Using Structural Diagenesis to Infer the Timing of ...

4 downloads 0 Views 917KB Size Report
crack-seal and blocky textures that record fracture opening and sealing. Other core ... York (Fig. 1) and in cores from Washington County, Pennsylvania (Fig. 2).
SPE 168770 / URTeC 1580135

Using Structural Diagenesis to Infer the Timing of Natural Fractures in the Marcellus Shale Laura Pommer, Julia F. W. Gale*, Peter Eichhubl, András Fall, Stephen E. Laubach Bureau of Economic Geology, University of Texas at Austin Copyright 2013, Unconventional Resources Technology Conference (URTeC) This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, 12-14 August 2013. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the respon sibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited.

Summary Economic production of oil and gas from mudrocks such as the Devonian Marcellus Shale relies on hydraulic fracture stimulation. The orientation, size, porosity, and strength of subsurface natural fracture systems can influence the growth of hydraulic fractures by conducting fluid, opening, or slipping during treatment. Knowledge of the orientation, size, porosity, and other attributes of natural fractures in the Marcellus Shale is based on core and outcrop data. Fractures in outcrop and core may not be the same age, however, and uncertainty in knowledge of fracture timing and origin impedes use of outcrop data for subsurface applications. Previous studies of fracture timing correlated fracture strikes in outcrop with inferred paleostress directions from past tectonic events. Fractures in the subsurface typically share common orientations with those observed in outcrop, but most fractures in outcrop are barren joints, whereas most of those in the subsurface are lined or sealed with cement. We compare rare fracture cements in outcrop with subsurface examples to test the hypothesis that some fractures in outcrops are equivalent to subsurface fracture systems. We compare fracture cement morphology, texture, mineralogy, and geochemistry from a suite of outcrop samples from Union Springs, New York, with fractures in four cores from a currently producing reservoir in southwest Pennsylvania. Light-microscope petrography and cold cathodoluminescence of calcite in outcrop and some core samples reveal crack-seal and blocky textures that record fracture opening and sealing. Other core samples have fibrous calcite fill and other mineral phases. Using aqueous and hydrocarbon fluid inclusions from synkinematic fracture cements, we can tie fracture growth to burial history. Stable isotopes in calcite fracture cements from different fracture types in cores and outcrop range from -21.5‰ to +4.4‰ δ13C PDB and -8.0‰ to -12.0‰ δ18O PDB. Assuming burial history predicts thermal history, isotopic composition together with fluid inclusions suggest calcite formed at 50-100°C, and that fracture timing was Acadian or early Alleghanian, forming during burial. The outcrop fractures tested in this study appear analogous to subsurface fractures, although other fracture types are present in the cores that are not observed in outcrop; additionally, the orientations in outcrops and the subsurface do not always match. Introduction Hydraulic fracturing of low-permeability (< 0.1 md) unconventional reservoirs such as the Marcellus Formation is necessary for economic production of hydrocarbons (Law and Curtis, 2002). Natural fractures may reactivate during hydraulic fracturing and can either adversely affect the stimulation or positively contribute to production volumes (Warpinski and Teufel, 1987; Gale et al., 2007; Dahi-Taleghani and Olson, 2011). The mechanics of the interaction depend on the geometry and attributes of the natural fracture network (Zhang et al., 2007; Dahi-Taleghani and Olson, 2011; Zhang and Ghassemi, 2011) and an improved knowledge of the natural fracture system can further our ability to predictively model its effect on hydraulic fracturing. Key fracture attributes include orientation, length and height, spacing, kinematic aperture, fracture-plane strength, fracture cement, mineral composition, and the degree of fracture cement infill.

