YANG, WAN
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Abstract: Imbrication Patterns, Depositional Dynamics, and Reservoir Compartmentalization of Barrier-Bar Systems in the Eocene Jackson Group, Duval County, South Texas

The Eocene Cole and Mirando barrier-bar-lagoon systems penetrated by 300 wells in a 20-Mi² area, developed along Rio Grande Embayment during regional sea-level rise and stillstand. Each system is 100-300 ft thick and contains cycles of thin lagoonal shales and thick barrier-bar sandstones. Stratigraphic hierarchy is defined by three orders of cycles, termed major, intermediate, and minor, in which sandstone thickness varies from 10-250 ft, 0-100 ft, and 0-50 ft, respectively. Cycle correlation established a cyclostratigraphic framework within which 3-D geometry and stacking patterns of barrier bars were delineated. Areally, barrier bars converge from two abandoned deltaic headlands to interdeltaic area. They accreted lagoonward during transgression and seaward during regression, consistently oblique to regional depositional strike, forming left-lateral or right-lateral en echelon patterns. Stratigraphically, retrogradational and aggradational bars at the base and top of major cycles are thin and persistent, whereas progradational bars are thick, thickening and thinning away from headlands and down longshore current.

The intricate, imbricated patterns depict the evolution of these barrier-bar systems. Sand sources include deltaic headlands, offshore bars, and small contemporary fluvial-deltaic systems. Sand redistribution is assessed in terms of mass conservation. It occurred through barrier-bar formation, destruction, and migration. Minor redistribution formed tidal-channel and washover deposits. Growth faults effectively impeded sand transportation, and depositional topography, affected by growth faulting and differential compaction dictated local lateral thickness and facies changes. Imbrication patterns, depositional dynamics, and sand volumetrics controlled reservoir heterogeneity, as supported by production histories.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas