Geology and Hydrocarbon Potential of the Richmond Basin, Virginia
Published:December 01, 2015
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Robert C. Milici, James L. Coleman, Jr., 2015. "Geology and Hydrocarbon Potential of the Richmond Basin, Virginia", Petroleum Systems in “Rift” Basins, Paul J. Post, James Coleman, Jr., Norman C. Rosen, David E. Brown, Tina Roberts-Ashby, Peter Kahn, Mark Rowan
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The Richmond basin, a rift basin of Late Triassic to Early Jurassic age in east-central Virginia, produced the first coal mined in the United States in the early 1700s. These Triassic coal beds are thick and gas-rich, and fatal explosions were common during the early history of exploitation. Since 1897, at least 38 confirmed oil, natural gas, and coal tests have been drilled within the basin. Although shows of asphaltic petroleum and natural gas indicate that active petroleum systems existed therein, no economic hydrocarbon accumulations have been discovered to-date.
The Richmond basin has been assessed by the U. S. Geological Survey (USGS) as one composite total petroleum system, in which the hydrocarbon potential of the source beds (both coal and dark shale) and potential reservoirs have been combined into a single continuous tight gas assessment unit within the Chesterfield and Tuckahoe groups (Upper Triassic). Sandstone porosities are generally low (<1 % to 14 %). Thick, dark-colored shales have total organic carbon (TOC) values that range from <1% to 10%, and vitrinite reflectance (%RO) values that range generally from about 0.3 to 1.1%, which indicates that the submature to super mature shales appear to be the source of the hydrocarbons recovered from some of the boreholes. The stratigraphic combination of these potential source rocks, tight sandstones, and hydrocarbon shows are the basis for the current USGS assessment of the technically recoverable undiscovered hydrocarbon resources of the basin. Mean values for these resources are 211 billion cubic feet of gas (BCFG) and 11 million barrels of natural gas liquids (MMBNGL).
In 2011, the U.S. Geological Survey conducted a quantitative assessment of the hydrocarbon resources of five east coast Mesozoic basins, including the Deep River basin in North Carolina, the Dan River-Danville basin in North Carolina and Virginia, the Richmond basin in Virginia, the Taylorsville basin in Virginia and Maryland, and the south Newark basin in Pennsylvania and New Jersey (Milici et al., 2012). Collectively, these basins were assessed to contain 3,869 BCFG of technically recoverable gas at the mean.
Of that amount, the Richmond basin was assessed as a composite total petroleum system (TPS) and continuous gas Assessment Unit (AU) to contain 211 BCFG at the mean. The TPS consisted of multiple selfsourced continuous reservoirs of coal beds and shale rich in organic matter that were interbedded with potential reservoirs of siliciclastic strata that range in texture from boulder beds to tight sandstones, siltstones and shales. The hydrocarbon potential of the eastern Mesozoic basins was assessed previously by Schultz (1988) and then by Milici (1996) as part of the 1995 USGS national assessment. Schultz (1988) concluded that the basins had potential for producing hydrocarbons, but did not assess them quantitatively. In 1995, the USGS assessment team (Milici, 1996) assessed all of the eastern U.S. Mesozoic rift basins collectively and concluded that at the mean at their combined technically recoverable undiscovered gas resources was 348 BCFG.
The Richmond basin, a Mesozoic rift basin of Late Triassic to Early Jurassic age, is located entirely within the Piedmont physiographic province of central Virginia (Roberts, 1928). Although coal mining in the Richmond basin may have commenced in the mid-to late 1600s, the first commercially produced coal in the United States was from the Triassic strata of the Richmond basin sometime in the mid-1700s. The coal beds are generally gassy, as attested to the hundreds of miners killed by gas and coal dust explosions during the early history of mining in the area.
