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Crustal stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms
380A
IN SITU S T R E S S : C A U S E S
958167 Site-response models from high-resolution seismic reflection and refraction data recorded in Santa Cruz, California R. A. Williams, E. Cranswick, K. W. King, D. L. Carver & D. M. Worley, US Geological Survey Professional Paper, 1551-A, 1994, pp 217-242. The study compares earthquake site-response estimates derived from digital recordings of aftershocks recorded in Santa Cruz, with synthetic site-response estimates calculated from models of the sites' seismic-impedance structure. The impedance structure of each site is determined, first, from interpretations of high-resolution P-wave seismic-reflection and P- and S-wave seismic refraction data, and, second, from published data on the regional geology and geophysics. This study is motivated by an assumption that the impedance structure of a given site, and its seismic response, can be estimated from geophysical and geologic data other than direct recordings of weak or strong ground motion. The results suggest only limited success in predicting the seismic response and point out the critical importance of highly accurate seismic-velocity data. (from Authors) 958168 Source time functions of nuclear explosions and earthquakes in central Asia determined using empirical Green's functions Yingping Li, M. N. Toksoz & W. Rodi, Journal of Geophysical Research, 100(B1), 1995, pp 659-674. Relative source time functions (RSTF) have been estimated for four underground nuclear explosions and seven earthquakes in Central Asia using broadband P waveforms of nearby smaller events as empirical Green's functions (EGF). The study demonstrates the power of the EGF method for retrieving RSTFs and reveals that differences in RSTFs and source parmaters can be used to distinguish large nuclear explosions from moderate to large earthquakes (rob > 5.5). (from Authors)
Identification and classification 958169 Rock mass rating systems: to use or not to use? P. J. Tarkoy, Tunnels & Tunnelling, 27(5), 1995, pp 38-40. Rock mass ratings systems were developed as design aids for conventionally excavated drill and blast tunnels and underground openings. However, their use on TBM-bored tunnels gives ambiguous results in the assessment of the support need for these tunnels. The use of rock mass rating systems is discussed with reference to case histories. Recommendations for the use of rock mass rating systems are outlined. (I'.M.Taylor) 958170 Approaches to predicting reservoir quality in sandstones S. Bloch & K. P. Helmold, American Association o f Petroleum Geologists Bulletin, 79(1), 1995, pp 97-115. Despite limited understanding of the details of many diagenetic processes, empirical techniques can be used effectively to predict reservoir quality prior to drilling. The predictive approach depends mainly on the availability of empirical data in the area of interest. In frontier basins, mean or maximum porosity of a potential sandstone reservoir can be estimated for a given compositon and level of thermal exposure (or burial depth). The reliability of the estimates is constrained by input data. Approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area. (from Authors)
& NATURE
IN SITU STRESS
Causes and nature of in situ stress 958171 The influence of natural fractures, faults and earth stresses on reservoir performance - geomechanlcal analysis by numerical modelling K. J. Heifer, N. C. Last, N. C. Koutsabelouiis, H. C. M. Chan, M. Gutierrez & A. Makurat, Publikasjon - Norges Geotekniske Institutt, 192, 1994, 13 pp; reprinted from: Proc. North Sea oil and gas reservoirs - III, (Kluwer, for Norwegian Institute of Technology), 1994, pp 201-211. Many aspects of the development of hydrocarbon fields are influenced by earth stresses, for which it is only possible to obtain sparse field measurements. This paper describes a preliminary programme to investigate the utility of numerical modelling of stress variations within a reservoir. This includes the spatial variations of pre-development in-situ stresses which occur across a reservoir caused by the fault structure; and also the temporal variations caused by commonplace processes of field development. Prediction of spatial variations is illustrated by reference to a case study of a field on the UK Continental Shelf in which conductive natural fractures are important to well productivity, and in which a horizontal well was planned. (from Authors) 958172 Crustal stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms C. D. Lindholm & 6 others, Terra Nova, 7(1), 1995, pp 5159. The regional stress field in the northern North Sea has been studied through the acquisition and analysis of directions of maximum horizontal compression (~r0 as extracted from borehole breakouts and from earthquake focal mechanism solutions. The results indicate that the regional stress field is dominated by NW-SE compression. The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin. The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. Possible sources for this anomaly are discussed, and include postgiacial uplift and/or lateral variations in the physical properties of the crust. (from Authors) 958173 Quantitative models of very high fluid pressure: the possible role of lateral stresses O. Bour, I. Lerche & D. Grauls, Terra Nova, 7(1), 1995, pp 68-79. Because of the occurrence of very large observed overpressure in a well located in SE Asia, numerical modelling has been undertaken to evaluate evolution of the sediments. It is suggested that the tectonic regime is related to the generation of high excess pressure. To verify this hypothesis, a 2D fluidflow/compaction model (GEOPETII) was used to test whether high excess pressure could be obtained without considering the effect of lateral stresses. The migration of fluids cannot be reproduced because of the extremely low permeability of the sediments. Accordingly, the model has been modified to allow for the effects of lateral stresses. Finally, the role of faulting or fracturing is discussed as a possible mechanism responsible for the upward transfer of fluid which can lead to an increase of excess pressure at depths shallower than the depth where excess pressure was firstgenerated. (from Authors)
