Accepted Manuscript Pore structure, gas storage and matrix transport characteristics of lacustrine Newark shale R. Fink, A. Amann-Hildenbrand, P. Bertier, R. Littke PII:
S0264-8172(18)30278-2
DOI:
10.1016/j.marpetgeo.2018.06.035
Reference:
JMPG 3404
To appear in:
Marine and Petroleum Geology
Received Date: 13 December 2017 Revised Date:
29 June 2018
Accepted Date: 29 June 2018
Please cite this article as: Fink, R., Amann-Hildenbrand, A., Bertier, P., Littke, R., Pore structure, gas storage and matrix transport characteristics of lacustrine Newark shale, Marine and Petroleum Geology (2018), doi: 10.1016/j.marpetgeo.2018.06.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pore structure, gas storage and matrix transport characteristics
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of lacustrine Newark Shale
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R. Finka,*, A. Amann-Hildenbranda, P. Bertierb, R. Littkea
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Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources
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Group (EMR), RWTH Aachen University, Lochnerstr. 4-20, D-52056 Aachen, Germany
Department of Clay & Interface Mineralogy, Energy and Mineral Resources Group (EMR), RWTH
Aachen University, Bunsenstr. 8, D-52072 Aachen, Germany *
[email protected]
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Abstract
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Shale gas production in the U.S. fuelled research activities in unconventional
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reservoir rock characterisation. Most studies focused on organic-rich shales of
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marine origin, while disregarding lacustrine sequences. In this study, thirteen
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lacustrine shale samples from the Newark Basin, NJ, USA are comprehensively
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characterised in terms of pore structure, gas storage and matrix transport
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characteristics. These thermally overmature (VRr 1.4 to 2.7 %) shales have a Na-
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rich, heterogeneous mineralogy with TOC contents of up to 3.6 %.
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Methane sorption capacity and pore structure parameters as identified with low-
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pressure N2 physisorption (microporosity, BET surface area) are neither
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interrelated with each other nor with any shale components (e.g. clay content,
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TOC).
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In contrast, porosity shows a positive correlation with TOC content, which is also
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typical
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Correspondingly, porosity and TOC positively correlate to bedding parallel
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matrix permeability coefficients (between 2 and 80 nD at 40 MPa effective
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stress). In contrast, permeability coefficients perpendicular to bedding are two
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to three orders of magnitude lower.
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Compared to previous studies on marine lithotypes, Newark shales have rather
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poor gas storage properties with average He-porosities of 2.3 % and average
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methane sorption capacities of 0.047 mmol g-1.
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Keywords: Shale Gas; Permeability; Porosity; N2 Physisorption; CH4 Sorption
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for
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overmature
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marine
shale
sequences.
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1 Introduction Knowledge of pore structure is critical for understanding both, fluid flow and
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storage mechanisms in shale (Chalmers & Bustin 2012, Clarkson et al. 2013, Slatt
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& O’Brian 2011). Porosity characteristics of shale are very complicated due to
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the large variability in pore sizes (micro-, meso- and macroporosity),
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microstructure, mineralogical composition, organic carbon (TOC) content and
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the geologic history of each system. As a result, advanced pore structure
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investigation techniques from other fields were applied to gain new insights into
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microporous (< 2 nm, see IUPAC classification (Sing et al. 1985)) and
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mesoporous (2 – 50 nm, IUPAC classification (Sing et al. 1985)) characteristics of
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shale. These methods can be divided into fluid invasion and radiation methods.
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Fluid invasion methods include high-pressure mercury intrusion (MIP) and low-
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pressure gas adsorption (N2 and CO2) (e.g. Clarkson et al. 2012, 2013, Kuila &
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Prasad 2013, Ross & Bustin 2009). Radiation methods include field emission
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scanning electron microscopy (FE-SEM) and transmission electron microscopy
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(TEM) after preparation of high-quality surfaces by argon ion beam milling (e.g.,
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Klaver et al. 2016, 2015, 2012, Loucks et al. 2009, Milliken et al. 2013 and
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references therein) and small and ultra-small angle neutron scattering
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(SANS/USANS) (Bahadur et al. 2015, Clarkson et al. 2013, King et al. 2015,
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Mastalerz et al. 2012, Sun et al. 2016). Application of these methods on marine
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gas shales revealed that porosity, pore size distributions, specific surface area
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and pore morphology may be related to TOC, mineralogy, diagenesis and
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microstructure. The controls are, however, poorly constrained and strongly
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differ between geologic formations.
