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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Improved Characterizationof the Compartmentalized and Overpressured Vicksburg Sandstone Reservoirs Using Integrated
Sequence Stratigraphy, Diagenesis, and Petrophysics
By
Oklahoma State University,
Stillwater, Oklahoma
The natural gas-bearing transgressive systems tract (TST) Vicksburg interval in TCB Field, South Texas, was the focus of a comprehensive petrophysical evaluation. This interval was deposited in a transgressive shallow marine setting and is composed of thinly bedded and laminated silty, very fine-grained sandstones, siltstones, and shales. The TST interval was selected for its specific sedimentary features and compositional complexity that are typical of low-resistivity/low-contrast shaly sandstone reservoirs.
The goal was to develop improved formation
evaluation techniques for this type of reservoir.
Cores were sampled for thin-section, X-ray,
SEM, φ-k, capillary pressure, and cation
exchange capacity (CEC) analyses. Array
induction, gamma ray, neutron-density, micro-imaging, and
nuclear magnetic
logs were calibrated to cores. High-resolution
logs allowed the recognition of reservoir and non-reservoir facies
thicker than 1 ft, while micro-imaging resolved beds down to the
one-half inch scale. High-resolution density porosity measurements
were very close to measured core porosity values and used
to estimate total porosity. The low- resistivity signatures of sandstones
are mainly generated by the abundance of clays, especially
illite/smectite mixed-layer clays. The Pickett crossplot technique
was used to derive m, n and Rw values for water saturation (Sw)
calculation. Resistivity modeling using Waxman-Smits and
Modified Dual-Water methods also yielded m and n values that
agreed with those from the Pickett technique. Reliable results
were achieved using the simple Archie equation modified using
the newly estimated m, n and Rw values. Modified Archie,
Waxman-Smits and Modified Dual-Water
methods yielded Sw values that correlated
best to those from core analyses.
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