IN SITU S T R E S S : C A U S E S
958167 Site-response models from high-resolution seismic reflection and refraction data recorded in Santa Cruz, California R. A. Williams, E. Cranswick, K. W. King, D. L. Carver & D. M. Worley, US Geological Survey Professional Paper, 1551-A, 1994, pp 217-242. The study compares earthquake site-response estimates derived from digital recordings of aftershocks recorded in Santa Cruz, with synthetic site-response estimates calculated from models of the sites' seismic-impedance structure. The impedance structure of each site is determined, first, from interpretations of high-resolution P-wave seismic-reflection and P- and S-wave seismic refraction data, and, second, from published data on the regional geology and geophysics. This study is motivated by an assumption that the impedance structure of a given site, and its seismic response, can be estimated from geophysical and geologic data other than direct recordings of weak or strong ground motion. The results suggest only limited success in predicting the seismic response and point out the critical importance of highly accurate seismic-velocity data. (from Authors) 958168 Source time functions of nuclear explosions and earthquakes in central Asia determined using empirical Green's functions Yingping Li, M. N. Toksoz & W. Rodi, Journal of Geophysical Research, 100(B1), 1995, pp 659-674. Relative source time functions (RSTF) have been estimated for four underground nuclear explosions and seven earthquakes in Central Asia using broadband P waveforms of nearby smaller events as empirical Green's functions (EGF). The study demonstrates the power of the EGF method for retrieving RSTFs and reveals that differences in RSTFs and source parmaters can be used to distinguish large nuclear explosions from moderate to large earthquakes (rob > 5.5). (from Authors)
Identification and classification 958169 Rock mass rating systems: to use or not to use? P. J. Tarkoy, Tunnels & Tunnelling, 27(5), 1995, pp 38-40. Rock mass ratings systems were developed as design aids for conventionally excavated drill and blast tunnels and underground openings. However, their use on TBM-bored tunnels gives ambiguous results in the assessment of the support need for these tunnels. The use of rock mass rating systems is discussed with reference to case histories. Recommendations for the use of rock mass rating systems are outlined. (I'.M.Taylor) 958170 Approaches to predicting reservoir quality in sandstones S. Bloch & K. P. Helmold, American Association o f Petroleum Geologists Bulletin, 79(1), 1995, pp 97-115. Despite limited understanding of the details of many diagenetic processes, empirical techniques can be used effectively to predict reservoir quality prior to drilling. The predictive approach depends mainly on the availability of empirical data in the area of interest. In frontier basins, mean or maximum porosity of a potential sandstone reservoir can be estimated for a given compositon and level of thermal exposure (or burial depth). The reliability of the estimates is constrained by input data. Approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area. (from Authors)
& NATURE
IN SITU STRESS
Causes and nature of in situ stress 958171 The influence of natural fractures, faults and earth stresses on reservoir performance - geomechanlcal analysis by numerical modelling K. J. Heifer, N. C. Last, N. C. Koutsabelouiis, H. C. M. Chan, M. Gutierrez & A. Makurat, Publikasjon - Norges Geotekniske Institutt, 192, 1994, 13 pp; reprinted from: Proc. North Sea oil and gas reservoirs - III, (Kluwer, for Norwegian Institute of Technology), 1994, pp 201-211. Many aspects of the development of hydrocarbon fields are influenced by earth stresses, for which it is only possible to obtain sparse field measurements. This paper describes a preliminary programme to investigate the utility of numerical modelling of stress variations within a reservoir. This includes the spatial variations of pre-development in-situ stresses which occur across a reservoir caused by the fault structure; and also the temporal variations caused by commonplace processes of field development. Prediction of spatial variations is illustrated by reference to a case study of a field on the UK Continental Shelf in which conductive natural fractures are important to well productivity, and in which a horizontal well was planned. (from Authors) 958172 Crustal stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms C. D. Lindholm & 6 others, Terra Nova, 7(1), 1995, pp 5159. The regional stress field in the northern North Sea has been studied through the acquisition and analysis of directions of maximum horizontal compression (~r0 as extracted from borehole breakouts and from earthquake focal mechanism solutions. The results indicate that the regional stress field is dominated by NW-SE compression. The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin. The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. Possible sources for this anomaly are discussed, and include postgiacial uplift and/or lateral variations in the physical properties of the crust. (from Authors) 958173 Quantitative models of very high fluid pressure: the possible role of lateral stresses O. Bour, I. Lerche & D. Grauls, Terra Nova, 7(1), 1995, pp 68-79. Because of the occurrence of very large observed overpressure in a well located in SE Asia, numerical modelling has been undertaken to evaluate evolution of the sediments. It is suggested that the tectonic regime is related to the generation of high excess pressure. To verify this hypothesis, a 2D fluidflow/compaction model (GEOPETII) was used to test whether high excess pressure could be obtained without considering the effect of lateral stresses. The migration of fluids cannot be reproduced because of the extremely low permeability of the sediments. Accordingly, the model has been modified to allow for the effects of lateral stresses. Finally, the role of faulting or fracturing is discussed as a possible mechanism responsible for the upward transfer of fluid which can lead to an increase of excess pressure at depths shallower than the depth where excess pressure was firstgenerated. (from Authors)