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- Volume 13, Issue 3, 2007
Petroleum Geoscience - Volume 13, Issue 3, 2007
Volume 13, Issue 3, 2007
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Static characterizations of reservoirs: refining the concepts of connectivity and continuity
Authors Joseph M. Hovadik and David K. LarueStatic descriptive measures can be used to quantify characteristics of a 3D reservoir model. These static measures may have implications for the prediction or interpretation of dynamic performance and can draw attention to geological uncertainties that may impact flow behaviours. This study reviews, modifies and introduces techniques to characterize the spatial distribution of permeability in reservoir models, with emphasis placed on connectivity and continuity analysis. Topics include: the relationship between connectivity and percolation theory; definition of types of reservoir connectivity; methods of measuring connectivity; connectivity as a function of distance; connectivity maps; categorical classifications of connectivity; types of reservoir path lengths; and continuity lines. The key factors controlling reservoir connectivity are identified. Static measures can be used to locate regions of higher sweep efficiency and lower tortuosity that are connected to the wells.
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Exploring below the basalt, offshore Faroes: a case history of sub-basalt imaging
Authors Joseph W. Gallagher and Peter W. DromgooleDiscoveries in the Faroe–Shetland Basin to date have been made in areas not covered by extensive basalt flows as these flows seismically obscure the geology and structure of the underlying section. The industry has been tackling this problem for many years and it is known that high frequencies penetrate only a short way into the basalt before being scattered, generating high frequency noise. Recent seismic data in Faroes Licence 006 show a dramatic improvement in sub-basalt imaging in this problem area. The improvement comes primarily from processing the data, removing high frequencies (dominantly noise) early in the processing to concentrate on the low frequency data and using careful multiple removal at all stages of the processing. Velocity analysis must be performed as an iterative process and take into account the geological model. Deep towing the source and cable also improves the data by avoiding the higher frequencies that generally cause noise and thereby enhancing the lower frequency signal. Applying these processing techniques to earlier 2D surveys also shows significant improvement in imaging the sub-basalt structure. Data examples show that the sub-basalt structure can now be mapped on the improved data and sub-basalt prospects defined.
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Diagnosis of the Upper Cretaceous palaeokarst and turbidite systems from the Iranian Persian Gulf using volume-based multiple seismic attribute analysis and pattern recognition
Authors Pourdad Farzadi and Jonny HesthammerDepositional facies variability with a secondary overprint of Turonian palaeokarst controls the quality of the Cenomanian Mishrif reservoir, the main oil producer in the southeastern Persian Gulf. Subsequent drowning of the prominent area and successive deepening of the basin during the Coniacian and Santonian enabled the deposition of pelagic marls of the Coniacian Laffan Formation and the development of a carbonate turbidite system within the overlying Santonian Ilam Formation. The Ilam Formation occurs within slope and pelagic carbonates and consists of oil-bearing channel-reworked limestone facies.
Although high-quality 3D seismic data exist over the Sirri C/D oil fields studied in this paper, it is a real challenge to map the palaeokarst and turbidite deposits in 3D space using conventional seismic interpretation procedures.
This work describes a procedure using Paradigm's Seisfacies software for seismic facies classification and uses this to develop a volume-based interpretation of palaeokarst geobodies and sedimentary patterns of the carbonate turbidite. A hierarchical facies classification technique combined with principal component analysis (PCA) is used to analyse a set of seismic attribute volumes that capture the seismic stratigraphic patterns inherent in the data. PCA as a data reduction algorithm greatly optimized the analysis of the attribute volumes while preserving the essential features of seismic character. A hierarchical facies classifier recognized enough variability within the seismic data to reveal details of the associated geological features. This classification method uses multiple 3D volume attributes as input and generates a single 3D seismic facies volume (a synthesis of different attributes). Using this method, interpretative work can focus directly on geological features in 3D space. This study gives new insights into the internal variability of palaeokarst and carbonate turbidite systems in the Sirri C/D oil fields (SE Persian Gulf) that are essential for the estimation of reservoir volume, connectivity and variability.
