Facies Variation Within Salt Constrained Shelf Margin Deltas – a New Model for the Texas Shelf

McPherson, David G.¹, Richard E. Kilby² (1) Shell Exploration & Production Company, Houston, TX (2) Shell International Exploration & Production, Houston, TX

The Early and Middle Miocene shelf margin delta (SMD) systems offshore Texas contain prolific petroleum reservoirs that have been actively explored for over 40 years. During this time, the linked structural-depositional model for these systems has evolved, following advances in seismic imaging and structural analysis.

The earliest models for the SMD systems were characterized by large, listric, growth faults, terminating on a shale-based detachment. The current model suggests these faults detach on a regional salt weld surface. This weld is the remnant of a regional-scale paleo-salt sheet.

The interpretation of recent well data and 3-dimensional seismic sections suggests that the position of the earliest SMD sediments, and therefore the shelf margin, was controlled by the up-dip edge of an extensive allochthonous salt sheet or sheets. A basinward gradient and loading of these sheets caused salt withdrawal, creating space for the deposition of sediments. As sedimentation continued, growth faults developed at the up-dip margin of the salt sheet – in this way the position of the shelf margin was pinned by the underlying salt.

The shelf margin sediments typically exhibit a rapid basinward progression from deltaic sand to pro-delta silt and mud. This rapid facies change indicates that the withdrawal zone was a trough approximately 5-10 km wide, paralleling the paleo-coastline. Exceptions to this depositional pattern occur where remnant salt pillows created structural relief at the time of sedimentation.

The recognition of these salt sheets adjacent to, and controlling the position of the shelf margin, suggests that proximal slope basins may also have existed down-dip from the sand-rich deltas, and may present attractive reservoir targets across the Texas Shelf.