Subsequently, the basin became a target for oil and gas exploration, and at least 38 test wells have been drilled in the basin since 1897 (Wilkes, 1988; Milici and Wilkes, 2003; McCartney, 2010). Several of the hydrocarbon tests yielded shows of asphaltic petroleum as well as natural gas, which indicate that active petroleum systems have existed within the basin. Owing in part to the large amount of extensional faulting within the basin, there has been no commercial oil and gas production (including coal-bed methane, CBM) to-date (Knox et al., 1992). The Richmond and Taylorsville basins and surrounding Piedmont terrane appear to have been uplifted since Triassic rifting (Malinconico, 2003), so that any large conventional accumulations of petroleum were at least partially devolatilized, fractionated, and leaked off, leaving behind only residual asphaltic petroleum and natural gas in “continuous” (i.e., not conventional) accumulations, in addition to the hydrocarbons still resident within the source rock shales and coals.
Nevertheless, there are relatively thick sequences of dark-colored shales and coal beds that may serve as self-sourced reservoirs for hydrocarbons. The stratigraphic potential of continuous accumulations of hydrocarbons in these source rocks and from interbedded tight sandstone reservoirs, together with shows of hydrocarbons in several wells are the basis for the current assessment of the technically recoverable undiscovered hydrocarbon resources of the Richmond basin by the USGS. This recent assessment concludes that the potential mean values for the technically recoverable undiscovered hydrocarbons in the basin are 211 BCFG and 11 MMBNGL.
Location and Extent
The onshore Mesozoic basins of the eastern United States extend from the southeastern United States northeastward into Canada (Davis and Evans, 1938; Froelich and Robinson, 1988; Withjack et al., 1998; LeTourneau, 1999, undated;). These basins are exposed within the Piedmont, Blue Ridge, and New England physiographic provinces of Fenneman (1938), and several lie beneath the Cretaceous and Tertiary strata of the adjacent Atlantic Coastal Plain province. Only a few of the basins extend westward into the adjacent Paleozoic sedimentary strata of the Appalachian Valley and Ridge physiographic province. In addition, numerous basins occur on the adjacent Atlantic outer continental shelf (Schlee and Klitgord, 1986; Bayer and Milici, 1987; Benson, 1992; Wilkes et al., 1989).
The strata within the Mesozoic Basins that are buried beneath the Coastal Plain are separated from overlying younger deposits by the postrift unconformity of Schlee and Klitgord (1986). This unconformity reflects approximately 100 million years of erosion of the pre-existing early Mesozoic topography (Carnian) prior to the deposition of the non-marine deposits of the Lower Cretaceous Potomac Group (Aptian). For assessment purposes, all 17 of the basins that were considered to have some potential for petroleum production have been grouped within the Atlantic Coastal Plain assessment province by the USGS assessment team. The formation of these basins apparently reflects the regional heating, uplift, and extension of the continental crust prior to the early opening of the Atlantic Ocean about 227 million years ago in middle Carnian (Late Triassic) time (Manspeizer and Cousminer, 1988).
In general, the onshore basins are filled with continental siliciclastics derived from adjacent uplands, and several of the south-central basins contain extensive coal deposits. The lithologies of the siliciclastic sedimentary rocks contained in these Mesozoic basins range from boulder beds that were deposited along basin margins to arkosic and calcareous sandstones, red siltstones, mudstones, and gray and black shales that were deposited in fluvial-lacustrine environments within the interiors of the basins (Froelich and Robinson, 1988; Smoot, 1991).
The 17 onshore basins that were considered to have potential for containing hydrocarbon resources are shown on Figure 1A. Of these, there was sufficient data to assess the technically recoverable undiscovered hydrocarbon resources within five basins, and the Taylorsville1, Richmond, and Danville-Dan River2 basins are the three that were judged to have the greatest potential for generating and producing technically recoverable hydrocarbons in Virginia (Enomoto, 2010, 2013; Milici et al., 2012). Of the three, the Richmond basin is the smallest. It is about 33 mi long, 9.5 mi wide at its maximum width, and covers an area of approximately 190 sq mi (Goodwin et al., 1985, 1986). The Richmond basin and several other Mesozoic rift basins are within the eastern part of the Piedmont physiographic province in central Virginia, near the Piedmont-Atlantic Coastal Plain boundary (Fig. 1B). The basin is bounded by the Mississippian-age Petersburg Granite and metamorphosed Paleozoic volcanic rocks of the Piedmont on its eastern side, and by the upper Paleozoic cataclastic rocks of the Hylas fault zone on the west (Bobyarchick and Glover, 1979; Virginia Division of Mineral Resources, 2003) (Fig. 2). The regional geology of the Piedmont physiographic province in the vicinity of the Richmond basin is discussed by Marr (2002).
