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Abstract

Two continuous gas assessment units (AU’s) are present in the Late Triassic (Norian) onshore rift basins of North Carolina and south-central Virginia. Continuous AU’s are the USGS classification/nomenclature for the oil and gas rich resource plays industry has been pursuing and exploiting throughout the continental United States. “Continuous gas assessment units” include tight gas sandstone as well as two resource plays—coal-bed methane and shale gas/oil. The USGS assessed the East Coast Mesozoic rift basins as continuous gas AU’s primarily as tight gas AU’s because oil and gas have been found (although not produced) from tight (i.e., low porosity and permeability) sandstones, coal beds, and shale beds/intervals. The source rocks are lacustrine shales that were deposited in freshwater lakes that were near the paleo-equator after the onset of Pangea rifting.

These two rift basins, the Deep River basin wholly within North Carolina, and the Dan River-Danville basin, located in north-central North Carolina and south-central Virginia have been assessed numerically as part of the USGS’s National Petroleum Resource Assessment (Fig. 1). The name ‘Dan River-Danville basin’ is used by the U.S. Geological Assessment team for assessment, and the name, ‘Dan River basin’ is used herein following stratigraphic revision and formal basin naming in 2015 (Olsen et al., 2015). These two rift basins are part of a series of larger continental series rift basins that formed during the Permian to Early Jurassic extension in central Pangea as the supercontinent began to fragment.
Figure 1.

Map of the Eastern United States showing the location of the five quantitatively (volumetrically) assessed East Coast Mesozoic basins (in red), the nine basins that were not volumetrically assessed (in orange), and the U.S. Geological Survey province boundaries. Each basin includes a single continuous gas assessment unit (Milici et al., 2012).

Figure 1.

Map of the Eastern United States showing the location of the five quantitatively (volumetrically) assessed East Coast Mesozoic basins (in red), the nine basins that were not volumetrically assessed (in orange), and the U.S. Geological Survey province boundaries. Each basin includes a single continuous gas assessment unit (Milici et al., 2012).

These continuous gasprone AUs each have a single total petroleum system (TPS). The Deep River basin continuous AU has an estimated mean gas content of 1,660 billion cubic feet of gas (BCFG) and an estimated mean of 83 million barrels of natural gas liquids (MMBNGL). Noble gases have been documented from two shut-in wells in the Deep River basin by the North Carolina Geological Survey in a separate study (Reid et al., 2015c). The Dan River-Danville basin continuous AU has an estimated mean gas content 49 BCFG and no natural gas liquids from data available in 2011 assessed by the U.S. Geological Survey (Milici et al., 2012) (Table 1).

Table 1.

East Coast Mesozoic basin assessment results (Milici et al., 2012). The Deep River basin composite TPS and the Dan River-Danville basin composite TPS assessment results and data for other basins. Note - MMBO, millions of barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids; TPS total petroleum system; AU, assessment unit. Results in the table are fully risked estimates. For gas accumulations, all liquids are included as NGL (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. Gray shading indicates not applicable (Milici et al., 2012).

The Dan River basin stratigraphy has been clarified by Olsen et al. (2015). A continuous 1,477-foot-deep stratigraphic core hole drilled in 2015 by the North Carolina Geological Survey penetrated a 323-ft-thick unconventional lacustrine shale reservoir containing a 3-ft-thick coal having gas shows in the coal and lower siltstone and then drilled through an underlying siliciclastic formation containing previously unknown thin organic strata, to basement at a depth of 1,451.2 ft below the surface.

The Cumberland-Marlboro ‘basin,’ a large, strike-parallel and seaward negative aeromagnetic anomaly that is buried beneath thin unconsolidated coastal plain sediments, also was drilled and cored (three Rotasonic holes) in 2015 by the North Carolina Geological Survey. Metasedimentary Paleozoic(?) basement rock was recovered; no Triassic strata were present.

Additional information that accompanies this extended abstract is found in Appendices 1–3.

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