The Overlooked Basins?
The natural gas rush associated with the Marcellus Shale and Utica Shale has drawn an enormous amount of attention to the Appalachian Basin. Most rocks east of that basin have been subjected to so much heat and pressure that their oil and natural gas were destroyed. An exception are rocks in a series of small Mesozoic-age basins that parallel the eastern edge of the continent. Perhaps the small size of these basins and their lack of previous oil and gas production caused them to receive very little attention in the early days of the Appalachian shale gas rush.
Starting as early as 1891, the oil and gas industry has drilled exploratory wells into these basins and has encountered shows of oil and natural gas - but this work has not resulted in important production. Most of it was done before hydraulic fracturing and horizontal drilling were successful at producing natural gas from the shales that are abundant in these basins.
A recent USGS assessment reports that five of these basins hold a natural gas and natural gas liquids resource. The USGS assessment has called public and industry attention to these basins. Now the oil and gas industry must decide if the basins merit the drilling required to determine if the resource can be economically developed.
Where are the Mesozoic Basins?
During the early opening of the Atlantic Ocean in the Mesozoic Era, numerous extensional basins formed along
the eastern margin of the North American continent from Florida northward to New England and parts of adjacent
Canada. The basins extend generally from the offshore Atlantic continental margin westward beneath the Atlantic
Coastal Plain to the Appalachian Mountains. See Figure 1.
The provinces, which contain these extensional basins, extend across parts of Georgia, South Carolina, North
Carolina, Virginia, Maryland, Delaware, Pennsylvania, New Jersey, New York, Connecticut, and Massachusetts.
The basins formed along the continental margin in response to the regional uplift, extension (rifting), and crustal
thinning that occurred during the early opening of the Atlantic Ocean in middle Carnian (Late Triassic) time,
approximately 227 million years ago.
See Geologic Time Scale.
Using a geology-based assessment method, the U.S. Geological Survey estimated a mean undiscovered natural gas
resource of 3,860 billion cubic feet and a mean undiscovered natural gas liquids resource of 135 million barrels
in continuous accumulations within five of the East Coast Mesozoic basins: the Deep River, Dan River-Danville, and
Richmond basins, which are within the Piedmont Province of North Carolina and Virginia; the Taylorsville basin,
which is almost entirely within the Atlantic Coastal Plain Province of Virginia and Maryland; and the southern part
of the Newark basin (herein referred to as the South Newark basin), which is within the Blue Ridge Thrust Belt
Province of New Jersey. [USGS conducted the assessment on just five of the fourteen Mesozoic basins - the ones
that were thought to have the greatest oil and natural gas potential.]
Sedimentation and Source Rocks
The basins were filled with a variety of sediments as they formed, including boulder beds, coarse-grained fluvial
to deltaic sandstones, red siltstones, mudstones, gray and black shales, and coal. These deposits represent the
diverse fluvial to deltaic and lacustrine environments that existed within the basins. The rifting ended early in the
Jurassic with the onset of regional volcanism and intrusion of diabase dikes and sills.
The source rocks for oil and gas within the Mesozoic basins include the gray and black shales and the coal beds.
The shales accumulated in nearshore deltas, in interdistributary bays, and in the deeper portions of the lakes that
occupied the basins. The majority of the coal accumulated in marshes and swamps that developed around the basin margins
early in the depositional history of the basins. In addition, smaller amounts of coal accumulated locally within the
overlying deltaic deposits.
Organic Content and Thermal Maturity
Black shale beds may range in thickness from a few feet to several hundred feet. Kerogen in these beds generally consists
of material derived from vascular plants and algae, which are the types of kerogen that are prone to yield both gas and oil.
With respect to the generation of liquid hydrocarbons, the thermal maturation values of the source rocks range widely from basin
to basin, from within the zone of dry gas to immature. The potential reservoirs in these basins are continuous accumulations
that encompass a variety of lithologies ranging from boulder conglomerates and very coarse sandstones to mudstone, shale,
and coal. The shale beds that are interbedded with coarser grained strata may act effectively as seals.
The Resource Assessment
The assessment of the East Coast Mesozoic basins is based on the geologic and geochemical characteristics of the individual
total petroleum systems that were recognized within the basins. For the petroleum source rock, the characteristics include the
source rock richness, thermal maturation, timing of petroleum generation, and migration; for the reservoir rocks and seals,
they include their stratigraphic position and content and petrophysical properties.
Using this gasgeologic framework, the USGS defined a composite total petroleum system and an assessment unit for continuous
accumulations in each of the 14 major East Coast Mesozoic rift basins. The basins are present both onshore and offshore
(in State-administered waters) of the Eastern United States. See Figure 1.
The USGS quantitatively assessed the technically recoverable, undiscovered resources within five of the total petroleum systems
and associated assessment units that demonstrated the most potential for generating and accumulating hydrocarbons.
Several assessment units appear to contain continuous accumulations of gas, within both the shale and coal source rocks and
within the adjacent coarser grained, siliciclastic strata. Small amounts of oil and petroliferous hydrocarbons, such as
asphalt, have been detected in some of the wells drilled in these basins.
Potential Resource Summary
For the total of the continuous resources, the USGS estimated a mean of 3,860 billion cubic feet of gas and a mean of 135
million barrels of natural gas liquids. See Table 1. The available data indicate that, of the five basins that were
quantitatively assessed, the Deep River, Taylorsville, and South Newark basins appear to possess the potential to produce
the most hydrocarbons. See Tables 1 and 2.
The nine basins that did not receive quantitative assessments were the Hartford, North Newark, Gettysburg, Culpeper, Delmarva,
Cumberland-Marlboro, Florence, South Georgia and North Florida basins.
More information about the East Coast Mesozoic basins and the assessment methodology, as well as other oil and gas assessments,
total petroleum systems, and assessment units may be found at this USGS Web site: http://energy.usgs.gov.
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Figure 1: Map of the Eastern United States showing the locations of the five quantitatively (volumetrically)
assessed East Coast Mesozoic basins, the nine basins that were not volumetrically assessed, and the U.S. Geological
Survey province boundaries.
|Mesozoic Basin Natural Gas (billion cubic feet)
|South Newark Basin
|Deep River Basin
|Dan River - Danville Basin
|Table 1. Results shown are fully risked estimates for natural gas. F95 represents a 95-percent chance of at least the amount tabulated; other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation.
|Mesozoic Basin Natural Gas Liquids MMBNGL
|South Newark Basin
|Deep River Basin
|Dan River - Danville Basin
|Table 2. Results shown are fully risked estimates for natural gas liquids. F95 represents a 95-percent chance of at least the amount tabulated; other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation.
|Mesozoic Basin Assessment Team
|The East Coast Mesozoic Basin Assessment Team consisted of Robert C. Milici (firstname.lastname@example.org), James L. Coleman, Elisabeth L.
Rowan, Troy A. Cook, Ronald R. Charpentier, Mark A. Kirschbaum, Timothy R. Klett, Richard M. Pollastro, and Christopher J. Schenk.
The East Coast Mesozoic Basin Assessment Team acknowledges the Directors and staff of the State geological surveys of North Carolina,
Virginia, Pennsylvania, and New Jersey for providing geological information and assisting with the basin-analysis workshops, which
were held in preparation for the assessment.