Stratigraphy of the Richmond Basin
In general, basal conglomerate and sedimentary breccia deposits along the basin margins grade laterally into the marsh and swamp sediments that had accumulated within the poorly drained lowlands that initially occupied much of the basin interior. In places, peat accumulated within or near the basal siliciclastic sedimentary deposits of the basin, and upon lithification was transformed into mineable coal beds some tens of feet thick (Wilkes, 1988). With continued subsidence, the marshes and swamps were covered by muds and silts that appear to have been deposited within lakes adjacent to the prograding deltas. The lake deposits, in turn, were covered by widespread delta-front sediments consisting of interbedded shale, siltstone, and sandstone turbidites, which were then overlain by red siltstones and coarser grained sandstones of deltaic and fluvial origin (Lucas et al., 1981; Luttrell, 1989; Smoot, 1991; Milici and Wilkes, 2003).
Early workers in the Richmond basin divided the Upper Triassic (Carnian) stratigraphic sequence within the Richmond basin into two groups of strata, a basal Tuckahoe Group and an overlying Chesterfield Group (Shaler and Woodworth, 1899; Olsen et al., 1978; Goodwin et al., 1985; Fig. 3), which together are classified as being within the Newark Supergroup of Olsen (1978). The stratigraphic terminology used herein (Fig. 3) generally follows that of Goodwin et al. (1985) and Wilkes (1988) and correspond to the geologic quadrangle maps and reports of the Richmond basin that were published by Goodwin (1970, 1980, 1981) and Wilkes (1988).
The formations of the Tuckahoe Group overlie the crystalline rocks of the Piedmont. The lower part of the Tuckahoe Group, which is commonly described as the Boscobel “boulder beds” (herein called the Boscobel breccia), generally consists of breccia and conglomerate, which in turn are overlain by sandstone, siltstones, and shale of the lower barren (non-coal-bearing) beds and then by the coal-bearing productive coal measures. In many places the boulder beds and lower barren beds are missing and the productive coal measures or overlying Chesterfield Group lie directly upon Paleozoic crystalline rocks along the borders of the Triassic graben (Goodwin, 1970, 1980, 1981; Fig. 3). The productive coal measures are about 500 ft thick in the northeastern parts of the basin, where they are overlain by 1,400 to 1,500 ft of dark gray to black Triassic shale (lower Vinita beds), which in turn grade laterally into and are overlain by about 2,500 ft of interbedded Triassic sandstone and shale (upper Vinita beds) (Goodwin, 1970; see his cross sections for estimated thicknesses).
Boscobel boulder breccia and lower barren beds
The Boscobel boulder breccia (Figs. 2 and 3) consists of sedimentary breccias and conglomerates that contain large boulders of cataclastic rock (derived mainly from the Hylas fault zone) up to 5 ft in diameter, that are within a matrix of sandstones (Shaler and Woodworth, 1899; Goodwin, 1970; Goodwin and Farrell, 1979; Goodwin et al., 1985). In general, the breccias and conglomerates are restricted to narrow outcrop belts of variable widths along the northwestern and southwestern margins of the basin (Fig. 3), and Goodwin et al. (1985) have suggested that they may have been talus deposits that accumulated along the fault scarps that originally defined the basin. Judging from the outcrop pattern and approximate angle of dip, the maximum thickness of the Boscobel boulder breccia in the northern part of the basin is about 2,000 ft (Goodwin, 1970).
Although the lower barren beds are commonly less than 400 ft thick, Heinrich (1878) compiled a stratigraphic section for the formation in the Midlothian District that contains 36 ft of basal conglomerate that is overlain by 496 ft of carbonaceous to calcareous gray sandstones, and black or brownish-black fossiliferous shales that are calcareous and/or rich in organic matter (Lucas et al., 1981; Goodwin et al., 1985).
