Paul R. Knox1
The Miocene-age Matzen Sandstone is typical of many transgressive sandstones in that it is thick (~50 m) and massive appearing with blocky to bell-shaped SP expression, rests on a basal unconformity, is said to be "shoreface" in origin, and is a prolific reservoir (>250 MMbbl produced). However, facies analysis, high-frequency stratigraphic correlation, 3-D seismic interpretation, and production behavior indicate that the Matzen, and perhaps other transgressive sandstones, are more complicated than expected.
The dominant facies in the Matzen Sandstone is interpreted as shallow shelf deposits and consists of sharp-based structureless, upward-fining, strongly bioturbated, bimodally sorted, sandstone beds 1 to 2 m thick commonly containing shell fragments. They are hypothesized to be resedimented delta front or mouth-bar deposits that were transported short distances by density flows that originated as storm-surge relaxation currents. Another (rare) sandy facies is low-angle crossbedded unburrowed sandstones lacking shell fragments, which typically occurs only at the base of the unit and is interpreted as preserved delta front/mouth bars of a braid or fan delta. Backstepping high-frequency depositional cycles, which lap out or thin over paleohighs, are capped by flooding surfaces characterized by muddy, highly bioturbated sandstones with abrupt upper contacts and Thalassinoides burrows infilled with sand of the overlying bed. Vertical permeabilities in these capping sandstones are typically one to three orders of magnitude lower than underlying sandstones, creating barriers or baffles to vertical fluid movement. A reexamination of other "massive" transgressive sandstones may find similar complexities in architecture that impact management of these commonly large reservoirs.
1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713; e-mail: email@example.com.