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- Volume 21, Issue 7, 2003
First Break - Volume 21, Issue 7, 2003
Volume 21, Issue 7, 2003
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Algerian licensing round may offer opportunity for exploration plays in deep offshore frontier
By M.J. CopeMichael J. Cope, senior exploration geologist, WesternGeco, reviews the geological prospectivity of Algeria’s offshore margin in the wake of the recently announced licensing round. Algeria’s offshore margin is one of the few remaining deep water exploration frontiers around Africa. With only one deep exploration well drilled in the 1970s, the area is set for a more extensive exploration effort following the announcement of a new offshore licensing round by the Algerian government. WesternGeco, in collaboration with Sonatrach, recently acquired a 9127 km 2D survey covering the entire 108 000 km2 designated area of the Algerian continental shelf out to water depths of 2700 m. The survey was designed to image below the widespread Messinian salt layer which had not been possible with earlier seismic surveys. The new data have allowed the identification of plays and prospective structures in the post-salt Pliocene, underlying pre-salt Miocene basins and the pre-Miocene subcrop.
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One man's mission to publicise Russian petroleum geology research
The Russian version of this article was first published in 2001 in Geologiya Nefti I Gaza, No 4, pp. 20-24. It is reproduced here thanks to the work of editor and publisher Dr James Clarke, who includes the article in the latest edition of Petroleum Geology, a digest of Russian language papers on petroleum geology from independent states of the Former Soviet Union. The journal, published privately on a non-profit basis in the interest of the science of petroleum geology, was a quarterly publication up until Volume 36 last year, but will from 2003 be published annually. The 2003 Volume 37 consists of 30 articles covering the Barents Sea, Sea of Okhotsk, West Siberia, Caspian depression, Dnieper-Donets basin, Black Sea, Russian craton, Kazakhstan, Armenia, Siberian craton, Sakhalin, Yenisey-Khatanga basin, Cis-Caucasus, Cis-Urals foredeep, Bashkortostan, and other regions.
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Question of oil-gas prospects in northern regions of the Barents-Kara shelf (Franz-Viktor Trough)
Authors P. Shkatov and A.G. StarkThe significant hydrocarbon potential of the shelf areas of the Barents and Kara Seas has been demonstrated by the discovery of very large gas and gas condensate fields in their southern areas. Against this background the increase in oil reserves has not been so significant. Nevertheless there is a geologic basis for predicting large zones of oil accumulation in the north of the Barents-Kara basin, which is an area of border highs. First of all, the depth to source beds is not great, and this provides for the generation of mostly liquid hydrocarbons. The neotectonic environment was also more favourable for formation and preservation of oil pools. Areas of this type are characterised by large zones of oil and bitumen accumulation related to lateral migration of hydrocarbon fluids from more subsided parts of oil-gas basins. All this is substantiated by much geochemical data derived from study of bitumen shows on Franz Josef Land and bottom samples from the surrounding offshore areas. The bitumen shows are largely viscous and solid natural bitumens. Their content in host rock is comparatively insignificant at fractions of a percent. All this indicates migration of predominantly liquid hydrocarbons from deeper horizons of the sedimentary cover as well as absence of significant surface precipitation accompanying destruction of oil pools. In this light the region may be regarded as favourable for oil. It is therefore important that geochemical anomalies be recognised in areas of local positive structures in the north of the Barents-Kara shelf. Information has been gathered in recent years in the Franz-Viktor Trough that indicates a large swell-like structure of north-south trend in its southern part (see Figures 1 & 2). It is designated as the Ladin High in connection with recovery of sediments of the Ladin Stage at station 52 (Figure 2). Reflection and common depth point seismic profiling show the Ladin High as clearly expressed in the relief of the Precambrian (Vendian) basement. Its dimensions are 110 km x 20-40 km. Its base is outlined by the 3 km structure contour, and the crest is at a depth of 1.5-1.7 km. Ladin High retains its inherited form in the Phanerozoic cover, although more gentle, particularly in its upper part. Small cupolas that measure 10-20 km in cross section and with amplitudes of 0.4-0.6 km have been mapped on the reflection profiles. These coincide in direction with the trend of the Ladin High (See Figure 3).
