How was Wyoming coal formed?

WSGS Circular No. 44. Click To Enlarge.

Thick Wyoming coal beds are indicative of past swampy environments. Cretaceous coal mires were adjacent to a long beach with a shoreface sand barrier that prevented salt water from invading the swamps. The exception of short-term storm surges and transitional mangrove swamps of brackish origin where salt water mixed with fresh degraded coalification. Coal deposits of brackish origin reflected times of major storm surges, resulting in higher ash and sodium (Na) content in some Cretaceous coal deposits. As the seaway retreated to the northeast corner of the state, the wetland environments migrated in a northeast direction behind the advancing beachfront. The coal swamp consisted of organic material from trees, palms, algae, moss, and leaf matter that composted together under freshwater (with a pH <4.5) to form peat. The climate was hot, the relative sea level was high, and the paleo-latitude was semi-tropical.

WSGS Circular No. 44. Click To Enlarge.

The coal swamps accumulated organic material until they were buried by sand and mud carried from inland rivers or from the ocean. The organic material became submerged under water where it was exposed to aerobic decay (decay by underwater organisms, bacteria) creating humic matter, a material formed by microbial degradation of dead plant matter, such as lignin. The underwater section of the material from freshwater systems then underwent a gelification process of anaerobic decay that changed the peat into a hydrophilic gel-creating a partially decayed peat substance called “gytta.” The burial water was subsequently removed from this material by heat, which is the second part of the coalification process. Temperatures are very important to the formation of coal types, measured by the ‘rank’ of coal. Thermal alteration or bituminization of the coal is the end result (Jones, 2010).

The more carbon found in coal maceral, the higher the rank of coal. Initially, the organic material is thermally altered to lignite through the coalification process. Lignite or brown coal is a low rank coal, about 60 percent carbon. Next is subbituminous coal (black) at 70 percent, bituminous coal at 85 percent, and anthracite coal is 95 percent carbon. Most of Wyoming’s Tertiary coal is subbituminous, and most of the Cretaceous coal is subbituminous to bituminous. Generally, older and deeper coals are higher in rank than the younger coals in Wyoming basins. Consequently, the deeper the Powder River Basin coal mines go the higher the rank of the coal that is extracted. Exposure to high heat sources deep in the Earth’s crust can upgrade the coal rank in many Rocky Mountain coal regions. Some coals in New Mexico and Colorado, for instance, were thermally upgraded to metallurgical-grade coking coal by intrusive and extrusive Tertiary volcanics.

Wyoming’s coal quality

There are many physical parameters that make up a coal deposit's quality. Most important are the heat value, sulfur, and ash contents of the coal. The heat value is reflected in the rank of the coal, which is measured in British thermal units per pound (Btu/lb). In southwestern Wyoming, the heat values average 10,000 Btu/lb (as-received raw coal sample, not heat dried or baked). Powder River Basin (PRB) coals range from 7,710 Btu/lb to 9,410 Btu/lb, averaging 8,580 Btu/lb. The ash and sulfur contents are not rank dependent but are measured by percent. They are related to the depositional environments of the coal mire. As the coal swamp is covered by overburden sediment, it mixes with the top layers of the organic material, increasing the inorganic percentage or ash in the coal.

Sulfur content in most Wyoming coals is organically bound. Seawater is several orders of magnitude higher in SO4 than river water. As the sea retreated or prograded away from Wyoming during Cretaceous and Tertiary time, sulfur concentrations decreased. Cretaceous Wyoming coals have 0.4 to 2.0 percent sulfur, and Tertiary coals have 0.2 to 0.9 percent sulfur. Most of the coal mined in the PRB today is 0.2 to 0.3 percent sulfur, which is why it is the very lowest sulfur and naturally the “cleanest coal” in the United States. Some Wyoming coal is ‘super compliant’ meaning its sulfur quality is <0.25 percent.

Moisture is also an important parameter in Wyoming coal. Lower rank (younger) subbituminous PRB coals have much higher moisture than the higher rank (older) bituminous coals of the Green River Basin region. Moisture content of PRB coals is typically between 20 and 30 percent, while in the Green River Basin Cretaceous coals the moisture content is <15 percent. Volatile matter and fixed carbon contents also vary throughout Wyoming and are also rank dependent. Calcium and sodium concentrations in Cretaceous coals are up to six times greater than those found in Tertiary Wyoming coals. Wyoming coals also have very low concentrations of trace elements such as mercury and arsenic, when compared to Eastern U.S. coals on a national level.

Reference: Jones, N.R., 2010, Genesis of thick coal deposits and their unique angular relationships: Powder River Basin, Wyoming: Wyoming State Geological Survey Report of Investigations No. 60, 85 p.

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