ABSTRACT

The Frio Formation is one of several basinward-thickening wedges of sand and shale in the Texas Gulf Coast subsurface; the Frio is less than 500 feet thick updip and greater than 10,000 feet thick downdip near the coast. The basinward thickening is the result of slow regional subsidence and of more rapid local movement along major growth faults. The objective of this paper is to illustrate the regional sand-shale facies distribution and relationships; preliminary data needed to evaluate the geopressure-geothermal resources of the Frio Formation along the lower and middle Texas Gulf Coast.

On the basis of foraminifer zonation, major depositional patterns, and structure, the Frio is subdivided on regional dip and strike cross sections into six subunits. From these cross sections and sand percent and net sand maps three main depositional patterns were recognized: (1) The updip portion of each unit consists of thin linear dip-oriented sand bodies separated by thick shales and local lignites, representing channel and overbank facies of a fluvial system; (2) In the southern part of the study area and downdip of the fluvial system, thick sand bodies broadly oriented in a dip direction are separated by thick shales. These sand bodies were deposited in a high-destructive wave-dominated delta system; (3) North of the delta system, thick strike-oriented sand bodies accumulated in an extensive strandplain system.

Contemporaneous faulting, which resulted from sediment loading, allowed vertical stacking of thick sand bodies in the Frio and reduced basinward progradation throughout much of the lower and middle Texas Gulf Coast.