- Home
- A-Z Publications
- First Break
- Previous Issues
- Volume 23, Issue 12, 2005
First Break - Volume 23, Issue 12, 2005
Volume 23, Issue 12, 2005
-
-
Hydrocarbon potential of western Greece
More LessAngelos Mavromatidis, Schlumberger Middle East, contributed this optimistic assessment of the prospects for hydrocarbon exploration and production in western Greece. Greece is a net energy importer. Most hydrocarbon-related activity is concentrated on the downstream energy sector. The upstream sector is very limited. Specifically, hydrocarbon exploration in Greece has not managed to build a respectable hydrocarbons reserve accumulation over the years. However, there is a great potential for companies to develop exploration projects in the country. The paper reviews the country’s energy status, and describes drilling activity in Greece speculating about the causes of the unsuccessful drilling rate in western Greece and, finally, emphasizes the need for a new exploration strategy.
-
-
-
High resolution of streamer seismic data: obtaining optimal results
By A. LongAndrew Long, PGS Marine Geophysical, defines the requirements for successful high resolution 3D marine seismic surveys and how these can be met. High resolution of seismic data results when high frequency geological features in close proximity can be precisely focused without any contamination from artefacts or noise (i.e. high signal-to-noise ratio). In general, recoverable frequency bandwidth is an important factor for vertical resolution, and dense 3D spatial sampling is an important factor for spatial resolution. Vertical and horizontal resolution is linked by the success of high frequency focusing and imaging. Without pursuing a theoretical treatise, the following section discusses the geophysical principles that are relevant for high-resolution seismic imaging. High resolution of 3D seismic data is achieved by the application of rigorous geophysical principles. In particular, the systematic pursuit of 1) uniform target illumination, 2) dense spatial sampling of the reflected wavefield, and 3) careful processing and imaging, will collectively deliver an optimum result. Note that the final step can never be successfully completed without the platform of proper target illumination and wavefield sampling. Several 3D data examples are used to demonstrate how flexible survey design and implementation can robustly achieve very high resolution target imaging for all target depths and challenges.
-
-
-
Calibrated seismic measurements for improved reservoir definition: a UKCS case study
Authors S. McHugo, J. Bacon, A. Furber and S. PickeringSteve McHugo, John Bacon, Andrew Furber, and Steve Pickering of WesternGeco present a UK North Sea case study to show the benefits of recent advances in marine seismic imaging. In 2003, WesternGeco acquired a new high-resolution seismic survey for Chevron over the Captain field, UK North Sea. The seismic data were acquired using pointreceiver technology and processed using deterministic data-processing techniques. The objective of the new survey was to provide improved seismic imaging of the field, improve confidence in the detailed mapping of known reservoir zones, and identify new targets. The new survey took advantage of significant technological advances made in seismic exploration. These advances included calibrated point-receiver acquisition, streamer steering, high-accuracy acoustic positioning, and recording of near-field source signatures allowing for much more reliable and detailed images of the subsurface. This article focuses on the impact that sampling and near-field source measurements have on the quality of the final dataset. Background Rose (1999) describes some aspects of the reservoir characterization of the Captain oilfield and refers to issues associated with the seismic data used during appraisal and development. The field, in UKCS Block 13/22 (Figure 1) in the Western Moray Firth, produces heavy oil (19 to 21o API) from two Lower Cretaceous deepwater turbidite reservoirs and an Upper Jurassic shallow marine reservoir. The field was discovered in 1977 and was brought into production in 1997 using closely spaced long-reach horizontal wells. Field appraisal was performed using more than 25 vertical delineation wells; these, combined with existing 1990 vintage seismic data, yielded a top reservoir structure map that has been used to land many long-reach high net-togross horizontal development wells. The 1990 seismic survey, however, provided poor overall imaging and depth estimates of the reservoir zones because of weak signal-to-noise ratio and limited resolution at the key target reflectors. This was primarily caused by known geophysical problems associated with the relatively weak seismic reflectivity of the target sands and the complex geology of the overburden. The new seismic survey was charged with overcoming these geophysical problems.
