Page 148 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 129

this waning energy by vertical changes in texture and sedimentary structures. Res-

ervoirs in grain flow deposits and turbidites are subject to the same limitations on

porosity as debris flows and slumps. These facies commonly include high matrix
content that blocks pore throats and limits depositional porosity. In many cases,
reservoir quality in deep slope deposits depends on burial diagenetic dissolution to
create or open pores and pore throats.

5.2.11 Basinal Environments
The word “ basinal ” conjures images of cold, dark, forbidding depths. Images like
those of the great ship Titanic lying miles beneath the North Atlantic Ocean or of
mysterious sea creatures revealed in photographs taken in the abyss through
windows of special submarines. From the practical viewpoint of geoscientists and
engineers, the basinal environment is simply the end of the marine environmental
spectrum that began at the strandline and ended at the deepest part of that particu-
lar sedimentary basin. There is no unique depth that identifies the “ basinal ” environ-

ment. There is not even a rigid definition of basinal environment or basinal facies.
In fact, the greatest depth that exists in one basin may be the same measured depth
as the shallow subtidal regime in another basin. For example, the maximum depth
in the modern Persian Gulf is only about 200 m, but the maximum depth in the
Mariana Trench is about 11 km. The 200 - m depth, as we have mentioned earlier, is

the arbitrary limit used by oceanographers to define the outer limit of the neritic
(shallow subtidal) environment. It is commonly used to define the edge of the “ con-

tinental shelf ” or “ continental terrace ” as well, but in the Persian Gulf it represents
basinal depth and the basinal environment. This lack of a unique and single defi ni-
tion for basinal environment and basinal facies also holds true for basinal deposits
in the fossil record.
Epicontinental seas may not have been more than a few tens of meters deep on
average, while the centers of the world oceans may have had average depths of
several kilometers. For carbonate sediments to accumulate in the basinal environ-
ment, water depths must be shallower than the carbonate compensation depth
(CCD) because below the CCD, carbonate particles dissolve. This depth varies
depending mainly on water temperature and partial pressure of CO 2 . It is also dif-
ferent for aragonite than for calcite because of their different solubilities. In the
major world oceans today the CCD is at depths of several kilometers and it is shal-
lower for aragonite than for calcite because aragonite is more soluble. We have
already established that the optimum zone for carbonate sediment production, or
the carbonate factory, extends from the surface to about 200 m. Below about 200 m,
even in temperate environments with heterotrophic biota, there is little or no in situ
sediment production; therefore deep - water deposits consist of imported sediment
shed from platform tops, from slopes, and from the rain of pelagic organisms that
live and die in the water column. Typical basinal sediments may include both car-

bonate and siliciclastic muds and fine sands, planktonic skeletal remains such as
foraminifera, cocolithophoridae, discoasters, radiolarians, calpionellids, and tintinids,
sponge spicules, and the odd coarser sediments carried in by density or turbidity
currents.
Restricted basinal environments have limited water circulation. The Black Sea
is a silled basin with such restricted circulation that the bottom sediments and