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Geological modelling and finite difference forward realization of a regional section from the Zagros fold-and-thrust belt
Authors Behzad Alaei and Steen A. PetersenThis paper provides detailed geological modelling and finite difference realization results of a 2D regional section from the Zagros fold-and-thrust belt, Iran. Different scale geological detail has been included in the model. The modelling approach is based on a hierarchical procedure and the model is made up of cells representing the geometry and properties (such as seismic P-wave velocity) of parts of the model. By using a hierarchical procedure for building the model, the time and space consistency of the geological model is preserved. The constructed model is 81 × 17 km and crosses several drilled and undrilled structures. The model geometry is controlled by surface, subsurface and seismic interpretation results along the model. The model building blocks were chosen based on the deformation history of the area and the modelling was carried out in two stages. First, a regional model was constructed including the major structural elements. At the second stage (fine tuning of the regional model), structural and stratigraphic details, such as onlap, truncations, pinch-outs and thickening/thinning of salt beds, were added to the regional model. The velocity and density models were extracted from well logs, check shots and seismic processing-derived velocity data. A few shots were generated using an acoustic finite difference method to see the effect of the inclusion of detailed internal geometry in the model and the seismic wave complexity around three main anticlines of the model. The seismic response shows that the detailed modelling of stratigraphy and structure gives results that simulate the real data better. It shows the existence of complex diffracted and non-hyperbolic events, suggesting the poor image quality of real data is not related entirely to acquisition problems caused by irregular topography. Advanced depth-imaging methods (e.g. reflection tomography and wave-equation-based extrapolation or migration algorithms) are required to get better images. The sharp lateral velocity changes associated with major thrust faults cause the generation of diffractions that can compromise reliable velocity analysis, especially at conventional offsets. This effect could be considered as one of the causes of the poor seismic response observed on real data. The use of larger offsets, therefore, can distinguish between diffractions and primary reflections during velocity analysis in time and depth.
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The importance of constraining regional exhumation in basin modelling: a hydrocarbon maturation history of the Ghadames Basin, North Africa
Authors R. Underdown and J. RedfernEstimation of eroded overburden at unconformities is essential to accurately constrain burial histories and predict the timing of hydrocarbon maturation. In the Ghadames Basin, three independent techniques, palaeo-isopach construction, sonic velocity and vitrinite reflectance analysis, were employed. The resultant basin model suggests that only the two most significant unconformities, the Hercynian (Late Carboniferous) and Alpine (early Eocene), have a major control on timing of hydrocarbon charge.
Modelling indicates only one period of generation from the Lower Silurian shales on the western margin of the basin, with 95% hydrocarbon generation prior to Hercynian exhumation. The central basin and southern margin experienced maximum burial during Eocene time. The Middle–Upper Devonian mudstones are the main source rocks and they did not generate significant volumes of hydrocarbons over the basin centre until Cretaceous time; they are currently within the peak oil generation zone.
In Libya, on the eastern/northeastern flank of the basin, results indicate Cenozoic maximum burial, followed by up to 2000 m of Alpine exhumation. The magnitude of this exhumation has not been recognized previously, although it is a critical component of the basin model as it has a major impact on potential hydrocarbon charge in this area. Maturation models predict that the Lower Silurian source underwent two generative phases: (1) pre-Hercynian (Carboniferous) generation; and, significantly, (2) post-Hercynian (Late Jurassic–Cenozoic) generation. The identification of late hydrocarbon generation offers potential for oil and gas to migrate into post-Hercynian traps. Over the western, northern and eastern flanks, the Devonian source rocks remain immature/ marginally mature at present day.
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Assessment of the petroleum generation potential of Lower Carboniferous coals, North Sea: evidence for inherently gas-pronesource rocks
Authors Henrik I. Petersen and Hans P. NytoftThe petroleum generation potential of Lower Carboniferous coastal plain coals in the Danish Gert-2 and Norwegian 2/11-9 wells in the Central Graben, North Sea, has been assessed in order to determine the source-rock quality of the coals in connection with deeper plays. The coals are vitrinite-rich, have H/C ratios from 0.75–0.79, and hydrogen index (HI) values from 171–219, which are very close to the HImax values attained during thermal maturation. The thermal maturity of the Gert-2 coals is close to the start of the effective oil window (oil expulsion), c. 0.90–0.95% vitrinite reflectance. The proportion of aliphatic chains >C18 in the coals is negligible or very low: the C19–35 aliphatic chains constitute only c. 6% of the total amount of C12–35 aliphatic chains in the coal structure. Aliphatic moieties are characterized by shorter-chain aliphatics and isolated CH2. The generated liquid petroleum is trapped in the coals and the saturated hydrocarbon fraction is dominated by shorter-chain aliphatics. Upon solvent extraction, the measured HI values of the Gert-2 coals can, on average, be reduced by 30%, indicating that the trapped petroleum contributes to the measured generation potential. The Lower Carboniferous coals are thus inherently gas-prone, a feature they have in common with Carboniferous coals in general and which is not maturity related. Inefficient expulsion of the saturated hydrocarbons may enhance the gas-proneness by intra-source rock oil-to-gas cracking. In a deep exploration context in the North Sea area, Lower Carboniferous coal/coaly sections can therefore only constitute a gas source rock.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)