Productive coal measures
The Productive coal measures formation of the Tuckahoe Group is distinguished from the lower barren beds formation of the Tuckahoe Group by the presence of mineable coal beds. The name does not refer to the coal-bed methane (CBM) productive potential of the Tuckahoe Group. Coal was first discovered in the Richmond area by French Huguenot settlers in the late 1600s or early 1700s (McCartney, 2010; Wilkes, Virginia Department of Mines, Minerals, and Energy, written communication, 2014). They were observed using coal by 1701 (Wilkes, 1988). Commercial coal production apparently was developed by 1748 (Wilkes, 1988), although it may have been earlier, and the coal was marketed in the nearby northeastern United States. The coal mines were gassy, and explosions resulted in the deaths of many miners (Wilkes, 1988). Coal production peaked in the mid-1800s and declined steadily thereafter until the last major coal mine in the Richmond basin was closed in the mid-1900s (Wilkes, 1988; Milici and Wilkes, 2003; McCartney, 2010). The Productive coal measures formation of the Tuckahoe Group overlies the lower barren beds over much of the basin. The base of the unit is commonly placed at the base of the stratigraphically lowest mineable coal bed.
The Chesterfield Group overlies the Tuckahoe Group unconformably and consists of a thick section of shale and interbedded shale and sandstones, the Vinita beds at the base, which are overlain by the Otterdale Sandstone (Fig. 3). Detailed geologic mapping of the northern and central parts of the Richmond basin by Goodwin (1970) has shown that the Triassic shales and sandstones of the Vinita beds commonly overlie the basement of Piedmont crystalline rocks, the Triassic border conglomerates, and the Triassic coal measures in a complex facies relationship (Fig. 3).
The Vinita beds overlie the Productive coal measures, barren beds, and crystalline basement, and consist generally of up to 6,000 ft of sandstone, siltstone, and dark gray to black shale. Sedimentary environments generally range from fluvial-deltaic around the western basin margins to lacustrine in the central part of the basin (Shaler and Woodworth, 1899; Goodwin et al., 1985).
The overlying Otterdale Sandstone crops out in the south-central part of the Richmond basin. In places the formation consists of channels filled with crossbedded arkosic sandstones that commonly contain granules, pebbles, or cobbles of quartz, granite, and gneiss. Fine-grained sandstones may be interbedded with siltstones and shales. Goodwin et al. (1985) interpreted the depositional environments to be dominated by fluvial deposits together with associated braided stream and overbank deposits. In general, the Otterdale Sandstone consists of a large lobate mass of relatively coarse-grained sedimentary rock that dominates the center of the Richmond basin. A well drilled into the Otterdale Sandstone near the basin’s western border penetrated 1,514 ft of conglomerate and sandstone before ending in diabase (Goodwin and Farrell, 1979).
Early workers, including Fontaine (1883), Russell (1892), and Shaler and Woodworth (1899), concluded “that the Triassic sediments that filled the Richmond basin were formed in a shallow depression that deepened as filling progressed and ended in faulting and folding caused by lateral compression” (Goodwin, 1970, p. 32). In general, the Richmond basin is faulted along its western margin by east-dipping extensional faults, and the hanging wall has moved downward to the east during basin extension and filling. Extension continued together with sedimentation, so that the first strata that were deposited within the basin were deformed progressively by extensional faulting as the basin widened. Extensional faults, thus, have enhanced the development of fracture porosity in the older Mesozoic rocks and in places have resulted in folding of the strata within the basins. West-dipping faults within the basin are subsidiary to the eastwarddipping master set. Progressive faulting during deposition apparently has resulted both in the compartmentalization of potential reservoirs, especially within the deeper parts of the basin and in some places may have reduced the effectiveness of available seals.
Since we do not have any evidence of post-Upper Triassic/pre-Cretaceous stratigraphy within the Richmond basin, it is hard to pick a time when hydrocarbon generation effectively ceased in the upper part of the section due to uplift. It may have been associated with the initial tectonic inversion of the basin sometime in the Jurassic. Malinconico (2008) shows that the Horner No. 1 well (Table 1) is barely thermally mature at %Ro = 0.49 at 94.5 ft. Her data for the Bailey well (%Ro = 0.6 at 2685 ft) suggests that it is still thermally mature at depths exceeding 2000 ft depth (%RO > 0.5).
