Page 53 - Fundamentals of Gas Shale Reservoirs

GEOGRAPHIC DISTRIBUTION OF ORGANIC MATTER‐RICH SHALES 33
occur around the former Tethys and North Atlantic Oceans High productivity
and on Indo‐Pacific seamounts that occupied a periequatorial 50 A
position at the time of deposition are actually black cherts.
Black, organic matter‐rich cherts are not always laminated. B
Most of the skeletal debris produced by planktonic 10
organisms never reaches the seafloor, and most of the debris
that does reach the seafloor is nonetheless dissolved. This is Anoxic
particularly the case for siliceous skeletal material, because
seawater is undersaturated with respect to biogenous silica at Corg (%)
all water depths. Silica corrosion is greatest in surface waters 1
due to elevated temperature, whereas carbonate dissolution
is greatest at depth (Berger, 1974). There is a negative
correlation between silica and carbonate distribution patterns A
on the seafloor (Fig. 2.5), which has been attributed to Open-marine oxic
opposing chemical requirements for preservation (Correns, 0.1
1939): increasing productivity leads to decreasing preserva-
tion of calcite and to increasing accumulation of silica. 0.1 1 10 100
Seawater is also undersaturated with respect to all forms of Sedimentation rate (cm/kyr)
calcium carbonate. At a critical level of undersaturation, disso-
lution rates of calcium carbonate increase rapidly and, below FIGURE 2.7 Correlation between marine organic carbon and
the CCD, which is the level at which the rate of supply of car- sedimentation rates. The three fields, A, A′, and B are based on data
bonate is balanced by its rate of dissolution, calcium carbonate derived from Neogene and Quaternary sediments deposited in open
ocean environments (A), upwelling zones (A′), and anoxic environ-
does not accumulate on the seafloor. High fertility along the ments (Black Sea and Mediterranean sapropels and modern Black
equator in the Pacific leads to a depression of the CCD by some Sea sediments). The stippled area in field A′ indicates coastal
500 m (Seibold and Berger, 1996). Increased fertility leads to upwelling and the open area at the lower end of the field indicates
an increased supply of calcareous skeletal debris to the seafloor equatorial upwelling. Based on figures 2 and 3 in Stein (1986).
in excess of the increased supply of organic matter, because
while the calcareous debris transits to the seafloor, organic seafloor on the shelf and upper slope, but only 1 gC reaches the
matter tends to be destroyed on its way down. Along continental deep seafloor (Berger et al., 1989). Most of the sinking material
margins, however, high productivity raises the CCD (Seibold is oxidized and remineralized. Deep‐sea sediments have low
and Berger, 1996). In fertile areas along continental margins, concentrations (C < 0.25%) of organic matter. Below the
org
the high supply of organic matter to the relatively shallow sea- equator, the concentrations increase slightly, because upwelling
floor leads to increased benthic activity and to the development along the equator results in a greater supply of organic matter
of much CO in sediment pore waters, which produces carbonic to the seafloor. The highest concentrations of organic matter,
2
acid. For this reason, carbonate debris is dissolved even at however, are linked to coastal upwelling along continental
depths of a few hundred meters on continental slopes, and margins (Fig. 2.6). The large supply of organic matter along
pericontinental black shales tend to contain little carbonate. the continental margin, as well as the physiography of the
In the ocean, most biogenous material is produced in the basin, generates an oxygen minimum zone as a consequence of
top layers of the water column and arrives at the seafloor as high oxygen demand and, in the case of restricted basins, low
a rain of particles. These particles are mostly aggregates, and oxygen replenishment (Fig. 2.3), and this may result in anoxic
a large fraction of these consists of fecal pellets of various conditions on the seafloor. Under such conditions, the
sizes and in various stages of disintegration. Aggregates sink preservation of organic matter is enhanced because anaerobic
faster through the water column than their constituent bacteria are less efficient in destroying organic matter.
particles, which would take years to settle to the average The relationship between organic matter content and
depth of the seafloor. Because all biogenous particles are sedimentation rates in oxic and anoxic Neogene marine sedi-
subject to dissolution and/or remineralization in the water ments gives insight into the processes that control the organic
column, if it were not for the aggregation mechanism, most carbon content of marine sediments (Fig. 2.7). There is a lack
would never reach the seafloor. Despite this mechanism, the of correlation between sedimentation rate and C in anoxic
org
proportion of primary production that leaves the photic zone conditions, and a good correlation in oxic conditions (Stein,
(export production) is small. Along the continental margin, 1990). In the case of anoxic sediments, if there is sufficient
higher export factors and shorter distances to the seafloor input of organic matter, the organic matter is preserved even
enhance the burial of organic matter. when sedimentation rates are low. In the case of oxic sediments,
Sediment trap studies indicate that for every 100 gC that is relatively rapid burial is required to preserve a significant
produced in the sunlit layer of the ocean, about 30 gC reach the portion of the organic matter delivered to the seafloor. Rapid