Veins, Fluid Migration and Hydrocarbon Generation in the Utica Shale, Northern Appalachian Basin, New York

The Upper Ordovician Utica Formation of the Appalachian Basin is a potential target for natural gas development. This black shale outcrops throughout the Mohawk Valley in Central New York State. The lower Flat Creek Member is characterized by E-W Mode 2 (strike-slip) fractures, bedparallel thrusts, and N-S Mode 1 (tensile) fractures. E-W fractures and dilational jogs host the majority of calcite veins, some with hydrocarbon staining and methane-dominated low-salinity aqueous fluid inclusions. Mode 1 fractures host calcite veins and sand injectite dikes sourced from sand and dolomite sourced from underlying Paleozoic strata and Proterozoic basement. Volcanic ash beds from within the Utica are also a source of material for sand injectites. These features promote the hypothesis that faulting was active at the time of burial and seismic pumping may have allowed vertical migration of fluids from overlying and underlying units. Horizontal veins also promote this hypothesis, as low confining pressures and/or high fluid pressure would allow fracturing and vein precipitation. The migration of multiple fluids is evidenced by stable isotope data for carbon and oxygen (δCcalcite =-11 to +15 PDB; δOcalcite = -6 to -12 PDB) and fluid inclusion data (Th ≈105-185°C, TMice = -0.5 to -4.5 C), and indicate that vein generation occurred during hydrocarbon maturation, and that vein-forming fluids were mainly derived from within the Flat Creek Member. Multiple events and mixtures of fluids caused the variation in isotope values and suggest mixing of seismically pumped fluids in fault systems during the evolution of the Taconic foreland basin. The upper Utica (Indian Castle Member) and overlying units show different fracture patterns than the Flat Creek Member and generally lack mineralization, suggesting relatively early burial likely caused the fracturing and fluid expulsion in the Flat Creek Member. Basement-derived hydrothermal fluids may have facilitated hydrocarbon maturation during the early burial and diagenesis in the Flat Creek Member. Later burial and fracturing events in the Utica occurred after deposition of Silurian sandstone strata and permitted up-migration of dry gas into sandstone reservoirs. Introduction The late Ordovician Utica Formation outcrops in central New York and extends into the subsurface through much of the Appalachian Basin (Figure 1). Deposition occurred during the downward flexure of the Appalachian Foreland during the onset of the Taconic Orogeny (Bradley and Kusky 1986). The Utica is a high total organic carbon (TOC) (>3.0%) black shale, making it a potential target for natural gas development (Martin, 2005). The potential zone of Utica production (fairway) includes portions of south-central New York. In the eastern and central Mohawk Valley region, the Utica Formation is separated into two members: the basal Flat Creek and upper Indian Castle (Figure 2). The Flat Creek Member overlies Trenton Group limestones. Overlying the Indian Castle Member is the Frankfort Formation, which is unconformably overlain by the Oneida Formation in the central Mohawk Valley (Brett and Baird, 1982). The Flat Creek Member is replaced westward by the Dolgeville Member, which in turn is replaced by platform limestones in the Tug Hill Region. The highest TOC intervals are 1 Michaels: Veins, Fluid Migration and Hydrocarbon Generation in the Utica Sh Published by Digital Commons @ Colgate, 2012 293 found in the lower Flat Creek and basal Indian Castle (Nyahay and Martin, 2008). Successful well stimulation of gas shale reservoirs is enhanced by an understanding of the orientation of natural fractures in the shale (e.g. Engelder et al, 2009). The character of natural fractures, whether open or mineralized is also critical to successful gas shale development. Figure 1: Map of Utica Formation outcrop in New York (Selleck, 2010) 2 Colgate Academic Review, Vol. 9, Iss. 1 [2012], Art. 12 http://commons.colgate.edu/car/vol9/iss1/12 294 Figure 2: Stratigraphic column of central New York during the late Ordovician (Selleck, 2010) Fractures and mineralized veins are present in the Utica Formation. These features record the stress history and fluid evolution, key factors in understanding the hydrocarbon potential of the Utica. The systematic patterns of both the fractures and veins are of importance to the petroleum industry, as fractures and faults often serve as conduits for hydrocarbon migration. Since these fractures offer access to hydrocarbons present in the Utica, drilling horizontally and at an angle perpendicular to the systematic fractures at depth could reduce or eliminate the need for high volume hydraulic fracturing, a practice not currently permitted in the New York. Analysis of regional fracture trends demonstrates that the Flat Creek underwent an early stress history different from the overlying units (Colborne, 2011). In this study, the orientation and compositional aspects of mineralized veins in the Flat Creek Member are used to define the relationship between fracturing, fluid sources, and hydrocarbon potential. This paper describes the characteristics of mineralized veins in the Flat Creek Member from east to west across the Mohawk Valley and the relationship between vein formation and hydrocarbon maturation. These results show that the Flat Creek member fracture and mineralized vein systems in the Mohawk Valley evolved during relatively shallow