Richmond Basin Composite Total Petroleum System
The Richmond basin was assessed as a composite total petroleum system (TPS) in which the hydrocarbon potential of the source beds (coal beds and black shales), and potential reservoirs (coal beds, shales, siltstones, sandstones, and conglomerates) were combined and assessed together as a single continuous tight gas assessment unit (AU) (Milici et al., 2012). The absence of any clear indicators of conventional accumulations together with oil and gas shows in tight sandstones, coal beds, and shale intervals led the assessment team to establish the Richmond basin AU as a continuous tight gas AU. Although petroleum was generated as the basin matured, the tight oil potential of the basin was not considered by the team because of the lack of evidence of widespread continuous accumulations. The area assessed for each Composite TPS and AU was defined by the general outline of each of the basins assessed.
Richmond Basin Continuous Gas Assessment Unit
The boundaries of the Richmond basin Continuous Gas Assessment Unit (AU) are the geologic boundaries that enclose the Triassic sedimentary rocks of the basin (Dicken et al., 2008; Fig. 2) within the igneous and metamorphic rocks of the Piedmont Province. Much of the basin is in fault contact with the surrounding crystalline basement (Fig. 2), and the western boundary of the basin is marked by a continuous network of extensional faults that generally follow the Hylas mylonite zone (Bobyarchick and Glover, 1979).
Potential continuous oil and gas resource reservoirs within the Richmond basin are the coals, shales, and sandstones within the Chesterfield and Tuckahoe groups. The coals are well cleated and average 21 to 24 ft in cumulative thickness (Knox et al., 1992). Wilkes (1988, p. 3) estimates that the coal-bearing interval averages 100 ft thick “but may be up to 400 ft thick.” The coal beds range in rank from sub-bituminous A to anthracite to natural coke; although much of the coal apparently is high–volatile bituminous in rank (Robbins et al., 1988).
In the three wells discussed by Knox et al. (1992), desorption tests indicate gas contents range from 9.0 to 12.8 m3/ton; methane content is between 91 and 94%; ethane content ranges from 0.13 to 2.62%; nitrogen levels are between 3.8 and 6.1%; and carbon dioxide content is between 0.3 and 0.5%. Overall gas quality ranges from 950 to 999 gross British thermal units per standard cubic foot of gas. Knox et al. (1992) estimated that the unrisked resource base is 1.3 trillion cubic ft. of gas (TCFG). However, coal-bed methane development was not pursued by Amoco because the core drilling found that the thicknesses of the individual coal seams and total net coal thickness were variable and unpredictable. Examination of the surface geology in conjunction with the well and core hole data indicates that the basin is structurally complex, which contributes to the stratigraphic variability of the potential coal bed reservoirs. The basin also deepened substantially away from its margins so that any potential development area would be limited in size. Compounding the technical development issues is the presence of a rapidly spreading urban and suburban footprint over the potential development area in the basin’s center (Knox et al., 1992).
Potential tight gas sandstone reservoirs within the Richmond basin are present within the Otterdale Sandstone and other sandstones in the Chesterfield and Tuckahoe groups. Based on examination of wireline logs from the Virginia Division of Gas and Oil, densityneutron log porosity measurements near the total depth in the Horner No. 1 well indicate that the sandstone porosity is very low, in the range of 0% to 8%. The Bailey well has slightly higher porosity values, ranging from 0 to 14% in sandstones of the Turkey Branch Member and Tuckahoe Group.
The hydrocarbon potential of the Richmond basin consists of petroleum sourced from coal beds and from shale rich in organic matter. The total organic carbon content of potential shale reservoirs within the Vinita Beds (Chesterfield Group) ranges from 0 to 10%. Although the maximum thickness of the measured organic shale beds is only about 6.6 ft, the maximum organic richness typically occurs only within a small fraction of those intervals (Whiteside et al., 2011). Several exploratory wells have yielded shows of bitumen, asphalt, and natural gas, which indicate that there may be some potential for hydrocarbon generation in the basin. The most likely source rock for hydrocarbons in the Richmond basin is the lacustrine gray to black shale within the Upper Triassic Vinita beds. Natural gas may also be sourced from the Vinita beds as well as from the coal beds in the Tuckahoe Group.
