FISSURE FORMATION AND SUBSURFACE SUBSIDENCE IN A COALBED FIRE Taku S. Ide 1, David Pollard 2, and Franklin M. Orr, Jr. 3 1,3 Dept. of Energy Resources and Engineering, 2 Dept. of Geological and Environmental Sciences Stanford University, Stanford, CA 94305 e-mail: [email protected] was started in May of 1962 when the local government Abstract decided to burn an unregulated trash dump in an abandoned strip mine to reduce trash volume and Coalbed fires are uncontrolled subsurface fires control rodents. The fire ignited an anthracite outcrop, that occur around the world. These fires are believed to eventually connected to and spread through be significant contributors to annual CO emissions. 2 underground tunnels, and has been burning since. Although many of these fires have been burning for Fissures created by the coal fire emit assorted hot gases, decades, researchers have only recently begun to some of which are toxic. A combination of subsidence investigate physical mechanisms that control fire and emissions from fissures has caused the town of behavior. One aspect of fire behavior that is poorly Centralia to be abandoned (DeKok, 1986, GAI characterized is the relationship between subsurface Consultants, 1983). combustion and surface fissures. At the surface above The particular fire examined in this study, called many fires, long, wide fissures are observed. At a the North Coalbed Fire, to distinguish it from other coalbed fire near Durango, CO., these fissures form active fires in the region, was discovered in 1998 on the systematic orthogonal patterns that align with regional Southern Ute Indian Reservation when sets of fissures joints in the Upper Cretaceous Fruitland Formation. that are orthogonal to each other—similar to those at Understanding the mechanisms that form these fissures other coal fires around the world—appeared at the is important, as the fissures are believed to play vital surface (Williamson, 1999). Anecdotal evidences roles in sustaining the combustion in the subsurface. In provided by local Southern Ute Tribe members (Ide, some of the coalbed fire simulation models available 2007) suggest that the fire may have been smoldering today, these fissures are treated as fixed boundary for decades prior to the reported date of discovery. The conditions. We argue, using data collected, field fire continues to burn today. The research effort observations and simulation result, that there exists a described here is an attempt to understand whether coal relationship between the location and magnitude of combustion followed by subsurface subsidence can subsidence caused by the fire and the opening of produce fissures with systematic patterns at the surface. fissures. The results presented suggest that fissures are Subsidence can occur when a burned coalseam loses its believed to open when subsurface subsidence gives rise structural integrity and collapses under the weight of to tensile stresses around pre-existing joints. the overburden. Understanding the formation of fissures is important, as they appear to foreshadow the direction Keywords: of the combustion front propagation and may play a key coalbed fire, coal fire, subsidence , pre-existing role in sustaining the underground fire. joints, fissures, numerical modeling, CO 2 First, we summarize the San Juan Basin geology, highlighting key features in the NW section of the Introduction basin, where the coalbed fire is located. Second, we characterize the geological features and the surface Uncontrolled subsurface fires in coalbeds account anomalies in the vicinity of North Coalbed Fire. for significant releases of CO to the atmosphere. One 2 Surface topography, images of fissures overlying the of the world’s largest active coalbed fires has been coalbed fire, and measurements of fissure orientations documented in Wuda, China (Dai et al., 2002), where are presented. We also outline the process of digitizing the estimated annual loss of coal is around 200,000 features over the North Coalbed Fire and describe how tons, equivalent to a yearly emission of ~1.5Mt of CO 2 they were combined with the subsurface information (Kuenzer et al., 2005). Coalbed fires are burning in obtained from the wells drilled in the area. In the third many locations in China, Indonesia, India, and the section, the field data and previous geological surveys United States (Stracher and Taylor, 2004). They can be of the area are used to suggest how subsidence can open started naturally by forest fires that burn near an pre-existing joints in the area, leading to the formation outcrop, by lightning strikes, by human activities, or by of surface fissures over the combustion region. Finally, spontaneous exothermic reactions of pyrites (DeKok, we model this phenomenon using a simple boundary 1986). Forest fires in Indonesia in 1997 and 1998 element numerical code, and explore relationships ignited hundreds of coal fires at outcrops (Brown, among key variables that contrast subsidence activities 2003). A subsurface fire near