Warren F. Agena

Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles

Interpretation of Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) seismic reflection profiles indicates that the Midcontinent (Keweenawan,1100 Ma) rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km (about 10.5-s reflection time) along profiles crossing western, central, and eastern Lake Superior and the northern end of Lake Michigan. This area may overlie the greatest thickness of intracratonic rift deposits on Earth. Times to Moho reflections vary along strike from 11.5 to 14 s (about 37-46 km depth) in the west, to 17 s (about 55 km) in the center, and 13 to 15 s (about 42-49 km) in the eastern end of Lake Superior. The prerift crust, however, was thinned 25-30 km beneath the central rift (compared with its flanks), providing evidence for crustal extension by factors of about 3-4. The Midcontinent rift system differs from Phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding (presumably unextended) regions.

Pre- and poststack migration of GLIMPCE reflection data

Abstract GLIMPCE deep Seismic reflection profiles across the Midcontinent Rift System beneath Lake Superior reveal a central asymmetric rift with an enormous thickness of volcanic and sedimentary rocks. True amplitude cmp-processing, poststack and prestack migration and forward modelling are used to improve images of steeply dipping faults, unconformities and other discontinuities in the deep Seismic data. With prestack migration important steeply dipping structural features of the Lake Superior data set are revealed for the first time. Improved images of high-angle normal faults, later reactivated as reverse faults, provide key structural information for understanding the evolution of the rift system. Results illustrate that structural interpretations of complex deep reflection records, such as those recorded by GLIMPCE, should always be based on migrated data. Furthermore, depth-migrated sections provide useful starting models for forward velocity modelling of Seismic data.

Amplitude blanking in seismic profiles from Lake Baikal

Abstract Imaging of the deepest sedimentary section in Lake Baikal using multichannel seismic profiling was hampered by amplitude blanking that is regionally extensive, is associated with water depths greater than about 900 m and occurs at sub-bottom depths of 1–2 km in association with the first water-bottom multiple. Application of a powerful multiple suppression technique improved the quality of occasional discontinuous, dipping primary reflections, but failed to substantially alter the non-reflective character of the blanking zone. Detailed analysis of amplitudes from original data and synthetic models show that the threshold for detecting primary energy in deep water of Lake Baikal occurs when the primary is about 14–20 dB less than the multiple energy. The blanking occurs because of anomalously low reflectivities of the deep sediments coupled with this 20 dB limitation in real data processing. The blanking cuts across seismic stratal boundaries, and is therefore probably unrelated to depositional lithologies. The deepest, early rift deposits, inferred to come from a mixed fluvial and lacustrine setting, do not easily explain the widespread and uniform character of the blanked deposits. More likely, blanking occurs because of processes or phenomena that physically alter the deposits, causing them to be non-reflective and/or highly attenuating. No single process explains all the observations, but a combination of diagenesis, overpressure, and the presence of dispersed free gas at sub-bottom depths of 1–2 km, offer plausible and possible conditions that contribute to blanking.

Successful gas hydrate prospecting using 3D seismic - A case study for the Mt. Elbert prospect, Milne Point, North Slope Alaska

Statistical analysis of the data indicates that the gas hydrate prospects in the Milne Point area, North Slope Alaska, may hold about 668.2 billion cubic feet (BCF) of gas. Detailed analysis and interpretation of 3-D seismic data, along with seismic modeling and correlation with specially-processed well log data, has led to the development of a viable method for identifying subpermafrost gas hydrate prospects within the gas hydrate stability zone (GHSZ). The Mt. Elbert prospect was shown to have an estimated 93 BCF gas-in-place prior to drilling the Mt. Elbert test well. Within the Mt. Elbert prospect, two regional reservoir-quality sands, identified as zones “C” and “D,” were identified within the zone of interest between the base of ice-bearing permafrost and the base of the gas hydrate stability zone. The Mt. Elbert prospect was selected for a vertical hole, stratigraphic test to confirm the validity of the seismic interpretation and modeling, and was chosen because it was the best-defined prospect identified in the study. Additionally, the Mt. Elbert prospect is unique due to its large areal extent, structural compartmentalization, potential for multiple pay zones, and its proximity to existing infrastructure. The test well was drilled in February of 2007 and penetrated thick “C” and “D” Unit gas hydrate reservoirs, as predicted in this study.