Relationships between lineal fracture intensity and chemical composition in the Marcellus Shale, Appalachian Basin

Abstract

Within mudrock reservoirs, brittle zones undergo failure during hydraulic stimulation, creating numerous artificial fractures which enable hydrocarbons to be liberated from the reservoir. Natural fractures in mudrock reduce the tensile strength of the host rock, creating planes of weaknesses that are hypothesized to be reactivated during hydraulic stimulation. Combined, brittleness and natural fractures contribute to creating more abundant and complex fracture networks during hydraulic stimulation. Research efforts toward quantifying rock brittleness have resulted in numerous mineral-/compositional-based indices, which are used during petrophysical analysis to predict zones most conducive to hydraulic stimulation. In contrast, investigations on the relationship between chemical composition and core-scale natural fractures are limited. For this study, we collected high-resolution energy-dispersive X-ray fluorescence (XRF) data, calibrated with a wave-dispersive XRF, from a Marcellus Shale core. Additionally, we characterized corescale natural fractures in terms of length, width, in-filling material or lack thereof, and orientation. Following the characterization, we transformed the natural fracture data into a continuous P10 (lineal fracture intensity) curve, expressed as the number of fractures per a one-half foot window. Using these data sets, we investigated the relationship between rock composition and natural fracture intensity. Regression analyses recorded positive relationships between natural fracture intensity and calcium, silicon/aluminum, and total organic carbon (TOC), and negative relationships with silicon and aluminum. Aluminum recorded the strongest (negative) relationship (r2 1⁄4 0.379) with natural fracture intensity. To access the degree to which natural fractures can be predicted based on chemical composition, we applied a partial least-squares analysis, a multivariate method, and recorded an r2 1⁄4 0.56. Our study illustrates that although numerous factors are responsible for natural fracture genesis, such fractures predictively concentrate in areas of similar chemical composition, largely in zones with low aluminum concentrations. Introduction The term “mudrock” will be used following Blatt et al. (1972), who uses mudrock as a broad term for fineto very-fine-grained rocks (<62.5 μm in diameter) dominated (>50% of grains) by varying degrees of silt, mud, and clay concentration and include fissile and nonfissile rock. When further divided, fissile and nonfissile mudrocks are termed “shale” and “stone” respectively, and they are modified by the dominant grain size, such that a silt-rich, nonfissile mudrock would be termed a siltstone. Mudrock is a useful term for the Marcellus Shale specifically because it contains a spectrum of such fineto veryfine-grained rocks. Mudrock reservoirs are characterized by nanometerto micrometer-size pores and nanoto microdarcy permeability (Loucks et al., 2012; Peng and Loucks, 2016; Milad and Slatt, 2018). Such reservoir characteristics significantly impede the ability for hydrocarbons to flow through the formation into the borehole. Advances in hydraulic fracturing technology over the last decade have allowed for the exploitation of hydrocarbons from mudrock reservoirs by generating artificial fractures, which create permeability for liquid and gaseous hydrocarbons to flow through rock into the borehole. Rock brittleness and ductility are key variables in determining a rock’s conduciveness to hydraulic stimulation. A ductile rock will absorb a high amount of energy before fracturing, whereas a brittle rock will fail and more readily form fractures (Zhang et al., 2015). Research efforts have attempted to quantify brittleness through mechanical testing, mineral content, elastic parameters, and sedimentary characteristics (Chang et al., 2006). Within silica-rich Woodford Shale intervals, anisotropic features, such as laminations and bedding, were characWest Virginia University, Department of Geology and Geography, Morgantown, West Virginia 26506, USA. E-mail: [email protected]; [email protected]; [email protected]. Formerly West Virginia University, Department of Geology and Geography, Morgantown, West Virginia 26506, USA; presently California State University, Bakersfield, Department of Geological Sciences, Bakersfield, California 93311, USA. E-mail: [email protected]. Manuscript received by the Editor 26 November 2018; revised manuscript received 31 January 2019; published ahead of production 11 June 2019; published online 9 September 2019. This paper appears in Interpretation, Vol. 7, No. 4 (November 2019); p. SJ33–SJ43, 8 FIGS. http://dx.doi.org/10.1190/INT-2018-0221.1. © 2019 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved. t Special section: Petrophysical analysis for shale reservoir evaluation Interpretation / November 2019 SJ33 D ow nl oa de d 09 /1 8/ 19 to 1 57 .1 82 .1 54 .1 87 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg / terized by lower resistance to fracture and tensile strength, indicating that anisotropic features may contribute to the fracability of a unit (Molinares et al., 2017). In addition to anisotropic features, mineral composition plays a large role in the overall brittleness of a rock (Jarvie et al. 2007; Slatt and Abousleiman, 2011). It is now widely held that enrichment of silica contributes to brittleness, whereas an enrichment of total organic carbon (TOC) and clay increase ductility. Jarvie et al. (2007) conclude that silica was the only contributor to brittleness, whereas clay and carbonate increased ductility. Wang and Gale (2009) find similar results, but they differentiate between dolomite and calcite, claiming that although calcite does contribute to ductility, dolomite has a brittle behavior. More recently, Jin et al. (2014) conclude that quartz, feldspars, brittle mica, and carbonate all contribute to brittleness. Within unconventional mudrock exploration, units with high TOC are desirable. The fact that TOC contributes to ductility complicates locating zones that are organic rich and also conducive to artificial stimulation. Using Passey’s method to derive TOC and the brittleness equation of Wang and Gale, (2009) and Verma et al. (2016) confirm that TOC has a negative relationship with brittleness. In addition to rock brittleness, natural fractures play a significant role in the effectiveness of hydraulic stimulation (Olson et al., 2001; Rijken, 2005; Gale et al., 2007). It has been hypothesized that natural fractures form as a result of potentially one or a number of mechanisms, including tectonic events that cause local and regional stress changes, uplift, differential compaction, strain from the accommodation of large structures, and catagenesis (Neuzil and Pollock, 1983; Jowett, 1987; Price, 1997; Gale et al., 2007; Engelder et al., 2009; Rodrigues et al., 2009; Milad and Slatt, 2018). Natural fractures, either mineralized (sealed) or open, can reactivate during hydraulic stimulation and create more complex fracture networks than could be achieved with a single hydraulic fracture (Gale et al., 2007). This is largely because natural fractures, in some cases, can act as a plane of weakness. Within the Barnett Shale, Wang and Gale (2009) observed that calcite-filled fractures were half as strong as an unbroken rock due to the lack of chemical bonding between the calcite in-filling the natural fracture and the fractured host rock. In addition to academic studies, empirical observations made during analysis of the Marcellus Shale core show that breakages/fractures along mineralized fractures and anisotropic planes occur at a rate much higher than a fractureless or more massive cored interval. Brittleness and natural fractures play a role in the effectiveness of hydraulic stimulation. Although knowledge of the relationship between chemical composition and rock brittleness have advanced, studies addressing the relationship between chemical composition and core-scale natural fracture presence are lacking. To address this shortcoming, we use high-resolution X-ray fluorescence data and detailed natural fracture data collected from the study well core to define the relationship between chemical composition and natural (open and mineralized) fractures. Questions that will be addressed include 1) Do natural fractures preferentially concentrate in rock of certain chemical composition in a statistically meaningful way? 2) If so, what chemical composition is most likely to have a positive or negative relationship with the natural fracture presence? 3) Are the relationships strong enough for natural fracture prediction based on chemical composition? 4) If so, can existing mineral-based brittleness indexes also be used for natural fracture prediction? 5) If not, can findings be used to create a chemicalbased natural fracture index for natural fracture prediction? Middle Devonian stratigraphy of the Appalachian Basin The Marcellus Shale is divided into in three members, namely the Oatka Creek, Cherry Valley or Purcell Limestone, and the Union Springs members (Lash and Engelder, 2011). The Marcellus Shale is overlain by the Skaneateles Formation and overlies the Onondaga Limestone. The Union Springs Member, informally referred to as the lower Marcellus, is thinnest in western New York and thickens to the east and southeast, reaching a thickness exceeding 160 ft (49 m) in northeastern Pennsylvanian (Lash and Engelder, 2011). This member is highly organic, calcareous, and it is characterized as a dark-gray to black mudrock containing skeletal material (Sageman et al., 2003). The lower portion of the Union Springs Member records a particularly high gamma-ray response, reaching upwards of 650 API units. This interval is characterized by a decrease in clay content and a significant incr