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
Bo Henk, Pacheron Group, McKinney, TX, Adjunct Assistant Professor, TCU, Ft Worth, TX; Robert G. Loucks, Bureau 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.
AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section
Meeting, Fort Worth, Texas, March 1-4, 2003