Using core properties and seismic reflectivity to estimate pore pressure in an active decollement fault

Author(s): Tobin, Harold J.; Moore, J. Casey
Author Affiliation(s): Primary:
Stanford University, Department of Geophysics, Stanford, CA, United States
University of California at Santa Cruz, Santa Cruz, CA, United States
Volume Title: American Association of Petroleum Geologists 1996 annual convention
Source: Annual Meeting Expanded Abstracts - American Association of Petroleum Geologists, Vol.5, p.141; American Association of Petroleum Geologists 1996 annual convention, San Diego, CA, May 19-22, 1996. Publisher: American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States. ISSN: 0094-0038 CODEN: APGAB2
Note: In English
Summary: In the decollement zone of the Barbados accretionary prism, a 3-D seismic image exhibits patchy high-amplitude negative polarity reflections, which have been attributed to large overpressures confined to the fault zone. We collected laboratory P-wave velocity and porosity vs. pore pressure data, using core samples from and adjacent to the decollement zone at ODP Site 948. Logs constrain density and velocity through the decollement zone at Site 948. We use these data to calibrate the reflectivity of the fault zone to pore pressure through waveform and amplitude models of the fault plane reflections. Modeling of the positive polarity Site 948 reflection indicates that it can be explained by a lithologic boundary coincident with the decollement, without anomalous fault properties. By contrast, the dominantly-negative polarity waveform of the reflection approx. 2 km arcward (beneath Site 947) is best modeled by inserting a 16-19 m thick zone of extremely low impedance into the Site 948 impedance structure, with a gradational return to "normal" impedance just above the positive boundary. Relative amplitudes in this reflection indicate a larger impedance contrast than can be accounted for at sub-lithostatic fluid pressure, based on the core properties data. We conclude that lithostatic pore pressure with attendant hydraulic dilation of the fault zone is required to generate the negative-polarity reflections. Mapping of these reflections thus delineates zones of elevated fluid content and zero effective stress in the fault zone.
Year of Publication: 1996
Research Program: ODP Ocean Drilling Program
Key Words: 16 Structural Geology; Accretionary wedges; Active faults; Amplitude; Antilles; Barbados; Body waves; Calibration; Caribbean region; Core; Cores; Decollement; Elastic waves; Fault zones; Faults; Geophysical methods; Geophysical surveys; Impedance; Leg 156; Lesser Antilles; Models; ODP Site 948; Ocean Drilling Program; Overpressure; P-waves; Pore pressure; Porosity; Pressure; Properties; Reflection methods; Seismic methods; Seismic waves; Surveys; Three-dimensional models; Velocity; Waveforms; Well logs; West Indies
Coordinates: N153128 N153134 W0584354 W0584356
Record ID: 1997017792
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by American Association of Petroleum Geologists, Tulsa, OK, United States