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Results
ABSTRACT Borehole geophysical logging was conducted in several holes at the Barber-Larder gold prospect, Larder Lake, Ontario, to map the stratigraphy, structures and alteration zones associated with gold mineralization. Natural gamma ray spectral logging, induced polarization (IP), resistivity, self potential (SP) and magnetic susceptibility logging were done. Gold mineralization at the Barber-Larder prospect occurs within altered basic volcanic flows, tuffs and agglomerates which are overlain by metasediments (greywacke) and underlain by fuchsite bearing carbonate breccia. Alteration in the area includes pyritization, silicification, carbonatization and sericitization. Gold mineralization is mainly associated with pyritization. There is, however, a genetic as well as spatial relationship with the other types of alteration processes. A major feature within the area associated with the gold-bearing horizon is an extensive graphitic zone. The stratigraphic units are defined by the natural gamma ray spectrometry, IP, SP and magnetic susceptibility methods. The greywacke exhibits high radioactivity, low IP response, low susceptibility and no SP anomalies. The overall radioactivity within the volcanic rocks is lower than that observed in the greywacke. The graphitic zone can be easily delineated as a low resistivity, high IP zone at the contact between the greywacke and the volcanic rocks. The graphitic zone also exhibits a high negative SP anomaly. Within the altered volcanic flows, tuffs and agglomerates, the sericitized zones are characterized by high natural gamma ray activity due to an increase in potassium. Pyritized zones are characterized by high IP response and zones with more than 10 percent pyrite have a corresponding low resistivity response. The silicified and carbonatized zones are characterized by low gamma ray activity, low susceptibility and in some cases high resistivities. Generally the magnetic susceptibilities observed were fairly low. The basic volcanic flows showed higher susceptibilities than the greywacke. The susceptibility lows within the volcanic flows reflect carbonatized and pyritized zones.
- Geology > Mineral > Native Element Mineral > Gold (1.00)
- Geology > Geological Subdiscipline > Volcanology (1.00)
- Materials > Metals & Mining > Gold (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.95)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (0.91)
ABSTRACT In traditional drilling practice, the real time overpressure indicators are generally of very short notice, especially in complex growth fault systems, which is the case in the Niger delta. The conventional prediction from regional correlation and surface seismic interpretation has some limitation which are difficult to overcome in case of dipping horizons, as well as in complicated fault systems. The VSP iterative modeling inversion gives a more accurate predictive profile of acoustic impedance even when recorded at several hundred meters above the overpressured zones. The results are valid even for non-horizontal pressure fronts. This inversion procedure is independent of the low frequency content in the seismic signal, contrarily to other inversion methods based on integration. It gives the change of compaction trend (which is indicative of overpressure) as well as some other details (e. g. sand shale sequences). A number of cases (three in all) in various geological conditions are presented. Results in every case are confirmed by drilling, within less than a hundred meters. The method is valid assuming a continuity in the trend of acoustic velocities in the well, down to the top of overpressure.
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Near-well and vertical seismic profiles (1.00)
ABSTRACT The application of crosswell acoustic measurements to gas sands research has been explored through surveys conducted in the Mesa Verde formation at the Department of Energy Multi-Well Experiment (MWX) site near Rifle, Colorado. The borehole tools used in the survey are similar in concept to those used in commercial service for sonic logging, but they are especially adapted for the stringent requirements of crosswell shooting in hot gas wells. Important information about the geologic structure between wells can be extracted from crosswell scans without resorting to elaborate processing. A useful representation is a display of the travel time of P-waves in terms of the cylindrical coordinates of the transmitter referenced to the receiver. This is known as a gamma-depth (Y-Z) plot. Such a representation may yield distinctive patterns, which can be interpreted based on the successful replication of the pattern through computer simulations. The apparent seismic Q of P-waves transmitted through the sands at the MWX site is derived using two methods. The first applies to crosswell surveys in which signals can be acquired over a significant range of source-receiver distances. A Q of 15 between well pair MWX l/2 is derived in this manner. The second method makes use of signals transmitted between wells in a three-well complex and provides an estimate of seismic Q for the rocks bounded by each well pair. Q estimates derived from this technique are 18, 30, and 28 for well bores MWX-l/2, MWX-2/3 and MWX-3/l, respectively. Channel waves propagate through the MWX coals. Evidence suggests that tube waves launched in the transmitter well give rise, under appropriate conditions, to channel waves, which in turn excite tube waves in nearby wells that penetrate the same channel. Although the sequence of conversions is weak, the resulting waveforms are coherent enough to resolve the channel waves through stacking.
- North America > United States > Colorado (1.00)
- North America > United States > New Mexico (0.88)
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (1.00)
- North America > United States > New Mexico > San Juan Basin > Mesaverde Formation (0.99)
- North America > United States > Colorado > San Juan Basin > Mesaverde Formation (0.99)
- North America > United States > Colorado > Piceance Basin > Williams Fork Formation (0.99)
- (3 more...)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
ABSTRACT Cenozoic marine sediments were drilled and cored by the Deep Sea Drilling Project at Site 613 in the Baltimore Canyon Trough, and four channel sonic waveforms were recorded downhole from 125.4 to 581.7 meters below sea floor. Compressional wave velocities and spectra were calculated from the sonic waveforms. Compressional velocity increased as porosity decreased with depth. However, the energy loss between near and far receivers increased and the peak crosspower frequency decreased with depth, suggesting that attenuation increases over the same interval. The observed changes in energy and frequency cannot be explained solely by changes in the elastic properties of the sediments. We calculate the compressional wave Q at 10-m depth intervals using the spectral ratio technique. Mean compressional wave Q decreases from about 69 above to 28 below a diagenetic boundary at 442.0 m. Attenuation estimates at the peak frequency are lower than the spectral ratio values, but clearly increase below the diagenetic boundary. Sonic wave attenuation can be explained by a surface stress relaxation mechanism in thin fluid-filled pores. A decrease in pore aspect ratio and an increase in pore surface area due to diagenetic effects may explain the observed increase in attenuation with depth at Site 613. These diagenetic effects are not unique to conditions at this site.
- Geology > Sedimentary Geology > Depositional Environment > Marine Environment (0.72)
- Geology > Geological Subdiscipline > Geomechanics (0.48)
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Sedimentology (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)