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Davies R.J.,Durham University | Mathias S.A.,Durham University | Swarbrick R.E.,Durham University | Swarbrick R.E.,Mountjoy Research Center | Tingay M.J.,Curtin University Australia
Journal of the Geological Society | Year: 2011

A new method for estimating the duration of a mud volcano eruption is applied to the LUSI mud volcano in East Java. The estimate is based upon carbonates at depths in the range 2500-3500 m being the water source, with an estimated area of 100-600 km2, thickness of 0.2-1.0 km, porosity of 0.15-0.25, an initial pressure between 13.9 and 17.6 MPa, and a separate, shallower source of mud (c. 1200-1800 m depth). The resulting 50 percentile for the time it takes for flow to decline to <0.1 Ml day-1 is 26 years. By analogy with natural mud volcanoes it can be expected to continue to flow at lower rates for thousands of years. Assuming subsidence rates of between 1 and 5 cm day-1, land surface subsidence of between c. 95 and c. 475 m can be expected to develop within the 26 year time period. © The Geological Society of London.

Swarbrick R.E.,Mountjoy Research Center | Lahann R.W.,Mountjoy Research Center | Lahann R.W.,Indiana University Bloomington | O'Connor S.A.,Mountjoy Research Center | Mallon A.J.,Durham University
Petroleum Geology Conference Proceedings | Year: 2010

A high magnitude of overpressure is a characteristic of the deep, sub-Chalk reservoirs of the Central North Sea. The Upper Cretaceous chalk there comprises both reservoir and non-reservoir intervals, the former volumetrically minor but most commonly identified near the top of the Tor Formation. The majority of nonreservoir chalk has been extensively cemented with average fractional gross porosity of 0.08, and permeability in the nano- to microDarcy range (10218-10221 m2), and sealing properties comparable to shale. Hence deeply buried chalk is comparable to shale in preventing dewatering and allowing overpressure to develop. Direct pressure measurements in the Chalk are restricted to the reservoir intervals, plus in rare fractured chalk, but reveal that Chalk pressures lie on a pressure gradient which links to the Lower Cenozoic reservoir above the Chalk and the Jurassic/Triassic reservoir pressures below. Hence a pore pressure profile of constantly increasing overpressure with increasing depth is indicated. Mud weight profiles through the Chalk, by contrast, show many borehole pressures lower than those indicated by these direct measurements, implying wells are routinely drilled underbalanced. The Chalk is therefore considered the main pressure transition zone to high pressures in sub- Chalk reservoirs. In addition to its role as a regional seal for overpressure, the Base Chalk can be shown to be highly significant to trap integrity. Analysis of dry holes and hydrocarbon discoveries relative to their aquifer seal capacity (the difference between water pressure and minimum stress) shows that the best empirical relationship exists at Base Chalk, rather than Base Seal/Top Reservoir, where the relationship is traditionally examined. Using a database of 65 wells from the HP/HT area of the Central North Sea, and extending the known aquifer gradients from the Fulmar reservoirs via Base Cretaceous to Base Chalk, leads to a risking threshold at 5.2 MPa (750 psi) aquifer seal capacity. Discoveries constitute 88% of the wells above the threshold and 36% below, with 100% dry holes where the aquifer seal capacity is zero (i.e. predicted breached trap). This relationship at Base Chalk can be used to identify leak points which control vertical hydrocarbon migration as well as assessing the risk associated with drilling high-pressure prospects in the Central North Sea. © Petroleum Geology Conferences Ltd. Published by the Geological Society, London.

Wilkinson M.,University of Edinburgh | Haszeldine R.S.,University of Edinburgh | Hosa A.,University of Edinburgh | Stewart R.J.,University of Edinburgh | And 10 more authors.
Energy Procedia | Year: 2011

In the UK, by far the largest CO2storage opportunities lie offshore. The North Sea in particular has a long and complex geological history, with potential reservoirs geographically widespread and occurring at multiple stratigraphic levels. Diverse storage estimates have been made, using a range of working methods, and yielding different values, e.g. SCCS (2009) [1]; Bentham (2006) [2]. Consequently the UK Storage Appraisal Project (UKSAP), commissioned and funded by the Energy Technologies Institute (ETI), is undertaking the most comprehensive assessment to date, using abundant legacy seismic and borehole data. This study has a remit to use best current practice, consistent between locations, to calculate the CO2 storage capacity of the entire UK Continental Shelf (UKCS) within saline aquifers and hydrocarbon fields. The potential storage formations have been subdivided into units for assessment, and filtered to remove units with only a small estimated storage capacity to concentrate resources on more viable units. The size of potential storage units approximate to a power law distribution, similar to that of hydrocarbon fields, with a large number of small units and a small number of large units. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd.

