Frequencies are fault finding factors

Looking Low Aids Data Interpretation

Numerous examples have circulated among the geophysical community that illustrate how some geologic targets can be better seen by constraining the reflected seismic wavefield to a particular narrow range of frequencies.

The exact frequency range that produces an optimal image of a target varies, depending on target size, depth, thickness and impedance properties.

The data discussed here document an example where frequency-constrained seismic data provided improved images of deep fault systems.

These seismic data come from a 3-D seismic survey acquired in West Texas. The principal objective was to image deep gas reservoirs at depths of approximately 20,000 feet (6,000 meters).

The seismic grid traversed an area where the exposed surface layer had large variations in impedance and thickness caused by the dissolution of exposed salt and anhydrite and the infill of younger, unconsolidated sediment.

This variable-velocity surface layer made static corrections of the seismic data difficult. Because of this static-calculation issue and the great depth of the targets, seismic data quality was not as good as desired for reservoir characterization and drill site selection.

Across the study area, the deep reservoir interval was traversed by numerous faults, making accurate fault mapping one of the keys to exploiting the reservoir system.

One example of a seismic profile crossing a key structural feature is shown as figure 1:

  • The display on panel (a) shows the image that was created by attempting to preserve the maximum frequency bandwidth of the data.
  • The display on panel (b) shows the data after the post-migration image was filtered to preserve only the first octave of the illuminating wavefield (8 to 16 Hz).
  • Panel (c) is added to show several of the faults (not all faults) that can be interpreted from the frequency-constrained data (panel b) and that are more difficult to recognize on the broad-frequency image (panel a).

In all profiles that traversed the study area, it was found that deep faults were consistently better defined by data that were frequency constrained to emphasize only the low-frequency response.

To illustrate this point, a second profile across the geologic target is shown on figure 2, using the same sequence of data panels used in figure 1.

Again, the low-frequency image is a better depiction of the deep faulting pattern.

What is the message?

If you are confronted by the problem of interpreting faults in limited-quality seismic data, try viewing the fault system with the low-frequency portion of the data bandwidth.

If the fault throws are significant– as in these examples –data that are constrained to the first octave of the frequency spectrum may allow the faults to be better seen and interpreted.

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Geophysical Corner

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We reviewed the tectonostratigraphic evolution of the Jurassic–Cenozoic collision between the North American and the Caribbean plate using more than 30,000 km (18,641 mi) of regional two-dimensional (2-D) academic seismic lines and Deep Sea Drilling Project wells of Leg 77. The main objective is to perform one-dimensional subsidence analysis and 2-D flexural modeling to better understand how the Caribbean collision may have controlled the stratigraphic evolution of the offshore Cuba region.

Five main tectonic phases previously proposed were recognized: (1) Late Triassic–Jurassic rifting between South and North America that led to the formation of the proto-Caribbean plate; this event is interpreted as half grabens controlled by fault family 1 as the east-northeast–south-southwest–striking faults; (2) Middle–Late Jurassic anticlockwise rotation of the Yucatan block and formation of the Gulf of Mexico; this event resulted in north-northwest–south-southeast–striking faults of fault family 2 controlling half-graben structures; (3) Early Cretaceous passive margin development characterized by carbonate sedimentation; sedimentation was controlled by normal subsidence and eustatic changes, and because of high eustatic seas during the Late Cretaceous, the carbonate platform drowned; (4) Late Cretaceous–Paleogene collision between the Caribbean plate, resulting in the Cuban fold and thrust belt province, the foreland basin province, and the platform margin province; the platform margin province represents the submerged paleoforebulge, which was formed as a flexural response to the tectonic load of the Great Arc of the Caribbean during initial Late Cretaceous–Paleocene collision and foreland basin development that was subsequently submerged during the Eocene to the present water depths as the arc tectonic load reached the maximum collision; and (5) Late Cenozoic large deep-sea erosional features and constructional sediment drifts related to the formation of the Oligocene–Holocene Loop Current–Gulf Stream that flows from the northern Caribbean into the Straits of Florida and to the north Atlantic.

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This article describes a 250-m (820-ft)-thick upper Eocene deep-water clastic succession. This succession is divided into two reservoir zones: the lower sandstone zone (LSZ) and the upper sandstone zone, separated by a package of pelitic rocks with variable thickness on the order of tens of meters. The application of sequence-stratigraphic methodology allowed the subdivision of this stratigraphic section into third-order systems tracts.

The LSZ is characterized by blocky and fining-upward beds on well logs, and includes interbedded shale layers of as much as 10 m (33 ft) thick. This zone reaches a maximum thickness of 150 m (492 ft) and fills a trough at least 4 km (2 mi) wide, underlain by an erosional surface. The lower part of this zone consists of coarse- to medium-grained sandstones with good vertical pressure communication. We interpret this unit as vertically and laterally amalgamated channel-fill deposits of high-density turbidity flows accumulated during late forced regression. The sandstones in the upper part of this trough are dominantly medium to fine grained and display an overall fining-upward trend. We interpret them as laterally amalgamated channel-fill deposits of lower density turbidity flows, relative to the ones in the lower part of the LSZ, accumulated during lowstand to early transgression.

The pelitic rocks that separate the two sandstone zones display variable thickness, from 35 to more than 100 m (115–>328 ft), indistinct seismic facies, and no internal markers on well logs, and consist of muddy diamictites with contorted shale rip-up clasts. This section is interpreted as cohesive debris flows and/or mass-transported slumps accumulated during late transgression.

The upper sandstone zone displays a weakly defined blocky well-log signature, where the proportion of sand is higher than 80%, and a jagged well-log signature, where the sand proportion is lower than 60%. The high proportions of sand are associated with a channelized geometry that is well delineated on seismic amplitude maps. Several depositional elements are identified within this zone, including leveed channels, crevasse channels, and splays associated with turbidity flows. This package is interpreted as the product of increased terrigenous sediment supply during highstand normal regression.

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