GEOLOGY OF THE DEEP TUSCALOOSA (UPPER CRETACEOUS) GAS TREND IN LOUISIANA
Authors: Smith Guy W. 
Published: October 2019

Drilling activity in the Deep Tuscaloosa Gas Trend has experienced a significant decline since reaching a peak in 1980. An important factor in this decline is the current oversupply of gas, but, in addition, the Trend has entered a mature stage of development. The deep Tuscaloosa Gas Trend in Louisiana is approximately 25 miles in width and extends northwesterly from Lake Borgne on the east to the Texas state line, a distance of nearly 200 miles. Since the Trend discovery in 1975, exploratory drilling has discovered 24 fields. Proved and potential reserves are estimated at five trillion cubic feet of gas and 100 million barrels of condensate. Virtually all the significant reserves have been established in the vicinity of Baton Rouge, Louisiana.

Reservoirs in the deep Lower Tuscaloosa Trend occur in an Upper Cretaceous wedge of clastics, which rapidly expands downdip from a carbonate shelf break of Early Cretaceous age. The occurrence of commercial oil and gas in the Trend is dependent on many factors. Foremost are paleoenvironment, syndepositional tectonics, source rock, original sand mineralogy, and burial diagenesis.

Reservoir distribution, thickness, and quality are closely related to the depositional environment. Commercial discoveries have been made in deep Lower Tuscaloosa sandstones deposited in fluvial, distributary channel, distributary-mouth bar, offshore bar, and prodelta environments.

Three types of productive structures have been mapped in the deep Lower Tuscaloosa. The most common structures are east-west trending closures on the downthrown side of large down-to-the-basin growth faults, which developed in response to sediment loading. A second type of structure, which is low-relief with only minor faulting, occurs north of the expanded trend. The third type of structure is defined by uplift and closure related to salt piercement.

The total organic carbon content of the Lower Tuscaloosa shales ranges between 0.5 and 2.3 wt.% and averages 0.9 wt.%. The very mature kerogen is a mixture of oil and gas prone types.

Reservoir sandstones reflect a wide range of lithologies and textures. Features that many of the reservoirs share include a high quartz and low feldspar content and ubiquitous, grain-rimming authigenic chlorite. The high quartz content, indicative of a mature mineralogy, is caused by extensive dissolution of metastable, silicate grains.

Although local variations are encountered, two diagenetic sequences are recognized. The sequences are similar in several respects. In both areas, physical compaction, accompanied by precipitation within the pore system of authigenic clay, was followed by syntaxial quartz overgrowths. This episode of porosity reduction was temporarily reversed by a process of dissolution and replacement of metastable grains. The sequences differ in the amount and type of authigenic clay, the degree of quartz cementation, and the quantity, quality, and genesis of the secondary porosity.

Primary, intergranular pore space normally dominates the pore system in the deep Lower Tuscaloosa reservoirs. Secondary porosity, created by grain dissolution and replacement, is also common and, in many reservoirs, comprises 20% of the total porosity. Reservoir porosities exceeding 20% are common where the primary and secondary pore systems are combined.

It is anticipated that additional fields will be discovered in the deep Tuscaloosa Gas Trend. An awareness of the factors that contribute to oil and gas accumulations in the deep Tuscaloosa will aid in their discovery, development, and exploitation.