Regional Tectonic and Hydrocarbon-Trap Modeling Along the Louisiana Offshore
Authors: Lowrie A. , Sullivan N.M. , Hoffman K.S. 
Published: October 2008

In order to understand the formation of related hydrocarbon traps and to aid in the location of these traps, three dynamic, regional tectonic models are presented for salt extrusion southward into the Gulf of Mexico. Each model is characterized by a different depositional setting for the original salt: continental, transitional, and oceanic crust. Regional salt movements, lateral and vertical, are calculated assuming isostatic salt adjustment to sedimentation. Computations include average and highstand/lowstand rates. Regional geologicprocesses, such as upward salt migration, growth fault movement, growth of intraslope bas ins, and regional subsidence, are estimated both qualitatively and quantitatively.

Potential hydrocarbon trap types in offshore Louisiana are viewed within the context of the regional salt-extrusion models. The trap types are listed according to paleophysiographic province of original deposition: coastal plain, shelf, shelf break, upper slope, middle slope, and lower slope. Characteristics of each trap type are categorized in terms of: tectonics, regional and local sedimentation rates and types, position within an evolving sequence as determined by sequence stratigraphy, duration of reservoir and/or trap creation, and sea level position.

Salt Occurrence Along the Northern Gu1f of Mexico

Upper middle Jurassic (Callovian) Louann salt is presently found overlying continental, transitional, and ocean crust. Overlying continental crust, extensive salt deposits are found in central and northern Louisiana and southern Arkansas. The basins containing them include the Mississippi Interior, East Texas, and North Louisiana basins, the Tampa Embayment, and the more regional Gulf CoastGeosyncline. Along the latter, salt is found west of the Santa Rosa Arch, 86°W, and generally east of the San Marcos Arch, 97°W, and locally within the Rio Grande Embayment. These salts appear to be autocraneous.

Overlying transitional crust are the extensive salt deposits generally in the Louisiana offshore, with some salt in the eastern Texas offshore to approximately 96°W, and in the Alabama offshore to approximately 87°W. This salt may be autocraneous, or it was laterally extruded southward from the north (the author's opinion). (See Humphris, 1978; Jackson, 1989, among others.)

Overlying oceanic crust, salt extends from approximately 87°W to 96°W. The southern boundary is the bathymetrically significant Sigsbee Escarpment. The northern boundary is transitional, with salt overlying transitional crust. This salt is generally held to have been extruded from the north. See Humphris, 1978; Jackson, 1989, among others. Mention is made by Lopez, 1989, of a stock-like feature underlying this salt extruded wedge near 27°N, 90° 20'W. This feature, confirmed by seismic data, may be an isolated salt dome rising to the extruded salt wedge. Also, this dome may have been derived from normal seafloor spreading processes.

Models for Regional Salt Deposition

Here we will explore various depositional scenarios in which salt may be deposited regionally. The intent is to find the or those scenarios that could explain the massive deposition of Louann salt in the Gulf of Mexico area. Thus, whatever scenario(s) are selected, the capacity for massive salt deposition is a paramount criteria.