URTeC 1580135

2

Natural fractures in outcrops of Devonian shales of the Appalachian basin have been used as analogs for fractures in the Marcellus Shale play (Engelder and Geiser, 1980; Engelder et al., 2009). There are two main sets of fractures in the Marcellus Formation outcrops, known as the J1 and J2 joint sets. These are tall, steeply dipping, and sometimes cemented (Engelder and Geiser, 1980). The J1 set trends east-northeast throughout the region, within a few degrees of the present-day maximum horizontal stress (Zoback, 1992), with the J2 set at a high angle to it, but rotating in strike, following the change in trend of the Alleghanian deformation front (Engelder and Geiser, 1980). Fractures in outcrop and core need not be the same age, however, and uncertainty in knowledge of fracture timing and origin impedes use of outcrop data for subsurface applications. Knowledge of fracture timing might allow inferences about the diagenetic state of the host rock and the broad paleostress regime at that time, and such inferences might inform prediction of the character of the natural fracture system beyond the limits of the observed fractures in outcrop and core. In this study we use stable isotope and fluid inclusion analyses of mineral cement (calcite) in natural fractures in core and outcrop to determine the temperature of precipitation of those cements, and thereby constrain fracture timing. We compare the results from outcrop and core. Natural fracture orientations and cement fill attributes Sealed fractures in the J1 and J2 orientations were sampled in outcrop at the Wolfe Quarry near Union Springs, New York (Fig. 1) and in cores from Washington County, Pennsylvania (Fig. 2). In this paper we focus mainly on the subvertical fracture sets, although a wider study of other fracture types was completed by Pommer (2013). Fracture orientations were measured in outcrop for 52 J1 fractures and 42 J2 fractures (Fig. 1b). The orientations, ENE (J1) and NNW (J2), are consistent with the orientations published in Engelder et al. (2009). Vertical fracture orientations were determined for one of the cores in southwest Pennsylvania (Range Resources Paxton Isaac Unit #7) through image log analysis of the cored interval. The J2 set is clearly represented, but the interpretation of two other sets trending ENE and NE is less clear. It is likely that one of these sets represents J1.

The tall, vertical fractures in this study contain calcite mineral fill, with blocky, sub-euhedral, or fibrous crystal growth. Fracture walls are planar but host rock grains are commonly included in the cement. In many outcrop samples of J1 fractures, these host-rock grains align with fluid inclusions in closely spaced planes parallel to the fracture wall and separated by layers of incremental fracture cement (Fig. 3). These textures are termed crack-seal texture (Ramsay, 1980; Gale et al., 2010). In the J1 fracture outcrop samples, cements typically have crack-seal texture with up to eight increments at the fracture margins with a central portion of blocky, sub-euhedral calcite cement (Fig. 3). Some fractures have crack-seal on both margins, whereas others have crack-seal only on one margin. J2 fractures do not have crack-seal texture and have only blocky calcite cement. Definitive crack-seal texture is not present in the subsurface fracture samples, but repeated opening may be marked by separate zones of cementation with small crystals along the fracture margins and coarser, blocky crystals in the center. Fractures with one increment of cementation are more common in the subsurface than are those with multiple increments. Fractures with fibrous calcite cement typically have a planar zone of host-rock inclusions in the central portion of the fracture.

Stable Isotope Analysis of Fracture Cements We used stable isotope analysis (δ18O and δ13C) of calcite cements in 27 samples of fractures from outcrop and all four cores to constrain the cement compositions and to estimate temperatures of cement precipitation. Stable isotopes in calcite fracture cements from different fracture types in cores and outcrop range from -21.5‰ to +4.4‰ δ13C PDB and -8.0‰ to -12.0‰ δ18O PDB. Oxygen isotope data were converted from values relative to PDB into values relative to standard mean ocean water (SMOW) (Coplen, 1988). These values were then input into Friedman and O’Neil’s (1977) experimentally determined equation for calculating precipitation temperatures. Considering the range of oxygen isotope measurements for calcite and assuming a marine pore water δ 18O isotopic composition of 0‰ SMOW (given the open marine basin depositional environment of the Devonian Marcellus), calcite precipitation temperatures are calculated to be between 49.39 and 89.74°C. Outcrop fracture calcite cements have precipitation temperatures of 59.27–80.45°C, mean 66.9°C, lower than those in the subsurface: 59.27–80.45°C, mean 66.9°C.

URTeC 1580135

(a)

3

(b)

(c) Figure 1: Calcite-sealed fractures in Marcellus Shale outcrop at Wolfe Quarry. (a) Two sets present in quarry floor. (b) Rose diagrams of fractures trends in set A and set B; n = number of fractures measured. (c) Relative timing of sets indicated by 1 cm of dextral offset of a J1 fracture on a J2 fracture plane.