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Natural gas in shale is stored in both free and adsorbed states. The adsorbed gas
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capacity is strongly controlled by microporosity due to its large internal surface
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area. This is why several studies found that sorption capacity is often positively
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correlated with TOC, which in turn often positively correlates to microporosity
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(e.g. Chalmers & Bustin 2007, Ross & Bustin 2009). However, also clay minerals
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may significantly contribute to adsorbed gas storage capacity (Gasparik et al.
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2012, Ji et al. 2012, Merkel et al. 2016, 2015, Ross & Bustin 2009). In contrast,
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only few studies report experimental gas permeability data and relate the
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ACCEPTED MANUSCRIPT transport properties to pore structure characteristics (Ghanizadeh et al. 2015,
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Ismael & Zoback 2016).
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Lacustrine shales have not been in the focus of shale gas research, because the
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currently developed shale gas systems in the U.S. are exclusively of marine
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origin. In contrast, as lacustrine shale is widely distributed in the sedimentary
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basins of China (Ordos, Bohai, Songliao and Sichuan), there is an increasing
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interest in its transport and storage properties for shale gas and shale oil
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development (e.g. Fu et al. 2015, Guo et al. 2014, Ji et al. 2014, Liu et al. 2015,
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Wang et al. 2015, Xiong et al. 2016).
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Lacustrine sequences may significantly differ from marine sequences and it is
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therefore important to asses their shale gas potential individually. Among the
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key differences is their greater sensitivity to high frequency climatic variability
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leading to rapid lake level fluctuations that result in rapid facies changes.
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Although this significantly reduces the core unconventional target area, the
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thickness of lacustrine shales may significantly exceed the thickness of marine
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shales (Katz & Lin 2014). If favourable reservoir conditions are present, the
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greater thickness and the lack of carrier systems in many lacustrine system may
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lead to a lower expulsion efficiency increasing the potential viability of these
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unconventional plays,
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2 Newark basin Geology and Stratigraphy
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The Newark basin is located in Eastern North America and developed in a huge
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rifting zone (~8000 km long), which resulted in the break-up of Pangea and
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formation of the central part of the Atlantic Ocean from the Late Permian to the
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Early Jurassic. Normal fault systems with significant hanging wall subsidence
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(1000s of meters) lead to the development of individual rift valley basins that
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were most of the time filled by great lakes, far from access to the sea (Van
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Houten 1964, Olsen 2010, 1997, Olsen et al. 1996, Schlische 2003).
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The sediment infilling of the Newark half graben system is of continental, largely
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lacustrine origin ranging from early Triassic (Carnian) to Early Jurassic
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(Hettangian) age (Olsen 1986) (Figure 1). The fluvial Stockton formation is
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followed by highly cyclical lacustrine strata with a thickness of > 3000 m. They
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overlain by the mostly red Passaic Formation. Both of them preserve the longest
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unbroken record of orbital forcing that was studied in detail using the Newark
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Basin Coring Project cores that were also used for this study (e.g. Olsen & Kent
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1996, 1990, Olsen et al. 1996) (Figure 1).
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Cyclical lacustrine strata formed due to lake level fluctuations in a tropical
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climate controlled by orbital parameters. The sedimentary rocks are subdivided
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based on lithology and sedimentary structures into “depth ranks” from 0 – 5
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related to the relative water depth of the lake during deposition. Rank 0 consists
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of red mudstone produced by playas or dry lakes whereas rank 5 is comprised of
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microlaminated black organic-rich calcareous mudstones formed in deep lakes
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with perennial chemical stratification. High depth rank (3 – 5) units have
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elevated organic carbon contents (> 1 %) and are usually gray or black (Olsen
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1986, Olsen & Kent 1996, Olsen et al. 1996).
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3 Samples
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For this study, we selected lacustrine mudstone samples from the Lockatong and
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Passaic formations. Overall thirteen locations were sampled, one of them from
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outcrop (Palmyra, Pennsylvania) and twelve from the Newark Basin Coring
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Project cores (NBCP) (Figure 1 and Table 1). The NBCP recovered over 6770 m
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(22200 ft) of continuous core of excellent quality by offset drilling between
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seven carefully selected sites. Details on the NBCP are published in Olsen et al.
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(1996) and Olsen & Kent (1996).
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Sampling was focused on black mudstones to cover a highly variable TOC
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sequence and on a maximal variation in composite depth of > 2500 m (8600 ft)
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to cover maturity changes. To analyse the effect of facies changes, we
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furthermore sampled different facies at location 8, 9 and 10 in a very narrow
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depth interval of around 3.5 m (11.4 ft) (Figure 1).
