ABSTRACT: Determining the Depositional Environment of a Braided Channel Sand Using Formation MicroScanner Images, North Vacuum Field, Lea County, New Mexico

Dan E. Burnham

Paleoenvironmental reconstruction analysis characterize the Atoka sands of the North Vacuum field, Lea County, New Mexico, as being deposited in a braided fluvial system. The channels trend from northwest to southeast. In the past the only sure way of identifying the depositional environment of a sand was by cutting a whole core. With the addition of the Formation MicroScanner (FMS) images, now one can interpret the depositional environment with greater confidence and without the cost of a shale core. The depositional environment was determined for Atokan sands by using the FMS in two wells. The results were verified by whole core and FMS in one well.

The FMS data interpreted on Schlumberger's Sun workstation indicated that the sand was deposited in a braided channel system. The following sequence was observed: (1) erosion of the underlying shale, (2) then coarse-grained sands were deposited in a downflow direction until the eroded channel was filled, (3) then the sands were laterally accreted perpendicular to flow direction, producing the thickest sand bodies, and (4) the final stage occurred during sea level transgression when fine-grained sand and shale lenses capped the massive sands with depositional current bedding in a landward direction indicating fluvial-marine influences.

The conclusions based on the FMS images were supported by a whole core interpretation of a complete sandstone sequence in one well. Several additional locations have been chosen in the channel trend based on this model.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990