Figure 2: Calcite-sealed fractures in four cores from southwest Pennsylvania. Fractures are typically subvertical and less than 1 mm wide.

URTeC 1580135

4

Figure 3. Alternating calcite cement and host rock/fluid inclusion layers in a J1 outcrop fracture (Sample WQ4). Crack-seal texture occurs at both margins and blocky, coarse, subhedral growth occurs in the center.

Fluid inclusion analysis We made petrographic descriptions of fluid inclusion assemblages and determined which of these were suitable for microthermometry, following the methods indicated by Bodnar (2003). Fluid inclusion petrography revealed abundant secondary inclusions but few primary inclusions. The secondary inclusions formed along microfractures that postdated the main fracture opening and initial cement precipitation. Based on this observation, we used the microthermometry data on the secondary inclusions to constrain the temperature at the time of secondary events. Homogenization temperatures of aqueous inclusions where the ranges of Th within individual FIAs lie within 5°C give mean Th of 110°C –120°C. This gives the minimum trapping temperature, given that pressure corrections would elevate temperatures further. Many secondary oil inclusions were observed, indicating that hydrocarbons migrated through the system at some time after initial fracture formation and cementation, with UV fluorescence of blue/white indicating an API gravity around 40°: light oil. We plotted the two groups of data (precipitation temperatures from O18 data, and fluid inclusion homogenization temperatures) onto a burial history curve that Evans (1995) had constructed for a well close to our sampled wells (Fig. 4). Fracture cement precipitation must predate the formation of the secondary fluid inclusions in that cement. Given a precipitation temperature range of 50°C to 100°C and homogenization temperatures greater than 100°C, the fractures must have formed on the burial part of the curve. Furthermore, the fractures must have all formed during the Acadian or in the early part of the Alleghanian Orogeny (Fig. 4). The fractures in outcrop clearly have a different burial history, given their distance from the subsurface fractures, but the natural hydraulic fracture model (Engelder et al., 2009) for fractures across the upstate New York region suggests that the outcrop rocks were buried sufficiently to have hydrocarbon generation. This indicates that they would have undergone similar processes to those in the subsurface. According to East et al. (2012), measured vitrinite reflectance of the Devonian Shales in Washington County, Pennsylvania, and in outcrop near Union Springs, New York, is between 1.3%Ro and 2%Ro, which is within the gas generation window and is consistent with a maximum burial temperature shown in the burial history curve from Evans (1995) of around 165°C.

URTeC 1580135

5

Figure 4. Inferences of timing of fracture formation, fracture cement precipitation, and fluid inclusion entrapment plotted on Evans’ (1995) burial history curve from the PA-2 well in southwest Pennsylvania. This curve represents the burial history for fractures in the subsurface sampled in this study. Minimum homogenization temperatures plotted in blue, inferred temperatures from O18 isotope values plotted in red, and fracture opening plotted in green.

Conclusions Calcite cement in natural mode I (opening) fractures in outcrop and core samples can be used to constrain the timing of fracture formation by combining analyses of oxygen isotopes and fluid inclusions in the cement with a burial history curve. Assuming burial history predicts thermal history, oxygen isotopic compositions suggest calcite formed at 50°C to 100°C. Secondary fluid inclusions that postdate the calcite give homogenization temperatures in excess of 100°C, indicating the fractures must have formed during burial rather than uplift. These data constrain fracture timing to be Acadian or early Alleghanian Orogeny. The outcrop fractures tested in this study appear analogous to subsurface fractures, but other fracture types are present in the cores that are not observed in outcrop; additionally, the orientations in outcrops and the subsurface do not always match.

