includes soft sediment deformations, sand injections, shale diapirs and mud volcanoes -- is more widespread than previously thought. The ever-increasing resolution of subsurface data yielded many new observations of SSM, not only from regions obviously prone to sediment remobilization, such as an active tectonic setting or in a region with exceptionally large sediment supply, but also from tectonically quiescent areas. Until now, all the different aspects of SSM have largely been treated as separate phenomena. There is very little cross-referencing between, for example, studies of mud volcanoes and those of sand injections, although both are caused by sediment fluidization. Divisions according to sediment type, mobilization depth or triggering mechanism make little sense when trying to understand the processes of SSM. There is a gradation in mobilization processes that cause considerable overlap between categories in any classification. Hence, it is necessary to integrate our understanding of all types of SSM, regardless of scale, depth, location, grain size or triggering mechanism. In addition, polygonal faults are important in this context, as this nontectonic structural style is closely associated with sedimentary injections and may also reflect large scale mobilization. The main goal of this volume is to help develop a more integrated understanding of subsurface sediment mobilization. It contains specific case studies and a number of overview papers about the mechanisms of sediment mobilization in the subsurface (Maltman & Bolton), about polygonal faulting (Cartwright) and about shale diapirs (Morley). Other recent review papers were published about sand ... This 250-word extract was created in the absence of an abstract.

Description: Clastic dykes and sills witness that subsurface sediment mobilization is often controlled by the brittle failure of units sealing' overpressured and liquidized sediments. Brittle failure also imposes a limit on the buoyancy pressure that can be exerted by hydrocarbon columns. Conventional understanding of brittle failure induced by increasing pore pressure (Pp) assumes that total minimum horizontal stress ({sigma}h) is unaffected by changes in pore pressure. However, total minimum horizontal stress increases from shallow, normally pressured sequences to deeper, overpressured sequences. Data from the Canadian Scotian Shelf, the North Sea and the Australian North West Shelf demonstrate such Pp/{sigma}h coupling, with the minimum horizontal stress increasing at approximately 60-80% of the rate of pore pressure (i.e., {Delta}{sigma}h/{Delta}Pp = 0.6-0.8). Hence, a greater increase in pore pressure can be sustained prior to brittle failure of units sealing overpressured compartments than would be predicted by conventional, uncoupled failure models. Furthermore, because total vertical stress is not similarly coupled to pore pressure, differential stress ({sigma}1-{sigma}3) reduces as pore pressure increases in normal fault regime basins. Thus, the mode of rock failure can not be inferred from differential stress in the stable state and Pp/{sigma}h coupling promotes tensile over shear failure.

Description: Fault sealing is one of the key factors controlling hydrocarbon accumulations and trap volumetrics and can be a significant influence on reservoir performance during production. Fault seal is, therefore, a major exploration and production uncertainty. We introduce a systematic framework in which the geologic risk of faults trapping hydrocarbons may be assessed. A fault may seal if deformation processes have created a membrane seal, or if it juxtaposes sealing rocks against reservoir rocks, and the fault has not been reactivated subsequent to hydrocarbons charging the trap. It follows from this statement that the integrated probability of fault seal can be expressed as {1 − [(1 − a )(1 − b )]} × (1 − c ), where a , b , and c are the probabilities of deformation process sealing, juxtaposition sealing, and of the fault being reactivated subsequent to charge, respectively. This relationship provides an assessment of fault-seal risk that integrates results from the critical parameters of fault-seal analysis that can be incorporated into standard exploration procedures for estimating the probability for geologic success. The integrated probability of fault seal for each prospect can be visualized using the fault-seal risk web, which allows rapid comparison of success and failure cases through construction of prospect risk web profiles. The impact of uncertainty ( U ) and the value of information (VOI) for each aspect of fault sealing on the overall fault-seal risk may be determined via the construction of data webs and the relation U = [1 − {(∑ nw ) / n }] × 100, where nw is the value given to each data web parameter and n is the number of data web components. For example, the data web components required to assess fault reactivation risk are the orientation and magnitude of the in-situ principal stresses, pore pressure, fault architecture, and the geomechanical properties of the fault. Risking of the Apollo prospect, Dampier subbasin, North West shelf, Australia is presented as a worked example. Fault-seal risking for the Apollo prospect has been conducted on 10- and 100-ft oil columns to allow integration with volumetric probabilistic statements. The critical parameter for fault-seal risking at the Apollo prospect is the ability of disaggregation zone faults (low shale gouge ratio fault gouge) to support increasingly large hydrocarbon columns. Evaluation of the individual components for Apollo fault sealing indicates a = 0.3 (10-ft column) and a = 0.1 (100-ft column), b = 0.2, and c = 0.1. The overall probability of the Apollo trap-bounding fault sealing a 10-ft oil column is 0.4 or 40% (seal condition moderately unlikely). The likelihood that the fault seals oil columns greater than 100 ft is 0.3 (seal condition unlikely). Data web error margins for the Apollo prospect are 20% (juxtaposition uncertainty), 26% (fault-rock process uncertainty), and 27% (fault reactivation uncertainty). Recalculating each parameter by its uncertainty, for a 10-ft oil column, the upper value of integrated fault-seal risk is 0.5 (seal condition intermediate), and the lower value is 0.3 (seal condition unlikely). The upper value of integrated fault-seal risk for a 100-ft oil column is 0.3 (seal condition unlikely), and the lower value is 0.2 (seal condition very unlikely). The variation in the Apollo final risk calculation reflects the lack of prospect-specific data. The greatest VOI benefit for Apollo fault-seal prospectivity is sedimentary architecture data. Richard joined Woodside in September 2000 and is currently working in the Trap and Pressure Team, New Ventures. He graduated with a joint B.Sc. degree (with honors) in geology and economics from Keele University (1992) and a Ph.D. from Keele University (1996). He has worked extensively in the area of fault and top seal evaluation and has been involved with seals research programs in Europe, the United States, and Australia. Current interests include structural modeling, seal evaluation, wellbore stability, and Liverpool FC. Richard is a member of AAPG, PESA, and PESGB.Richard holds the State of South Australia Chair in Petroleum Reservoir Properties/Petrophysics at the National Centre for Petroleum Geology and Geophysics (NCPGG), Adelaide University. He graduated with a B.Sc. degree (with honors) from Imperial College (London, 1985), and a Ph.D. from the University of Edinburgh (1989). After seven years at Adelaide University's Department of Geology and Geophysics, Richard joined the NCPGG in 1999. His main research interests are in petroleum geomechanics and sedimentary basin tectonics. He has published approximately 50 papers and has consulted to many Australian and international oil companies in these topics. Richard is a member of AAPG, American Geophysical Union, Australian Society of Exploration Geophysicists, European Association of Geoscientists and Engineers, Geological Society of America, Geological Society (London), Petroleum Exploration Society of Australia, Society of Exploration Geophysicists, and Society of Petroleum Engineers.