Page 57 - Fundamentals of Gas Shale Reservoirs

ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS 37
of sediment to the Atlantic per year (Milliman and Meade, breaks down, and shallow water deposits remote from river
1983). Most of this sediment is mud and a large fraction input of terrigenous material and freshwater may have sedi-
moves along the South American coast (Meade, 1994), part mentological, geochemical, and paleobiological signatures
in suspension and part as large migrating mud banks (Rine more characteristic of “deep” water (Hallam, 1967).
and Ginsburg, 1985). Despite several modern examples The organic matter content of sediments on the shelf
(e.g., Anthony, 2008; Frey et al., 1989; Nair, 1976; Rine seafloor is typically higher than in the deep sea (about three
and Ginsburg, 1985), ancient equivalents of muddy open times higher, at present). Indeed, organic matter‐rich deep‐
coastlines are not well documented in the literature. Walker sea sediments are usually redeposited organic matter‐rich
(1971) studied Devonian marine mudstones in Pennsylvania shelf sediments (e.g., Dean et al., 1984; Degens et al.,
that pass upward into mudstones with rootlets and mud 1986). The water column of the shelf receives nutrients
cracks without a sand body at the paleoshore. from runoff and coastal upwelling and thus is thus very
fertile. Because water depths are relatively shallow, the
export path for organic matter is much shorter and organic
2.5.3 Shallow Marine Depositional Environments
particles are less likely to be extensively oxidized en route
Mud‐dominated facies are the most abundant of all ancient to the seafloor. Sedimentation rates on the shelf are high in
shallow marine deposits (Johnson and Baldwin, 1996). On comparison to those of the deep ocean, which aids in the
the shelf, siliciclastic sediment is supplied from adjacent preservation of organic matter in relatively well‐ oxygenated
land and, away from the mouths of large rivers, by environments, but retrogradation of clastic systems and
reworking of seafloor sediment. Skeletal debris is an addi- trapping of terrigenous sediment in nearshore environments
tional sediment source on shallow marine environments, during transgressions and early relative sea level highstands
but it only becomes dominant when siliciclastic sediment minimize excessive dilution of organic matter settling to
supply is low and biogenous sediment supply high. In the the seafloor of the shelf.
geologic record, shallow marine mudstones covered large Shelf sediments below areas with strong upwelling are
epicontinental areas in response to sea‐level rise. Examples typically rich in organic matter and opal. For example,
of this type of setting include Paleozoic sequences of sediments on the SW African shelf, particularly off Walvis
Africa, Europe, and North America, the upper Jurassic Bay, Namibia, have up to 20% organic carbon and up to
Kimmeridge Clay, and the Mesozoic of the Western 70% opal from the frustules of diatoms (Seibold and
Interior Seaway of North America (Johnson and Baldwin, Berger, 1996). Fish debris and other vertebrate phosphate
1996, and references therein). remains are also abundant. Upwelling is due to a
Shallow marine, neritic, or shelf environments are those combination of the cold, coastal Benguela Current with
in which the seafloor is within the photic zone and persistent offshore winds blowing in a northwest direction
periodically reworked by storms, that is, shallower than ca. (Fig. 2.3c). Oxygen‐poor, nutrient‐rich water ascends from
200 m. Most Neogene continental shelf sediments are relict a depth of about 200 m and mixes in the photic zone with
in composition but modern in texture (Emery, 1968b; oxygenated water causing high productivity along a narrow
Milliman et al., 1972); the sediments were brought to the coastal strip (Demaison and Moore, 1980). A zone of
shelf during lowered sea levels associated with glacial oxygen depletion, parallel and close to the coastline, is
episodes but have been reworked by present‐day currents. created on the shelf by the high oxygen demand due to the
The seafloor of the pericontinental shelf dips seaward from decomposition of large quantities of planktonic organic
the shoreface to the shelf break at low angles of 0.1–1°, and matter resulting from the Benguela Current upwelling.
the width of the shelf varies from tens to hundreds of kilo- However, not all areas of upwelling and high productivity
meters. Because of their gentle slope, shelves are greatly in the ocean are associated with the intensification of oxygen
influenced by changes in sea level. minima and with the deposition and preservation of organic
A large proportion of preserved ancient marine sediments, matter on the seafloor. This is the case in areas where oxygen
including many black shales, was deposited in epicontinental supply exceeds the biochemical oxygen demand, for
marine environments. Epicontinental black shales are the example, offshore Antarctica, offshore southeastern Brazil,
typical subjects of shallow versus deepwater origin debates. and offshore Japan and the Kuril Islands (Summerhayes
At present, most shallow marine environments consist of et al., 1976).
relatively narrow pericontinental shelves, and there is a strong Shelf bathymetry can favor the preservation of organic
relation between water depth and distance to land. Shallower matter‐rich sediments. Bottom waters can become isolated
deposits thus show stronger terrigenous signatures than deeper from the well‐mixed surface layer in bathymetric lows and
deposits. During times of higher sea level in the geologic past, may become oxygen‐deficient through aerobic oxidation of
however, wide epicontinental shelves were common and organic matter. Moreover, topographic lows are traps for low‐
covered vast areas of the continental crust. In epicontinental density organic matter. During winter, however, effective
shelves, the relation between water depth and distance to land wave base is deeper and the water column is better mixed than