Facies Variability and Reservoir Quality of Incised-Valley Fill and Shallow-Marine Sandstones in the Upper Cretaceous Tuscaloosa Formation, Central Louisiana
This study by the Deep Shelf Gas Consortium, using lithology, porosity, and permeability data from five whole cores, defines systems tracts by integrating lithology 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 occurs in sandy, nonconglomeratic 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, collectively composing up to ~400-ft (~122-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 and conglomeratic braided-stream systems that grade upward into mixed-load meanderbelt deposits overlain by a regionally continuous mudstone interval (10- to 25-ft [3- to 7.6-m]) recording a period of valley inundation and subsequent development of estuarine systems. Permeability values in Tuscaloosa bedload-fluvial channel-fill deposits are as much as 1,000 md in fine- to coarse-grained, nonconglomeratic sandstone beds. In contrast, permeability values are significantly lower in chert-clast conglomerates that contain limited sandy matrix between chert pebbles.
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 and (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. 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.