Page 149 - Geology of Carbonate Reservoirs

130 DEPOSITIONAL CARBONATE RESERVOIRS

interstitial waters are anoxic and reducing. The Permian sea in the Delaware Basin
of West Texas had limited circulation and high salinity that allowed evaporite miner-
als to crystallize from interstitial waters on the basin fl oor. Evaporite crystal molds

are easy to find on bedding planes in today ’ s outcrops of these ancient basinal
deposits. If basinal depths are greater than about 200 m or if water turbidity is high,

little or no sunlight can penetrate to the seafloor. Depending on the amount and
kind of current circulation in a given basin, bottom waters may be stagnant, anoxic,
dark, cold, and above normal salinity. However, if the basin has open connections
to the world oceans and circulation is vigorous, bottom waters will be in motion,
oxygen will be more abundant, the Eh may be neutral to positive, and salinity may
be closer to normal. Of course, if the depth is greater than the CCD, the preserva-

tion potential for fine carbonate sediments is low. Clearly, the number of environ-
mental variables in basinal environments is great enough to prevent making rigid
descriptions of basinal environmental characteristics.
Basinal environments, even those only a few hundreds of meters deep, are gener-
ally protected from wave action, surface currents, and ordinary tidal effects. But they
are not immune from being swept by geostrophic and density currents. These deep -
water currents can transport fine sediments and create sedimentary structures.

Current ripples are common in basinal sediments, along with a variety of current -
related markings such as flute casts and tool marks. In deeper basins, the environ-

ment is more hostile to life and only the best - adapted organisms colonize the
seabed. Less hostile basinal settings may host a variety of benthic organisms that,
in turn, produce many different kinds of tracks, trails, burrows, and other forms of
trace fossils . Trace fossils have been studied extensively and a comprehensive litera-
ture exists about them. For purposes of reservoir studies, trace fossils are useful in
identifying and subdividing basinal facies types when used in conjunction with
physical stratigraphic and sedimentological criteria. In general, basinal settings can
be characterized as cool, dark, high - pressure environments that may be dysoxic to
anoxic, and that may have negative to neutral Eh. They usually have low taxonomic
diversity, they may exhibit distinctive trace fossils, spiculiferous chert, and they may
exhibit sedimentary structures produced by geostrophic and density currents.

5.2.12 Depositional Rock Properties in Basinal Environments

Slope facies overlap basinal facies. If slope - derived debris and grain fl ows are

excluded, basinal deposits commonly consist of fine - grained carbonate and silici-
clastic mud and sand, particulate organic matter, and pelagic microfossil remains. If

the source of fine sediment is relatively constant and the basinal setting does not
receive sediments from nearby slopes, the typical facies will be dark colored, thin
bedded (millimeter scale), fi ne grained, sparsely fossiliferous, organic - rich deposits
called laminites . Sets of millimeter - scale laminae are common and may be bundled
in 5 – 30 - cm beds alternating with centimeter - scale shale layers. Lamina sets are com-
monly graded from base to top. Coarser grained millimeter - scale laminae grade
upward to finer ones. These graded beds are known as rhythmites because they may

be cyclically repeated through many meters of vertical thickness. Occasionally tur-
bidites may extend long distances from their source, typically shelf - slope areas, and
become incorporated in the basinal facies melange. Ramps characteristically have
low slopes that are not conducive to the initiation of turbidity currents and slope