Paradox basin

Fig 1 shows the location of the Paradox basin with respect to other geologic structures in the area.

Introduction

Fig 2 depicts a salt dome cross section in the paradox basin.

The Paradox Basin is a foreland basin located predominately in Utah and Colorado, but also branching into Arizona and New Mexico in the area known as the “four corners”. The basin is around 33,000 square miles in total area. The basin is bordered by the Uncompahgre Uplift to the East and the San Rafael Uplift to the Northwest. Several rivers flow through the basin including the Dolores River, the Colorado River, the Green River, and the San Juan River.

History

The Paradox Basin was formed primarily in the Pennsylvanian time period. The basin was a result of tectonic activity between South America, Africa, and North America. These continents were colliding with one another, which sparked several geologic events including the uplift of the ancestral Rocky Mountain range. These geologic events also resulted in the formation of the Uncompahgre Uplift, which in turn resulted in a depression or basin being created on the other side of a reverse fault on the Southwest side of the uplift. The basin is the Paradox basin, and that geologic activity is why the Paradox basin is considered a foreland basin. The basin bears the name of the Paradox Valley, a structure found within the basin. The name “Paradox” was given to the geologic structures in the area in 1875, when geologist Albert Charles Peale discovered and surveyed the valley. He chose the name Paradox due to the fact that the Dolores River had a “desire to preform strange and unexpected things”.

Geologic Structure

The basement rocks of the Paradox Basin consist of several sequences formed during the Proterozoic period. The oldest rocks in the basin are gneiss and schist that were formed during the Early Proterozoic, followed by plutonic igneous rocks formed during the Middle Proterozoic. Metasedimentary sequences formed after, during the later part of the Middle Proterozoic. Clastic and carbonate Cambrian formations formed an unconformity over to basement rocks. Another unconformity formed during the Upper Devonian period where the Devonian lies on top of the Cambrian strata. Yet another unconformity formed separating the Mississippian from the Devonian, where several formations formed, including the Leadville Limestone and the Redwall Limestone. There aren’t many Ordovician or Silurian rocks found in the Paradox Basin. These sequences of strata make up the Pre-Pennsylvanian formations. The Pennsylvanian period is the time period responsible for the deposition of organic rich shale, dolomite, limestone, anhydrite, and halite. The shales deposited during this time period are the primary petroleum source rocks of the basin. Large deposits of salt also occurred during this period, a feature that defines the basin. Most of the rocks formed after the basement period (Proterozoic) were actually deposited into the basin during the Pennsylvanian period. These deposits include the Lower and Middle Cambrian Tintic Quartzite, the Upper Cambrian Ignacio Quartzite, Middle Cambrian Maxfield Limestone, Upper Cambrian Lynch Dolomite, Upper Devonian Aneth and Elbert Formations, Ouray Limestone, and the aforementioned Mississippian Leadville Limestone. The Hermosa group makes up most of the rocks formed during the Pennsylvanian period. Sedimentary formation continued to occur during the Jurassic and Triassic periods, and even occurred during the Crustaceous periods.  

Geologic Risk and Uncertainty

Thick salt deposits in the paradox basin compromise most of the geologic risk and uncertainty. The salt deposits occurred during the Middle Pennsylvanian time period, and result in the Paradox basin being considered an evaporite basin. Salt tectonics have played a large role in shaping the basin, as thick deposits make up roughly 12,000 square miles of the basin (equivalent to about 40% of the entire basin). The salt deposits typically occur at depths of around 5,000 feet, however there are areas where the salt deposits are much shallower, specifically on the northeast side of the basin, near the reverse fault that borders the Uncompahgre Uplift. These salt tectonics helped lead to the formation of the Paradox valley, which is where the salt is often found close to the surface. The salt beds range in thickness 2,500 to 14,000 feet thick. These salt deposits are present in anticlines which form salt domes throughout the basin. These salt domes can lead to uncertainty when drilling and mining for economically viable resources in the area.

Petroleum Elements

The Paradox Formation Total Petroleum System is the petroleum rich formation present in the Paradox basin. There are both conventional and unconventional reservoirs present in the formation. The formation contains black dolomitic shales which can be traced back to the Middle Pennsylvanian period, which are interbedded with the thick salt deposits discussed in the previous section.

Source Rock

The petroleum source rock is the Middle Pennsylvanian black dolomitic shale.

Conventional Reservoirs

The paradox formation contains three conventional reservoirs which are distinguished by their well defined boundaries, hydrocarbon-water contacts, and their porosity and permeability. The conventional reservoirs include Leadville McCracken, Pennsylvanian Carbonate Buildups and Fractured Limestone, as well as Upper Paleozoic - Mesozoic Reservoirs. The Leadville McCracken Reservoir is made up of Mississippian limestone and Devonian Sandstone, and the transportation of the hydrocarbons occurred through the faults within the basin. The Pennsylvanian Carbonate buildups and Fractured Limestone primarily contain algal mounds made up of limestone. The upper Paleozoic - Mesozoic Reservoirs are made up of Late Pennsylvanian-Jurassic clastics.

Traps

Traps within the Leadville McCracken and Pennsylvanian Carbonate Buildups and Fractured Limestone are primarily stratigraphic, while the Upper Paleozoic - Mesozoic's traps are primarily salt domes and ridges and walls.

Seals

The seals for all of the reservoirs are composed of non-reservoir quality units.

Migration

Migration of hydrocarbons in the basin occurs through faults and fractures. The fracture networks are closely connected with the salt deposits present throughout the formation.

Unconventional Reservoirs

There are four unconventional reservoirs present in the Paradox Formation, these include 1) Cane Creek Shale Oil, 2) Cane Creek Shale Gas, 3) Gothic, Chimney Rock, Hovenweep Shale Oil, and 4) Gothic, Chimney Rock, Hovenweep Shale Gas. These reservoirs contain diffuse boundaries and don't have obvious traps and seals present.

Petroleum Potential and Engineering

The most recent US Geological Survey assessment of the Paradox Basin occurred in 2011 and they estimated 560 million barrels of undiscovered oil, as well as almost 13 trillion cubic feet of natural gas that has yet to be discovered in the Paradox basin. The most historically profitable play in the basin is the Aneth Field, which was discovered in 1956 and has produced around 450 million barrels since its discovery. The most profitable natural gas fields are the Ute Dome and the Barker Dome, which have produced 6.9 and 4.7 trillion cubic feet (respectively) to date. The Cane Creek Reservoirs have produced interest from several companies, however the unconventional properties have made it difficult to produce and scale. There is currently research being done on how to efficiently produce from some of the unconventional plays in the basin. The current project being conducted by National Energy Technology Laboratory, University of Utah, and Zephyr Energy will take four years to complete and will result in the the testing to determine optimal stimulation techniques using at least one horizontal well. If the project is successful it could lead to a boom in production of the Paradox unconventional plays. There are several major pipelines within the basin including the Northwest Pipeline, Southern Trails Pipeline, and TransColorado pipeline for natural gas; the Western Refining pipeline for crude oil; and the Rocky Mountains pipeline owned by Enterprise for natural gas liquids.

External Readings

NETL ADVANCING NEW STRATEGIES TO EXTRACT OIL FROM UTAH’S PARADOX BASIN

Information about the Paradox Basin

Identifying Potential Oil Zones in Tight Reservoirs