Centralia, Pennsylvania, Stanford Rock Fracture Project Vol. 20, 2009 C-1 and surface deformation. The results and the limitations the coalbed fire, formations above the dotted line—the of applicability of this simulation model are discussed. Kirtland Shale and most of the Fruitland Formation— have been removed by weathering and erosion. The San Juan Basin Geology Kirtland Shale and the Fruitland Formation lie atop of the Pictured Cliffs Sandstone (PC), which was The San Juan Basin is an asymmetric, coal bearing deposited as regressive marine sands (Condon, 1988) basin that covers approximately 16,800 – 19,400 square parallel to the shoreline stretching northwest-southeast kilometers, stretching approximately 145km west-east (Fassett, 2000). The Fruitland Formation is a mixture of and 160km north-south (Fassett, 2000, Kelso et al., mudstones, siltsones, carbonaceous shales and coals 1988). It is located near the Four Corners, and spans deposited landward and parallel to this shoreline across northwest New Mexico and southwest Colorado (Fassett, 2000). Coalseams in the Fruitland Formation as shown in Figure 1. The basin is well characterized are often referred to as Lower Coal, Middle Coal and due to the abundance of both coal and coal-bed Upper Coal, and the thickest, most continuous coalbeds methane resources in the Upper Cretaceous Fruitland are found in the Lower Coal Zone in the northeastern Formation (Figure 2). One study has estimated that a region of the San Juan Basin (Sandberg, 1988). The coal-bed methane reserve of nearly 1.4 x 10 12 m 3 (50 x Lower Coal is burning at the North Coalbed Fire. 10 12 ft 3) adsorbed onto 219 x 10 9 metric tons of coal that underlies the San Juan Basin (Kelso et al., 1988). The flat, Central Basin is bounded by several key geologic features, which are described in detail in previous geologic surveys of the area (Fassett, 2000, Kelso et al., 1988, Lorenz and Cooper, 2000). The western and northwestern regions of the basin are circumscribed by the Defiance and the Hogback monoclines, respectively, and the Nacimiento uplift borders the basin on the east side (Lorenz and Cooper, 2000). As Figure 1 shows, the North Coalbed Fire is located along the Hogback Monocline in the northwestern portion of the basin. The Hogback monocline is believed to have formed either due to the shortening of the Cretaceous strata that induced a right- lateral strike-slip motion along the western and eastern margins during the Laramide orogeny (Lorenz and Cooper, 2000), or through reactivation of western dipping thrust faults underlying the Hogback monocline that resulted in the uplift (Taylor and Huffman, 1988). In the former view, the shortening can be attributed to the Zuni uplift thrusting northward and north northeastward into the San Juan Basin from the south, and the San Juan uplift indenting southward into the Figure 1: San Juan Basin and its characteristic basin (Lorenz and Cooper, 2000). Today, only the geologic features. The North Coalbed Fire location forelimb of the Hogback Monocline is exposed and is highlighted in the box in the northwestern corner some of the formation members of the Upper of the basin along the Hogback Monocline. The Cretaceous are exposed on the western side of the green area denotes outcrops of Pictured Cliffs basin. Along the northern perimeter of the Basin, sandstone. Figure reproduced from Lorenz and Cooper, 2003. including areas affected by the North Coalbed Fire, thick coalseams crop out along the Hogback monocline At the North Coalbed Fire, 14 boreholes were (Kaiser et al., 1991). drilled in 2007 over an area of 600m x 200m. The high Formations that make up the Upper Cretaceous density of boreholes allowed reliable subsurface rocks in the San Juan Basin are described by Molenaar correlations to be made at the site. Both the PC and the (Molenaar and Baird, 1992). The Fruitland Formation, Fruitland Formation rise in a step-like fashion from the which includes the coalbed fire, and adjacent geologic southwest to the northeast with respect to the units are depicted in the stratigraphic column in Figure isochronously deposited Huerfanito Bed in the Lewis 2. The left column is representative of the entire San shale, representing a migrating regression-transgression Juan Basin. The right column shows the top 25m of cycle over 1.2 million years (Fassett, 1971, Sandberg, rock found directly over the North Coalbed Fire. Above 1988). The deposition pattern suggests that the Stanford Rock Fracture Project Vol. 20, 2009 C-2 subsurface correlation along the shoreline in the approximately 600 m x 200m. There are signs of northwest-southeast direction is warranted, as this is the coalbed fires underlying the bare patch of land south trend of the long axis of most coal deposits (Fassett, southwest of the North Coalbed Fire, but only minor 1988). The echelon geometry of coal deposits can make surface deformation is observed; thus this area is not subsurface correlations difficult in the transverse included in the surveys.