Page 45 - Fundamentals of Gas Shale Reservoirs

STRATIGRAPHIC DISTRIBUTION OF ORGANIC MATTER‐RICH SHALES 25
result in a significantly decreased oxygen supply to deep water, (e.g., Degens et al., 1986; Ghadeer and Macquaker,
that reduction would be balanced by a reduction in oxygen 2012; Macquaker et al., 2010a). Moreover, large fluxes of
demand (Meyer and Kump, 2008). Other studies have metabolizable organic matter favor processes of natural vul-
suggested that a more vigorous circulation resulted in a more canization, which lead to the creation of resistant geobio-
productive mid‐Cretaceous ocean (Hay and Floegel, 2012; polymers (Lallier‐Vergès et al., 1997; Sinninghe Damsté
Southam et al., 1982; Topper et al., 2011; Trabucho‐Alexandre et al., 1989). However, too much of any component will
et al., 2010; Wilson and Norris, 2001). “mask” others, especially if they are present in low absolute
The Black Sea is often used as a model for ancient sluggish amounts in the sediment, as is typically the case for organic
or stagnant oceans. However, evidence suggests that this matter. Dilution of organic matter by inorganic material, ter-
enclosed basin is not properly described as stagnant. rigenous and/or biogenous (skeletal), is an important control
Radiocarbon dating indicates a mean residence time of 935 in the accumulation of organic matter in sediments (Bohacs
years for deep Black Sea water, whereas mass balance calcu- et al., 2005; Tyson, 2001). High dilution rates can result in
lations indicate a shorter residence time of 475 years (Östlund, organic matter‐lean sediments even under regions of high
1974). Brewer and Spencer (1974) calculated a present‐day surface productivity. In deltaic settings, for example, organic
upward advective velocity of 0.5 m a in the interior of the carbon contents are feeble where elevated sedimentation
−1
Black Sea (in Degens and Ross, 1974). These results suggest rates of terrigenous sediment dilute the organic component
that the rates of vertical exchange in the Black Sea are of the of sediments (Dow, 1978).
same order of magnitude as those in the modern open ocean, Along continental margins, the calcite compensation
and that euxinia, which refers to the presence of free hydrogen depth (CCD) is raised due to higher primary productivity
sulfide in the water column, in the Black Sea represents a and consequent respiration of organic matter in sediments,
dynamic balance (Southam et al., 1982). which releases metabolic CO and thus increases carbonate
2
In the open ocean, the fraction of the organic matter dissolution (Berger, 1974; Seibold and Berger, 1996). As a
produced in surface waters that reaches the seafloor is result, dilution of organic matter by calcareous skeletal
inversely proportional to water depth (Hedges and Keil, debris is minimized. On the other hand, dilution can also be
1995; Müller and Suess, 1979; Suess, 1980). All other vari- too low. The fraction of organic carbon that reaches the
ables being equal, the preservation of organic matter is basin floor is a function of water depth and of bulk
favored where the seafloor is relatively shallow, namely, on sedimentation rate (Müller and Suess, 1979), but more than
the continental shelf and upper slope and on the top and 90% of the organic matter that does reach the seafloor is
flanks of seamounts. Long transit times through mildly nonetheless remineralized (Emerson and Hedges, 1988).
−1
oxidizing water (e.g., ca. 3 ml l O in the deep modern Where sedimentation rates are low, the preservation of
2
North Pacific, Southam et al., 1982) and slow sedimentation organic matter is reduced because the sediments are kept
−1
rates (ca. 2 m Myr ) are sufficient to result in the deposition within the mixed sediment layer for too long, where they
of organic matter lean, red/brown pelagic clay on the deep are exposed to active microbial reworking and oxidants in
ocean floor. A significant part of the vertical flux of pore waters, as well as erosion and transport (Bohacs et al.,
particulate organic matter from the photic zone is in the form 2005, and references therein). In condensed sequences in
of organomineralic aggregates. Because they are larger than the Mesozoic of Alabama, United States, for example,
their constituent mud particles, these aggregates settle much organic matter was not preserved probably due to low
faster through the water column, and transit times to the sea- sedimentation rates (Mancini et al., 1993). In conclusion,
floor are within weeks. The preservation of organic matter is there is not just a single combination of variables that
thus greatly favored. will yield organic matter‐rich sediments, but optimum
Dilution is a consequence of the mixing of siliciclastic, organic enrichment occurs where production is maximized,
skeletal, and organic material, because the composition of a destruction minimized, and dilution optimized (Bohacs
sediment is a zero‐sum game; an increase in one component et al., 2000; Tyson, 2001).
must be accompanied by a relative decrease in the others.
The input of siliciclastic material, which is a key control in
the composition of a shale, may be included in dilution. Up 2.3 STRATIGRAPHIC DISTRIBUTION
to a certain point, an increase in the input of siliciclastic and/ OF ORGANIC MATTER‐RICH SHALES
or skeletal material, that is, an increase in sediment
accumulation rates which leads to relatively rapid burial, Although shales are a ubiquitous component of the stratigraphic
favors the preservation of organic matter. Indeed, the record, the distribution of black shales in the Phanerozoic is
preservation of organic matter, particularly in oxidizing predominantly limited to six stratigraphic intervals (Fig. 2.2),
environments, is favored by sedimentation processes that which together represent about one‐third of Phanerozoic time
deliver large quantities of sediment to the seafloor, including (e.g., Bois et al., 1982; Klemme and Ulmishek, 1991; North,
metabolizable organic material, in a short period of time 1979; Tissot, 1979). The petroleum source rocks in these