-
-
-
3D geologic modelling of channellized reservoirs: applications in seismic attribute facies classification
By R. WenRenjun Wen, president and CEO, Geomodeling Technology, presents a new methodology for modelling stratigraphic heterogeneity in channellized reservoirs. Geological models are usually used qualitatively in seismic interpretation. This paper illustrates that quantitative representations of detailed geological models can significantly enhance seismic attribute interpretation through facies classification. When applying seismic attribute classification to reservoir facies mapping, one often faces such typical questions as: ■ Which attributes should be used as input to classification? ■ How many classes should be used in the unsupervised classification method? ■ How many levels of hierarchy should be selected in the hierarchical classification method? ■ Does the seismic facies correspond to the geological facies? ■ How can attribute-derived facies models be validated? There are no unique and easy answers to the above questions. In this study, we aim to create a more accurate representation of the reservoir by using 3D synthetic Earth models to guide seismic attribute classification. We consider a channellized reservoir for which seismic attribute analysis has proven to be very useful, but results can be difficult to interpret. The next section describes a 3D stratigraphic modelling approach for the channellized reservoir. The major channel components and parameterizations are illustrated with examples. This is followed by a summary of seismic attribute analysis and classification workflow applied to a synthetic seismic volume. Results of attribute classifications using a self-organized map (SOM) (Kohonen, 1989) and waveform correlation maps are compared in relation to different input attributes and classification parameters. The lessons learned from this synthetic example are summarized and the selection of attributes for facies classification is discussed. 3D stratigraphic models of channellized reservoirs There are several computer-based methods to build 3D reservoir model flow simulations, such as object-based methods or cell-based geostatistical methods (Dubrule and Damsleth, 2001). However, none of these methods are able to reproduce stratigraphic heterogeneity patterns at sub-seismic scale, which can be major controlling factors for fluid flow and spatial variations of acoustic properties. In this paper we report a new modelling method to generate 3D stratigraphic architectures of channellized reservoirs. The method is an extension of the bedding structure modelling method developed by Wen et al. (1998) and is being further developed in the SBED Joint Industrial led by Geomodeling Technology. The stratigraphic features within channellized reservoirs to be modelled in this study are below the resolution limit of conventional seismic data. The cell size is about 20 x 20 x 1 m3. At such a modelling scale, detailed geological features must be modelled, based on their formation process so that their 3D structures can be correctly represented in the geological model.
-
-
-
Migliorini and the Three Pillars of Geological Wisdom
Authors F. di Cesare, F. Guidi and R. CasnediFranco di Cesare, Francesco Guidi, and Raffaele Casnedi bring to life the career of Carlo Ippolito Migliorini, one of Italy’s most influential 20th Century geologists, and examine his legacy. One of the wisest remarks made by Carlo Ippolito Migliorini was the simple observation that ‘curiosity not ambition should be the mainspring of research’. It was a maxim to which he adhered throughout his distinguished scientific career. Migliorini is acknowledged as one of the most important figures in the pantheon of Italian geologists, noted for what we refer to as the Three Pillars of the Geological Wisdom. He was also influential in his role as head of exploration for the first 20 years of the Italian national oil company Agip. He was born in Bibbiena in the Arezzo Province (Tuscany) on 13 August 1891, the progeny of an Anglo-Italian union. His father Migliorotto Migliorini was a lawyer and his mother Elène Fowke was from a notable British family. Her uncle was Major General Sir George Henry Fowke. The young Migliorini apparently showed an early interest in geology, so much so that at the age of 15 he had enrolled as a member of the Italian Geological Society and was also a frequent visitor to the Geological Institute of Florence. On completing school studies in Florence, he went to Cornwall, England to attend the Camborne School of Mines where he graduated in 1912, at the age of 21, as a mining engineer. He immediately launched into a career of geological activity, first in Cornwall and later in Portugal, only for the war to interrupt. He served in the Italian army with some distinction being honoured with a bronze medal and a Distinguished Service Cross. As soon as the war was over he returned to his chosen career and was soon involved in a geological mission to Anatolia where he earned some recognition for his tectonic interpretations. In 1920 he took a post at the Italian Institute of Colonial Agriculture in Rhodes, Greece where he remained until 1934. It was during this period that he carried out some important studies in a number of the Dodecanese Islands mainly focused on quarrying and hydrology. Also during his stay in Rhodes, he met the love of his life. In 1926 he married Vera Sanine Ivanovna, a reportedly charming lady from a noble Russian émigré family who had escaped from the revolution in Russia. Among those in attendance at the engagement ceremony in Rhodes were Sir George Fowke along with Lord and Lady Mountbatten, who presented Migliorini’s fiancé with a beautiful ring.
