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Abstract

Nearly 2,000 feet of continuous core, combined with well log and 3-D seismic data, afford a rare opportunity to document variations of stratal architecture and related processes of shelf-edge, growth faulted deltaic systems through active and inactive periods of growth faults at the scale of 4th-order sequences in the Frio Formation in Corpus Christi Bay.

The growth fault history, examined by expansion ratio and T-Z plots, is divided into three different periods: (1) development (rapid subsidence, growth ratio > 2, steep and positive slope of T-Z plot); (2) less active to inactive (no growth, flat to negative slope of T-Z plot); and (3) maintenance (slow subsidence, growth ratio ~ 1, flat and positive slope of T-Z plot). These periods can be correlated to aggradational, repetitive fourth-order sequences of shoreface deposits, a transgressive unit composed of backstepping shoreface deposits, and several high-frequency progradational fourth-order sequences represented by wave-dominated, fluvial-influenced deltaic deposits.

Varying subsidence rates serve as a dominant process in stratigraphic development of the Frio Formation, whereas sediment supply and eustatic sea-level changes are subordinate. The decameter-scale fourth-order sequences within the hanging wall section are grouped into two categories based on stratal architecture: (1) T-R cycles within development period of growth faults and (2) R cycles within maintenance period of growth faults. T-R cycles have complete transgressive and regressive intervals of similar thickness (R-T thickness ratio ~ 1.7), created by a balance between rapid sediment supply and rapid accommodation rate caused by high subsidence rate and low sea-level drop rate. This balance permits the possibility of preserving of both regressive and transgressive units and provides more time for modification of deltaic deposits by wave-storm processes. R cycles are dominated by regressive intervals containing minor transgressive intervals (R-T thickness ratio > 6). R cycles are the results of rapid progradation stacking as rapid sediment supply, slow subsidence and rapid sea-level drop.

The reservoir quality of wave-dominated deposits is highly controlled by the percentage of the bioturbated beds that constitute permeability barriers. Within the period of development of growth faults, a high subsidence rate associated with high sediment rate may indicate a relative high energy environment with fewer bioturbated beds and better reservoir quality as documented by porosity and permeability data. Relatively more antithetic faults are present during the growth fault development period, making the interval structurally complex with potentially greater number of reservoir compartments.

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