burial in response to a generally E-W directed shear stress field. Veins formed as fluids migrated from the dewatering of shale, as documented by fluid inclusion and stable isotope data. Fracture and vein systems also provided connections to fluids derived from underlying Cambrian and Ordovician strata, and faulted Proterozoic basement (Jacobi, 1981; Brett and Baird, 1982). These fluids mobilized and transported sand from the basement and underlying sedimentary units to form sand dikes that fill primarily N-S Mode 1 (extensional) fractures. Volcanic ash beds within the Flat Creek were also mobilized and mixed with transported detrital grains and precipitate within vein material to form carbonate cemented sand injectite dikes. Extensive hydrocarbon staining, hydrocarbon inclusions, and stable isotope patterns indicate that fracturing and vein development was coincident with organic maturation. 3 Michaels: Veins, Fluid Migration and Hydrocarbon Generation in the Utica Sh Published by Digital Commons @ Colgate, 2012 295 Previous Studies Why Study the Utica Formation? The Utica Formation is of interest to the petroleum industry. There has been active exploration in Ohio and Ontario within the Utica. Wickstrom et al. (2011) notes that 44 wells have been drilled into the Utica Formation (and Point Pleasant Formation, a western Appalachian Basin equivalent) in the state of Ohio, and thus the Utica has proven to be a productive gas shale. Artificially fractured wells in Quebec target the Dolgeville Member of the Utica Formation, which is over 200 feet thick in some areas and has proven to be productive gas shale reservoirs (Marcil et al., 2011). There are potential plays in the southern tier of New York, in Otsego County (Oil and Gas Journal, -Ross 1 Well Test Report). Regional Stratigraphy and Tectonics The regional tectonic setting for Ordovician-age sedimentary units of New York State is key to understanding the depositional history of the Utica Formation. Bradley and Kidd (1991) describe the creation and deepening of the Appalachian Foreland Basin during the Taconic Orogeny. The Flat Creek Member was deposited in the foreland basin that developed in response to subsidence as Laurentia collided with, and was overridden by, a prism-arc complex in the late Middle Ordovician. As summarized by Joy et al. (2000), “The accretionary load flexed the eastern Laurentian margin, resulting in structural deepening in the Taconic foredeep and a rapid increase in the rate of accommodation space growth.” The overriding arc-prism caused a flexural deepening of the basin in the eastern Mohawk Valley and accommodated sedimentation. “Locally dominating forces of differential subsidence created by collision tectonism, sediment loading, and resulting lithospheric flexure” created a progressive east to west deepening of the Taconic foreland basin (Joy et al 2000). The Taconic foreland basin received fine-grained, organic-rich mud relatively poor in terrigenous clastic material. Bradley and Kusky (1986) noted that downward flexure of the Taconic foreland was accomplished as, “former continental shelf was being cut by high-angle faults.” This syndepositional faulting influenced sedimentary facies patterns, and continued through deposition of the Utica Formation. This tectonic influence is supported by Smith et al. (2011) as, “Deposition and preservation of sediments is strongly influenced by syndepositional active faulting.” Normal faults allowed for extension of the basin and an increase in accommodation space, allowing fine-grained sediments to accumulate, resulting in deposition of the organic-rich Flat Creek Member. Even as the Utica was deposited in the calm-water setting which is necessary for black shale development, the uplift of the Taconic Orogen produced an increase in terrigenous clastic sediment supply. This terrigenous clastic sediment is the source of the units overlying the Utica, including the Frankfort and Schenectady Formations (Figure 2). The Frankfort is a coarsening-upward sequence of cross-bedded, submarine-fan deposits that records the initiation of erosion of the Taconic allochthon (Bradley and Kidd 1991). “Late Ordovician Frankfort Formation mud, silt and sand represent rapid filling of the Taconic foreland basin” (Selleck 2010). The filling of the basin by clastic sediments occurred rapidly, as little organic material is found in the Frankfort Shale and shallow marine clastics of the Lorraine and Pulaski Formations overly the Frankfort to the west. It is likely that the Frankfort was deposited at higher sedimentation rates, and the coarsening-upward pattern is related to both sediment accumulation and local sea-level fall resulting from relaxation of compressive stresses and lithospheric flexure as the Taconic Orogeny drew to a close. The rapid burial of organic material 4 Colgate Academic Review, Vol. 9, Iss. 1 [2012], Art. 12 http://commons.colgate.edu/car/vol9/iss1/12 296 allowed its preservation before oxidation may have occurred. The lack of oxidation may also be influenced by oxygen-depleted, or even anoxic, conditions in the bottom waters of the Appalachian Basin. The Utica and Frankfort formations are cut by N-S trending normal faults that were active prior to deposition of the overlying Silurian and Devonian strata in the Mohawk Valley region. These east-dipping faults accommodate normal, dip-slip displacement (Bradley and Kusky 1986). Few dip-slip indicators were noted in this study apart from three faults at Flat Creek locality that exhibited