Reservoir
Quality and Pore-Type Evolution in Deep to Ultradeep Tertiary Sandstones of the
Northern Texas Gulf of
Mexico Coast
Dutton, Shirley P.1, Robert G.
Loucks2 (1) Bureau of Economic Geology, Austin, TX (2) Bureau of
Economic Geology, Jackson School of Geosciences, The University of Texas at
Austin,
Exploration in Tertiary reservoirs of the
Gulf
of Mexico
focuses increasingly on depths below 15,000 ft, where reservoir quality is a
critical risk factor. To evaluate controls on deep-reservoir quality, we
conducted petrographic analysis on deep Eocene to Pliocene sandstones of the
northern Texas Gulf Coast; samples come from Texas coastal areas and
Federal waters. With increasing burial depth, total volume of porosity
decreases, and the proportion of different pore types changes. Mean porosimeter
porosity of samples from 10,000 to 15,000 ft is 18.9%, compared with 12.6% in
samples from 15,000 to 20,000 ft and 9.6% in samples from >20,000 ft.
Thin-section point counts demonstrate a pronounced change from a mix of
primary, secondary, and micropores (P36S24M40)
at shallower depths to predominantly secondary pores and micropores in deeper
sandstones (P5S20M75). Primary pores are lost
mainly by quartz cementation, which begins significantly at ~9,000 ft in Wilcox
sandstones and ~11,000 ft in Frio sandstones, and compaction.
Secondary-pore volume decreases more slowly with depth than primary-pore
volume. Below 15,000 ft, quartz cement begins to precipitate in secondary
pores. Micropores appear most resistant to compaction and cementation at these
depths.
Decrease in permeability with depth
correlates mainly with loss of primary pores. Geometric mean permeability of
samples from 10,000 to 15,000 ft is 8.6 md, compared with 0.4 md in samples
from 15,000 to 20,000 ft and 0.1 md from >20,000 ft. At deeper burial
depths, permeability is lower per porosity unit than at shallower burial depths
because secondary pores and micropores dominate.