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- Volume 21, Issue 4, 2003
First Break - Volume 21, Issue 4, 2003
Volume 21, Issue 4, 2003
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How crossing the Irish Sea proved a faultless move
By A. McBarnetWhen the Fault Analysis Group (FAG) moved home nearly three years ago from the University of Liverpool to University College, Dublin, it was a transfer without precedent in the geoscience academic world. But, as Andrew McBarnet reports, the group’s focus and output has continued uninterrupted with important contributions to research covering faults and other types of fracture with applications not just in the field of hydrocarbons but also coal and minerals.
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Graph theory and DFN analysis: a case example of the impact of network behaviour analysis on well location assessment
Authors L. Thompson, R. Smallshire and M. BonoraNaturally fractured reservoirs account for an increasing proportion of operator’s portfolios, (McKeown, 2001, submitted) and techniques for objectively assessing the properties of Discrete Fracture Networks (DFNs) are therefore of great interest. This paper presents some of the latest techniques available for the analysis of DFN behaviour. Of particular importance to the hydrocarbon industry are those factors affecting the ability of the network to transmit fluid and the likely patterns of transmission across the network. These patterns of behaviour will significant impact the risk assessment of well placements and productions strategies. The first section of this paper presents the theory and technique of connectivity mapping using graph theory. The utility of this technique is clearly shown by the case study presented in the second section. The example is from a complex thrust trap in Argentina and clearly illustrates both the need to understand the evolution of an area (in order to produce realistic DFNs) and also the need for rapid, interactive assessment of the impact of significant variables (fracture length, dilatancy state, etc) on the DFN behaviour. The evolution of the study area was modelled and the strain imposed on the reservoir was modelled. Elements of the strain history were used as a guide in developing the DFN. The impact of changing stress regimes on connectivity behaviour is demonstrated.
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The UK Atlantic Margin: are big prizes still out there?
Authors R.W. Gatliff, H. Johnson, J.D. Ritchie and K. HitchenR.W. Gatliff, H. Johnson, J.D. Ritchie and K. Hitchen of the British Geological Survey in Edinburgh follow the previous article's assessment of recent drilling by offering some geological evidence . The first well drilled on the UK Atlantic margin was spudded in 1972. Since then, drilling activity has fluctuated (Figure 1) with peaks of activity in 1977 (when the Clair oilfield was discovered), 1984-86 (deep water step-out into the Faroe-Shetland Basin), 1991 (mainly licence round related) and 1994-1996 (related to the Foinaven and Schiehallion discoveries). Initially, exploration tested the major structural highs of the West Shetland and North Rona basins and the Rona Ridge beneath the shallow water shelf areas. Subsequently, exploration stepped out into the deeper water associated with the Faroe-Shetland and Rockall basins. Exploration interest in the area seemed to be waning until the Foinaven and Schiehallion discoveries gave the area renewed impetus. Within the last few years the agreement of the international boundary between the UK and the Faroe Islands has resulted in some renewal of drilling activity in both countries close to the newly agreed median line. This latest phase of exploration seemed to be stalling after a series of disappointing tests across a range of ‘play types’ in the Faroe-Shetland Basin and the Rockall Basin. However, it remains to be seen whether the Amerada Hess Marjun discovery in Faroese waters (well 6004/16-1Z) and particularly the recent Dooish discovery by Enterprise in Irish waters (well 12/2-1) within the Rockall Basin will lead to another surge in drilling activity. The Marjun discovery, with a 170 m hydrocarbon column, is in a Paleocene structural trap (Smallwood 2002). Details of the Dooish discovery have yet to be released but the Petroleum Affairs Division of Ireland (2002) describes it as a ‘significant’ hydrocarbon accumulation. It is speculated that the trap may comprise a pre-rift succession within a tilted fault block on the eastern margin of the Rockall Basin. This discovery establishes that a working petroleum system is present within the Rockall Basin and consequently that the potential for further hydrocarbons discoveries is significant. Tilted block structures have already been identified on both east and west flanks of the Rockall Basin (Walsh et al. 1999).
