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USGS Releases La Plata County Natural Gas-Seep Study Findings
Released: 6/2/1997

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Heidi Koehler 1-click interview
Phone: 303-236-5900 x317

The U. S. Geological Survey has released Open-File Report 97-59 entitled: Geology and Structure of the Pine River, Florida River, Carbon Junction, and Basin Creek Gas Seeps, La Plata County, Colorado. The results of the study reflect a collaborative effort between the USGS, Bureau of Land Management (BLM), the Colorado Oil and Gas Conservation Commission (COGCC), La Plata County, and four gas-producing companies operating in the northern part of the San Juan Basin in La Plata County, Colo.

Increased population growth in Durango and La Plata County has occurred at the same time as large-scale development of natural gas from Fruitland Formation coal beds. As a consequence, the public has become concerned about safety issues related to natural gas seeps where Fruitland coal beds crop out. The USGS was asked to study Fruitland coal beds in the seep areas at the surface and near the coal outcrops in the subsurface as part of a multifaceted study to evaluate the nature of the seeps.

"The goal of the La Plata County study was to determine if man-made activities like drilling wells and producing natural gas near coal outcrops could be causing or contributing to the seepage of natural gas at coal outcrops in La Plata County, " said James Fassett, geologist and USGS project team leader.

In the Pine River seep area, where the subsurface geologic data were most complete, geologic evidence indicated that the gas seeps were probably not related to the presence of nearby gas wells. In the Florida River, Carbon Junction and Basin Creek seep areas, subsurface geologic data were not detailed enough to warrant positive conclusions regarding the relations of the gas seeps to nearby producing gas wells.

A seep is defined as a place where fluids, such as water, petroleum, or natural gas emanate from the Earth’s surface. Most seeps are natural, but in some unusual circumstances man-made activities can create new seeps or change the rate of flow of natural seeps. Because gas seeps can constitute a serious hazard, the BLM and COGCC were concerned about the source of the gas seeps. If nearby gas-producing wells were contributing to the seeps, these regulatory agencies would have to consider passing new regulations regarding proximity of gas wells to the outcrop, mandate mitigation measures, or in extreme cases, order the shutting-in of producing gas wells.

La Plata County officials required detailed geologic maps of outcropping coal beds in the major seep areas in order to precisely target possible hazardous areas where building development would have to be restricted. Gas-producing companies were concerned because they wanted to operate in an environmentally responsible manner and regulatory actions resulting in shut-in wells would be very costly. The companies wanted any such regulatory actions to be based on a totally objective scientific analysis of all available data. The USGS was selected to conduct the geologic study because of its long and well-established reputation for providing credible, reliable, and impartial Earth-science information to nearly 2,000 partners at the local, state, and regional level.

The report was prepared by USGS geologists J.E. Fassett, S.M. Condon, A.C. Huffman, and D.J.Taylor and consisted of four parts; 1) detailed surface mapping of Fruitland Formation coal outcrops in the seep areas, 2) detailed measurement of joint and cleat patterns in the seep areas, 3) detailed coal-bed correlation of Fruitland coals in the subsurface adjacent to the seep areas, and 4) studies of deep-seated seismic patterns in those seep areas where seismic data were available. The open-file report is divided into three chapters containing the results of the subsurface coal-bed correlation study, the results of the surface geologic mapping and joint measurement study, and the results of the deep-seismic study. The report is 126 pages long and contains seven folded plates consisting of the geologic maps of the seep areas and measured geologic sections of the Fruitland Formation. Copies of the report are available from USGS Information Services, Open-File Reports, P.O. Box 25286, Denver, CO 80225; phone, (303) 202-4200. It can also be viewed on the Internet at http://energy.cr.usgs.gov:8080/. Click on "Oil & Gas", then "La Plata County Gas Seeps Report". The report can be downloaded in its entirety. For La Plata County residents, the report can be reproduced for printing cost at the Bureau of Land Management office in Durango, U.S. Federal Building, 701 Camino Del Rio, Durango, CO 81301.

