San Juan Basin


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 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

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, the landmasses of Laurentia and Gondwana collided to form the giant landmass of Pangea. This continental collision resulted in several pivotal orogenic 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 Uncompahgre Mountain Range, which bound the San Juan Basin on the northeast.

Mesozoic subduction

During the late Jurassic, continental collision of the Farallon and North American plates resulted in low-angle subduction beneath the western margin of the United States. Pressure on the underlying lithosphere resulting in a "depression" of the continents interior, and this allowed for the formation of the Inner Cretaceous Seaway. This began the transition from terrestrial sedimentation during the late Paleozoic and early Mesozoic to a shallow marine basin, as waters from the Arctic and Gulf regions poured into the center of the continent.

Cenozoic evolution

During late Cretaceous to early Tertiary, compressional forces continued to act and caused the uplift of the modern Rocky Mountains via the Laramide Orogeny. Early-Tertiary tilting towards the northwest resulted in over of erosion in the southeast. As compression shifted to extension and the formation of the Rio Grande Rift began, volcanism dominated the area throughout much of the Eocene and Oligocene. Uplift in the northwest and continued deposition brought the basin to its current-day configuration.

Components

The San Juan Basin is an asymmetrical syncline with three components: the Central Basin Platform, the Four Corners Platform, and the Chaco Slope. The basin is bound on the northwest by the Hogback Monocline, on the northeast by the Archuleta Anticlinorium, on the east by the Nacimiento Uplift, and on the south by the Zuni Uplift.

Depositional history

Paleozoic

Prior to collision, Mississippian- and Pennsylvanian-aged units were deposited during various marine environments, e.g. the Leadville Limestone and the Pinkerton Trail Formations. Once the supercontinents collided, the subsidence of the Paradox Basin and the uplift of the Uncompahgre highlands allowed for enormous amounts of sediment to shed off the highlands via Permian fluvial systems. The Rico Formation represents the transition from Pennsylvanian marine deposits to Permian terrestrial deposits of the Cutler Formation. The Permian continued to be a time of terrestrial deposits, including late-Permian eolian deposits.

Mesozoic

The Cretaceous was a time of three major transgressive-regressive cycles, as eustatic changes in sea-level caused fluctuations on the Western Interior Seaway's shoreline. The San Juan Basin was conveniently located in the western margin of the seaway and recorded these cycles in the stratigraphy. The westernmost extent of the seaway was recorded by the Lewis Shale, which eventually graded into the Pictured Cliffs and the Fruitland Formation as the shoreline made its final retreat.

Cenozoic

The Western Interior Seaway's shoreline regression resulted in ample swamps, lakes, and flood plains; this resulted in the coal-rich formations of the late Mesozoic/early Cenozoic . Eocene/Oligocene volcanism resulted in large volcanic aprons that covered thousands of square-kilometers, and these volcanic fields sourced the Cenozoic units the Ojo Alamo, and the Animas and Nacimiento'' formations. Uplift in the northwest, and continued deposition brought the basin to its current-day configuration.

Stratigraphy

Precambrian

Little is known about Precambrian units due to poor outcrop exposure and poor well-control. Precambrian rocks consist of quartzite, schist, and granite, and rocks are overlain unconformably by younger Paleozoic units.

Paleozoic

Little is known about the stratigraphy of the Paleozoic. Of the >40,000 wells drilled in the San Juan Basin, only about 12 have penetrated deep enough to come in contact with Paleozoic units. Additionally, poor outcrop exposure and lateral facies changes complicate the defining and correlation of these units.