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The MEGATEM fixed-wing transient EM system applied to mineral exploration: a discovery case history
Authors R. Smith, D. Fountain and M. AllardRichard Smith and David Fountain of Fugro Airborne Surveys (Ottawa, Canada) and Michel Allard of Canadian mining company Noranda (Laval, Quebec) describe the development of a new airborne electromagnetic survey system for mineral exploration. In the mid 1980s, Geoterrex (now part of Fugro Airborne Surveys) introduced the GEOTEM system, a fully digital airborne electromagnetic (AEM) receiver, using the same transmitter that had previously been used, very successfully, on the INPUT system (Annan, 1990). The GEOTEM system is mounted on a twin engine CASA 212 aircraft, and the transmitter loop is wound around the nose, wing tips and the tail of the aircraft (Figure 1). This transmitter excites eddy currents in the subsurface with periodic pulses of the ‘primary’ magnetic field. The decay of these currents is measured with a receiver towed behind the aircraft in a ‘bird’. When the eddy currents decay slowly, this is generally indicative of material in the subsurface that is conductive. Figure 2 shows a schematic diagram of the system showing the towed receiver bird. Airborne electromagnetic systems typically also measure the intensity of the Earth’s geomagnetic field. For the GEOTEM system the magnetometer is in a second bird. also towed behind and below the aircraft. Having magnetic data is useful for mineral exploration, as it can help to distinguish between conductive suphides (which are often associated with magnetic sulphides – for example pyrrhotite) and other conductive features like clay, graphites and shear zones (which are not magnetic). The magnetometer towed-bird location puts the magnetometer sensor close to the ground and provides high-resolution data. One measure of the strength of an AEM system is the transmitter dipole moment, the product of the current flowing in the transmitter loop, the area of the loop and the number of turns of wire in the transmitter. For the GEOTEM system, the peak dipole moment has been 0.69 million Am2 since the early 1990s. The system has been used in many different types of terrain and for many applications (Smith and Annan, 1997). It was recently attributed with discovering the Storliden deposit in Sweden (Posey, 1999). In the mid 1990s, the GEOTEM system on a CASA was deployed by BHP (now BHP Billiton) for exploration at relatively high altitudes (less than 2400 m) in the Altiplano regions of the Andes Mountains of South America. By 1996, BHP was interested in exploring in areas above 2400 m altitude.
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Country risk: the good, the bad and the ugly
More LessIan E. Marshall, specialist in country risk assessment for the minerals and petroleum industry and a director of Transparency International Canada, provides a template for weighing the potential investment hazards of exploration and production activity in certain countries, with examples from Indonesia and South Africa. The text is an amended version of a presentation given in March to the Prospectors & Developers Association of Canada (PDAC) Annual Convention. Everybody who has worked in a developing country has an opinion on the risks involved in making an investment in that country. A potential investor can also access a wealth of relevant information through the internet and other sources. This paper begins with some general observations on country risk and then presents a template to assist assessment of opinions and organising information for country risk analysis. This approach will then be applied to two gold producing developing countries. Mining companies tend to conduct formal country risk evaluations at three times: ■ prior to exploration, ■ prior to the feasibility study and ■ prior to the investment decision. Companies are generally willing to assume more risk during the exploration stage. However, since there may be a 10 year period from the time a mining company first enters a country until it begins to realize a return on its investment, it must focus on long-term risk. Mining companies must look beyond the tenure of current governments and current officials and identify any factors in society which could lead to a change in direction, a shift in policy or a restructuring of the overall political system to the detriment of their investment.
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Impact of consolidation on the mining industry
By A. DavidsonAlex Davidson, senior vice-president, exploration of the major gold mining company Barrick Gold, in a presentation to the Producers & Developers of Canada (PDAC) Annual Convention last March, offered an analysis of the mining industry’s consolidation over the last few years which has an uncanny resemblance to the oil industry experience. We publish a printed version of the paper. The last half decade or so has been one of the harshest stretches for exploration that I’ve seen in 30 years’ experience. The main culprits are obvious: low metal prices across the board, whether in gold, copper, nickel or other metals, and the flight of speculative capital along with investor confidence. But these alone don’t account for all the changes we’ve been through. Consolidation is also impacting the picture, both in combination with low prices and apart from them. These factors have contributed to a decline in exploration spending that promises to have continued effect on production supply going forward. Let me start with an overview of consolidation and the forces driving it, how it’s reshaping the industry, and then assess its impact on exploration. As we’ll see, while low metal prices may have helped start the consolidation wave, rising prices don’t mean an end to consolidation, or its effects on exploration. While my observations can be applied to the mining industry in general, my examples will be mainly drawn from the gold industry, which I know best. This wave of consolidation started in 2000, and it started mainly with the base metal companies: Billiton bought Rio Algom, Phelps Dodge bought Cyprus Amax, Grupo Mexico got Asarco and Rio Tinto got North. The wave picked up momentum through 2001 with the BHP Billiton merger and then the gold companies picked up on the trend. Since mid-2001 there have been over 14 company mergers/acquisitions and over 10 mine transactions in the precious metals industry.
