Page 58 - Fundamentals of Gas Shale Reservoirs

38 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

in summer. Indeed, oxygen‐deficiency in Phanerozoic shallow the sediments are typically organic matter‐rich, because the
epi‐ and pericontinental shelves must have been seasonal. preservation of organic matter is favored (Fig. 2.4).
Organic matter‐rich shales may also form in epiconti- The occurrence of low oxygen water masses in the modern
nental carbonate platforms. There are no modern examples, ocean appears to be much less widespread than during the
but these carbonate platforms were widespread in the Mesozoic. There are probably many reasons behind this, but
geologic past. Some of these platforms may represent the two are likely to be the most important: seawater temperature
interior of carbonate shelves or ramps, while others were and paleogeography. The modern Atlantic Ocean, for example,
truly epeiric platforms covering areas in the order of is a corridor for meridional circulation of cold, oxygen‐rich
millions of square kilometers (Wright and Burchette, water formed in the polar regions; this thermohaline circulation
1996). Intraplatform basins in such settings are rimmed by counters the expansion and intensification of the oxygen
low gradient, ramp‐like margins, and water depths in these minimum layer. In comparison, the northwest Indian Ocean
basins were shallow (<150 m). During transgressive and/ contains very little oxygen at depths between 200 and 1200 m
or highstand phases, intraplatform basins became strati- (Southam et al., 1982). Where oxygen concentrations in the
−1
fied and cyclic suboxic or anoxic sediments developed in oxygen minimum layer fall below 0.5 ml l and the layer
the basin center. Organic matter‐rich sediments may form impinges on the continental slope, sediments on the seafloor
in these settings, and a number of examples of black shales are typically rich in organic matter (2% ≤ C ≤ 20%).
org
deposited in intraplatform basins are known, particularly The California continental borderland consists of a series
in the Jurassic and Cretaceous of the Middle East (Burchette, of basins of varied sizes, separated by submarine ridges,
1993; Droste, 1990). sills, and islands (Gorsline, 1978). High nutrient levels in
surface waters due to the combined effect of climate and
oceanography support high, but variable, primary produc-
2.5.4 Deep Marine Depositional Environments
tivity. An oxygen minimum zone, which partly results from
Deep marine environments extend from the shelf break to the oxidation of sinking pelagic organic matter, impinges
the abyssal seafloor. The deep marine continental margin, upon this structurally complex continental margin. The
that is, the slope and rise, consists of thick accumulations of basins are silled below a depth of about 500 m and contain
terrigenous sediment mixed with marine biogenous material; predominantly dysoxic water due to the interplay of oxygen‐
beyond the continental margin, the deep ocean is character- deficient deepwater flow and bottom topography (Savrda
ized by extensive facies belts dominated by biogenous et al., 1984). The preservation of significant thicknesses of
sediment (Fig. 2.5), and little terrigenous sediment reaches organic matter‐rich sediments in these basins is favored by
the deep sea under modern conditions (e.g., Meade, 1994, the impingement of their seafloor by the oxygen minimum
his figure 2.8). layer (Fig. 2.3b). The Miocene Monterey Formation of
In terms of depositional processes, there are three different southwestern California comprises a large volume of
facies in the deep sea: pelagites, turbidites, and contourites organic matter‐rich siliceous and phosphatic (hemi)pelagic
(Stow, 1985a); the processes that are responsible for the sediments. The sediments were deposited under similar
enrichment in organic matter and its preservation in deep‐sea conditions in relatively quiet deep water in a fault‐bounded
sediments vary according to facies. Black pelagites form complex of borderland basins separated by islands and
under regions of high surface productivity, where the rate of banks (Pisciotto and Garrison, 1981). Basins adjacent to
supply of organic matter exceeds aerobic oxidation in the shore received abundant terrigenous sediment, whereas
water column. Rapid downslope resedimentation of organic basins farther offshore are sediment starved.
matter‐rich shelf sediments favors the preservation of organic Seamounts and other oceanic rises affect ocean circulation
matter in black turbidites on the deep ocean floor (e.g., patterns and result in local high fertility of seawater and con-
Dean et al., 1984). High sedimentation rates associated with sequent high productivity (e.g., Boehlert and Genin, 1987).
sediment gravity flows lead to a rapid burial of organic matter The preservation of organic matter is generally favored on
and therefore to its removal from the upper oxygenated part the seafloor of seamounts because the length of the export
of the sediment column where degradation of organic matter path is reduced (Fig. 2.3d). The preservation of organic
is particularly aggressive (Stow et al., 2001). Low oxygen matter is also favored where the oxygen minimum layer
levels in pore water and low predator pressure in deep marine intersects the flanks of seamounts and other submarine topo-
environments result in the absence of deep burrowing, graphic highs (Fig. 2.4). Cretaceous black shales in the
characteristic of the Nereites ichnofacies, which also favors Pacific were deposited on seamounts, oceanic plateaus, and
the preservation of organic matter in black turbidites. other submarine topographic highs as a consequence of local
The consumption of oxygen by biochemical processes in a high productivity. Other black shales were deposited as a
layer of relatively small replenishment of oxygen by advective result of high productivity associated with the passage of the
movement results in an oxygen minimum layer (Wyrtki, submarine highs beneath the equatorial belt of high fertility
1962). Where this layer comes into contact with the seafloor, (Waples, 1983).