Three‐component (P, SH, and SV) expanding spread profiles (ESP), common‐midpoint profiles, and sparse three‐dimensional P wave data were collected over an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), in southeastern Georgia. Shear wave reflections from the SBS at 10.9 s (16 km depth), and possibly from the lower crust at 18.3 s (29 km depth), were recorded but required substantial source effort (stacking) and were too weak for reliable reflectivity measurements. Reflections on the ESPs delineate a 1.5‐km‐thick Atlantic Coastal Plain section whose seismic properties (Vp=2.53 km/s, Vs=1.51 km/s, Vp/Vs= 1.67) are consistent with quartz‐rich sandstones and siltstones, sitting atop a 15‐km‐thick upper crust (Vp=6.38 km/s, Vs=3.25 km/s, Vp/Vs=1.96), which in turn overlies a 15 km‐thick lower crust of slower material (Vp=6.02 km/s, Vs≈3.26 km/s, Vp/Vs≈1.84). The velocity inversion may result from underthrusting of upper crustal rocks during suturing of Florida to North America. Amplitude‐versus‐offset analyses, combined with an earlier reflection polarity test and waveform modeling, indicate that the SBS originates from a thin (∼80–120 m), high‐impedance layer, most likely a mafic dike or tectonically emplaced ultramafic body.
253 255 Index 271 Downloaded 06/25/14 to 134.153.184.170. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/ COEDITORS FOREWORD Publication of a monograph composed of papers on seismic shear waves and their application to petroleum exploration was proposed by W. E. Laing in an October 19, 1983 letter to $. H. Ward, then the SEG Editor. Mr. Laing's stated objective for the mom)graph was to develop interest in shear-wave exploration by exposing geologists and geophysicists engaged in seismic data interpretation to its potential for determining lithology and discovering stratigraphic traps. Following SEG Executive Committee approval, the assigned coeditors solicited submittals from authors of pertinent papers presented at a Shear-wave Exploration Symposium in Midland, Texas on March 31 and April 1, 1984 and at the 1984 S EG Annual Meeting in Atlanta, Georgia, December 2-6. Most presentations at the symposium were expanded versions of those given at the 1983 SEG Annual Meeting in Las Vegas, Nevada, September 11-15. From 17 papers solicited, 14 are in the monograph. Of the included papers, eight were presented at the Shearwave Exploration Symposium and six were presented at the 1984 SEG Annual Meeting. Two additional papers were prepared especially for the monograph: an introductory paper by the coeditors and a comprehensive classified bibliography of papers concerning shear-wave exploration seismology compiled by R. A. Ensley. This monograph is the first volume under Geophysical Developments series, one of seven SEG special publication series. The seven series, proposed by the SEG Publications Committee, were accepted in principle by the SEG Executive Committee in February, 1986. Volumes in the Geophysical Developments series summarize recent advances in specific areas of geophysical exploration. They are intended to provide the status of technology in developing exploration methods that are of immediate interest to geophysicists.The coeditors express their gratitude to the authors for preparing and submitting papers and for their patience and diligence in making revisions resulting from manuscript reviews. In addition, the coeditors are grateful for the support and assistance provided by the Publication Staff at the Tulsa SEG Headquarters. In particular, we wish to acknowledge the substantial effort by Lynn Griffin, ABSTRACT Applicati{)n ()1' shear (S)-waves in seismic pe-tr{)lcum expl{)rati{)n is in a critical stage of devcl(•pment. Pr()pagati•)n ()f these waves and of the hist()rically applied c()mpressional (P)-waves in a sedimentary secti(•n are affected differently by r(•ck physical pr(•perties. Principally, propagati()n vcl(>city and, in turn, reflection amplitude ()t' P-waves is affected by b{)th rock incompressibility and rigidity, whereas, that of S-waves isat'l'ccted by r(>ck rigidity only. Because of this difference it is p()ssible, for example, to verify P-wave reflection amplitude variation due to pore fluid change (e.g., brine to a gas-brine mixture), that a...
Fifty fields in the deepwater Gulf of Mexico (GOM) have been studied to determine the relationship between clastic lithologies, saturating fluids, and seismic response with source-receiver offset (AVO). The fields range from East Breaks / Alaminos Canyon to Viosca Knoll protraction areas. The goal is to predict theoretically and demonstrate with observed data, seismic attributes that are most diagnostic of reservoir sands and hydrocarbon saturation in diverse deepwater locations and at various depths. The findings of this paper will illustrate the general conclusions for GOM based upon over 3000 km of reprocessed long-offset seismic data. Detailed modeling of key wells has been carried out as well as petrophysical analysis of each well. The stacked and seismic AVO gathers are tied to synthetic data based upon the well logs recorded in each well. A general conclusion is that almost every field has Class 3 AVO anomalies associated with hydrocarbon reservoirs. These Class 3 reservoirs are often less productive than thicker pays that exhibit Class 2 AVO anomalies, which have low reflectivity on the near-offsets. Reservoirs with Class 2 AVO response often do not have anomalous amplitudes on conventional stacked data. Detailed analysis of AVO seismic attributes provides a more reliable picture of the reservoir quality, geometry, degree of saturation, and extent. In addition to the project wells in each field, dry holes drilled nearby on seismic amplitude anomalies are studied to determine which seismic attributes are indicative of brine saturation versus pay in that area. Modeled and observed examples of this are presented illustrating which attributes are most diagnostic. The role of offset data in identifying sands pinching out against salt is illustrated. Examples of gas-oil and oil-brine contacts in reservoirs are analyzed. The use of multiple attributes such as NI × PR (product), AVO crossplot, and gradient of amplitude envelope seem to be the most diagnostic in the general situation. Several analogs from all areas studied will be discussed. Improved seismic AVO attributes and calibration utilizing existing and yet to be drilled wells will provide improved reduction of risk as the deepwater GOM exploration becomes a more mature play. This study makes a contribution to aid explorationists in achieving this goal. 1.0 Background The objective of this project was to use both seismic and well data in the AVO / AVA domain to calibrate seismic data to well synthetic models. On a field-by-field basis, the study sought to learn how seismic attributes of reprocessed 2-D seismic data can be used to predict lithology and fluid properties of reservoirs. The seismic data used in the project were completely reprocessed from field tapes to provide a consistent set of data in which amplitude information was preserved. A detailed description of the acquisition and processing flow is provided in a companion paper (Verm et al., 2002). This study comprises 50 fields as shown on the map in Figure 1.1.
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