-
-
-
The Global Offset seismic approach: advantages and limitations
More LessSeismic acquisition and processing in thrust belt areas represent challenging tasks in geophysics. Rough topography, geological complexity and sharp velocity variations increase the difficulties of recording seismic data of good quality and decrease the possibility of obtaining satisfactory imaging. In these circumstances, application of pre-stack depth migration (PSDM) can help in improving the imaging. However, the success of PSDM depends on the range of offset used and on the accuracy of the adopted background velocity model (Jin and Madariaga, 1994). When significant lateral velocity variations are predominant (Lynn and Claerbout, 1982), the derivation of an accurate velocity model by standard velocity analysis can fail. In these cases, accurate tomographic techniques can be extremely helpful, especially if applied to redundant data sets recorded in an extended range of offsets. In this framework, the advantages and difficulties of recording useful data at offsets larger than 15-20 km, or more, are well known from refraction seismic widely applied for studies of the middle-deep portion of the terrestrial crust. Super-critical reflections and long offset turning rays generally contain a lot of information about the elastic properties of the investigated medium. On the other hand, the risk of introducing artefacts when stacking and migrating wide angle reflections can be high, especially in the case of complex geological settings. Typical artefacts can be produced by lateral events introduced in the final migrated section, or by reflected energy migrated in the wrong position due to wrong velocity field definition. Other artefacts can be created when the wide angle reflections are not properly distinguished and separated from refracted energy. How to reduce these risks without renouncing the benefits offered by long offset data is still an open question. During the last decade many experiments have been performed using the Global Offset seismic approach (Buia et al., 2002; Colombo et al., 2004a; Colombo et al., 2004b; Colombo et al., 2003; Dell’Aversana, 2003), in order to verify the contribution offered by this methodology for improving the velocity field definition and seismic imaging. The Global Offset acquisition layout is set in order to guarantee the same shot and receiver spacing typical of the standard near-vertical reflection seismic, but for an extended range of offsets. Experiments with offsets larger than 30 km and shotreceiver density comparable with that of the industrial reflection seismic have been performed in the last few years (Dell’Aversana et al., 2001). In this sense Global Offset bridges the gap between commercial reflection seismic (commonly used for exploration in the hydrocarbon industry) and long offset refraction seismic (often applied for academic studies, focused mainly on the middle-deep portions of the terrestrial crust). Near-critical and post-critical reflections and long-offset turning rays are included in the Global Offset data set. The advantage is that the wide-angle reflections can show a high signal-to-noise ratio even in those cases where complex overburden can prevent to record good near-vertical reflection data. Moreover, using long offsets, it is possible to undershoot high velocity ‘shallow’ layers that can produce a shield effect for the propagating wave-field (Dell’Aversana et al. 2003). In general, massive application of transmission-reflection tomography is the first step aimed at producing a reliable velocity model (Dell’Aversana et al., 2003; Improta et al., 2000a, 2000b; Operto et al., 2004; Ravaut et al., 2004;). This can be successively used for improving the pre-stack depth migration process (Dell’Aversana et al, 2002). In this paper we show, with real examples, how including high-fold long offset data can really improve the seismic imaging and interpretation process. At the same time we also discuss the pitfalls that can be caused if long offset data are used indiscriminately without the necessary limitations. Finally, we explain how these problems can be controlled and avoided using a series of ‘consistency’ criteria.