small-scale (cm to dm-scale) displacement and at the Town of Minden locality that showed oblique slip down to the west. East-dipping faults are well documented in the region (Jacobi, 1981). These N-S normal faults may be related to the N-S fracture pattern of the Flat Creek in the eastern part of the Mohawk Valley. It has been hypothesized that basement faulting and post-Taconic orogenic events (Acadian and Alleghenian) may have caused the NW-SE and NE-SW trending fractures of Utica Formation in western New York (Smith et al, 2011; Colborne, 2011). These later orogenic events may have caused some of the N-S Mode 1 fractures in the Utica Formation in the eastern segment of the Flat Creek member (Colborne, 2011). Fractures and Veins Lim, et al (2005) summarized the origins, temperatures, structural, and fluid characteristics of veins hosted by deformed Utica Formation equivalents in the distal portions of the Taconic frontal thrust zone (Figure 4). Most veins described by Lim, et al (2005) are calcitefilled with some minor quartz, and are “planar with continuity of <1 m to several meters or more,” and are a few millimeters to a few centimeters thick. Lim, et al (2005) cite homogenization temperatures for fluid inclusions in the Taconic vein samples between ~180250° C, while δO values ranged from -14.36 to -10.87 PDB ‰ and δC values ranged from 6.4 to 0.7 PDB ‰. In the veins closer to the Taconic Orogen, Lim et al (2005) proposed metamorphic sources for the fluids present while vein precipitation occurred. These values give some insight into possible fluids present in the Utica Formation during burial, however a complicated mixing of fluids is likely a cause for variation in stable isotopes. A preliminary analysis of fractures and veins in the Utica Formation by Selleck (2010a) proposed possible changes in fracture mode from east to west in the Mohawk Valley. Mode 2 (strike-slip) fractures, as well as filling of mineralized veins, are more common in eastern and central Mohawk Valley within the Flat Creek Member. The western portion of the outcrop belt displays a different overall fracture orientation with most fractures of Mode 1 origin. E-W oriented fractures in the eastern portion of the Flat Creek Member show slip indicators such as slickenlines, en echelon fracturing, dilational jogs, and step surfaces. N-S fractures are not often mineralized, are Mode 1 tensile fractures, and are associated with sand injectite dikes in a few rare cases (Selleck 2010a). Selleck’s (2010b) descriptions of veins at Town of Minden point out that, “The most abundant fracture sets trend ~N70W and N15E and are often mineralized with hydrocarbon-stained calcite.” This E-W fracture set is prevalent throughout much of the Flat Creek Member in the eastern and central Mohawk Valley and is described in detail in following sections and by Colborne (2011). Selleck et al. (2009) suggests that elevated δC values in some calcite vein samples, were due to microbial fermentation of bitumen in later mineralized veins. Many samples also have negative δOPDB values, interpreted as secondary precipitation of calcite from of fluids that were present in the sediments at the time of burial and were reaching oxygen isotopic 5 Michaels: Veins, Fluid Migration and Hydrocarbon Generation in the Utica Sh Published by Digital Commons @ Colgate, 2012 297 equilibrium with the rock reservoir (Figure 3). This preliminary study included subsurface core data, which was not available for this project. Figure 3: δ13C and δ18O plot samples from initial studies in the Utica Shale. Includes outcrop and core data (Selleck et al., 2009) Methods Field study of fracture and vein systems in the upper Ordovician Utica Formation and Frankfort Formation was accomplished in the summer of 2010. A total of eight localities in the Mohawk Valley were studied (Figure 4) to measure orientation of the veins and fractures and to characterize the fracture modes, as well as to assess crosscutting relationships of fractures and veins. 6 Colgate Academic Review, Vol. 9, Iss. 1 [2012], Art. 12 http://commons.colgate.edu/car/vol9/iss1/12 298 Figure 4: Map of Utica Formation outcrop in New York with generalized rose diagrams representing fracture patterns of near-vertical veins and fractures for localities of this study. Outcrop locations were recorded using coordinates from a Magellan GPS unit. The number of measurements taken at each locality was dependent on the quality of the outcrop hence the number of observations varies considerably (Tables 1 and 2). Strike and dip measurements were made with a Silva compass were compiled to create rose diagrams (Figure 4) using the software program Stereo32. These rose diagrams allowed interpretation of strike of near-vertical fractures and veins. Locality Number of Joint/Fracture Orientation Measurements Stratigraphic Unit Frankfort Gorge 317 Frankfort Flat Creek 412 Flat Creek Town of MindenOtsego Creek 94 Flat Creek Ilion Gorge 230 Frankfort 7 Michaels: Veins, Fluid Migration and Hydrocarbon Generation in the Utica Sh Published by Digital Commons @ Colgate, 2012 299 Hallsville 62 Indian Castle Wintergreen Park 36 Flat Creek Reall Creek 170 Indian Castle Total 1321 Table 1: Number of fracture and vein measurements taken in this study Locality Number of Joint/Fracture Orientation Measurements Stratigraphic Unit Fultonville 21 Flat Creek Delta Reservoir 32 Indian Castle Whetstone Gulf 47 Indian Castle South Chuctanunda Creek 42 Flat Creek Pixley Falls 31 Indian Castle Little Falls Exit 26 Flat Creek