Climatically Controlled Redox Cycles of the Upper Cretaceous Mooreville Formation in Mississippi

E. Heydari
Mississippi Office of Geology, Jackson, MS

The significance of the Selma Chalk as an oil and gas producer in Mississippi has been recognized for many years. In the salt basin, the potential reservoir rocks are often high in microporosity but low in permeability which has thwarted commercial gas production. In 1991, the Selma produced only 4725 MCFG in Gwinville field in Covington County. In 1993, Enron began to redevelop the Selma gas in Gwinville, and by 1999 the Selma was producing 3,450,826 MCFG per year and growing rapidly. This dramatic development emphasizes the chalk’s future possibilities. A core study of the Selma Chalk equivalent formations in the outcrop area is presented in order to unravel detailed lithofacies characteristics of this unit.

The Mooreville Formation in Noxubee County, Mississippi, is 81 m (268 feet) thick and consists of 51 redox cycles, ranging in thickness from 0.3 to 5 m (1-15 feet). Each cycle is composed of a basal laminated section indicating sedimentation under anoxic bottom water conditions, hostile to burrowing organisms. It grades upward into a gray, wispy laminated interval, suggesting that bottom waters became slightly oxygenated and organisms tolerant of low oxygen conditions were sparely present. Cycles are capped by a bioturbated layer indicative of deposition under fully oxygenated conditions.

Cycles were initiated during wet climatic periods with the introduction of copious amounts of siliciclastics and terrestrial organic matter into the basin. Oxidation of organic matter at the sedimentwater interface or in the uppermost portion of the sediment column resulted in oxygen depletion and bottom water anoxia, leading to the deposition of laminated strata. Riverine input decreased as climatic conditions became arid, limiting the input of terrestrial organic matter. Then, the amount of oxygen in bottom sediments increased to the point that deposition of the dysoxic section of the cycle began. Eventually, organic matter input decreased significantly that bottom waters became fully oxygenated and heavily bioturbated sedimentation resumed.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana