The San Juan Basin is a geologic structural basin located near the

Four Corners The Four Corners is a region of the Southwestern United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area ...

region of the

Southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural region of the United States that generally includes Arizona, New Mexico, and adjacent portions of California, Colorado, Ne ...

. The basin covers 7,500 square miles and resides in northwestern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...

, southwestern

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

, and parts of

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

and

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

. Specifically, the basin occupies space in the San Juan,

Rio Arriba Rio Arriba County is a List of counties in New Mexico, county in the U.S. state of New Mexico. As of the 2010 United States Census, 2010 census, the population was 40,246. Its county seat is Tierra Amarilla, New Mexico, Tierra Amarilla. Its nort ...

Sandoval Sandoval is a habitational surname of Spanish language, Spanish origin. It primarily originates from Sandoval de la Reina, Spain, earlier called ''Sannoval'', which is a blend word of Latin ''saltus'' (meaning 'grove' or 'wood') and Latin ''novalis ...

, and McKinley counties in New Mexico, and

La Plata La Plata () is the capital city of Buenos Aires Province, Argentina. According to the , it has a population of 654,324 and its metropolitan area, the Greater La Plata, has 787,294 inhabitants. It is located 9 kilometers (6 miles) inland from th ...

and

Archuleta ''Archuleta'' is a unique family name with origins in Spain. The surname ranked 2,762 out of 88,799 in the United States. Origin The name ''Archuleta'' is a Castillian derivation of Basque ''Arechuleta'', which ultimately comes from the place na ...

counties in Colorado. The basin extends roughly N-S and E-W. SJB US Map

The San Juan Basin is an asymmetric structural depression in the

Colorado Plateau The Colorado Plateau, also known as the Colorado Plateau Province, is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. This province covers an area of ...

province, with varying elevation and nearly in topographic relief. Its most striking features include

Chaco Canyon Chaco Culture National Historical Park is a United States National Historical Park in the American Southwest hosting a concentration of pueblos. The park is located in northwestern New Mexico, between Albuquerque and Farmington, in a remote c ...

(northwestern New Mexico, between

Farmington Farmington may refer to: Places Canada *Farmington, British Columbia * Farmington, Nova Scotia (disambiguation) United States *Farmington, Arkansas *Farmington, California *Farmington, Connecticut *Farmington, Delaware * Farmington, Georgia * ...

and Santa Fe) and Chacra Mesa. The basin lies west of the

Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not ...

, and its main drainage is the southwest- to west-flowing San Juan River, which eventually joins the

Colorado River The Colorado River ( es, Río Colorado) is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and northern Mexico. The river drains an expansive, arid drainage basin, watershed that encompasses parts of ...

in Utah. Climate of the basin is

arid A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ar ...

semiarid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi- ...

, with an annual precipitation of and an average annual temperature of around . The San Juan Basin has been a major producer of

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

and

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

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 Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Au ...

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 Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

, the San Juan Basin was part of the ancient landmass called ''

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

;'' this was a supercontinent that contained much of current-day

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. The ancient landmass called ''

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

'' contained most of the southern continents, e.g.

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

and

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. During the late- Mississippian (~320 million years ago), the landmasses of Laurentia and Gondwana collided to form the giant landmass of ''

Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

''. This continental collision resulted in several pivotal

orogenic Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent boundary, convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumpl ...

(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 Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. They ...

. 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 The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

, continental collision of the Farallon and

North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Ca ...

plates resulted in low-angle ("flat slab")

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

beneath the western margin of the United States. Pressure on the underlying

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

resulting in a "depression" of the continents interior, and this allowed for the formation of the ''Inner Cretaceous Seaway'' (

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...

). 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 The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

early Tertiary The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...

, compressional forces (continued subduction of the Farallon Plate) continued to act and caused the uplift of the modern

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

via the ''

Laramide Orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the o ...

''. 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 The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihua ...

'' began, volcanism dominated the area throughout much of the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

and

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

. Uplift in the northwest and continued deposition brought the basin to its current-day configuration.

Components

The San Juan Basin is an asymmetrical

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...

with three components: the Central Basin Platform, the Four Corners Platform, and the Chaco Slope (a.k.a. the Chaco ''Homocline''). The basin is bound on the northwest by the Hogback Monocline (separating the Central Basin and Four Corners platforms), 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 In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

, Mississippian- and Pennsylvanian-aged units were deposited during various marine environments, e.g. the ''Leadville Limestone'' and the ''Pinkerton Trail'' Formations. Once the

supercontinents In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leav ...

collided (see ''Tectonic Evolution'' above), the

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...

of the

Paradox Basin The Paradox Basin is an asymmetric foreland basin located mostly in southeast Utah and southwest Colorado, but extending into northeast Arizona and northwest New Mexico. The basin is a large elongate northwest to southeast oriented depression for ...

and the uplift of the Uncompahgre highlands allowed for enormous amounts of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

to shed of 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 The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

continued to be a time of terrestrial deposits, including late-Permian eolian deposits.

Mesozoic

The

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

was a time of three major transgressive-regressive cycles, as

eustatic Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

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 (see ''Stratigraphy'' below). The westernmost extent of the seaway (a.k.a. maximum transgression) was recorded by the

Lewis Shale The Lewis Shale is a geologic formation in the Western United States. It preserves fossils dating back to the Campanian to Maastrichtian stages of the late Cretaceous period. Description The Lewis Shale is an olive-gray marine shale with som ...

, which eventually graded into the Pictured Cliffs and the

Fruitland Formation The Fruitland Formation is a geologic formation found in the San Juan Basin in the states of New Mexico and Colorado, in the United States of America. It contains fossils dating it to the Campanian age of the late Cretaceous.

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 Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...

-rich formations of the late

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

/early

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

(e.g. ''Fruitland Formation'' and the ''Kirtland Shale)''.

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...

resulted in large volcanic aprons that covered thousands of square-kilometers, and these volcanic fields sourced the Cenozoic units the ''Ojo Alamo'' (sourced from the west), and the ''Animas'' and ''Nacimiento'' formations (sourced from the northeast). Uplift in the northwest (and subsequent erosion), and continued deposition (e.g. the ''San Jose Formation'') brought the basin to its current-day configuration.

Stratigraphy

Precambrian

Little is known about

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

units due to poor outcrop exposure and poor well-control. Precambrian rocks consist of

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

, and

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

, and rocks are overlain unconformably by younger

units.

Paleozoic

Little is known about the

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

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

units. Additionally, poor outcrop exposure and lateral facies changes complicate the defining and correlation of these units.

Devonian

* The

Ignacio Formation The Ignacio Formation (Ingacio Quartzite) is a geologic formation that crops out in the San Juan Mountains of southwestern Colorado. Long thought to be Cambrian in age, the formation is now thought to be upper Devonian in age, based on detrital ...

consists of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

, and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

layers. The unit is argued to be

Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...

in age and was deposited by an east-trending transgression (unconformably) 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 Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

-rich

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

, and black

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

layers. This Late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

-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 The Elbert Formation is a geologic formation that crops out in the San Juan Mountains of southwestern Colorado. The formation contains fossils indicating it is upper Devonian in age. Description The Elbert Formation is divided into the lower McC ...

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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

-rich (

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, whi ...

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...

crinoids Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

, etc.) limestone or dolomite beds that conformably overlie the previous unit. Fossil faunas are indicative of a Late Devonian (and possibly early Mississippian) marine environment.

Mississippian

* The

Leadville Limestone The Leadville Limestone is a Mississippian geologic formation in the western United States.Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

and

. * The

Molas Formation The Molas Formation is a geologic formation that is found in the Four Corners region of the United States. Its age is poorly constrained but is thought to be Namurian (late Mississippian to middle Pennsylvanian). Description The Molas Formati ...

consists of three members: ** The Coalbank Hill Member is a residual soil deposit that consists of red to brown

siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

, cherts, and conglomerates. It may rest conformably over the

Leadville Limestone The Leadville Limestone is a Mississippian geologic formation in the western United States.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 (north) and the Sandia (south) formations consists of grey

argillaceous Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...

fossiliferous A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

and

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

shale beds. The units were deposited during a southwest to westward transgressing sea. * The

Paradox Formation In geology, the Paradox Formation Is a Pennsylvanian age formation which consists of abundant evaporites with lesser interbedded shale, sandstone, and limestone. The evaporites are largely composed of gypsum, anhydrite, and halite. The formatio ...

contains complex, cyclic evaporite deposits of alternating porous salts and non-porous shales/limestones. These work as excellent

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...

traps for

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

. * The

Honaker Trail Formation The Honaker Trail Formation is a geologic formation in Utah. It preserves fossils dating back to the Carboniferous period. It is a member of the 3-member Hermosa Group and is located above the Paradox Formation in the dramatic site of Goose ...

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 The Madera Group is a group of geologic formations in northern New Mexico. Its fossil assemblage dates the formation to the middle to late Pennsylvanian period. Description The group consists primarily of marine limestones, and it is exposed in ...

. Its lower section consists of grey shales and limestones of the Gray Mesa Formation that grades into the upper, acrostic sandstone-rich

Atrasado Formation The Atrasado Formation is a geologic formation in New Mexico. Its fossil assemblage dates the formation to the Kasimovian age of the Pennsylvanian. It was formerly known locally as the Wild Cow Formation or the Guadelupe Box Formation. Descript ...

. 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 The Cutler Formation or Cutler Group is a rock unit that is exposed across the U.S. states of Arizona, northwest New Mexico, southeast Utah and southwest Colorado. It was laid down in the Early Permian during the Wolfcampian epoch. Descri ...

consists of alluvial fan deposits from north and northeastern sources (e.g., the Uncompahgre and the San Luis highlands). 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 Cedar Mesa Sandstone (also known as the Cedar Mesa Formation) is a sandstone memberThe Cutler Formation is sometimes classified as the Cutler Group, in which case Cedar Mesa Sandstone is classified as a formation rather than a member. of the Cutle ...

varies with location, but contains evaporite, fluvial, tidal-flat, and sabkha facies. ** The

Organ Rock Formation The Organ Rock Formation or Organ Rock Shale is a formation within the late Pennsylvanian to early Permian Cutler Group and is deposited across southeastern Utah, northwestern New Mexico, and northeastern Arizona. This formation notably outcrops a ...

contains siltstones and sandstones from coastal-plain and fluvial deposits sourced from the north. ** The

De Chelly Sandstone The Yeso Group is a group of geologic formations in New Mexico. It contains fossils characteristic of the Kungurian Age of the early Permian Period. Description The Yeso Group is lithologically complex, ranging from marine shelf carbon ...

consists of sandstones of eolian deposits. The unit is broken into lower and upper members based on sediment transport directions. * The

Yeso Formation The Yeso Group is a group of geologic formations in New Mexico. It contains fossils characteristic of the Kungurian Age of the early Permian Period. Description The Yeso Group is lithologically complex, ranging from marine shelf carbonate ...

is divided into two members: ** The

Meseta Blanca Sandstone Member The Yeso Group is a group of geologic formations in New Mexico. It contains fossils characteristic of the Kungurian Age of the early Permian Period. Description The Yeso Group is lithologically complex, ranging from marine shelf carbon ...

contains classic eolian deposits of cross-bedded, well-sorted sandstones. This formation may be equivalent to the

.* ** The San Ysidro Member contains gypsiferous sandstones interbedded with limestones and reflects complex, cyclic facies changes (i.e. eolian, coastal, shallow shelf). * The Glorieta Sandstone contains buff to white, silicious sandstones indicative of eolian deposits. * The San Andres Limestone (aka, Bernal Formation) contains thick limestone and dolomite beds interbedded with sandstone or shale.

Mesozoic

Triassic

* The

Moenkopi Formation The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Part ...

and the upper Chinle Formation unconformably overlie Permian rocks and consist of continental sandstone, siltstone, and mudstone.

Jurassic

* Rocks of this time period, e.g. the

Morrison Formation The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...

, contain continental sandstone and siltstone, and marine limestone and anhydrite deposits.

Cretaceous

The Cretaceous-aged units are the most well-understood and the most productive units in the San Juan Basin. The Inner Cretaceous Seaway's western extent was along the San Juan Basin, and the three major transgressive-regressive episodes that occurred during this time are recorded in the mid- to upper-Cretaceous stratigraphy. * The Dakota Sandstone Formation is an early Cretaceous unit consists of fluvial sandstones deposited unconformably over older units. These units grade into the overlying Mancos Shale (see next line). * The

Mancos Shale The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, ...

represents deeper marine deposits as the Inner Cretaceous Seaway made its first major transgression. This formation is divided into three main members: ** The

Graneros Shale The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period. It is defined as the finely sandy argillaceous or clayey ''near-shore/ ...

contains sandstone, bentonite, and limestone beds. ** Then

Greenhorn Limestone The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...

contains alternating beds of limestone and calcareous shale deposited during a time of maximum transgression. ** The Juana Lopez Member contains fossil-rich calcareous shale beds. * The

Mesaverde Group The Mesaverde Group is a Late Cretaceous stratigraphic group found in areas of Colorado, New Mexico, Utah, and Wyoming, in the Western United States. History The Mesaverde Formation was first described by W.H.Holmes in 1877 during the Hayden ...

was deposited as the Inner Cretaceous Seaway regressed to the northeast, depositing the

Point Lookout Sandstone The Point Lookout Sandstone is a Cretaceous bedrock formation occurring in New Mexico and Colorado. Description The formation consists of two informal members. The lower is a sequence of thinly bedded sandstone and shale, while the upper is a ...

, and then transgressed to the southwest again, depositing the Cliff House Sandstone. * The

contains grey shales interbedded with sandstone and limestone. These are deeper marine deposits as the seaway continued to regress to the southwest. This unit represents the westernmost extent of the Inner Cretaceous Seaway. * The Pictured Cliffs Sandstone is divided into two layers: the lower unit contains interbedded Lewis-like shales and sandstones as the seaway began to regress, and the upper unit contains massive sandstone beds as the seaway made its final regression. * The

consists of shale, siltstone, and (most importantly) coal deposited from swamps, rivers, lakes, and flood plains. * The

Kirtland Formation The Kirtland Formation (originally the Kirtland Shale) is a sedimentary geological formation. Description The Kirtland Formation is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain th ...

is divided into two layers: the lower unit consists of shale very similar to the upper Fruitland, but is absent of coal beds (and thus, separated from the Fruitland), and the upper shale to sandstone units deposited by aggrading stream channels.

Cenozoic

* The

Ojo Alamo Formation The Ojo Alamo Formation is a geologic formation in New Mexico spanning the Mesozoic/Cenozoic boundary. Non-avian dinosaur fossils have controversially been identified in beds of this formation dating from after the Cretaceous–Paleogene extinctio ...

consists of arkosic conglomerates and sandstones most likely sourced from the west (noted by an eastward decrease in pebble size) that unconformably overlies older units. * The

Animas Formation The Animas Formation is a Formation (geology), geologic formation in Colorado. It preserves fossils dating back to the Paleogene Period (geology), period. See also * List of fossiliferous stratigraphic units in Colorado * Paleontology in Colorad ...

of the north gradually grades into the

Nacimiento Formation The Nacimiento Formation is a sedimentary rock formation found in the San Juan Basin of western New Mexico (United States). It has an age of 61 to 65.7 million years, corresponding to the early and middle Paleocene. The formation has yielded an ...

of the south. The units are volcanic in origin, sourced from the San Juan Volcanic Field, and contain conglomerates and andesite clasts. * The Eocene-aged San Jose Formation consists of arkosic sandstones and shales.

Hydrocarbon Plays

The San Juan Basin contains ample fuel resources, including oil, gas, coal, and uranium. The basin has produced from over 300 oil fields and nearly 40,000 wells, most of which are sourced from Cretaceous-aged rocks. Furthermore, 90% of the wells have been drilled in the state of New Mexico. As of 2009, cumulative production reached 42.6 trillion cubic feet of gas and 381 million barrels of oil.

History

The first documented oil play in the San Juan Basin occurred in 1911 on the Chaco Slope. The well was drilled to a depth of 100 m and produced only 12 barrels of oil per day. The first documented gas play occurred ten years later in the Central Basin Platform. The well was 300 m deep and resulted in a gas pipeline to carry and market gas to nearby cities. The following years resulted in many oil and gas discoveries that subsequently spiked interest in San Juan resources. The 1930s brought upon the first pipeline to transport gas outside of the basin. The 1980s brought upon the discovery of the coal-bed methane resources, resulting in a drilling spike during the 1980s and 1990s. Production has since leveled out, but the basin is still actively producing today.

Paleozoic Fields

While the majority of production has occurred in Cretaceous-aged units, the Paleozoic rocks of the Four Corners Platform have successfully produced from over two-dozen fields from Devonian, Mississippian, and Pennsylvanian-aged units. The Paleozoic units deepen in a northeast direction where they cross from the oil- to the gas-window; subsequently, Paleozoic fields yield gas in the northeast and oil in the southwest. Furthermore, Paleozoic field locations roughly align with the northeast-trending Hogback monocline. Future Paleozoic plays will target natural gas, and these will include untested carbonates in the Central Basin Platform and potentially undiscovered plays in the Four Corners Platform.

Mesozoic Fields

Cretaceous-aged units account for the majority of gas and oil production in the San Juan Basin, i.e. nearly 250 of the >300 fields source Upper Cretaceous units. Major oil plays in the San Juan Basin target the Dakota Sandstone, the Gallup Sandstone, the Tocito Sandstone, and the El Vado Sandstone Member. The source rock for these units was the black, organic-rich marine shale of the stratigraphically lower ''Mancos Formation''. Most of the oil fields described below are at or nearing depletion. Major gas plays in the San Juan Basin target the Dakota Sandstone, the Point Lookout Sandstone, and the Pictured Cliffs Sandstone. Plays consist of stratigraphic traps mostly concentrated in the Central Basin Platform.

Oil Plays

* The Dakota Sandstone has nearly 40 oil fields throughout the Four Corners and Central Basin platforms, each having produced millions of barrels of oil. * The Gallup Sandstone has about four oil fields on the Chaco Slope. Sandstones units have yielded tens of thousands to millions of barrels of oil. * The Tocito Sandstone Lentil of the Mancos Shale has about 30 fields throughout the Four Corners and Central Basin platforms. The Tocito beds are the best Cretaceous reservoirs, having produced over 150 MBO (million barrels of oil) from a variety of structural and stratigraphic traps. * The El Vado Sandstone Member of the Mancos Shale has produced from over nearly 40 fields mostly concentrated in the Central Basin Platform. This member alone has produced over 40 MBO.

Gas Plays

* The Dakota Sandstone stores gas in offshore marine sandstones trapped by marine shales. Fracturing is required to source these units. * The Pictured Cliffs Sandstone consists of regressive-marine deposits, where gas is stored in porous sandstones and trapped by mud- or siltstones. Production depends on natural fractures throughout unit. * The Point Lookout Sandstone (see the Pictured Cliffs Sandstone above).

Coal-bed Methane Plays

* The Fruitland Formation encompasses the San Juan Basin's gracious supply of methane-rich coal beds. Methane is found within thousands of coal beds throughout the Fruitland Formation. Similar to Paleozoic gas fields, there is a trend of increasing gas content (and thermal maturity) in a northeast direction. Cumulative production (2009) is 15.7 trillion cubic feet of gas, making this the largest coal-bed methane field in the world.

Methane cloud

In 2014

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

researchers reported the discovery of a

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

cloud floating over the Basin. The discovery was based on data from the

European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...

’s Scanning Imaging Absorption Spectrometer for Atmospheric Chartography instrument from 2002 to 2012. The report concluded that "the source is likely from established gas, coal, and coalbed methane mining and processing." The region emitted 590,000 metric tons of methane every year between 2002 and 2012—almost 3.5 times the widely used estimates in the

European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been des ...

’s Emissions Database for Global Atmospheric Research.

References

{{coord, 36.27, -107.90, display=title, region:US-NM Structural basins of the United States Cenozoic New Mexico Colorado Plateau Geologic provinces of Colorado