Abstract

The oil-water contact at Wasson field tilts to the northeast due to hydrodynamics. Capillary effects on the oil-water contact elevation are present but relatively minor due to intermediate wettability of the reservoir. A southwest-thickening wedge of residual oil underlies the tilted contact. The Residual oil zone has an approximately flat base, which is interpreted as the original, hydrostatic oil-water contact for Wasson field prior to initiation of hydrodynamics.

Contact tilting is a response to Late Cenozoic hydrodynamic flow in the San Andres aquifer. Flow is caused by uplift in eastern New Mexico and discharge in west central Texas. OWC tilt angle is consistent with the magnitude and direction of the hydrodynamic gradient.

Most of the oil formerly trapped in the Wasson area spilled through Ownby field to the northeast in response to the tilt. Spilled oil migrated along the anticline formed by drape over the underlying shelf edges toward Slaughter-Levelland field. The remigrated oil charged this field, and oil spilled updip to the west along pinchouts in the lower San Andres to fill the Slaughter trend stratigraphic traps, many of which formed with hydrodynamic tilts. Tertiary migration was therefore responsible for significantly reducing the size of one accumulation (Wasson) while either forming or increasing the size of other accumulations (Slaughter trend fields).

Hydrodynamically driven, tertiary migration explains some of the puzzling aspects of San Andres petroleum geology on the Northwest Shelf: common occurrence of tilted oil-water contacts, many wildcat wells with great shows but marginal or no production, and large uncharged areas surrounded by productive trends. Tertiary migration in response to hydrodynamics also provides a mechanism for petroleum charge long after the time of petroleum generation.