URTeC 1580135

6

Acknowledgments Funding for this project is provided by RPSEA through the Ultra-Deepwater and Unconventional Natural Gas and Other Petroleum Resources program, authorized by the U.S. Energy Policy Act of 2005. RPSEA (www.rpsea.org) is a nonprofit corporation whose mission is to provide a stewardship role in ensuring the focused research, development, and deployment of safe and environmentally responsible technology that can effectively deliver hydrocarbons from domestic resources to the citizens of the United States. RPSEA, operating as a consortium of premier U.S. energy research universities, industry, and independent research organizations, manages the program under a contract with the U.S. Department of Energy’s National Energy Technology Laboratory. Range Resources– Appalachia, LLC provided core and well data. Terry Engelder showed JFWG some key outcrops and provided valuable discussion in the field. The Fracture Research and Application Consortium at The University of Texas at Austin provided additional funding. Publication authorized by the Director, Bureau of Economic Geology. Isotope analyses were paid for by an AAPG Grants in Aid award to LP. References Bodnar, R.J. 2003. Introduction to Aqueous-Electrolyte Fluid Inclusions. In Fluid Inclusions: Analysis and Interpretation, v. 32, ed. I. Samson, A. Anderson, and D. Marshall, Chap. 4, 81–100. Vancouver, Canada: Mineralogical Association of Canada. Coplen, T.B. 1988. Normalization of Oxygen and Hydrogen Isotope Data. Chemical Geology: Isotope Geoscience Section 72 (4): 293–297. Dahi-Taleghani, A., and Olson, J. 2011. Numerical Modeling of Multistranded-Hydraulic-Fracture Propagation: Accounting for the Interaction Between Induced and Natural Fractures. SPE J. 16: 575–581. East, J.A., Swezey, C.S., Repetski, J.E. and Hayba, D.O. 2012. Thermal Maturity Map of Devonian Shale in Illinois, Michigan and Appalachian Basins of North America. U.S. Geological Survey Scientific Investigations Map 3214, 1 sheet. (Also available at http://pubs.usgs.gov/sim/3214/.) Engelder, T. and Geiser, P. 1980. On the Use of Regional Joint Sets as Trajectories of Paleostress Fields During the Development of the Appalachian Plateau, New York. Journal of Geophysical Research 85: 6319–6341. Engelder, T., Lash, G.G., and Uzcategui, R.S. 2009. Joint Sets That Enhance Production from Middle and Upper Devonian Gas Shales of the Appalachian Basin. AAPG Bulletin 93: 857. Evans, M.A. 1995. Fluid Inclusions in Veins from the Middle Devonian Shales: A Record of Deformation Conditions and Fluid Evolution in the Appalachian Plateau. Geological Society of America Bulletin 107: 327. Friedman, I., and O’Neil, J.R. 1977. Compilation of Stable Isotope Fractionation Factors of Geochemical Interest. USGS Professional Paper 440-KK. Gale, J.F.W., Reed, R.M., and Holder, J. 2007. Natural Fractures in the Barnett Shale and Their Importance for Hydraulic Fracture Treatments. AAPG Bulletin 91: 603. Gale, J.F.W, Lander, R.H., Reed, R.M., and Laubach, S.E. 2010. Modeling Fracture Porosity Evolution in Dolostone. Journal of Structural Geology 32: 1201-1211. Law, B.E. and Curtis, J. 2002. Introduction to Unconventional Petroleum Systems. AAPG Bulletin 86: 1851–1852. Pommer, L. E. 2013. Natural Fracture Cementation in the Marcellus Formation. MS thesis, University of Texas at Austin, 318 p. Ramsay, J.G. 1980. The Crack-Seal Mechanism of Rock Deformation. Nature 284:135–139. Warpinski, N., and Teufel, L. 1987. Influence of Geologic Discontinuities on Hydraulic Fracture Propagation. J. Pet Tech 39: 209–220. Zhang, Z., and Ghassemi, A. 2011. Simulation of Hydraulic Fracture Propagation near a Natural Fracture Using Virtual Multidimensional Internal Bonds. International Journal for Numerical and Analytical Methods in Geomechanics 35: 480–495. Zhang, X., Thiercelin, M., and Jeffrey, R., Effects of Frictional Geological Discontinuities on Hydraulic Fracture Propagation, in Proceedings SPE Hydraulic Fracturing Technology Conference, 2007. Paper SPE 106111. Zoback, M.L. 1992. First- and Second-Order Patterns of Stress in the Lithosphere: the World Stress Map Project. Journal of Geophysical Research 97 (B8): 11, 703–711, 728.