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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Title: Seismic
Data: Potential and Guidelines from a Tertiary Sandstone-Shale Sequence Model, Powderhorn Field, Calhoun County, Texas
Author(s):
Abstract:
We generated a set of three-dimensional (3-D) seismic
models of a lower Miocene progradational microtidal shore-zone system at the Powderhorn field, Calhoun County, Texas. These models were based on detailed lithological mapping of 250 m of stratigraphic section over an area of 13 × 9 km. The mapping was guided by detailed facies analysis based on wireline logs from 115 wells. Fourteen sandy depositional units averaging 3-30 m in thickness are encased in 15 shale units. Density-neutron and spontaneous potential (SP) logs in five recently drilled wells were used to calculate effective porosity and shaliness, which correlate well with P-wave velocity and bulk density logs. Those properties were then extended to older wells lacking acoustic, density, and other porosity
ogs through regression with SP and resistivity logs to build 3-D properties models, including models of transit time and impedance. The impedance model was mapped from x, y, z to x, y, t, where t is two-way vertical time. The impedance model was converted to primary reflection coefficient series that were then convolved with zero-phase wavelets of different frequencies to produce the
seismic
models. The results show how the depositional facies at reservoir scale might be illustrated by 3-D
seismic
data, and how the
seismic
resolution of depositional facies changes with
seismic
frequency, stratigraphic position, and facies patterns.
The Powderhorn reservoir sequence is interpreted as a barrier bar/lagoon depositional system associated with coastal stream plains, bayhead deltas, and small wave-dominated deltas corresponding to frequent relative changes of sea level. To detect these types of deposits, the seismic
frequency should be selected such that the
seismic
data are tuned to the maximum thickness to establish a linear relationship between
seismic
amplitude and facies, and to achieve the best signal-to-noise ratio. The facies associated with the environments where main sandstones are surrounded by muddy deposits, such as coastal stream plain, bayhead delta, delta plain, and backbarrier/lagoon transition, are well illustrated in our
seismic
model. Lack of muddy deposits, for example in a wave-dominated delta, c
uld result in obscure facies boundaries and therefore unclear lithofacies patterns. In this study, poorly compacted clean sandstone and pure shale have similar impedances, and consequently produce similar
seismic
responses. The resulting ambiguity may be reduced by careful analysis of facies relationships if some well data are available, and probably by the amplitude vs. offset (AVO) method. The stratal resolution of
seismic
imagery depends not only on wavelet frequency, but also on the stratigraphic position of a given reservoir, or the magnitude of geological interference.
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