William A. Ambrose, Robert G. Loucks, and Shirley P. Dutton
Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924
Sequence-Stratigraphic and Depositional Controls on Reservoir Quality in Lowstand Incised-Valley Fill and Highstand Shallow-Marine Systems in the Upper Cretaceous (Cenomanian) Tuscaloosa Formation, Louisiana, U.S.A.
Onshore Gulf of Mexico Exploration I (GRBCC, Assembly Area B)
Tuesday, September 22, 2015, 10:45 am
Upper Cretaceous (Cenomanian) lowstand incised-valley systems in the Tuscaloosa Formation in central Louisiana and southwestern Mississippi and equivalent strata in the Woodbine Group in the East Texas Basin are significant stratigraphic features in the Gulf of Mexico. This study, using lithology, porosity, and permeability data from five whole cores, defines systems tracts by integrating lithologic data and log stacking patterns and documents sequence stratigraphic and facies controls on reservoir quality in lowstand incised-valley and underlying highstand deltaic systems in the Tuscaloosa Formation in central Louisiana. Greatest reservoir quality exists in sandy, non-conglomeratic bedload-fluvial deposits within these incised-valley systems. However, underlying highstand deltaic systems also contain good reservoir quality in proximal-delta-front sandstones at the top of progradational successions.
Tuscaloosa incised-valley-fill systems in central Louisiana, collectively composing up to ~400 ft (~120 m) of predominantly fluvial deposits, record multiple episodes of incision into shallow-marine strata. These amalgamated valley-fill systems are locally >40-mi (>64-km) wide and contain a variety of fluvial and estuarine facies. The lower half of the incised-valley-fill succession is composed of coarse-grained, including conglomeratic, braided-stream systems that grade upward into mixed-load meanderbelt deposits overlain by a regionally-continuous mudstone interval, 10 to 25 ft (3.0 to 7.6 m) thick), recording a period of valley inundation and subsequent development of estuarine systems.
Coarse-grained bedload-fluvial facies in the lower half of the Tuscaloosa incised-valley fill are composed of thick (commonly >100 ft [>30 m]) multistoried and aggradational successions of chert-clast conglomerates interbedded with medium-grained sandstone and pebbly, coarse- to very coarse-grained sandstone beds. They are commonly comprised of multiple 4 to 10 ft (1.2 to 3.0 m) thick, upward-fining intervals that record high-energy, downstream migration of channel-floor gravel and sand bars mantled by fine-grained sandstone representing waning-flow, bar-top deposits. Interchannel facies consist of heterogeneous upward-coarsening sections of very fine- to fine-grained, ripple-stratified sandstone beds with abundant soft-sediment deformation and dispersed organic material.
Tuscaloosa bedload-fluvial channel-fill deposits have moderately-blocky vertical permeability profiles, with greatest values as much as 1000 millidarcys (md) in fine- to coarse-grained, nonconglomeratic sandstone, mainly in the middle part of channel-fill successions. Median values (~45 md) in Tuscaloosa sandy bedload-fluvial channel-fill deposits are one order of magnitude greater than those in mixed-load fluvial deposits near the top of the incised-valley fill (~4 md). These mixed-load fluvial deposits display complex vertical permeability trends that are principally related to increasing mudstone content from channel-floor to upper-point-bar facies. In contrast, upward-increasing permeability trends in the basal section of the transgressive systems tract at the top of the incised-valley-fill succession are controlled by an upward decrease in muddy matrix that record increasing depositional energy and winnowing from wave and storm processes.
Knowledge of porosity and permeability variations within facies, as well as contrasting values between facies, can be used to infer controls on reservoir quality in the Tuscaloosa Formation. Significant vertical contrast in permeability occurs between Tuscaloosa facies, including (1) highstand deltaic sandstones locally truncated by low-porosity and low-permeability chert-clast conglomerates or clay-clast-rich sandstone beds at the base of lowstand incised-valley-fill deposits, (2) heterogeneous and muddy estuarine deposits above sandy bedload-fluvial deposits, and where sandy, marginal-marine deposits occur near the upper part of the Tuscaloosa incised-valley-fill succession. High porosity and permeability values (100s of md) are common in individual thin (<1 ft [0.3 m]) Tuscaloosa sandstone beds in highstand distal-delta-front facies, but their reservoir potential is limited because of numerous mudstone layers between sandstone beds. Although the greatest reservoir quality in the Tuscaloosa Formation in central Louisiana occurs within lowstand incised-valley bedload-fluvial deposits and decreases upward along with decreasing average grain size, significant variation in permeability occurs between conglomeratic and sandy bedload-fluvial facies. These fluvial facies are commonly interbedded at fine scales (<2 ft [<0.6 m]) that may be difficult to differentiate from wireline logs in the absence of core data.