Detailed information follows:

Subsurface coal-bed correlation

Continuity of Fruitland Formation coal beds in the gas seep areas is quite variable. The best subsurface control is in the Pine River gas seep area. There, several monitor holes were drilled very close to the active gas seeps to determine if Fruitland coal beds were indeed serving as conduits for gas migrating updip from greater depths. Subsurface correlations indicate that the thick coal beds that are major gas producers in the subsurface are not continuous to the outcrop in the Pine River gas seep area. The lowermost Fruitland coal beds that do produce gas downdip, thin, split, and grade into high-ash coal to carbonaceous shales in the gas-seep area. Furthermore, down-hole videos in the monitor holes closest to the seeps show that gas is escaping primarily from upper, discontinuous Fruitland coal beds (probably by desorption) whereas, no free gas is entering the monitor holes from lower Fruitland coal beds. This would seem to indicate that these lower coal beds are not conduits for methane liberated by gas-producing wells at depth.

In the other gas-seep areas studied, no monitor holes have been drilled close to the coal outcrops, thus, the continuity of coal beds from the subsurface to the outcrop is less certain. It appears, however, that there is a good probability that at least some of the major coal beds that produce coal-bed methane at depth do extend to the outcrop in these gas-seep areas.

Fracture patterns in rocks near gas seeps

A study of outcrops of the Upper Cretaceous Pictured Cliffs Sandstone and Fruitland Formation was conducted at the four gas-seep areas in La Plata County, Colo., to determine the degree of fracturing and the orientation of fractures. In general, there is one main joint set (J1)in each geologic unit in each area, and a second subordinate set (J2) that is oriented about 90º to the first set. The J1 and J2 sets are the most common and pervasive in all the areas studied, however, a third and fourth set are present in some areas. Comparable fractures in coal beds are referred to as cleats, which also occur in two sets, a face cleat and a butt cleat.

Over the entire project area there appears to be a clockwise rotation from northwest to northeast of the J1 joint set in the Pictured Cliffs and in sandstone beds of the Fruitland. Face cleats display a slight counterclockwise rotation from north-south to northwest in the same area. The time of cleat development in Fruitland coals might have been shortly after burial, during coalification. The time of joint propagation in sandstones of the Pictured Cliffs and Fruitland was most likely during the Laramide orogeny in response to rotation of the Colorado Plateau. Joint and cleat trends observed at the outcrop probably extend southward into the subsurface of the San Juan Basin. Possible migration paths of methane would be north to northwest through cleats in coal and northwest to northeast through joints in sandstone.

Deep seismic study

Analysis of several reflection seismic lines made available by Amoco, Maxus Energy, and KN Energy, Inc. revealed the presence of a number of inactive faults in the subsurface. The principal fault in the area is the large thrust fault that underlies the Hogback Monocline along the northern and western margins of the San Juan Basin. This large fault offsets the entire sedimentary rock section as well as the underlying crystalline rocks and may cause significant fracturing in the immediate vicinity of the fault but is typically not visible at the surface. Other faults, projecting from the basin-margin fault into the basin, have broken and offset some of the younger sandstone, shale, and coal units but also have only rarely been observed at the surface.

There is no evidence that there has been any significant movement on any of these faults for millions of years. Studies of seismic lines in the vicinity of the Pine River gas seeps indicate little or no measurable offset of the coal-bed methane producing interval (Fruitland Formation) attributable to deep seated faults other than the basin margin thrust fault itself.

The U.S. Geological Survey provides the nation with reliable, impartial information to describe and understand the Earth. This information is used to minimize the loss of life and property from natural disasters and manage water, biological, and energy and other mineral resources in the wisest way possible.

For additional information contact:
Jim Fassett
U.S. Geological Survey
P.O. Box 25046, MS 939, DFC
Denver, Colo. 80225
(303) 236-0609

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