The Upper Eocene Yegua Formation expands dramatically across a regional flexure generally twelve to fifteen kilometers wide. During each of several postulated Yegua sea level drops, this flexure became a focal point for deltaic deposition of excellent reservoir-quality sands. From the western edge of the Houston Salt Dome Basin to the San Marcos Arch, this trend has yielded, since 1982, at least five noteworthy discoveries - Toro Grande and Lost Bridge Fields in Jackson County and Black Owl, Shanghai, and El Campo Fields in Wharton County.

Shanghai and El Campo Fields are located within what the authors have labeled the Shanghai Delta Complex. Integration of seismic and well data in this vicinity shows a marked increase in the expansion indices of growth faults as well as other structural and stratigraphic patterns characteristic of deltas deposited on Gulf Coast Tertiary shelf margins.

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