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The AAPG/Datapages Combined Publications Database
Fort Worth Geological Society
Abstract
ABSTRACT: Ichnology and Sedimentology of Two Reservoir
Sands: a Shelf-Edge Delta and an Incised Valley Fill within the Frio Fm, West
Mustang Island 470-ARCO 45-47#4 Well, Corpus Christi Bay, Nueces County, Texas
By
1Pacheron Group, McKinney, TX,
Adjunct Assistant Professor, TCU, Ft Worth, TX
2Bureau of
Economic Geology, Jackson School of Geosciences, The University of Texas at
Austin, Austin, TX
Integrating ichnology (biogenically derived structures) and
sedimentology (physical sedimentary structures) in a core description of
bioturbated shallow-marine
sandstones greatly facilitated interpreting the
depositional environment of Oligocene age, Frio Formation reservoir sandstones.
Within a 1200-foot continuous cored interval, in the West Mustang Island 470
ARCO 45-47 #4 well, two separate fine- to medium-grained, quartz-rich subarkosic,
sandstone reservoirs were described in detail for their physical and biogenic
structures.
The N30 sandstone is interpreted as an incised valley fill
consisting of brackish water deposits of subtidal mixed sand and mud flats with
flaser bedding and a low-diversity suite of diminutive Planolites and Arenicolites,
overlain by a fully-marine
subtidal sand body with a robust ichnoassemblage of Ophiomorpha,
Asterosoma, Conichnus, Cylindrichnus, and Palaeophycus.
The isopach of this fully
marine
sand body has a shoreline perpendicular trend
and is interpreted as part of an incised valley fill system.
The N40 sandstone, characterized by an almost total absence of
marine
ichnogenera, has an abundance of primary sedimentary structures including
planar cross stratification, and low-angle cross bedding and lies directly above
a fully
marine
silty shale section. It is interpreted as a shelf-edge
delta-front sand body. The ichnogenera within the shelf sediments include distal
suites of Anconichnus, Helminthopsis, Palaeophycus heberti, Zoophycos
and Terebellina. Soft-sediment deformation structures are present at the
base of the sandstone suggesting rapid loading onto middle to outer shelf
deposits as the delta front sand body advanced seaward.