Page 43 - Fundamentals of Gas Shale Reservoirs

PROCESSES BEHIND THE DEPOSITION OF ORGANIC MATTER‐RICH SHALE 23
depositional environments in which they formed. Indeed, water column and can be transported as bedload over a
shales can be deposited by a variety of processes in almost wide range of flow velocities (e.g., Richter, 1926; Schieber
any environment (e.g., Schieber, 2011; Stow et al., 2001; et al., 2007; Trusheim, 1929; van Straaten, 1951).
Trabucho‐Alexandre et al., 2012b). Mud can also accumulate in the presence of current
velocities that exceed the threshold of mud erosion if
−1
suspended sediment concentrations exceed 1 g l . Fluid mud,
2.2 PROCESSES BEHIND THE DEPOSITION which is a highly concentrated aqueous suspension of mud
OF ORGANIC MATTER‐RICH SHALE in which settling is hindered by particle proximity, forms
when the amount of mud entering the near‐bed layer is greater
Shales are the end product of the processes that control the than the dewatering rate of the high density suspension
production, erosion, transport, deposition, and diagenesis (McAnally et al., 2007). Fluid mud is a common feature of
of mud. The composition of shales is a product of the river, lake, estuarine, and shelf environments in which water
interaction of three key variables: sediment input, removal is laden with fine‐grained sediment. Along coastlines with
(or destruction), and mixing (or dilution). Diagenetic abundant mud supply, fluid mud dampens waves (Wells and
processes act on the sediment and result in changes to its Coleman, 1978) and allows mud deposition in relatively high
composition and/or texture. Although organic enrichment energy environments (Rine and Ginsburg, 1985). Mud drapes
of shales is always a function of the same basic variables, can therefore be formed over significant portions of the tidal
which combinations will yield organic matter‐rich sedi- cycle, rather than just at slack water; if fluid mud layers
ments depend on depositional environment. persist, mud can accumulate continuously over multiple tidal
cycles (MacKay and Dalrymple, 2011). Large volumes of
fluid mud can be transported downslope advectively by high
2.2.1 Processes Behind the Transport and energy events across low‐gradient shallow marine environ-
Deposition of Mud
ments as wave‐enhanced sediment gravity flows (e.g.,
Mud may be transported to its final resting place by Macquaker et al., 2010b).
gravitational settling, by advective processes, that is, mud The fabric of freshly deposited mud that resulted from the
transport resulting from net horizontal water movement, and gravitational settling of individual mud particles has a stable
by sediment gravity flows, that is, mud transport by density subparallel structure with comparatively little water; whereas,
currents for which excess density is produced by the presence the fabric of aggregated mud deposited in the same way is
of suspended solids. open with a water content in excess of 90% by volume
The deposition of particles smaller than about 10 µm is (Hedberg, 1936; Migniot, 1968; Terwindt and Breusers,
controlled by gravitational settling, that is, settling from 1972). The density and shear strength of aggregated mud
suspension under the force of gravity toward the deposi- deposits are therefore lower. However, if aggregated mud is
tional interface. For particles larger than about 10 µm, transported as bedload to its final resting place, the deposits
depositional processes are dominated by shear stress at the are denser, less porous, and contain less water than the
depositional interface, and silt has a bedform succession deposits produced by gravitational settling of aggregated
similar to that of sand finer than ca. 80 µm (Mantz, 1978; mud (J. Schieber, personal communication). The fabric of
Southard, 1971). settling mud particles and of freshly deposited mud is difficult
In freshwater, mud is mostly present as individual to observe directly, and burial of mud tends to obscure initial
particles, because the excess negative charge present on the sedimentary fabrics (Allen, 1985, p. 144, fig. 8.5) unless
surface of fine mud particles keeps them from flocculating. there is early cementation of the sediment. Flocs are crushed
In paralic and marine environments, aggregates are formed and rearranged by the accumulating overburden (Migniot,
due to changes in the chemical environment, namely an 1968), while water loss and compactional processes normally
increase in salinity, and due to the activity of organisms, destroy the pelletal character of fecal pellet mud (Pryor,
and mud tends to be present as flocs, fecal pellets, pseu- 1975). For this reason, fine‐grained sediments sampled from
dofeces, and other organominerallic aggregates (e.g., recent or fossil deposits and analyzed in the laboratory may
marine snow). Although salt flocculation is an important show a textural composition quite different from the original
mechanism, particularly in environments where water or in situ material (e.g., de Boer, 1998).
masses of different salinities mix, biogenic aggregation is
probably the most important process controlling the 2.2.2 Production, Destruction, and Dilution:
behavior of mud in paralic and shallow marine environ- The Many Roads to Black Shale
ments (Eisma, 1986; Pryor, 1975). Despite their lower
density, the behavior of aggregates is comparable to silt‐ Production is the synthesis of organic compounds from
and sand‐sized particles. Consequently, mud in paralic and nutrients, carbon dioxide, and water by terrestrial and aquatic
marine environments settles relatively quickly through the organisms through photo‐ and chemosynthesis, that is, using