Page 41 - Fundamentals of Gas Shale Reservoirs

2

ORGANIC MATTER‐RICH SHALE DEPOSITIONAL
ENVIRONMENTS

João Trabucho‐Alexandre 1,2
1 Department of Earth Sciences, University of Durham, Durham, UK
2 Institute of Earth Sciences Utrecht, Utrecht University, Utrecht, The Netherlands

2.1 INTRODUCTION protozoans (agglutinated forams) build their tests by
cementing terrigenous and other particles with an organic
Shale is the most abundant rock type available at the ligand. Some quartz silt aggregates in shales represent the
surface of our planet and makes up about two‐thirds of collapsed or compacted tests of these organisms (Pike
the stratigraphic record (Garrels and Mackenzie, 1969). and Kemp, 1996; Schieber, 2009). When a shale is char-
1
The term “shale” refers to all sedimentary rocks com- acterized by an organic matter content higher than the
2
posed predominantly of mud (>4ϕ or <0.0625 mm) par- average marine shale (ca. 0.5%, Arthur, 1979), it is
ticles (cf. Tourtelot, 1960, p. 342). Mud particles may be referred to as an organic matter‐rich shale or, more com-
3
terrigenous, biogenous, or hydrogenous. Terrigenous—or monly, a black shale. Hydrogenous mud precipitates
siliciclastic—mud is always detrital, that is, produced by out of solution directly, either from seawater or from
weathering and erosion of preexisting rocks, and comes interstitial water during diagenesis, and it includes oxides
into the depositional environment as individual particles and hydroxides, silicates, for example, zeolites and
and/or as aggregates. Biogenous mud is made by organ- clay minerals, heavy metal sulfides, sulfates, carbonates,
isms, and includes a skeletal and an organic component. and phosphates. Clay minerals are therefore not only
Skeletal mud may be calcareous or siliceous. Some benthic terrigenous, but they may also be hydrogenous, that is,
formed in situ. Biogenous and hydrogenous mud may be
detrital, that is, recycled from older deposits, in which
1 Although the original meaning of the word “shale” is “laminated clayey case their origin is biochemical, but their texture will be
rock,” our historical usage of the word has been that of “general class of clastic. The composition of the detrital fraction of a shale
fine‐grained sedimentary rocks” (Tourtelot, 1960). There is no reason why
we should restrict our usage of the word shale to laminated and/or fissile depends on the petrology of its source areas and on
fine‐grained sedimentary rocks. “Lamination” has a descriptive and a ge- the intensity and effectiveness of chemical weathering.
netic definition with distinct sedimentologic implications (cf. McKee and
Weir, 1953; Campbell, 1967), and fissility is a secondary property largely
related to weathering (e.g., Ingram, 1953). If we define shale as a fissile 3 Modern organic matter‐lean muds may also be black when their iron sul-
fine‐grained sedimentary rock, then “there are no shales in the subsurface, fide content is high (e.g., Potter et al., 2005); however, they become light‐
only potential shales” (Weaver, 1989, p. 6). Some rocks which are referred colored on lithification as the sulfide changes into either marcasite or pyrite
to in the literature as shales are actually metasedimentary rocks produced by (Twenhofel, 1939). Black shale is the general term for any dark‐colored,
regional low‐grade metamorphism, and should therefore be called slates. fine‐grained, organic matter‐rich sedimentary rock. In the words of Stow
2 Mud is the name given to particles or collections of particles smaller than et al. (1996, p. 403): “[m]any black shales are hemipelagites; others, such as
sand, that is, smaller than 62.5 µm, that is, silt and clay, which typically black cherts and organic matter‐rich limestones, are pelagites; whereas still
occur together. Some authors talk about “mud and silt,” perhaps making others are fine‐grained turbidites.” This de facto usage has also been noted
“mud” a synonym of “clay.” Operationally, the mud/sand boundary may be by Arthur (1979) who states that “the term ‘black shale’ is used in a general
defined based on sieve sizes around this value. Because mud is a term sense to refer to relatively organic carbon‐rich […] mudstone and marlstone
related to grain size, it has no connotations as to composition. which may or may not be ‘shale’ in the classical sense.”

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.