-
-
-
Improved P-wave imaging with 3D OBS data from the Clair field
Authors J. H. Kommedal, S. Fowler and J. McGarrityMulti-component ocean bottom seismic (OBS) data acquisition is often motivated by the ability to record converted shear waves, but such acquisition also has the potential to produce Pwave imaging superior to conventional streamer data. This paper presents such a case, where a significant improvement in S/N ratio, in large part due to improved multiple removal, has enabled us to reduce the uncertainty in the structural imaging both for the reservoir section and the overburden, and resulted in changing the geological model. Background At the Clair field, located west of the Shetland Islands, fracture characterization is crucial for planning the field development. The initial investigation into the fracturing was done by Smith and McGarrity (2001), and concluded that seismic data with full azimuth range is needed for such characterization. Consequently three 2D OBS lines were acquired in 2000 as a feasibility study in preparation for 3D OBS acquisition. The 2D programme produced good quality P- and converted S-wave data, and confirmed favourable sea floor conditions. The processed P-wave data produced imaging which seemed to be an improvement over existing streamer data, and thus improved structural imaging could be added to the list of main objectives for the 3D survey. Data The 3D OBS data was acquired by Petroleum Geo-Services (PGS) during the summer of 2002. Four multi-component cables (one hydrophone and three orthogonal, gimballed geophones at each receiver station) of 6000 m were laid 355 m apart. Over each deployment we shot lines 245 m apart and orthogonal to the cables with 2.4 km maximum offset from the cables. Both receiver and shot interval was 25 m. Twenty deployments were used to cover the survey area. Note how the cable direction is normal to the long axis of the elongated outline of field, and that the survey only covers the central part of the field. This acquisition geometry will give very high fold and uniform azimuth – offset distribution for the common depth points (CDPs). Figure 2 shows the total CDP fold for all offsets up to 10 km and including all patches. The acquisition fold footprint which appears are mainly due to fold variations from offsets of 2.5 km and greater. For the lower offsets, the fold distribution is evenly distributed down to offset ranges of 200-400 m, where the effect of cable and shot line spacing is seen. For the processing, we used offsets up to 4.5 km, which means that the average fold is about 140.
-
Volumes & issues
-
Volume 42 (2024)
-
Volume 41 (2023)
-
Volume 40 (2022)
-
Volume 39 (2021)
-
Volume 38 (2020)
-
Volume 37 (2019)
-
Volume 36 (2018)
-
Volume 35 (2017)
-
Volume 34 (2016)
-
Volume 33 (2015)
-
Volume 32 (2014)
-
Volume 31 (2013)
-
Volume 30 (2012)
-
Volume 29 (2011)
-
Volume 28 (2010)
-
Volume 27 (2009)
-
Volume 26 (2008)
-
Volume 25 (2007)
-
Volume 24 (2006)
-
Volume 23 (2005)
-
Volume 22 (2004)
-
Volume 21 (2003)
-
Volume 20 (2002)
-
Volume 19 (2001)
-
Volume 18 (2000)
-
Volume 17 (1999)
-
Volume 16 (1998)
-
Volume 15 (1997)
-
Volume 14 (1996)
-
Volume 13 (1995)
-
Volume 12 (1994)
-
Volume 11 (1993)
-
Volume 10 (1992)
-
Volume 9 (1991)
-
Volume 8 (1990)
-
Volume 7 (1989)
-
Volume 6 (1988)
-
Volume 5 (1987)
-
Volume 4 (1986)
-
Volume 3 (1985)
-
Volume 2 (1984)
-
Volume 1 (1983)