Page 52 - Fundamentals of Gas Shale Reservoirs

32 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

of silica‐secreting plankton in the upwelling biota. The oceanic environments and submarine highs, whereas the “red
production of siliceous skeletal material is maximal in coastal clay” facies is typical for the deep ocean (Fig. 2.5). Red clays,
regions, where productivities can exceed by a factor of 10 the derived mainly from eolian, volcanic, and cosmic sources
values in the subtropical gyres (Berger, 1974). These oceano- accumulate by default in distal, barren regions of the seafloor
graphic conditions are also favorable for the deposition of below the CCD. Siliceous ooze accumulates under surface
organic matter and the intensification of the oxygen minimum waters of high fertility, that is, along the margins of continents,
layer, which may impinge on the seafloor and enhance the along a periequatorial belt, and along the polar front regions
preservation of hydrogen‐rich organic matter‐rich sediments (Fig. 2.5). The composition of marine sediments is normally a
(Fig. 2.4). It is important to note here that anoxia is not a mixture of these components (Fig. 2.1). The main controls on
requirement for the preservation of organic matter in marine sediment composition on the seafloor are thus distance to
sediments; rather, under such conditions, more hydrogen is shore, water depth, and fertility of surface water (Fig. 2.6).
associated with carbon in the organic matter (Demaison, 1991; As mentioned before, the flux of biogenous sediment
Pedersen and Calvert, 1990), which means that the shales thus through the water column is mainly determined by two
produced have an enhanced hydrocarbon‐generating potential. variables: productivity and destruction. Together with
On the other hand, these conditions are adverse for the preser- processes that redistribute sediment on the seafloor, these two
vation of calcite, which minimizes the importance of carbonate variables also control the nature and distribution of sediments
as a diluent in high fertility settings. Dilution in such settings on the seafloor away from point sources of terrigenous
depends on the proximity of terrigenous sediment sources and sediment. The destruction of planktonic organic matter by
pathways, and on biogenous silica input and dissolution. bacterial oxidation during its transit through the water column
Oceanic sediments, that is, those deposited beyond the shelf dominates at depths of 300–1500 m. The supply of organic
break, on the continental margin generally consist of an matter depresses oxygen content due to decay in deep water
increasing proportion of biogenous material away from land and on the seafloor and an oxygen minimum layer develops.
due to decreasing dilution by siliciclastic material, which is The position of the oxygen minimum layer in the water
mostly trapped nearshore. Indeed, about half of the deep sea- column depends on ocean circulation (Wyrtki, 1962). The
floor area is covered by oozes, that is, by planktonic debris. oxygen minimum layer is further characterized by a maximum
The general outlines of sediment distribution on the seafloor in CO and nutrients. Upwelling of this water leads to high
2
are relatively simple. The main facies boundary in the ocean is primary productivity in surface waters and, therefore, to an
the CCD, that is, the boundary between calcareous and noncal- abundant supply of biogenous sediment to the seafloor,
careous sediments. Calcareous facies characterize shallower namely, siliceous skeletal debris and organic matter. The
oxygen minimum below upwelling zones is also more
intense. If the rates of organic matter supply are sufficiently
Increasing fertility high and oxygen levels sufficiently low, the preservation of
Pteropod ooze Black shale organic matter in the sediment is favored. Upwelling occurs
Pteropod ooze
Black shale
2 15 15 2 at divergent oceanic fronts, over submarine topographic
highs, and adjacent to continental margins, particularly on the
Depth (km) 3 10 Calcareous ooze 40 Diatom 3 4 western sides of continental masses. The distribution of
Calcareous ooze
40
Diatom
ancient open marine black shales has been shown to
15 15
10
and
4
and
Rad foram ooze
Rad foram ooze
terrigenous
Red clay CCD terrigenous correspond closely to the distribution of predicted upwelling
Red clay
Mn-ox
CCD
mud
5 (oxypelite) Mn-ox Rad ooze Diatom 100 mud 5 zones (Parrish, 1982, 1987; Parrish and Curtis, 1982), and the
(oxypelite)
100
Diatom
Rad ooze
2 2 deposition of black shales on the pericontinental shelves of
4 4 ooze the North Atlantic during OAEs has also been shown to be
ooze
Subtropical Oceanic Upwelling related to upwelling of nutrient‐rich water (e.g., Trabucho‐
convergence divergence area
Alexandre et al., 2010).
FIGURE 2.6 Distribution of major facies in a depth–fertility Nutrient‐rich surface waters have sufficient silica
frame based on sediment patterns in the Eastern Central Pacific available to support the production of siliceous skeletal
−1
(Berger, 1974). Numbers are typical sedimentation rates in m Ma . material. Siliceous ooze is an often forgotten component of
The CCD is the main facies boundary in deep‐sea deposits. The marine fine‐grained sedimentary rocks, and biogenous
composition of oozes varies according to water depth, fertility, and silica is typically not differentiated but grouped with clastic
latitude, and depends on whether surface ocean currents have a silica (e.g., quartz and feldspar) in ternary diagrams reflecting
tropical or polar origin. Black shales, or sapropelites, according to
other authors, occur in high fertility settings on the relatively shal- shale composition (e.g., Boak, 2012; Gamero‐Diaz et al.,
low seafloor of continental margins (shelf and upper slope). Other 2013; Passey et al., 2010). Because silica‐secreting plankton
black shale occurrences are related to oceanic topographic highs tends to proliferate in settings conducive to high organic
(viz. seamounts, rises, etc.) and to sediment gravity flows productivity, organic matter‐rich sediments are often
transporting fine‐grained material toward deeper water. siliceous (Hay, 1988). Many Mesozoic black shales that