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WAT-2011-02 – Geologic Model of the Purgatoire River Watershed within the Raton Basin, Colorado

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A report prepared for the Colorado Water Conservation Board (CWCB). 54 pages. Digital PDF download. WAT-2011-02D

From the Introduction

The coal resources of the Raton Basin of southeastern Colorado have been critical to the development and economy of this region since the late 1800s. When large-scale, commercial coal mining ceased in the mid 1990s, a new extractive industry, production of coalbed methane (CBM) started to boom. The production of natural gas (CBM) from coal seams in the Vermejo and Raton Formations in Las Animas County increased from 28 billion cubic feet (BCF) in 1999 to over 126 BCF in 2008 (Colorado Oil & Gas Conservation Commission). Groundwater coproduced by CBM wells in Las Animas County also increased dramatically f rom 22.9 million barrels in 1999 to a peak of 119 million barrels in 2007 (Colorado Oil & Gas Conservation Commission). The coal-bearing geologic formations critical to these industries are also part of the aquifers upon which residents rely for water supply. The mineral and water resources of the basin have and continue to play an integral part in the development and economy of the region. Furthering our understanding of the geologic formations that comprise these resources affords the best opportunity to effectively manage and protect them.

In 2009, the Colorado Water Conservation Board funded a severance tax grant request by the Colorado Geological Survey to study these resources. The purpose of this geologic investigation was to provide additional information and develop a geologic model to depict the stratigraphic and structural relationships of the coal-bearing formations in the Purgatoire River watershed of Las Animas County, Colorado (study area). The vast majority (nearly 90%) of the CBM production in the Colorado portion of the Raton Basin has been concentrated within the Purgatoire River watershed of Las Animas County. This study was undertaken because of the recent interest in and need to protect and manage both the mineral and water resources within these relatively shallow formations, and is facilitated by a large amount of recent subsurface data available from over 3,500 geophysical well logs acquired from CBM wells over the past fifteen years. The analysis and interpretations provided herein were made by integrating available surface and subsurface geologic data in a GIS environment to construct structural and stratigraphic cross-sections; develop maps depicting the structural surfaces of the Pierre Shale, Trinidad Sandstone, Vermejo, and Raton Formations; identify coal zones, and assemble a new digital composite surface geologic map from previous geologic investigations in the area. These formation surfaces, in digital format, provide for future geospatial analysis and model integration.