The objective of this regional study of the Wilcox Group and Carrizo Formation is to demonstrate that this stratigraphic succession in southeast Texas contains a greater variability in depositional systems, facies, and reservoir sandstone-body geometry than previously documented. Previous studies interpreted fluvial-dominated, wave-modified deltaic systems from thick (>600-ft [>183-m]), undivided intervals that encompass multiple depositional episodes. In contrast, this study, which integrates wireline-log, core, and outcrop data in a 13,770-mi²-area (~35,660 km²) updip and along the Wilcox shelf margin in southeast Texas, resolves and delineates a variety of net-sandstone geometries in tidally modified delta, wave-dominated shoreline, and inner-shelf systems.
Analysis of more than 2,000 ft (>610 m) of slabbed core in Leon County and outcrops near Bastrop, Texas, provides evidence for tidal processes throughout much of the Wilcox-to-Carrizo stratigraphic succession. Sandy depositional axes in many Wilcox sequences are narrow (<3 mi [<4.8 km]) and exhibit complex, anastomosing geometries consistent with deposits of tidally modified systems. Tidal signatures in the Wilcox Group, Sabinetown and Carrizo Formations occur at a variety of scales—microscale, mesoscale, and macroscale. Microscale tidal signatures in Wilcox cores and outcrops include rhythmically bedded, double-mud-draped ripple stratification and flaser-to-lenticular bedding, and systematically upward-thinning and upward-thickening sets of mud-draped bedforms and laminae. These features are common in Wilcox, Sabinetown, and Carrizo deposits interpreted as tidal-flat, crevasse-splay, marsh/swamp, interdistributary-bay, tidal-inlet, tidal-bar, and distributary-channel paleoenvironments.
Because of variability in depositional systems and sandstone-body geometries in the Wilcox-to-Carrizo stratigraphic succession, both the vertical and lateral complexities of reservoir systems are greater than previously inferred, implying a greater potential for reservoir heterogeneity and compartmentalization. This complexity should be considered in future reservoir development strategies in the Wilcox-to-Carrizo stratigraphic succession.
![Tidal deposits in the Calvert Bluff 2 sequence](/sites/default/files/media/0000/0557/Tidal%20deposits%20in%20the%20Calvert%20Bluff%202%20sequence.jpg)
Tidal deposits in the Calvert Bluff 2 sequence. A) Lenticular, mud-draped beds of very fine grained sandstone at 1,337.4 ft (407.7 m). Thickness of individual sandstone beds decreases upward, coupled with upward-thickening mud drapes within a 5-inch (12.7-cm) interval between arrows, suggesting this interval may be part of a tidal cycle recording upward-diminishing tidal energy. B) Very fine grained sandstone with erosional surface above steeply dipping beds at 1,329.0 ft (405.2 m). C) Mud-draped ripples composed of very fine grained sandstone at 1,320.0 ft (402.4 m). Many ripples are double-mud-draped. Scale of bedforms systematically decreases upward between arrows. Vertical features on left side of photograph are the result of core damage. D) Lenticular, mud-draped beds of very fine grained sandstone at 1,316.1 ft (401.3 m).
![Upper Calvert Bluff Formation at Pancake Rocks](/sites/default/files/media/0000/0558/Upper%20Calvert%20Bluff%20Formation%20at%20Pancake%20Rocks.jpg)
Upper Calvert Bluff Formation at Pancake Rocks along the Colorado River, southeast of Bastrop, Texas. A) Tidal flat and tidal channel deposits. B) Crossbedded tidal-channel facies, with location of photograph indicated in part A.
Research Program: STARR-30
Project dates: March 2019–January 2022
Project PI: William Ambrose
Other key personnel: Peter Flaig, Iulia Olariu, and Jinyu Zhang
Principal information contact:
William Ambrose
Email: william.ambrose@beg.utexas.edu
Telephone: (512) 471-0258
Fax: (512) 471-0140
Funding source: State of Texas
Other key partners/collaborators: Chris Denison (Astra Stratigraphics), Thomas Demchuk (RPS Group), and Jennifer O’Keefe (Morehead State University, Kentucky)
Geographic area of study: Southeast Texas Gulf Coast
General disciplines: Sequence stratigraphy, regional depositional systems and facies mapping/interpretation, play framework analysis
Keywords: Wilcox Group, Sabinetown Formation, Carrizo Formation, tidal systems, Gulf Coast Tertiary