Page 50 - Fundamentals of Gas Shale Reservoirs

30 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

(Broecker, 1969). However, the presence of “widespread” In continental environments, the composition of shales is
organic matter‐rich horizons in the deep sea is sometimes largely dominated by terrigenous material. In terms of
invoked as evidence for episodes of ocean‐wide stagnation composition and texture, shales deposited in continental
and/or anoxia, and much modeling effort has been put environments are typically shales sensu stricto, that is,
into creating increasingly complex models that attempt to laminated and/or fissile organic matter‐rich siliciclastic mud-
recreate global conditions that explain all occurrences of stones, and their organic matter is terrigenous. In the marine
organic matter‐rich sediments in ancient oceans. While environment, however, sediments only rarely come from a
certain stratigraphic intervals are characterized by frequent single source (Fig. 2.1). Most marine sediments are a mixture
and/or widespread black shale horizons, the correlation of of biogenous and terrigenous particles of various grain sizes,
individual layers is almost always questionable and the with an additional hydrogenous (authigenic) and/or (very
petrologic characteristics of the black shales varied. This minor) cosmogenous component. Biogenous debris, which
suggests that multiple processes are behind the deposition may be either calcareous or siliceous, may form a significant
of not exactly coeval organic matter‐rich sediments during proportion, if not the majority, of the inorganic fraction of a
OAEs (Hay, 1988; Trabucho‐Alexandre et al., 2011); black shale. The composition of marine shales follows a general
shales are the product of both local and global conditions pattern that is related to basin physiography, namely, water
(cf. Trabucho‐Alexandre, 2011). The danger in creating depth, and ocean circulation (Fig. 2.5).
“fully detailed models of complex systems is ending up The relationship between depositional setting and the tex-
with two things you don’t understand—the system you ture of shales is more complicated. For example, although
started with, and your model of it” (Paola and Leeder, 2011). we often think that sediment grain size is a function of
distance to shore, this is typically not the case, and there are
many examples of fine‐grained shores and relatively coarse‐
2.4.2 Controls on the Geographic Distribution grained deepwater deposits (e.g., Rine and Ginsburg, 1985;
of Black Shales
Stow, 1985b). In neritic environments below effective wave
The petrologic characteristics and widespread geographic dis- base, which are most affected by variations in terrigenous
tribution of black shales suggest that processes rather than input and in relative sea level, the variation in seafloor
environments control their accumulation. For this reason, a texture is the most unpredictable, because a significant
6
discussion concerning their geographic distribution should fraction of the seafloor is covered in either relict or palimpsest
focus on the processes that result in the deposition of organic sediment (Emery, 1968a; Shepard, 1932). The texture of
matter‐rich mud in each environment. In particular, we are shales is largely independent of environment and therefore
interested in linking the petrologic characteristics of shales, more difficult to predict in terms of its geographic distribution
that is, their varying composition and texture, with depositional than composition.
process and environment. This is important because regional The sediments of continental margins are different both in
changes in shale composition and texture due to paleoöceano- quantity and quality from those on deeper seafloor. Almost
graphic and paleogeographic controls affect, among other 90% of the total volume of all marine sediment is associated
things, the economic potential of a rock. Changes in organic with continental margins, that is, shelves, slopes, and rises,
matter content and in its nature affect the hydrocarbon genera- which constitute only about 20% of the ocean’s area. Neritic
tion potential of a shale, while changes in the relative amounts sediments, that is, those deposited on the continental shelf,
of siliciclastic mud and skeletal biogenous material, calcareous consist primarily of terrigenous material. Terrigenous
and siliceous, affect the mechanical properties of a rock and material is always ultimately derived from the continent, and
consequently its behavior when subjected to, for example, it is brought to the ocean by rivers, coastal erosion, and, to a
hydraulic stress. The nature of the phases that compose the lesser extent, wind. The immediate source of the terrigenous
rock is also important. Carbonate, for example, may be present component of a marine sediment, however, is often within the
as porous biogenous particles or as porosity‐filling diagenetic marine environment (Meade, 1972). Most terrigenous
crystals (e.g., dolomite). Likewise, silica may be present as sediment brought into the ocean by rivers is deposited where
detrital or authigenic quartz or as porous biogenous silica. The rivers meet the coastal ocean (e.g., Walsh and Nittrouer,
nature of the phases present in a rock depends on the processes 2009). Sediment that escapes paralic sediment traps and
and environments behind its formation. sediment from coastal erosion tend to travel along the shore
within a few kilometers of the coast rather than moving sea-
ward (e.g., Manheim et al., 1970; McCave, 1972). Sediment
6 An environment has been defined as “the complex of physical, chemical, resuspended from the shelf bottom and sediment transported
and biological conditions under which a sediment accumulates,” (Krumbein laterally from offshore constitute the main source of
and Sloss, 1963, p. 234) and as “a spatial unit in which external physical, suspended matter on the shelf away from the mouths of large
chemical, and biological conditions and influences affecting the development
of a sediment are sufficiently constant to form a characteristic deposit” rivers. Many ancient black shales were deposited on broad
(Shepard and Moore, 1955, p. 1488). continental shelves at times when sea levels were much