Robbins et al. (1988), from a study of the colors of pollen and spore from coal beds in the Richmond basin and the nearby Deep Run basin, documents that thermal maturation values range generally from submature to super mature with respect to oil generation. Although the thermal maturity of the basin is apparently primarily the result of the depth of burial of the strata within the basin, it also was affected locally by the heat that was generated by the intrusion of late stage dikes and sills. In the northern part of the Richmond basin, closely spaced surface thermal maturity values range from 0.66 to 1.07 %Ro, and in the nearby Deep Run basin outlier a sample has a reflectance value of 1.49 %Ro (Malinconico, 2008). Potential source rocks apparently reached petroleum generation maturity prior to the uplift and erosion of the basin after the early Jurassic.
Coal-Bed Methane Studies
Where the gassy coals are relatively thick and deeply buried in the Richmond basin, there is a potential for the development and production of coal-bed methane (Lucas et al., 1981; Knox et al., 1992), especially in areas where the land has maintained its rural character. In 1981, the Department of Mining and Minerals Engineering at Virginia Tech completed an extensive report on “The methane potential from coal seams in the Richmond basin, Virginia,” and concluded that the potential resource may range from 700 BCF to 1,400 BCF (Lucas et al., 1981). In 1989 and 1990, however, Amoco Production Company drilled six core holes to test this potential and concluded that it was highly unlikely that coal-bed methane could be commercially developed in the Richmond basin as discussed above (Knox et al., 1992).
Oil and Gas Exploration
In addition to the interest in coal-bed methane, the Richmond basin has been extensively explored for conventional oil and gas. Thirty-seven exploratory holes have been drilled in the basin between 1897 and 1990, (Wilkes, 1988, Oil and gas files, Virginia Department of Mines, Minerals, and Energy).
The Cornell/Gemini Oil Company No. l Bailey, the deepest hole drilled in the basin encountered granite basement at 7,110 ft and was drilled to a total depth of 7,438 ft (Wilkes, 1988). The hole encountered several coal beds and reported several oil and gas shows. Traces of oil in the cuttings occurred between 2,550 and 2,890 ft. Shows of asphalt (or “asphaltine”) were reported throughout the well between 3,120 and 6,900 ft, the strongest shows being between 3,680 and 5,700 ft and 5,890 and 6,470 ft. In addition, the well reported formation gas from 3,460 to 3,530 ft and 4,260 and 6,730 ft (Bailey No.1 mud log).
Of the remaining deep holes, fourteen had shows of liquid and/or gaseous hydrocarbons and/or coal. The Horner No. 1 well exhibited traces of coal in the samples from 6,340 to 6,360 ft and from 6,372 to 6,386 ft. Traces of oil in the cuttings began about 250 ft depth and continued to near total depth at 6,354 ft. Shows of asphalt occurred in the well from 2,810 to 6,270 ft. In addition, the well reported formation gas from 3,920 to 6,325 ft (Horner No.1 mud log). Seven wells drilled by Merrill Natural Resources Inc., exhibited oil and gas shows between 775 and 2,970 ft. The Shore Exploration No. 1 J. R. Hicks contained oil and gas shows at 2,894 to 2,908 ft and 3,048 to 3,056 ft, and the Richmond Syndicate No.7 had 11 petroleum shows between 960 and 1,846 ft, and gas was reported in the hole.
Figures & Tables
Petroleum Systems in “Rift” Basins
- coalbed methane
- natural gas
- Newark Supergroup
- organic compounds
- petroleum exploration
- potential deposits
- reservoir rocks
- Richmond Basin
- rift zones
- sedimentary basins
- sedimentary rocks
- source rocks
- stratigraphic units
- structural controls
- total organic carbon
- United States
- Upper Triassic
- Tuckahoe Group
- Chesterfield Group
- petroleum systems