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Post-well analysis of exploration drilling on UK Atlantic Margin provides clues to success
By N. LoizouThis paper is based on an updated and more detailed version to the article that was published in the January 2003 PESGB newsletter. The material presented here only forms part of a more confidential, comprehensive, in depth analysis on all exploration wells drilled on the UK Atlantic Margin from the end of 1995 (Figures 1 and 2). The analysis utilised available well, 2D/3D seismic data and other key geologic data. However, for reasons of confidentiality the paper presents a generalised overview of what has been learnt from exploration drilling. The main conclusion is that from the exploration wells analysed along the UK Atlantic Margin, less than 50% were located on what could be termed a ‘reliable, robust structure’. Moreover, the wells that were located on the more reliable, robust structures recorded a higher success rate of more than one in two.
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Accurate regional - residual separation by finite element approach, Bouguer gravity of a Precambrian mineral prospect in northwestern Ontario
Authors S. Kannan and K. MallickThe regional – residual resolution of the potential field continues to be a topic of considerable interest among geophysicists even to the present time. In spite of a large number of sophisticated analytical techniques both in the space and frequency domain (Coons, et al, 1967, Oliver, 1977; Jachens and Griscom, 1985; Simpson et al, 1986; Pawlowski, 1994; Chapin, 1996), there are instances where interpreters are not satisfied with the regional and residual components obtained by these methods (for example, Gupta and Ramani, 1980) and have resorted to the intuitive graphical approach. While processing the Bouguer gravity data for a mineral prospect in a Precambrian terrain in north Western Ontario, Canada, Gupta and Ramani (1980) were not fully satisfied by the regional components obtained by spectral factorization and upward continuation. These were still found to contain a portion of the shallower effects, thereby producing residual anomalies not quite suitable for gravity modeling. Taking geology and density of the formations into consideration, Gupta and Ramani (1980) carried out graphical smoothing for residualization. On the face of the reported unsatisfactory performances of three analytical techniques, namely trend surface analysis, upward continuation and spectral factorization, often used by many interpreters, we wish to illustrate that by employing a finite element approach - FEA (Mallick and Sharma, 1999), it is possible to obtain regional and residual anomalies that compare favourably with those intuitively assumed in Precambrian terrain in northwest Ontario, Canada. To prove our point, we have reprocessed the Bouguer gravity data of Gupta and Ramani (1980) by the new approach and present the contour maps and images of the regional and residual components. The new FEA approach is based on a finite element concept described in detail with a number of examples by Mallick and Sharma (1999).
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Anisotropic ambiguities in TI media
Authors I.F. Jones, M.L. Bridson and N. BernitsasIntroduction In recent years we have seen processing and imaging algorithms re-written to handle anisotropic effects. The most common type of anisotropy that one deals with in seismic data is polar anisotropy (transverse isotropy). Media with vertical (VTI), tilted (TTI), and horizontal (HTI) axes have been shown to exist as a result of sedimentary deposition or fracturing. The consequences of ignoring polar anisotropy vary depending on the degree and type of anisotropy. Typically, using isotropic imaging in an anisotropic medium results in mis-ties between the preSDM and the well depths. Such mis-ties, in some extreme cases, can exceed 10% of the true depth (for example, in the Franklin-Elgin field operated by TFE in the North Sea there is a 600m mis-tie at a depth of 5km). In addition to the vertical depth error, there is also a lateral shift, most pronounced for the steepest dips, and noticeable in fault surface reflections. Experience has shown that one cannot image simultaneously flat and steep dips with an isotropic velocity field. The difficulty in addressing anisotropy lies in the estimation of reliable parameters to be used with the processing or imaging algorithms. In this work, we assess the effects of errors in anisotropic parameter estimation for the case of polar anisotropy, and attempt to quantify the consequences of these errors with some specific synthetic examples.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)