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Vp/Vs ratio as a rock frame indicator for a carbonate reservoir
Authors F. Tsuneyama, I. Takahashi, A. Nishida and H. OkamuraCarbonates are understood to be the most difficult lithology to which to apply quantitative seismic analysis, because of their heterogeneity deriving from their complicated depositional systems and diagenetic processes. We were able to study a fairly complete set of property data for Cretaceous carbonates, consisting of wackestone, packstone and grainstone, which constitute the reservoir in the oil fields of a Middle Eastern offshore area. The database included 3D seismic, Dipole Shear Sonic (DSI) logs, core velocity measurements and lithology descriptions. Firstly, core measurement data were examined to identify parameters related to permeability. It was found that the Vp/Vs ratio carries information about permeability; a higher Vp/Vs ratio at higher porosities indicates a predominance of grainstone facies, with well-preserved inter-granular porosities closely related to higher permeability, in the reservoir in this area. We constructed rock-frame models to explain this relationship and verified it by theoretical calculations. This relationship was also observed in DSI well-log data in the area. Finally, in order to image the distribution of seismic facies of different rock frames, we calculated an innovative seismic attribute, 'pseudo Vp/Vs', from multiple limited-offset seismic volumes, using the AVO approximation given by Hilterman (1989).
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The potential of measuring fracture sizes with frequency-dependent shear-wave splitting.
Authors S. Maultzsch, M. Chapman, E. Liu and A. LitvinThe success of seismic anisotropy is its ability to provide subsurface fracture orientations as derived from the polarization of the faster shear wave, and spatial distributions of fracture intensity inferred from time-delays between the faster and slower shear waves (Crampin 1985, Li 1997). However, the reservoir engineers’ reluctance to accept seismic anisotropy as a routine technique for fracture characterization is partly because of its failure to provide information about sizes of fractures. Although both grain scale micro-cracks and macro-scale fractures are considered to cause seismic anisotropy, reservoir engineers are more interested in the latter as permeability in many hydrocarbon reservoirs is believed to be dominated by formation-scale fluid units (in the order of meters). The interpretation of anisotropic measurements made from seismic data requires theoretical models that relate measurable seismic parameters to macroscopically determined rock properties. Based on the assumption that the scale length associated with fractures is considerably smaller than the seismic wavelength, a description of the average properties of the medium will be sufficient. The two most popular models for the average properties are the Thomsen equant porosity model (Thomsen, 1995) and Hudson’s model (Hudson, 1981). Thomsen’s model assumes perfect fluid pressure equalization between the cracks and the surrounding rock while Hudson’s model assumes that the cracks are isolated with respect to fluid flow. Both models predict frequency independent behaviour. In both models the magnitude of the anisotropy is related to the crack density, although the precise dependence is different in each case. This crack density is defined as ε=Na3 where N is the number of cracks per unit volume and a is the crack radius. Unfortunately, radically different fracture distributions few large cracks can give the same crack density as many smaller cracks. Figure 1 demonstrates this concept. Both cases have identical crack densities, but the fluid flow response would be expected to be markedly different in each case. If we are constrained to work within the conventional equivalent medium approach, then we can only obtain crack density and orientation from the seismic data. To obtain information on fracture size which can be useful for permeability prediction we require a different approach. In this study we investigate the possibility of using the frequency dependence of anisotropy to derive such information. We begin by reviewing a new theory which models frequency dependent anisotropy. The theory is then calibrated and tested against laboratory data, before we conclude with an application to field data.
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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)