YANG, WAN, Bureau of Economic Geology, The University of Texas, Austin, TX
Abstract: Internal Geometry of Barrier-Bar-Lagoon Systems, Eocene Jackson Group, Duval County, South Texas
The Eocene Cole barrier-bar-lagoon system (150-250 ft thick), penetrated by 270 wells in a 25-mi2 study area, consists of cycles of transgressive barrier-bar sandstone and regressive lagoonal shale. Correlation of three orders of cycles (major, intermediate, and minor) establishes a cyclostratigraphic framework in which 3-D cycle stacking patterns and thickness and facies changes are delineated.
Major cycles (40-80 ft thick) represent major episodes of bar building. Cycles in the middle of a system can have aspect ratios (height:length) twice as small as those of the lower and upper cycles. Along depositional dip, the upper cycle surfaces are convex and undulating, having a shallow lee slope caused by well-developed washover fans; along depositional strike, the upper surfaces slope gently down longshore current but slope abruptly at facies transitions. Intermediate cycles (<20ft thick) are persistent. Mud-rich cycles are uniformly thick, but accreting sand-rich cycles in barrier cores are sigmoidal. Minor cycles, well developed in thick barriers, are sigmoidal and limited in extent.
Bar building occurred in three stages -- initiation, aggradation, and progradation. Small bars, symmetrical and triangular, form first in topographic lows. They are aggraded by thinner and more uniform cycles because of increased surface area. Once a bar attains a certain height (~40 ft), overlying cycles prograde laterally. Barrier cores of major cycles switch along depositional dip and/or strike for as much as 2 mi, depending on accommodation space and sand availability. Intermittent growth faulting also significantly controlled depositional loci and facies distribution. Facies geometry differs dramatically among three orders of cycles, indicating dominance of various processes at different scales. 3-D visualization cycle stacking patterns of barrier-bar / lagoon systems help in extrapolating facies geometry to discover untested reservoirs and hidden flow barriers.
AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana