From Bureau of Economic Geology, The University of Texas at Austin (
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2003 Symposium of Biostratigraphy of the Petroleum Exploration of Mexico, Reynosa, Tamaulipas, Mexico
June 19–21, 2003

Developing High-Frequency Chronostratigraphic Basinal Frameworks Using Sequence Stratigraphy Calibrated with Internationally Accepted Isotopic and Biostratigraphic Ages, Corpus Christ Region, Texas

L. Frank Brown, Jr., Robert G. Loucks, Ramón H. Treviño

A composite wireline log created by splicing unfaulted and relatively conformable log segments from several deep wells provides a stratigraphic record that captures the most complete succession of depositional and cyclic history. Site-specific, sequence-stratigraphic sections (S5 logs) are benchmarks that summarize available geologic information for a basinal site-specific area. Color-coded depositional sequences and component systems tracts are core information displayed on S5 logs. Sequence-bounding unconformities and internal maximum flooding surfaces delineated on composite benchmark logs, when correlated among other wireline logs, produce a chronostratigraphic framework for the area. This physical framework can then be calibrated with ages (Ma) of sequences and component surfaces. Ages are based on geologic time charts representing latest consensus from isotopic, polarity, and microfossil integration. Even without adequate microfossil data, site-specific sequences can be reliably calibrated to global cycle charts empirically by comparing erosional intensity of unconformities, magnitude of flooding events, third-order sequence thickness/stacking variations at second-order frequencies, third-order durations per estimated depositional rates, among other criteria. A single age-documented surface or microfossil marker within a major stratigraphic succession will link a site-specific sequence framework to an accepted age-dated cycle chart. Without age calibration, an S5 log will still support regional time-stratigraphic applications.

Color-coded sequences, systems tracts, and bounding surfaces on S5 wireline logs can be expanded into informational benchmark charts that temporally integrate biozones, ages of sequence surfaces, depositional settings and systems, stratigraphic nomenclature, sequence frequencies, related tectonism, identification of composite well segments, petroleum pay zones, local marker horizons, and sequence stratigraphic symbols. A myriad of data sets can be compared throughout a basin using the S5 log as a chronostratigraphic benchmark reference. For example, an S5 chart can integrate stratigraphy with porosity, permeability, velocities, petrophysical cross plots, source-bed quality, petrographic data, microfossil horizons, or any information in a wireline log format.

S5 logs expedite selection of isochronous surfaces for structural mapping and chronostratigraphic units for isopach, net lithologic, log facies, or other time-stratigraphic mapping. Composite benchmark logs can display various statistical parameters from well data sets ensuring basinwide isochroneity. Comparison of S5 logs among widely spaced or isolated sites provides a framework that temporally constrains regionally diachronous lithostratigraphic units.