Structural Styles of the Wilcox and Frio Growth-Fault Trends in Texas: Constraints on Geopressured Reservoirs

The wide variability in structural styles within the growth-faulted, geopressured trends of the Texas Gulf Coast is illustrated by detailed structural maps of selected areas of the Wilcox and Frio growth-fault trends and quantified by statistical analysis of fault compartment geometries. Structural variability is a key determinant of the size of geopressured aquifers in the deep subsurface.Two major structural styles exist in the Wilcox trend. (1) In southeast and Central Texas, the trend consists of continuous, closely spaced faults that have little associated rollover despite moderate expansion of section; the fault plane flattens little with depth. Where the trend crosses the Houston Diapir Province, growth faults are localized by preexisting salt pillows; however, the salt diapirs pierced the growth-faulted horizons after the main phase of faulting, so that salt-related movement deforms the growth faults. (2) By contrast, in South Texas a narrow band of growth faults having high expansion and moderate rollover lies above and downdip of a ridge of deformed, overpressured shale but updip of a deep basin formed by withdrawal of overpressured shale. Large antithetic faults are associated with this band of faults. Frio fault systems generally display greater rollover and wider spacing than do Wilcox fault systems; however, the Frio trend displays distinctive features in each study area. At Sarita in South Texas, shale mobilization produced shale ridges, one of which localized a low-angle growth fault. Shale mobilization at Corpus Christi produced a series of large growth faults, shale-cored domal anticlines, and shale-withdrawal basins, which become younger to the east. At Blessing, major growth faults show little progradation seaward. A major early-formed growth-fault system was deformed by later salt tectonism at Pleasant Bayou. At Port Arthur, low-displacement, long-lived faults formed on a sand-poor shelf margin contemporaneously with broad salt uplifts and basins. Most of the Frio growth faults, however, have a similar geometry, showing substantial rollover, expansion of section, and a moderate flattening of the fault zone with depth, possibly related to a deep decollement surface. Shale ridges are common but not ubiquitous. The local variability in style is related to the magnitude of Frio sedimentation and progradation and to the presence of thick salt or shale.Data on the sizes and shapes of fault compartments in the Texas Gulf Coast growth-fault trends confirm substantial differences between the two fault systems. Frio compartments generally show less elongation and higher absolute curvature than do Wilcox compartments. The compartment areas in both trends are distributed exponentially (small compartments outnumber large ones). There are more very large compartments than the best-fit exponential model predicts, however. The observed distribution of fault compartment curvature is consistent with a simple trapezoidal model of their geometry.Finding and developing a large geopressured aquifer require recognition of a favorable combination of sand-body geometry, reservoir quality, and fault compartment size and shape. In the Wilcox trend, reservoirs are predominantly dip-oriented sandstones of high-constructive deltaic origin. This geometry, together with the close spacing of faults and the characteristically low permeabilities, limits the size of geopressured reservoirs. The largest reservoirs in the Wilcox trend may be in areas between major fault trends or in salt- or shale-withdrawal basins. In the Frio trend, widely spaced faults and higher permeabilities are more favorable to reservoir development. Blocks showing dip reversal and salt- and shale-withdrawal basins were all found in the trend and are likely to contain large geopressured aquifers. Reservoir quality is the primary limitation.
Thomas E. Ewing

Ewing, T. E., 1986, Structural Styles of the Wilcox and Frio Growth-Fault Trends in Texas: Constraints on Geopressured Reservoirs: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 154, 86 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation