ABSTRACT

This study suggests that a significant volume of resources has been overlooked in inner-shelf sandstone reservoirs in the Frio Formation because of their complex architecture and the practice of focusing on thicker reservoirs during initial field development. Subregional and local geophysical well-log correlation, net sandstone and net pay mapping, petrophysical analysis, and mass-balance volumetric calculations were undertaken in West Fulton Beach field, Aransas County, Texas, to assess the reserve-growth potential of these reservoirs.

To identify individual reservoir bodies, it was necessary to break intervals down to fifth- and sixth-order genetic stratigraphic units. Inner-shelf sandstone bodies range in thickness from 2 to 8 ft and form shore-parallel lenses up to 1.6 mi long and 0.6 mi wide. These lenses, which may be associated with prograding shorefaces, transgressive disconformities, and retrogradational shelf sequences, form laterally stacked, commonly vertically overlapping, and potentially amalgamated bodies. They average up to 32 percent porosity and 990 md permeability. Internal heterogeneity is low, and drainage efficiency is moderate. Gas completions can drain areas of 150 to 300 acres, whereas oil completions drain from 20 to 50 acres.

Remaining resources identified within incompletely drained and untapped reservoirs of eight intervals in West Fulton Beach field total 11.8 billion cubic feet of gas and 315,000 barrels of oil, representing reserve-growth potential of 8 and 4 percent, respectively. Although much of this potential lies in small compartments containing from 50 to 500 million cubic feet each, several such targets can be identified in a vertically stacked arrangement such that a single wellbore can intersect 2 to 3 billion cubic feet of reserves. Projecting these results to the other fields of the play suggests that as much as 752 billion cubic feet of gas and 7 million barrels of oil remain as infill and recompletion targets. Because many of the Tertiary units of the Gulf Coast Basin share a similar depositional setting with the Frio Formation, significant reserve-growth potential is thought to exist in inner-shelf sandstones throughout the basin.