ABSTRACT

Late Paleocene and early Eocene lower Wilcox strata in southeast Texas are characterized principally by sandstone-rich deposits of fluvial and deltaic systems that prograded from a stable platform area into an unstable growth-faulted shelf margin setting. In contrast, incised valley systems, initiated during episodes of sea-level lowering and filled dominantly with mud during subsequent transgressions, punctuate several lower Wilcox intervals. Valley-fill sequences are known to provide seals and/or reservoirs for a number of stratigraphically trapped petroleum accumulations. Time-stratigraphic correlation of lower Wilcox strata provides improved differentiation of stacked valley-fill sequences and thereby more refined interpretation of depositional history and more accurate mapping for exploration purposes.

The key to differentiation of individual valleys is to recognize the top of each discrete valley-fill sequence where it has not been truncated by erosion at the base of a subsequent, younger valley. Identification of individual valley-fill sequences is possible where each cycle of downcutting and fill is followed by normal fluvial-deltaic deposition. We establish time-equivalence among these multiple-age valleys over a regional area through the relationships of the valley-fill tops to time-stratigraphic correlations carried in the surrounding and intervening lower Wilcox strata. The tops of successively younger valley-fill sequences are found progressively higher in the succession of markers. Through our technique of time-stratigraphic correlation we identify six valley-fill sequences; four have been mapped over a two county area, including the Lavaca Valley and the Hallettsville Complex.

The interpretation and exploration potential of lower Wilcox valley-fill systems may be broadly characterized as two types based on stratigraphic and sedimentologic evidence. Higher in the section, valley-fill sequences like the Lavaca Valley show characteristics of estuarine and bay environments associated with drowned shorelines similar to the Holocene Texas Gulf Coast. Relatively deep incisement of lower Wilcox valleys, however, is related to their formation near the seaward end of the progradational deltaic platform. Exploration potential exists where updip convex curvature of mudstone filled valleys or the intersection of primary and secondary drainages form lateral barriers to petroleum migration. These barriers can form stratigraphic traps or enhance structural closure.

Lower in the section, the Hallettsville Complex fills a shelf-edge canyon initially developed during a period of sea-level low stand and later modified by submarine processes during transgressive drowning. The extreme thickness of the down-dip Hallettsville sequence is closely associated with growth faulting and instability of the narrow shelf located at the margin of the stable platform. Exploration potential exists for channel-fill reservoirs that appear to be preserved most predictably at the intersection of the dip-oriented canyon trend with the strike-oriented trend of the growth faulted shelf margin.