The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah and Arizona. Specifically, the basin occupies space in the San Juan, Rio Arriba, Sandoval, and McKinley counties in New Mexico, and La Plata and Archuleta counties in Colorado. The basin extends roughly N-S and E-W. The San Juan Basin is an asymmetric structural depression in the Colorado Plateau province, with varying elevation and nearly in topographic relief. Its most striking features include Chaco Canyon (northwestern New Mexico, between Farmington and Santa Fe) and Chacra Mesa. The basin lies west of the Continental Divide, and its main drainage is the southwest- to west-flowing San Juan River, which eventually joins the Colorado River in Utah. Climate of the basin is arid to semiarid, with an annual precipitation of and an average annual temperature of around . The San Juan Basin has been a major producer of oil and natural gas since the early 20th century, with currently more than 300 oil fields and over 40,000 drilled wells throughout the area. As of 2009, cumulative production reached 42.6 trillion cubic feet of gas and 381 million barrels of oil. The area is especially known for gas reservoirs from its coal-bed methane formations. The San Juan Basin contains the largest coal-bed methane field in the world and ranks second in total gas reserves. Tectonic Evolution The Ancestral Rockies During the mid-Paleozoic, the San Juan Basin was part of the ancient landmass called Laurentia; this was a supercontinent that contained much of current-day North America. The ancient landmass called Gondwana contained most of the southern continents, e.g. South America and Africa. During the late-Mississippian (~320 million years ago), the landmasses of Laurentia and Gondwana collided to form the giant landmass of Pangea. This continental collision resulted in several pivotal orogenic (mountain building) episodes. The collision of supercontinents Gondwana and Laurentia resulted in the Alleghanian and Ouachita orogenies. The Alleghanian Orogeny was the collision of Africa with the current-day southeastern United States, and resulted in the Appalachian Mountains. The Ouachita Orogeny was the collision of South America with the current-day Gulf-region, and resulted in the Ancestral Rockies - a northwest trending intercontinental mountain belt mainly through Texas, New Mexico, and Colorado. The Ancestral Rockies gave way to the