Evidence for Sediment Fan Deposition on Outer Texas Shelf During Miocene Eustatic Sea Level Highstands

W. C. Riese, R. S. Olsen, R. N. Rosen

Four types of data were reviewed in an attempt to clearly define the environments of deposition for reservoir sands in the Matagorda 668 field: well log curve shapes, seismic amplitude responses, micropaleontology, and thin section sedimentary petrology. All four lines of evidence support the interpretation that these lower Miocene sands were deposited as fan complexes.

The sands exhibit three distinctive curve forms: coarsening upward, fining upward, and cylindrical-serrate. These represent respectively fringe settings around the perimeter of the fan where deposition from turbid flow dominates, proximal fan settings where settling from suspension is the more dominant process, and a collection of environments in medial settings intermediate between the two end members just described.

Seismic amplitude responses increase in areas where sand increases in abundance, i.e., higher energy environments of deposition. This is consistent with the responses we observe in other field analogs and with well data from this field.

Foraminiferal fauna present in the sands being investigated, as well as in the over- and under-lying shales, were examined and they indicate deposition in a deep, bathymetric setting. The benthic data also show evidence of microenvironmental zoning along the depositional strike of the sand systems we have investigated: calcareous species dominate the distal facies and arenaceous species dominate the proximal facies of the fan systems.

Petrographic information derived from analysis of sidewall cores supports a turbidite origin for the sands: they exhibit relatively coarse grain size, angular grains, moderate sorting, common intraclasts, common reworked fauna, well-developed planar laminations, and occasional thin silt and shale interbeds with ripple laminations.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.