Abstract

Recent discoveries in the Late Jurassic Haynesville and Bossier shales have dramatically increased unconventional gas exploration activity in the mature petroleum provinces of eastern Texas and northern Louisiana. The Haynesville and Lower Bossier shales comprise the uppermost units of a transgressive systems tract of a second-order supersequence, which spans the interval from the top of the Werner Anhydrite/Louann Salt equivalent to the upper Cotton Valley clastics. Depositional variations within the shales are a function of higher-order sequences that resulted from eustatic sea-level fluctuations, paleobasin physiography, and the interplay of local subsidence and sediment input rates. The antecedent topography shaped by underlying carbonates of the Gilmer (Haynesville) Lime (Forgotson and Forgotson, 1976) and subsequent sediment budgets strongly influenced (1) facies development and stacking patterns that vary along the northern rim of the young Gulf of Mexico (GOM) Basin during Haynesville and Bossier time, and (2) the depositional processes, total organic carbon richness, and preservation of the self-sourcing Haynesville and Bossier Shale units.

The Haynesville Shale depositional system is an example of a competing carbonate and clastic system that contains contemporaneous retrogradational and progradational facies. In the western part of the system, which is carbonate-dominated and fairly restricted from siliciclastic input, the time-equivalent Gilmer (Haynesville) Lime consists of backstepping carbonate facies. In contrast, to the east, strong progradational stacking patterns, comprised of mainly siliciclastic facies assemblages, dominate in northern Louisiana and western Mississippi because of increased sediment supply from the ancestral Mississippi River, which outpaced subsidence and eustasy. Hence, major bounding stratigraphic events such as higher-order maximum flooding surfaces and condensed sections critical for shale gas exploration appear to change facies laterally whereas the second-order maximum flooding surface, or the turn from retrograding to prograding stacking patterns, appears diachronous along depositional strike. During Bossier time, the youngest carbonates were drowned and siliciclastics became increasingly dominant, expanding westward from northern Louisiana into eastern Texas and ultimately across most of the northern GOM shelf as the Cotton Valley Sandstones and its distal shale equivalents. Depending on the paleophysiography of the depositional shelf setting, some areas of the Haynesville-Bossier system were restricted and relatively sediment starved. These correspond with areas of total organic carbon enrichment and, in turn, lower shale gas exploration risk.