Page 49 - Fundamentals of Gas Shale Reservoirs

GEOGRAPHIC DISTRIBUTION OF ORGANIC MATTER‐RICH SHALES 29
(1924), for example, suggested that the rate of supply of a global scale and the deposition of black shales
organic matter was important in oxygenated settings. Trask (cf. Chamberlin, 1906), most early authors thought of
(1932) analyzed a very large amount of samples and black shales as the product of local processes. Indeed, it
concluded that upwelling zones were favorable settings was not until Cretaceous black shales were recovered in a
for the deposition of organic matter‐rich sediments. number of Deep Sea Drilling Project (DSDP) sites in the
Brongersma‐Sanders (1971) also discussed in detail the 1970s that it became widely recognized that basin physiog-
effects of upwelling on sediment composition. Importantly, raphy was not a sufficient explanation for some ancient
Brongersma‐Sanders noted that upwelling is a countercurrent black shale successions. The discovery of widespread
system that creates a nutrient trap. This trap leads to high organic matter‐rich horizons in the deep sea represented a
fertility of a basin or coast and, where the subsurface water breakup with the notion that ancient black shales were the
ascends toward the photic zone, to high productivity. Parrish product of local conditions in marginal, restricted basins.
(1987) predicted the geographic distribution of ancient Bernoulli (1972) recognized the similarities between
upwelling zones and compared that distribution with the Tethyan Cretaceous sediments exposed on land in the
distribution of organic matter‐rich rocks. She concluded that Mediterranean region and sediments recovered by drilling
as many as half the world’s black shales may have been in the North Atlantic, and suggested that black shale
deposited in upwelling zones. horizons now exposed on land and horizons recovered by
In addition to the debate concerning the physiography of drilling are coeval. The discovery of Cretaceous black
ancient environments of black mud accumulation, which is shales of the same age in the Pacific greatly extended the
probably one of the longest running controversies in geology, geographic range of those horizons, and led to the suggestion
another intense debate arose, this time concerned whether that the deposition of black shales during the Early
unusually high primary productivity in the photic zone or Cretaceous might have been a worldwide oceanographic
unusual chemical conditions in the water column, namely, phenomenon (Jackson and Schlanger, 1976, p. 925). As
anoxia, provide the first‐order control on the accumulation a result, Schlanger and Jenkyns (1976) proposed that
of organic matter‐rich sediments in the ocean (Demaison, the occurrence of black shale horizons globally was due
1991; Pedersen and Calvert, 1990). As a result of this debate, to the expansion of the oxygen minimum layer in the ocean
models of black shale deposition are traditionally divided as a consequence of the Late Cretaceous transgression and
into two end‐member types: one of enhanced supply and the a reduced supply of oxygen to deep water due to an equable
other of enhanced preservation of organic matter. More climate (Fig. 2.4), the so‐called oceanic anoxic events
recently, however, some authors recognized the interdepen- (OAEs). The term OAE is unfortunate because it implies
dent roles of primary productivity, microbial metabolism, ocean‐wide anoxia, although it has been noted that this is
and sedimentation rates (e.g., Bohacs et al., 2000, 2005; not the spirit of the term (Arthur et al., 1990). Indeed, the
Sageman et al., 2003; Tyson, 2005). original concept (Fig. 2.4) included several environments
Although Van Waterschoot van der Gracht (1931) of black shale deposition. At first, the idea that the deep
proposed a link between changes in ocean circulation on sea could become anoxic was rejected by geochemists

O profile O 2 profile
2
0 5 ml/l 0 5 ml/l
Continent
Shelf
Rise

Barred basin

Upper mixed layer with high C xation Stagnant bottom water
Oxygen minimum layer C-rich sediments of various types
Terrestrial plant material
FIGURE 2.4 Ocean stratification during an oceanic anoxic event as proposed by Schlanger and Jenkyns in 1976 (their figure 2). The
oxygen minimum layer is expanded and intensified. The shoaling of the upper boundary of the oxygen minimum layer translates into a
geographic expansion of shallow seafloor impinged on by the oxygen minimum. The sinking of its lower boundary results in the seafloor at
the top and flanks of oceanic rises being impinged by the oxygen minimum layer. The concept included a barred basin setting in which
abundant terrestrial plant debris accumulated “in their early opening stages by rivers and turbidity currents.”