Page 44 - Fundamentals of Gas Shale Reservoirs

24 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

(sun) light or the oxidation of inorganic molecules as an zoöplankton produce large amounts of skeletal debris, viz.
energy source, respectively. Primary organic production frustules, which results in significant dilution of organic
from photo‐ and/or chemosynthesis is the first and foremost matter. To generate an organic matter‐rich sediment, the
prerequisite to generate an organic matter‐rich sediment. In destruction of organic matter must be minimized. Destruction
its broader sense, production also refers to the biomineral- refers to the remineralization of organic matter by organisms
ization processes by which aquatic organisms produce their (mainly bacteria) and oxidation in the water column. These
skeletons. The relationship between organic productivity processes can continue at the sediment–water interface and
and biomineral productivity is typically nonlinear; a possible to some depth within the sediment column. Destruction also
reason for this may be the effect of dissolution. includes the dissolution of skeletal material in the water
Organic matter in continental environments is terrigenous, column. Dissolution of calcareous skeletal material increases
that is, produced by land‐dwelling organisms, whereas in with water depth and with an increase in supply of organic
marine sediments organic matter may be either of marine or matter (Emerson and Archer, 1990), which lowers the pH of
terrestrial origin. On land, almost all primary production sediment interstitial waters unless sulfate‐reducing condi-
since the Devonian is by vascular plants. Land‐derived tions in the sediment prevail (Morse and Mackenzie, 1990).
organic matter, highly degraded and nitrogen poor, is brought Dissolution of siliceous skeletal material occurs throughout
into the ocean by rivers in dissolved and particulate forms. the water column, but is more intense in the warmer surface
Most terrestrial particulate organic matter, for example, layers of the ocean and shortly after deposition (Berger,
pollen, plant debris, and charcoal, is deposited in nearshore 1974). A minimization of the destruction of organic matter
environments, whereas the dissolved component escapes can be achieved by lowering dissolved oxygen content in the
removal and is carried out into the ocean. The bulk of water column, by making the export path and/or transit times
dissolved organic carbon in seawater is marine. Land‐ shorter, and/or by reducing sediment exposure time to bot-
derived dissolved organic matter entering the open ocean tom water after reaching the sediment–water interface.
must therefore be extensively oxidized back to CO (Emerson Oxygen levels in seawater depend on how much oxygen
2
and Hedges, 1988; Hedges and Keil, 1995; Hedges et al., seawater can hold and on oxygen supply and demand.
1997). Marine organic matter is produced largely by phyto- Oxygen levels are lower in warm climates due to the reduced
plankton, for example, cyanobacteria, diatoms, and dinofla- solubility of oxygen in warmer water. This is the case in a geo-
gellates, in the photic zone. Productivity on the continental graphic sense, that is, sea surface water at lower latitudes
margin is favored by a combination of fluvial, eolian, and contains less oxygen, and in a geologic sense, that is, seawater
offshore nutrient supplies. Nutrients carried by rivers to the during hot/greenhouse intervals contained less oxygen than at
ocean are consumed quickly within and immediately off present. Dysoxia, and—depending on the frequency, intensity,
river mouths (Piper and Calvert, 2009). Nutrients supplied and depth of mixing—anoxia, is favored in basins whose phys-
from the base of the thermocline by mixing and by upwelling iography (e.g., oxbow lakes and silled marine basins) and/or
are the main source of nutrients in highly productive areas of water column thermohaline structure (e.g., lakes) result in the
the ocean, and fuel about three‐quarters of the new produc- stagnation of (part of) its water column. Dysoxia develops
tion in the ocean (Eppley and Peterson, 1979). Although in response to runoff of nutrient‐rich water from rivers to
coastal regions have higher rates of photosynthesis than the lakes and oceans, and upwelling of nutrients and consequent
open ocean, most (ca. 80%) of the total photosynthetic pro- enhanced surface productivity in lakes (overturning) and
duction occurs in the open ocean (Emerson and Hedges, oceans. Oxygen depletion is more dynamic than commonly
1988), which accounts for about 90% of the total sea surface. assumed and depends on the interaction between lake/ocean
However, export production, that is, the amount of organic circulation, biological activity, and nutrient distribution (Meyer
matter that is not remineralized before it leaves the photic and Kump, 2008). Biochemical processes are ultimately
zone and sinks to the seafloor, is lower in the open ocean. At responsible for the consumption of oxygen, but ocean
present, for example, most export production is concentrated circulation is responsible for the distribution of dysoxic and
along the relatively shallow continental margins (Laws et al., anoxic water masses in the ocean (Wyrtki, 1962). Oxygen
2000; Walsh, 1991), where up to 90% of organic carbon depletion may be either local or regional, and it may be seasonal
burial takes place (Berner, 1982; Hedges and Keil, 1995). or permanent (e.g., Lake Tanganyika). It has been suggested
Although primary productivity is important (e.g., that the preservation of organic matter in mid‐Cretaceous
Pedersen and Calvert, 1990), it is not sufficient by itself. In marine sediments was favored by decreased oxygen supply to
the modern Southern Ocean, for example, areas associated deep water as a consequence of sluggish ocean circulation
with oceanic divergence are characterized by high primary (e.g., Bralower and Thierstein, 1984; Erbacher et al., 2001). In
productivity; yet, sediments below these fertile surface a stagnant ocean, the supply of nutrients to the photic zone
waters are organic matter lean (Demaison, 1991). This is would not be sufficient to sustain the elevated primary produc-
because the water column is well oxygenated, largely due to tivity required to support high oxygen demand in deep water
very low water temperatures, and because silica‐secreting (e.g., Hotinski et al., 2001). Whereas a sluggish ocean would