Control of Depositional Environments, Eustasy, and Gravity and Salt Tectonics on Sandstone Distribution in an Unstable Shelf-Edge Delta, Eocene Yegua Formation, Texas and Louisiana

EDWARDS, MARC B., Consulting Geologist, Houston, TX

Predicting the locations and geometry of downdip sandstone reservoirs in the Yegua trend of Texas and Louisiana is a key challenge to explorationists. In order to formulate and test depositional and stratigraphic models for this purpose, well-log, seismic, and biostratigraphic data were integrated over a distance of about 350 mi along strike. The resulting correlation framework, based on almost 4500 well logs, extends from updip on the stable shelf to the downdip limit of well control on the slope.

Well logs provide the highest stratigraphic resolution in this data set. On the stable shelf, the entire Yegua-Cook Mountain sand-bearing interval is about 1000 to 1500 ft thick. Shale markers which show tens of hundreds of miles of continuity along strike bracket progradation cycles 100-300 ft thick. These log-defined cycles comprise the basic mapping units used in this study. Seismic reflectors in this zone are essentially parallel. Downdip of the shelf, complications were introduced by growth faulting, erosion due to slope failure, mass wasting of sediments overlying failure-induced erosion surfaces, and complex patterns of basin fill seaward of the shelf margin. This complex zone thickens basinward to several thousand feet. All of these zones were further complicated by contempora eous and post-depositional salt movement in the Houston Salt Diapir Province. Biostratigraphic data were used to determine general ages for downdip section, to indicate prominent discontinuities, and to show environmental trends.

Large-scale slope failures occurred several times during Yegua deposition, resulting in regional erosional unconformities. It is not known if their origin is related to eustasy. Slope deposits that onlap extensive erosional surfaces are also well represented, but their precise age relationship to updip systems is unclear.

Paleoenvironments were interpreted from maps of log-derived interval isopach, net sand, percent sand, blocky sand, and log facies. Each mapping unit is bounded by regionally correlatable shale zones interpreted to have formed during highstands. Strata bounded by the markers represent systems tracts comprising upper delta plain, lower delta plain, mouth bar, distal mouth bar, and shelf. Inferred local shelf bypassing and channel incision and basinward shifts in facies suggest small but significant falls in sea level during deposition. Each of these cycles resembles "depositional sequences," but in terms of their duration they are described as high-frequency fourth-order events. Small-scale stratigraphic complexity within the mapped units suggests frequent autocyclic shifts of depocente s.
Effective sand prediction downdip to the shelf edge and upper slope is accomplished using transgressive shale ("condensed section" or "flooding surface") markers as a stratigraphic framework, while recognizing the significant effects of eustatic sea level changes. Although widespread erosional unconformities, possibly equivalent to sequence boundaries, could be mapped on the slope, neither unconformities nor correlative conformities could be identified sufficiently on the shelf to enable their detailed and regional mapping.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)