Downdip Sandstone Prediction; Yegua Trend of Texas and Louisiana

Abstract

Well log, seismic, and biostratigraphic data were integrated for 350 mi along strike to formulate and test depositional and stratigraphic models for predicting downdip sandstones in the Yegua trend. The correlation framework of almost 4,500 logs extends from the stable shelf downdip to the limit of well control of the slope. On the shelf, the Yegua sand-bearing interval is 1,000-1,500 ft thick. Shale markers formed during highstands are continuous for tens to hundreds of miles along strike and bracket progradational cycles 100-300 ft thick. Paleoenvironments were interpreted primarily from maps of log-derived interval isopach, net, percent and blocky sand, and log facies. Systems tracts range through delta plain, mouth bar, shelf, and upper slope. Downdip, complications were introduced by growth faulting, erosion due to slope failure, and complex patterns of basin fill. This zone thickens basinward to several thousand feet. Local shelf bypassing, channel incision, and basinward shifts in facies suggest small but significant eustatic falls. Each of the 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 shift of depocenters. Effective sand prediction downdip to the shelf edge and uppermore » slope is accomplished using transgressive shale markers as a stratigraphic framework, while recognizing the significant effects of eustatic sea-level changes. Although widespread erosional unconformities could be mapped on the slope, neither unconformities nor correlative conformities could be identified sufficiently to enable detailed and regional mapping on the shelf.« less