Lower Wilcox Submarine Canyon Channel Sandstones, Sheridan Field, Colorado County, Texas

MILLER, RANDALL S., Reservoirs, Inc., Houston, TX

Sheridan field was discovered in 1940 by Shell Oil Company. The field is located along the Wilcox fault zone and produces gas from Wilcox sandstones in an ovate, anticlinal structure on the downthrown side of a normal, down-to-the-southeast fault. Five wells in the field have been drilled below the main lower Wilcox sandstone sequence and reveal some enigmatic sandstones in an otherwise very thick shale sequence. This sequence is at least 2500 ft thick at Sheridan field and has been interpreted by regional correlation as part of the Lavaca submarine canyon fill.

Sedimentological analysis of several Sheridan field conventional cores, coupled with determination of vertical and lateral facies relationships, indicates that the canyon sandstones were deposited by mass-gravity flow mechanisms in channel and associated interchannel levee environments. Individual channel sandstones range from 3 to 30 ft thick but may locally form thicker sequences where they are stacked. These sandstones are very fine to fine grained, have sharp lower contacts, and are either (1) massive appearing with "floating" mudstone clasts or (2) exhibit a combination of cross-stratification and severely contorted bedding. Levee sandstones are thin bedded, horizontal planar stratified to ripple stratified, and form units similar to Bouma BCDE and CDE sequences. Associated levee mudstones are delicately graded laminites. All lithologies commonly exhibit soft-sediment deformation resulting from gravity-induced, mass movement such as slumping, flowing, and sliding. The above sedimentary features and inferred depositional processes support a continental slope setting.

The channel sandstones are potential exploration targets in the field and surrounding areas because of their enormous stratigraphic trap potential. They form the reservoirs in the Lavaca Shale interval at several fields in the Halletesville area. The sandstones have geometries that are elongate down depositional slope and truncate updip against adjacent canyon shale fill. In cross section, the sandstones are lenticular and terminate abruptly into laterally equivalent levee deposits. The recognition of these submarine canyon channel sandstones also implies that potential submarine fan reservoir sandstones may be present downdip at the mouth of the canyon.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)