Abstract

Genetic sequence stratigraphy of Upper Wilcox and Reklaw strata in the Northeastern Area of the Wilcox Producing Trend of South Texas indicates a regional marine transgressive systems tract that locally followed a southerly shift of Wilcox deltaic deposition. The marine transgression began here with a delta lobe shift before the Claibornian Stage, but a eustatic rise is necessary to explain the gulfwide regional character of the transgression which continued into the Claiborne and involves sediments of the Reklaw Formation. Such constraints suggest the following Upper Wilcox trend geohistory in the Northeastern Area:

A shift in point source of the Rosita delta system caused abandonment of the Wilcox Massive delta and established the Live Oak delta nearby but to the southwest. Sediment compaction and dewatering and/or a eustatic sea level rise resulted in a rapid rise in relative sea level. This rise allowed the deltaic Wilcox Massive Unit to fall below aggradational base level and become rapidly transgressed. A marine transgression thus began to cover the moribund delta lobe. Reworking of the uppermost deltaic units as well as the previously deposited Carrizo fluvial-dominated alluvial plain which lay shoreward resulted in deposition of a backstepping succession of marine transgressive parasequences over the Massive delta lobe and shoreward. These parasequences and the marine transgression itself culminate with a condensed section in the Marquez Shale member of the Reklaw.

The sandstones of the transgressive systems tract became the highly productive oil and gas reservoirs individually designated the Mackhank, Luling, Slick, Second, and First Reklaw sands in ascending order. Together with a lagoonal channel-filled equivalent of the First Reklaw locally known as the Atkinson Sand, these units of the Upper Wilcox-Reklaw marine transgression have provided the Northeastern Area of the South Texas producing trend with a majority of the Wilcox oil and gas reserves.

While classical lithostratigraphic units provide an orderly approach to the exploration for structural traps within the trend, genetic resolution of stratigraphic sequences offers an exciting new approach in the search for many subtle stratigraphic traps from these same prolific reservoirs together with a clearer understanding of the stratigraphic relationship between the Wilcox Group sediments and the overlying Reklaw strata.