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Samples 1,3, 4, 5, 7 and 11 are finely-microlaminated darkgrey-black mudstones
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with depth rank of 4-5 deposited during lake high-stands (Table 1). In contrast,
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samples 6, 12 and 13 with depth rank 3 are thin-bedded dark-grey mudstone
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deposited in a relatively shallow perennial lake and samples 2, 8 and 10 of depth
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lakes with saline and drying out episodes. Sample 9 is the only red massive
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mudstone of depth rank 0 which is typical for dry playa mudflat deposition
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(Olsen et al. 1996, 1986, Olsen & Kent 1996).
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From each location cylindrical sample plugs oriented perpendicular to bedding
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and a slab (length: 10 – 30 cm, width: 2 – 3 cm) were cut from the whole core
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using water as lubricant. Additionally, on three locations (3, 11 and 13) plugs
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were drilled parallel to bedding.
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4 Methods
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4.1 Sample preparation
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Cylindrical plugs of 28.5 mm diameter and 15 to 47 mm length were drilled
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either perpendicular or parallel to bedding. Plugs were dried in a vacuum oven
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for at least 48 hr at 105 °C until weight constancy and used for transport and
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porosity experiments.
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For high-pressure methane sorption experiments, total sulphur and Rock-Eval
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Pyrolysis, a subsample (20 to 40 g) of the core slab material was ground to a size
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of < 0.1 mm and dried for at least 48 hr at 105 °C in a vacuum oven.
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X-ray diffraction (XRD) and low-pressure physisorption experiments were
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performed on sieved size fractions prepared after manual crushing of another
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core slab subsample (10 to 20 g) in a mortar.
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4.2 Petrographic analysis and VRr
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Polished blocks used for petrographic analysis and vitrinite reflectance
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measurements (VRr) were produced following the procedure and using
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instruments described in Sachse et al. (2012).
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Vitrinite reflectance measurements were performed under oil immersion (ne =
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1.518) on a Zeiss Axio Imager microscope equipped with a tungsten-halogen
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lamp, a 50/0.85 Epiplan-NEOFLUAR oil immersion objective and a 546 nm filter.
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A cubic-zirconia standard with 3.125 % reflectance was used for calibration.
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ACCEPTED MANUSCRIPT Details of the analytical procedure and instrumentation are described in Littke et
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al. (2012).
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4.3 Geochemical and Mineralogical analysis
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4.3.1 TOC, TIC, TS and Rock-Eval Pyrolysis
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Total organic carbon (TOC) and total inorganic carbon (TIC) were measured on
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powdered samples with a liquiTOC II analyser. The instrument operates in a
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non-isothermal mode and analyses the released CO2 during heating with a non-
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dispersive infra-red detector (NDIR). TOC and TIC can be analysed during one
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temperature ramp without previous acidification.
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Total sulphur (TS) content was measured with a LECO S200 analyser and Rock-
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Eval pyrolysis measurements were performed on a Rock-Eval VI instrument
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following the procedure initially described in Espitalié (1977).
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4.3.2 Quantitative X-ray diffraction analysis
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Bulk mineralogical compositions were derived from diffraction patterns of
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randomly oriented powder preparates. Core slab material was crushed manually
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in a mortar to avoid strain damage. To ensure uniform crystallite sizes, the 400 –
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800 µm size fraction with an internal standard (Corundum, 20 wt %) was milled
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in ethanol for 15 min with a McCrone Micronising mill. The standard was added
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to evaluate the accuracy of the analysis.
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The measurement was done on a BrukerAXS D8 Advance diffractometer using a
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CuKα-radiation produced at 40 kV and 40 mA. Diffractograms were recorded
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from 2° to 92° 2θ in 0.02° steps.
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Quantitative phase analysis was performed by Rietveld refinement usig the
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BSGM software with customized clay mineral structure models (Ufer et al. 2008).
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Mineralogical compositions were related to the crystallite content of the
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analysed samples.
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4.4 Porosity and pore size distribution measurements
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4.4.1 Low-pressure N2 physisorption analysis
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Measurements were performed using a Micrometrics Gemini VII 2390t apparatus
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on the 63 – 200 µm grain size fraction. Approximately 1 g of sample material was
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least 24h.
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For nitrogen physisorption analysis, adsorption and desorption measurements
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were performed in a nitrogen bath (77.3 K) at 90 relative pressure steps (p/p0)
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between
