Page 56 - Fundamentals of Gas Shale Reservoirs

36 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

and in the stratification of the water column. The waters of Mud that escapes these sediment traps is transported
the lake are therefore increasingly dysoxic below 100 m, and offshore and deposited on the subaqueous prodelta, which
the sediments have up to 12% organic matter and hydrogen is the main site of mud accumulation in deltaic systems.
index values up to 600 (Potter et al., 2005). When lower density river water flows out over saltwater
(hypopycnal flow), suspended mud flocculates and is
deposited on the subaqueous prodelta. Flocculation and
2.5.2 Paralic Depositional Environments
biogenic pelletization result in mud aggregates that settle
Paralic environments are transitional environments on the rapidly and proximally. During periods of high discharge,
landward side of a coastline that are shaped by a complex sediment‐laden water may be denser than saltwater and
interaction of processes; they form an intricate mosaic of the river becomes hyperpycnal (Bhattacharya, 2010).
closely associated facies between land and ocean. The Prodelta hyperpycnites, the deposits of these bottom‐
morphology of a coast, which controls what sedimentary hugging flows, may represent more than half of shelf mud
environments are present, is a function of wave energy, tidal (Bhattacharya, 2009).
power/range, and relative sea level change (Boyd et al., Mud deposited in the prodelta is prone to resuspension
1992; Dalrymple et al., 1992). The quantity and type of during storms, but also by fair‐weather waves and tides. This
sediment supplied from land, alongshore, and offshore are resuspended mud may migrate along the shelf forming
also important. dilute, hyperpycnal geostrophic fluid mud belts
Much, if not most, terrigenous mud is trapped in paralic (Bhattacharya, 2010) and wave‐enhanced sediment gravity
environments; deltas, estuaries, lagoons, tidal flats, and so flows (Macquaker et al., 2010b). In saltwater basins, rapid
on, form the depocenters of many ancient shale successions. sedimentation of flocculated mud and methane formation in
Grabau (1913, pp. 483–484) thought that ancient marine organic matter‐rich sediments can result in high pore
black shales (his sapropelargillytes) with a wide geo- pressure and reduced mud density, which favor slope failures
graphic distribution, namely the early Toarcian black and resedimentation processes. Such failures occur at slopes
shales of northwest Europe, had been deposited in coastal as shallow as 0.2° (Potter et al., 2005).
lagoons and/or on extensive mudflats on marginal Mud is deposited behind barriers in wave‐dominated
epicontinental seas exposed to some extent at low tide (cf. estuaries and lagoons, and on the bottom of fjords and other
French et al., 2014). The abundance of mud in a paralic shallow‐silled basins (Fig. 2.3e and f). Mud also accumulates
environment depends on the relative magnitude of fine‐ in coastal swamps and mudflats along the sides of estuaries.
grained fluvial sediment input versus sediment reworking Estuaries act as mud traps because filter‐feeders remove
by nearshore currents. The distribution of organic matter‐ suspended organic and inorganic particles greater than 3 µm
rich sediments in paralic environments is typically patchy, from the water column, and convert them into dense, mucus‐
and the organic matter content is variable and often domi- bound fecal pellets and pseudofeces (Eisma, 1986; Newell,
nated by terrestrial components. The preservation of 1988; Pryor, 1975).
organic matter depends on local water column oxygen Climate is an important control on sedimentation in
deficiency and on relatively rapid burial. Excessive shallow, barred basins (Fig. 2.3e and f). In humid and
dilution by siliciclastic materials works against organic temperate climates, mud will be dark colored and rich in
enrichment in paralic environments. organic matter, including plant debris washed in by rivers. In
Mud‐rich deltas form where the work carried out by arid climates, mud will be lighter in color, organic matter
waves, tides, and associated nearshore current systems is lean, and alternate with evaporite beds.
insufficient to winnow mud. Although tide‐dominated Where nearshore wave power and associated nearshore
deltas can also trap some of the mud brought by the rivers current systems are high, mud is transported away from
associated with them, mud‐rich deltas are typically river‐ the river mouth along the shelf. Downdrift many modern
dominated. Mud in a delta is deposited in its subaerial large rivers, where the supply of mud is sufficient to
environments, namely, lakes, swamps, and abandoned dampen wave power and tidal currents, mud is deposited
distributaries, and as interdistributary bay fills. Mud in tide‐ on open coastlines. The Chenier Plain of southwestern
dominated deltas and in estuaries is trapped in broad Louisiana is a sediment wedge formed by the westward
mudflats that grade into marshes or evaporative flats moving mud stream of the Mississippi and Atchafalaya
depending on regional climate. In the upper delta plain, deltas (Gould and McFarlan, 1959). Variations in mud
interdistributary areas may contain coals or carbonaceous supply related to distributary channel activity (avulsions)
shales deposited in swamp and marsh environments. For cause shifts in the shoreline: the shoreline migrates sea-
example, in the Mahakam Delta, Indonesia, plant debris ward when supply is abundant and landward when supply
from palms and mangroves, supported by the tropical is reduced. The longest nearshore Holocene mud belt
climate, accumulates alongside mud in tidally influenced is located along the open, high‐energy northeastern coast
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interdistributary areas (Reading and Collinson, 1996). of South America. The Amazon brings more than 10 tons