Research paperRelationships between porosity, organic matter, and mineral matter in mature organic-rich marine mudstones of the Belle Fourche and Second White Specks formations in Alberta, Canada
Introduction
Recent success in extraction of hydrocarbons from tight unconventional-type reservoirs has triggered a dramatic increase in research on self-sourced systems, like hybrid source rock/reservoir mudstone plays. An unconventional oil/gas mudstone play (generally called a shale play) commonly consists of a sedimentary succession across a large geographic area with the following characteristics: dominantly clay- to silt-sized particles, high contents of silica and/or carbonates, high organic matter (OM) content, thermally mature, hydrocarbon-filled porosity, and low to very low permeability. Such plays require multistage fracture stimulation in horizontal wells to achieve economic production (Caputo, 2011). An evaluation of the unconventional hydrocarbon resource potential of source rock/reservoir formations must take advantage of multidisciplinary approaches involving porosity, OM, and mineral matter. This study uses the specific example of mature organic-rich marine mudstones within the Second White Specks petroleum system in Alberta to highlight strengths and shortcomings of various parameters, e.g., the vertical variation in porosity and pore types and facies-related controls on pore characteristics. A combination of analytical methodologies encompassing organic petrology, geochemistry, mineralogy, and porosimetry expands our understanding of organic-rich petroleum systems.
The Second White Specks Formation located within the Western Canada Sedimentary Basin (Fig. 1) is one of the emerging hybrid source rock/reservoir plays in North America that has attracted the attention of recent explorations (Clarkson and Pedersen, 2011, Kwan and Mooney, 2010). The Second White Specks and Belle Fourche formations were deposited within the Cretaceous Western Interior Seaway (a foreland basin) of North America (Bloch et al., 1999). Changes in the depositional paleoenvironment within the Seaway were responsible for lateral and vertical changes in the geochemistry and mineralogy within and between the formations. In turn, burial and compaction of sediment as well as diagenetic processes related to hydrocarbon generation influenced the poor preservation of porosity and permeability, making this play challenging in terms of exploration and production (Greff and Cheadle, 2012).
Extensive research has been done on the Second White Specks Fm.'s stratigraphy, lithology, and OM characteristics (i.e. organic petrography and Rock-Eval pyrolysis) from British Columbia, Alberta, Saskatchewan, and Manitoba (e.g., Bloch et al., 1993, Bloch et al., 2002, Kovac and Last, 1991, Macauley, 1984a, Macauley, 1984b, Schröder-Adams et al., 1996, Stasiuk and Goodarzi, 1988, Tyagi et al., 2007). Significantly less is known about the porosity systems. Bloch (1995) recognized that shales from the lower Colorado Group (including the Second White Specks, the Belle Fourche, and the Fish Scale formations; Fig. 2) were of economic interest as the shale reservoirs. However, at that time conventional wisdom considered the Second White Specks Fm. to be mostly a source rock that had expelled large volumes of hydrocarbons to adjacent sandstone reservoirs, e.g., the overlying Cardium Fm. (Mossop and Shetsen, 1994). Although several prolific wells have been completed in the Second White Specks Fm., many unsuccessful attempts to establish multi-well pools discouraged further drilling. Historically this formation has been mainly characterized as a natural fracture-controlled reservoir with high lateral heterogeneity in its primary intrinsic porosity distribution (Greff and Cheadle, 2012). Only recently this formation is being re-evaluated as a promising tight oil reservoir (characterized by very low matrix permeability) and also as one of the emerging tight oil/shale plays in North America. Assessment of unconventional formations, and hybrid source rock/reservoir plays in particular, requires parallel applications of several methodologies. Tight oil reservoirs are often highly heterogeneous strata within the oil window maturity and therefore require careful analyses of both OM and mineral matter, including the effects of kerogen transformation on porosity generation and preservation. Therefore, a set of organic geochemical techniques combined with mineralogical and porosimetry analyses had been applied to understand the pore types and their affiliations within these complex systems of the Second White Specks and Belle Fourche formations.
Section snippets
Sample material
Nineteen mudstone samples were collected at the ERCB Core Research Centre in Calgary, Canada, from well 07-19-45-6W5 (Fig. 2) that had been drilled in 1979 by the ‘Canadian Hunter’ company. Although long-term storage could possibly influence rock chemical properties, the core has value as a representative core for the two studied formations. The 77.5 m long core (depth 1793.5 m–1871.0 m) covers the middle and upper part of the late Cenomanian Belle Fourche Fm. and the lower part of the
Organic petrography and vitrinite reflectance
The liptinite maceral group, including alginite (from phytoplankton), liptodetrinite (small fragments of likely algal origin), and amorphous organic matter (AOM), accounts for 70–95% by volume of OM on a mineral-free basis. Vitrinite (dominantly reworked) and inertinite are only a minor addition to the total maceral composition. Secondary OM is represented by solid bitumen that characteristically fills small (1–2 μm) pores in isolated (elongated, lensoidal or round) fine-grained zones rich in
Characterization of the hybrid source rock/reservoir formations
The present day TOCpd concentrations between 1.74 and 3.62 wt. % indicate good to excellent organic richness (Peters, 1986). Organic matter in all mudstone samples is classified as marine kerogen Type II based on the prevalence of liptinites and limited input of terrestrial material. Although we cannot directly determine the origin of the AOM, the presence of marine alginite and marine planktonic microorganisms (foraminifera; Caldwell et al., 1978, Strong, 1972) strongly suggests that the AOM
Conclusions
Representative mudstones across a depth interval from 1793.5 m to 1871.0 m (77.5 m thick) from the Upper Cretaceous Second White Specks and Belle Fourche formations in central Alberta were analyzed to evaluate their potential as emerging hybrid source rock/reservoir formations. Organic petrography, bulk geochemistry, biomarkers, mineralogy, and porosimetry provide insights into the depositional paleoenvironment, mineralogical heterogeneity, maturity, and remaining hydrocarbon potential. The two
Acknowledgments
Funding for this research was provided by the Tight Oil Consortium that had been initiated by Chris Clarkson. This project was also partially supported by the U.S. Department of Energy, Basic Energy Sciences, Grant No. DE-FG02-11ER16246. Additional financial support was received from an ExxonMobil Global Geoscience Recruiting Grant 2011 for A. Furmann's research. We thank N. Zajac for assistance with the stratigraphy of the Second White Specks and Belle Fourche formations.
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