Comparison of isotopic and geochemical characteristics of sediments from a gas- and liquids-prone wells in Marcellus Shale from Appalachian Basin, West Virginia

Abstract

The Middle Devonian age Marcellus Shale contains one of the largest shale gas plays in North America. Hydrocarbon production in the eastern part of the play is mostly "dry gas," consisting of essentially pure methane. Production of natural gas liquids (condensate) increases toward the west, which is the area currently, being targeted by developers. Two Marcellus Shale cores from West Virginia were analyzed to compare the isotopic and geochemical characteristics of a liquids-prone well (WV-7) in Wetzel County with a gas-prone well (WV-6) in Monongalia County. The contrasts between the cores indicate that the conditions of the Marcellus Shale deposition were different between the two sites. The dominant organic matter preserved in each core is isotopically different; δ 13 C org values are lighter on average in WV-6 compared with WV-7. A possible explanation is that a larger fraction of terrestrial organic matter was preserved in the WV-6 core, whereas WV-7 may contain a greater percentage of marine organic matter.Clastic-influx proxies (e.g. Ti/Al, Ca/Al and Mg/Al) also suggest that the WV-6 core site received a higher siliciclastic input compared to WV-7, consistent with a more proximal location to dry land and the delivery of greater amounts of terrestrial organic matter. Depleted δ 13 C carb values, low concentrations of redox sensitive elements (e.g. V, Cr, Ni and U), and high 2 variability δ 15 N values in the WV-6 core all suggest the presence of higher dissolved oxygen concentration and short term shifts in an oxic/anoxic boundary near the sediment-water interface during deposition. These lines of evidence indicate that the depositional conditions were favorable for the accumulation of predominantly gas-prone Type III kerogen in the Marcellus Shale at the WV-6 site. In contrast, the Marcellus Shale at the WV-7 site was deposited in a more distal area that received a low terrestrial sediment supply, organic matter primarily derived from marine algae, and bottom water conditions that were dominantly anoxic. Such conditions were favorable for the accumulation of Type II kerogen that has a greater capacity to generate liquid hydrocarbons. Differences between the liquids-prone and gas-prone parts of the Marcellus Shale play have been largely ascribed to depth-of-burial and thermal maturation history; this study indicates that depositional environment and sedimentary facies may have played significant roles as well.

Testing Utility of Organogeochemical Proxies to Assess Sources of Organic Matter, Paleoredox Conditions, and Thermal Maturity in Mature Marcellus Shale

“…It can be inferred that probably WV-6 was deposited more toward basin margin, and therefore, it received a higher influx of mixed OM from both marine and the terrestrial origin, and hence, it is more gas prone. The isotopic and geochemical proxies also support this conclusion (Chen et al, 2015). Additionally, terrestrially derived OM contains more amount of oxygen as compared to the amount in marine-derived OM.…”

“…Both the cores were stored under room temperature conditions at the West Virginia Geological and Economic Survey, Morgantown, West Virginia. The majority of samples from lowermost shale interval for both WV-6 and WV-7 wells have high TOC values (>5%), whereas the majority of the samples in the overlying shale interval have relatively lower TOC values (<5%) (Chen et al, 2015). Simplified nomenclature of Chen et al (2015) was followed in this study where the lowermost Marcellus formation is defined as organic-rich (OR) zone and the overlying zone with lower TOC as organic intermediate (OI) zone (Figure 1).…”

“…The geochemical and isotopic characteristics of these two cores were extensively studied by Chen et al (2015), and authors proposed that the sediments in the gas-prone well WV-6 were deposited under alternating redox conditions close to ocean margin and received a greater influx of terrestrial OM. On the other hand, WV-7 sediments were deposited in a more distal anoxic environment dominated by marine OM influx (Chen et al, 2015). For this study, SRA and biomarker analysis was conducted on samples collected from the same two well cores, i.e., WV-6 and WV-7.…”

“…For this study, SRA and biomarker analysis was conducted on samples collected from the same two well cores, i.e., WV-6 and WV-7. Our aim was to test if the SRA and biomarker data supports the inferences drawn by Chen et al (2015) and to verify if these proxies provide meaningful interpretations for mature shales with VRo > 1. To our knowledge, this would be the first attempt to use biomarkers in conjunction SRA analysis to assess maturity, depositional environment, and source of organic matter in the mature shales like Marcellus.…”

“…The samples were collected from two mature Marcellus Shale well cores, an oilprone WV-7 well with VR o between 1 and 1.5 in Wetzel County and a gas-prone WV-6 well in Monongalia County with VR o >2.5 (Zagorski et al, 2012). The geochemical and isotopic characteristics of these two cores were extensively studied by Chen et al (2015), and authors proposed that the sediments in the gas-prone well WV-6 were deposited under alternating redox conditions close to ocean margin and received a greater influx of terrestrial OM. On the other hand, WV-7 sediments were deposited in a more distal anoxic environment dominated by marine OM influx (Chen et al, 2015).…”