Webster M.,Genesis Energy | O'Connor S.,Mountjoy Research Center | Pindar B.,Mountjoy Research Center | Swarbrick R.,Mountjoy Research Center
AAPG Bulletin | Year: 2011

Analysis of subsurface pressure data from Taranaki Basin using direct (e.g., repeat formation tester) and indirect measurements (drilling parameters and wireline log data such as sonic and resistivity) indicates the presence of three pressure zones: a near-hydrostatic regime (zone A) that extends across the entire basin and to varying depths; an underlying overpressured regime (zone B), with pressures approximately 1100 psi (7.584 MPa) above hydrostatic, that extends throughout the Manaia graben and north along the eastern basin margin at depths of 1900 to 4100 m (6234-13,451 ft); and a third regime (zone C), with approximately 2100 psi (14.479 MPa) overpressure, that directly underlies zone A and zone B in different parts of the basin (although well penetrations are limited). The primary cause of overpressure is interpreted to be disequilibrium compaction preserved in upper Eocene and Oligocène marine shales. In parts of the basin, hydrocarbon generation (and in particular cracking to gas at high maturities) is interpreted to contribute to overpressures. The overpressures drain laterally and vertically into permeable units. Intervening transition zones (seals) comprise lithologie boundaries, diagenetic zones, and fault planes. Oligocène carbonates, although commonly thin, provide an effective barrier to vertical hydraulic communication over much of the basin. The Manaia graben is a partially closed system, with overpressures retained by a complex combination of a top shale seal overlying a regional sequence boundary, lithologie barriers within fault compartments, fault planes, and subcropping sequences; episodic fault breach enables vertical transfer of fluids from zone B to zone A in a dynamic fault valve process. To date, all oil reserves have been found in zone A, a large proportion of gas-condensate reserves are within zone B, and no commercial reserves have been established within zone C. The spatial definition of these zones and the appropriate pressure regime is important for well design, drilling safety, determining hydrocarbon column heights and gas expansion factors, and for exploration migration analysis. Regional analysis of pressure regimes can identify subsurface barriers and seals. Faults, in particular, are key elements in fluid migration and the focusing of liquids at abrupt pressure transitions. The strength of fault planes and diagenetic zones is the likely control on dynamic fluid release. Zone C has been very lightly explored and may represent a potential for large dry-gas accumulations; the zone may be sealed by a diagenetic zone crosscutting lithologie boundaries (conventional mapping horizons). Copyright © 2011. The American Association of Petroleum Geologists. All rights reserved.

Rutter E.H.,University of Manchester | Green S.,Mountjoy Research Center
Journal of the Geological Society | Year: 2011

The Mam Tor landslide (Derbyshire, England) slips downhill at up to 0.5 m a-1, and offers an excellent opportunity to study time-dependent creep in mudstones under in situ conditions. Annual surveys show that creep rates increase following heavy rainfall, but to establish detailed correlation between creep and pore water pressure required spatially and temporally higher resolution monitoring. We acquired 3 years of such data, at 3 h intervals, from wire creep meters, borehole piezometers and rainfall monitoring, showing that creep is strongly focused into the winter months and correlates well with pore water pressure. Summer grass and fern growth strongly influence rainfall infiltration, recycling much of the rainfall back to the atmosphere, explaining the seasonal variability of creep. The dependence of creep rate on pore pressure can be recast as a rheological model relating shear strain rate to shear stress using the friction angle for the creeping mudstone to link effective normal stress to shear stress. The creep is very non-linear, following a power law with a stress exponent of 48. This flow law may apply also to shear failure in poorly consolidated mudrocks under high pore pressure conditions; for example, within some landslides, subglacial sediments, tectonic fault zones, tectonic mélanges and accretionary complexes. © The Geological Society of London.

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