From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Society of Independent Professional Earth Scientists, San Antonio, November 17, 2005

Application of 3-D Seismic and Sequence
Stratigraphy in Maturely Drilled Areas, Example from the Eastern Shelf of the West Texas Basin

L. F. Brown, Jr.

Abstract:

Sixteen depositional sequences (principally limestone transgressive and siliciclastic highstand and lowstand systems tracts) record paleogeography during Late Pennsylvanian Virgilian and Early Permian Wolfcampian Epochs (~275–296 Ma) on the Eastern Shelf and adjacent West Texas Basin, northwest and north-central Texas. Composing 16 depositional sequences, strata totaling 1,200 ft (365 m) on-shelf to more than 2,800 ft (853 m) within the eastern part of the basin were deposited under the influence of waning regional foreland tectonic elements, diverse source areas, variable subsidence, and glacio-eustatic sea-level variations.

To understand local sedimentary dynamics, regional tectonic events and sea-level cycles on the depositional history of these strata, we analyzed genetic depositional systems and chronostratigraphic sequence stratigraphy. Cross sections covering ~2,000 mi (3,200 km), extensive outcrop, and >5,000 wireline logs permitted (1) delineation and mapping of highstand and transgressive (TST/HST) packages; (2) documentation of sequential depositional history; (3) delineation and mapping of depositional systems and paleogeography for each of 16 depositional sequences; (4) analysis of depositional response to paleobathymetry and synsedimentary tectonics; and (5) construction of a regional sequence (chronostratigraphic framework) relative to sediment supply rates needed to understand relative changes of sea level.

Sedimentary cycles reflect interplay of differential subsidence and marginal uplifts, eustatic (glacially driven) sea-level cycles, and inferred growth faulting.

The fundamental and practical purpose of these kinds of regional sequence-stratigraphic frameworks is their application in searching large geographic areas for localized petroleum prospective areas. Analogs for further discoveries become obvious when a basic depositional and chronostratigraphic framework is available to explain the basis of currently producing fields. Such frameworks help to focus expensive 3-D seismic acquisitions where all known parameters indicate high odds for success on the basis of nearby successful wildcat discoveries within the same hydrocarbon system.

Trap, reservoir, and migration pathways explaining producing fields, as well as dry holes, can be inferred by sequence-stratigraphic criteria and then applied to stratigraphic analogs within the same hydrocarbon system. Regional net sandstone axes (actually sediment-transport pathways) do not precisely localize potential drill sites, but they provide the basis for reducing large potential prospect areas into highly promising localized and testable acreage for more expensive and detailed 3-D seismic acquisition. The chronostratigraphic framework integrates all available data and eliminates wasting the drill on obviously negative stratigraphic areas.