Page 42 - Fundamentals of Gas Shale Reservoirs

22 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

Increasing organic carbon
50%

20%
Oil shale
0.5%
Black shale
Terrigenous
mud
Increasing terrigenous content 75 Arls Marl Smarl Sarl
Mudstones
Mud

25
Oozes
Calc. siliceous calcareous
50
Biogenous
Biogenous Calcareous Siliceous Silic.
carbonate mud silica mud
FIGURE 2.1 Classification of shales based on composition. This scheme is based on the classification scheme proposed by Hay et al.
(1984, p.14). Shales are fine‐grained sedimentary rocks with varying relative proportions of terrigenous mud and mud‐sized biogenous com-
ponents. Shales may also contain up to 25% of terrigenous or biogenous grains coarser than mud (>62.5 µm). A shale with an organic matter
content higher than the average marine rock, that is, ca. 0.5%, is referred to as a black shale. Black shale is thus the general term for any
dark‐colored, fine‐grained, organic matter‐rich sedimentary rock.

Shales, particularly those deposited in marine environ- and places of quiet water deposition” (cf. Potter et al., 2005,
4
ments, are usually combinations of mud from different p. 75). Shales have been studied mostly for the unusual geo-
sources (Fig. 2.1). chemical signals they carry, and the composition of shales is
Although “black shale” is a useful term when referring often only known in terms of organic matter and elemental
to organic matter‐rich sedimentary rocks in general, it is a content. However, we should not attempt to explain the origin
collective noun that groups rocks of various types and of black shales by focusing solely on explanations for their
origins (Trabucho‐Alexandre et al., 2012b). In the petrologic high organic matter content, which typically constitutes but a
hierarchy of sedimentary rocks, the term “black shale” is few percent of the total rock volume.
equivalent to such terms as “sandstone” or “limestone,” rather It is now widely recognized that black shales show
than to more precise terms such as “lithic arenite” or “oöidal significant compositional and textural variability on a variety
grainstone.” However, shales are seldom subdivided based of scales (Aplin and Macquaker, 2011; Ghadeer and
on their composition and texture (Fig. 2.1). For this reason, Macquaker, 2012; Lobza and Schieber, 1999; Macquaker
we are often not able to distinguish subtle compositional and et al., 2007, 2010b; O’Brien, 1996; Plint et al., 2012;
textural differences that would otherwise allow us to subdi- Schieber, 1994, 1999; Schieber and Riciputi, 2004; Schieber
vide what were once considered monotonous successions et al., 2010; Trabucho‐Alexandre et al., 2011, 2012a), which
of shale and marl (cf. Lewan, 1978). reflects the diverse and dynamic nature of the processes and
Shales do not lend themselves well to study in the field or in environments behind their formation. The composition,
hand specimen. Sorby (1908, p. 196), who started the micro- textures (viz. grain size and fabric), structures, and fossil
scopic study of rocks, realized that the “examination of […] content of shales depend on the physical, chemical, and
rocks in a natural condition is enough to indicate that the biological processes responsible for their deposition and on
structure of clays differs enormously, and indicates formation their depositional environments. Patterns of vertical and
under very different conditions; but there is always some doubt lateral variability, which can be observed on a variety of
as to their true structure, when not made into thin sections.” scales in shale successions, preserve a record of the evolving
The study of shales has also been hampered by the ingrained
idea that shales always reflect deposition in quiet water.
“Aside from this it is hard to say anything definitive about the 4 “SEPM Strata, Terminology List,” accessed November 2, 2013. http://
environment of a shale since most environments have periods www.sepmstrata.org/TerminologyList.aspx?search=shale