Devonian

  • The Ignacio Formation consists of quartzite, sandstone, and shale layers. The unit is argued to be Late Devonian in age and was deposited by an east-trending transgression covering Precambrian rock throughout the present-day Four Corners Platform. Preservation of the Ignacio is poor and mostly limited to localized areas of down-thrown faults.
  • The Aneth Formation consists of dark limestone, clay-rich dolomite, and black shale or siltstone layers. This Late Devonian-aged unit has a depositional environment similar to that of the Ignacio Formation, though the two formations rest unconformably over one another. The Aneth Formation is found only in the subsurface, cropping out nowhere.
  • The Elbert Formation consists of two members:
  • * The McCracken Sandstone Member consists of poorly-sorted sandstones sourced from the east. Exposures can be found in the San Juan Mountains unconformably overlying Cambrian units.
  • * The unnamed upper member consists of green shales, white sandstones, and thin limestone or dolomite beds deposited in a tidal-flat environment.
  • The Ouray Formation consists of fossil-rich limestone or dolomite beds that conformably overlie the previous unit. Fossil faunas are indicative of a Late Devonian marine environment.

    Mississippian

  • The Leadville Limestone consists of shallow marine, open marine, and carbonate shelf deposits. This unit has yielded over 50 million barrels of oil in Colorado and Utah.
  • The Molas Formation consists of three members:
  • * The Coalbank Hill Member is a residual soil deposit that consists of red to brown siltstones, cherts, and conglomerates. It may rest conformably over the Leadville Limestone or unconformably over the Ouray Formation.
  • * The middle member consists of red-brown siltstones, sandstones, and conglomerates reflective of stream deposits. This unit rests unconformably over the Coalbank Hill Member.
  • * The upper member is similar to the preceding middle member, but also contains fossiliferous limestone indicative of a transgressing shoreline.
  • The Log Springs Formation is stratigraphically equivalent to the Molas Formation and lithologically similar to the Coalbank and middle members of the Molas Formation.

    Pennsylvanian

  • The Pinkerton Trail and the Sandia formations consists of grey argillaceous or fossiliferous limestone and calcareous shale beds. The units were deposited during a southwest to westward transgressing sea.
  • The Paradox Formation contains complex, cyclic evaporite deposits of alternating porous salts and non-porous shales/limestones. These work as excellent stratigraphic traps for hydrocarbons.
  • The Honaker Trail Formation consists of basal open marine limestones and dolomites overlain by arkosic sandstones from the northern Uncompahgre highland. The early 1,400' thick unit conformably overlies the Paradox Formation.
  • The southern equivalent of the Paradox and Honaker Formations is the Madera Group. Its lower section consists of grey shales and limestones of the Gray Mesa Formation that grades into the upper, acrostic sandstone-rich Atrasado Formation. Total thickness is nearly 1,300'.
  • The transition from Pennsylvanian marine units to Permian continental units is represented by the Rico Formation. It consists of conglomerates and arkosic sandstones interbedded with marine shales and fossiliferous limestones.

    Permian

  • The Cutler Group consists of alluvial fan deposits from north and northeastern sources. Deposits include arkosic sandstones, conglomerates, and minor siltstones and mudstones. The Cutler Group is divided into several formations:
  • * The Halgaito Formation consists of alternating marginal marine and fluvial sediments, and it conformably overlies the Rico Formation.
  • * The Cedar Mesa Sandstone varies with location, but contains evaporite, fluvial, tidal-flat, and sabkha facies.
  • * The Organ Rock Formation contains siltstones and sandstones from coastal-plain and fluvial deposits sourced from the north.
  • * The De Chelly Sandstone consists of sandstones of eolian deposits. The unit is broken into lower and upper members based on sediment transport directions.
  • The Yeso Formation is divided into two members:
  • * The Meseta Blanca Sandstone Member contains classic eolian deposits of cross-bedded, well-sorted sandstones. This formation may be equivalent to the De Chelly Sandstone.
  • * The San Ysidro Member contains gypsiferous sandstones interbedded with limestones and reflects complex, cyclic facies changes.
  • The Glorieta Sandstone contains buff to white, silicious sandstones indicative of eolian deposits.
  • The San Andres Limestone contains thick limestone and dolomite beds interbedded with sandstone or shale.