Stratigraphy and Selected Gas-Field Studies of North Louisiana

Abstract

Abstract North Louisiana is underlain by approximately 5,000-20,000 ft of latest Triassic-Jurassic to Eocene sedimentary rocks deposited in the northern part of the Gulf Coast geosyncline. Most of this section has commercial accumulations of hydrocarbons. These strata were deposited unconformably on an undetermined thickness of sedimentary rocks, so far nonproductive, which range in age from Paleozoic through Late Triassic. The oldest known beds of the Gulf Coast geosyncline are the redbeds of the Werner Formation and the overlying Louann Salt of latest Triassic-Middle Jurassic age. The salt was deposited in the Interior Salt basin, which is bounded on the west by the Sabine uplift—and thus is not continuous with the East Texas Salt basin—and which joins with the Mississippi Salt basin on the east. On the south, the Interior Salt basin may connect with the Coastal Salt basin of South Louisiana and coastal Texas through LaSalle, Avoyelles, and Rapides Parishes. Thirty-one piercement salt domes are known in North Louisiana, but only one of these is productive. This contrasts sharply with the more than 148 piercement domes of South Louisiana, of which 127 are productive. The Late Jurassic-Eocene section above the Louann Salt contains all the hydrocarbons thus far discovered in this region. In general the section thins northward into southern Arkansas and westward onto the Sabine uplift. It thickens south and east toward the coast and the Mississippi embayment, respectively. Facies changes accompany the thickness changes; these changes are particularly pronounced in the Late Jurassic-Early Cretaceous section which grades successively, from north to south, from littoral and continental facies, to shelf facies, carbonate reef facies, and, ultimately, basinal shale facies. Uplift and erosion affected much of North Louisiana before the Upper Cretaceous (Tuscaloosa) transgression. Positive movements of the Sabine uplift restricted the distribution of the basal Tuscaloosa sandstones. The Upper Cretaceous sea gradually transgressed the whole of North Louisiana. The sea retreated again from parts of Louisiana near the end of Cretaceous time and during Paleocene time. Paleocene-Eocene deposition was extensive, but the reservoirs which developed were deposited mainly in the Wilcox (late Paleocene-early Eocene) part of the sequence in eastern and southeastern North and Central Louisiana. The mobility of the underlying Louann Salt caused the formation of numerous traps during deposition of the Late Jurassic-Eocene geosynclinal sequence. Renewed vertical movements of the Sabine uplift—which was in existence before the deposition of the Louann Salt—and repeated tilting toward the Gulf of Mexico and Mississippi embayment caused the formation of numerous structural and combination stratigraphic-structural traps. Many wholly stratigraphic traps also formed. The deposition of the “Monroe Gas Rock” during Late Cretaceous time led ultimately to the accumulation of one of the largest gas deposits in North America—the Monroe field. Exploration of hydrocarbons in North Louisiana is in an advanced stage of development. To date 17 trillion cu ft of gas has been produced. Of this, Monroe field alone has produced more than 6 trillion cu ft. Estimated future gas reserves of North Louisiana are in the order of 8 trillion cu ft. However, Central Louisiana scarcely has been explored, and potentially a large and important reserve remains to be discovered in that region.