AGS / Energy / Lignite / Geology
Lignite occurs at shallow depths (less than 300 feet) in sedimentary strata of Eocene age in the West Gulf Coastal Plain and along Crowley’s Ridge in the Mississippi River Alluvial Plain. These rocks are divided, from oldest to youngest, into the Wilcox, Claiborne and Jackson Groups (Figure 1). These units contain sediments which were deposited in fluvial, deltaic and marine environments. Overlying these Eocene age units in many areas are gravels, sands, silts and clays of Quaternary age.
The group is composed predominantly of fine to very-fine sands, silty sands, and clays. The sands are generally light gray when unweathered. The clays are generally light shades of gray or brown and contain varying amounts of fine sand and silt. The sands and clays frequently contain enough carbonaceous material to make them dark brown to black. Lignite occurs in beds up to 10 feet thick and is generally lenticular in shape. The larger, more important lignite beds appear to be in the lower and middle Wilcox, while thinner beds are scattered throughout. The Wilcox is thought to be deposited in a fluvial environment.
The Claiborne is as much as 1,200 feet thick in Arkansas and consists predominantly of silty clays, fine to very-fine sands, and silts. The clays are light to dark gray, blue-gray and usually very silty or sandy. The sands are medium to dark gray when unweathered and alter to white, brown and red when weathered. The silts are medium to dark gray and clayey. Lignite beds in the Claiborne are lenticular, generally of limited areal extent, with a maximum reported thickness of over 10 feet. The lignite in the Claiborne was deposited in deltaic to near shallow marine environments which affected the areal distribution and the chemistry of the lignite.
|Figure 1. Generalized stratigraphic column of the Eocene and associated strata, southern and eastern Arkansas|
During the period of 1975-1985 the Arkansas Geological Commission (now the Arkansas Geological Survey) underwent a lignite project to (1) determine the geographical and geological distribution of lignite in Arkansas at mineable depths and (2) obtain heating values and chemical analyses of lignite. 1,843 test holes were drilled to an average depth of 157 feet for a total of over 290,400 feet. Of this total footage, 246,000 feet were drilled in lignite-bearing units.
Fifty-two (52) lignite samples from the Wilcox Group were collected for chemical analyses. Proximate and ultimate analyses show that on an as-received basis Wilcox lignites average 35.7% moisture, 17.6% ash, 0.57% sulfur and 5,759 Btu/1b. Moisture-free analyses show an average of 27% ash, 0.9% sulfur and 9,004 Btu/1b. On a moisture- and ash-free basis the Wilcox lignites have 1.2% total sulfur, and 12,226 Btu/1b. Total fixed carbon averages 44% with 56% volatile matter. Based on a moist, mineral-matter-free basis Wilcox lignites average 7,100 Btu/1b and are of Lignite A rank.
Fifty (50) lignite samples from the Claiborne Group were analyzed in this project. Proximate and ultimate analyses show that on an as-received basis Claiborne lignites average 38.7% moisture, 17.9% ash, 0.62% sulfur and 5,467 Btu/1b. Moisture-free values average 28% ash, 1.03% sulfur and 9,008 Btu/1b. On a moisture- and ash-free basis the Claiborne lignites have 1.48% total sulfur, and 12,524 Btu/1b. Based on a moist, mineral-matter-free basis Claiborne lignites average 6,764 Btu/1b and are of Lignite A rank.