Page 54 - Fundamentals of Gas Shale Reservoirs

34 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

burial removes the organic matter from the oxygen‐rich swamps, and phytoplankton (Talbot and Allen, 1996). Large
sediment–water interface, thereby enhancing preservation. lakes can contain a range of depositional environments
This mechanism appears to be particularly important for deep‐ including deltaic, coastal, and deepwater environments. Lakes
sea black shales. Sediment gravity flows triggered by tectonic are extremely sensitive to changes in climate and consequent
activity along the continental shelf, storms, and destabilization changes in accommodation space (Bohacs, 1998). The ratio of
of organic matter‐rich sediment as a consequence of gas accommodation space creation, that is, basin subsidence, to
generation in pore water transport large amounts of sediment sediment/water input, which is controlled by climate, is the
toward deeper water. Sediment gravity flows are probably the fundamental control on the stratigraphy of lakes (Carroll and
most important mechanism for moving large quantities of mud Bohacs, 1999). Accommodation space determines water
to distal parts of deep basins. The quick burial of remobilized depth, which is a factor behind oxygen deficiency, depositional
shallower water organic matter‐rich sediment enhances the environments, and facies; climate determines the biota,
preservation of the organic matter in fine‐grained turbidites. temperature, and salinity of a lake (Potter et al., 2005).
Carbonate is also often preserved in fine‐grained turbidites The processes that favor enrichment in organic matter of
despite having been deposited below the CCD (Stow, 1985a; lacustrine sediments depend on several factors that are
Trabucho‐Alexandre et al., 2011). At the present time, organic ultimately linked to the type of lake in which the sediments are
matter‐rich sediments are not accumulating in the central parts deposited (cf. Bohacs et al., 2000). A key factor is the
of major ocean basins. In the geologic past, however, this did availability of nutrients, which support primary production.
occur. Although part of the sediment is of biogenic derivation Nutrients are brought in by land surface flow from the lake
and settled vertically through the water column, much material catchment area and by eolian transport. In many lakes, seasonal
is redeposited (e.g., Degens et al., 1986; Stow et al., 2001; overturn recycles nutrients into surface waters. Permanently
Trabucho‐Alexandre et al., 2011; van Andel et al., 1977). stratified lakes require an external nutrient source to support
primary productivity. In alkaline lakes, such as lakes in tropical
Africa, productivity is enhanced due to the abundance of
2.5 ORGANIC MATTER‐RICH SHALE carbonate ions that are available for incorporation by primary
DEPOSITIONAL ENVIRONMENTS producers in addition to atmospheric CO (Kelts, 1988).
2
The oxygenation of lake waters occurs primarily via
Mud is everywhere, and life ubiquitous. Therefore, it is not exchange with the atmosphere, although some oxygen is a
surprising that black shales may be deposited in a wide byproduct of photosynthesis. When a lake is thermally or
range of sedimentary environments from the bottom of chemically stratified, oxygen in bottom waters cannot be
lakes to the abyssal plains of the ocean. The interpretation replenished (Fig. 2.3h). Oxygen is depleted by oxidation of
of ancient environments of black shale deposition has been sinking organic matter and the waters become anoxic, thereby
influenced by studies of modern environments where favoring the preservation of organic matter. The extent and
organic matter‐rich sediments are currently accumulating duration of bottom water anoxia in a lake depend on the fre-
(Fig. 2.3 and Table 2.1). However, most ancient black shales quency and intensity of mixing. In highly productive lakes,
appear to have been deposited in shallow marine epiconti- neither stratification nor permanently anoxic conditions are
nental environments for which we have no modern analogs needed for the preservation of organic matter (Talbot, 1988).
(cf. Arthur and Sageman, 1994). Bohacs et al. (2000) and Carroll and Bohacs (1999, 2001)
discuss the processes and environments of organic matter‐
rich sedimentation in different types of lakes in great detail.
2.5.1 Continental Depositional Environments
Lake deposits occur in several settings, but are most
Organic matter‐rich rocks deposited in continental environ- common in rift, intramontane, and foreland basins. Lake
ments account for more than 20% of current worldwide deposits of rift basins with rapid subsidence are more likely
hydrocarbon production (Bohacs et al., 2000; Potter et al., to be thick and well preserved (Potter et al., 2005). The lower
2005). Following the colonization of land by plants in the Permian Whitehill Formation, South Africa, contains
Devonian, organic matter becomes an important component in lacustrine black shales that are thought to originate from the
continental sediments. Mud in fluvial sequences mainly accu- accumulation of freshwater algae under anoxic, fresh‐to‐
mulates by vertical accretion as overbank deposits, which are brackish‐water conditions in a protorift basin in southwestern
produced when mud is deposited during floods in ephemeral Gondwana (Faure and Cole, 1999). Lake Tanganyika in the
ponds marginal to the main channel, and in oxbow lakes, which East African Rift is a good example of a modern lacustrine
are persistent lakes formed by abandonment of meander loops. environment of black shale deposition (Demaison and Moore,
Mud deposited in lakes often has organic matter contents 1980). It is a large, deep lake with sedimentary environments
that are significantly above the average for sediments in ranging from deltas and narrow carbonate platforms in
general. This organic matter comprises reworked terrestrial shallow water to deepwater fans. The equatorial location of
vegetation, from riparian environments, marginal macrophyte the lake and its great depth result in high surface productivity