Page 135 - Geology of Carbonate Reservoirs

116 DEPOSITIONAL CARBONATE RESERVOIRS

snails, and skeletons of land - dwelling animals may be present. It is rare to fi nd
skeletons and body fossils in borehole cores because their remains, if fossilized at
all, are so widely scattered within dunes.
We know that beach – dune successions may be attached to the mainland or they
may be detached as barrier islands. In either case, the distinguishing fundamental
rock properties are the same. The major difference for the reservoir analyst to rec-

ognize is whether or not the beach is detached because fine - grained, lagoonal sedi-
ments or evaporites might be present to form the updip trapping facies for barrier
island reservoirs. If the beach is attached, either dunes or continental facies will be
immediately updip and their trapping potential may not be good. If dunes are
present, they may have excellent potential as reservoir rocks but little value as trap-
ping facies. Unfortunately, most carbonate dunes lose their porosity and permeabil-
ity to diagenesis and rarely become productive reservoirs, according to Abegg et al.
(2001) . Barrier islands and barrier spits are formed when sand is moved parallel to
the shore by longshore currents. Bathymetric irregularities such as points on the
mainland or submerged shoals serve as nucleation sites for sand accumulation. As
the longshore drift system continues, the sand deposit is extended in up - current and
down - current directions, forming barrier spits or barrier islands. If the bathymetry

adjacent to a mainland shore is sufficiently monotonous and the shoreline is com-
paratively straight, the sand moved by longshore drift will accumulate directly on
the mainland beach. Beach sediment texture depends largely on the kind and origin
of sedimentary constituents. Ooids and peloids are common in the fine to medium

sand range, but a wide range of sizes may be typical of bioclastic beaches. As we

have already noted, the texture of bioclastic sands may reflect more about the skel-
etal anatomy of the constituent organism than it does about a history of abrasion
and transportation.
Before undergoing burial diagenesis, beach rocks may be subjected to marine,
freshwater, or vadose diagenesis, depending on local hydrology and on the location

of the beachrock sample on the beach profile. Samples taken above the storm high -
water level exhibit vadose diagenesis, those consistently saturated with seawater
exhibit marine diagenesis, and buried segments that are in contact with the fresh-
water table exhibit meteoric phreatic diagenesis. Mixing - zone diagenesis is some-
times evident in zones where seawater and freshwater are combined. Most
beachrocks show some degree of marine cementation. Diagenetic characteristics
indicative of freshwater, vadose, and marine environments and the distinctive verti-
cal arrangement of sedimentary structures provide the intrinsic evidence to distin-
guish beach deposits from dunes as well as from shelf margin sand waves and tidal
bars. The external relationships that distinguish beaches and barrier islands are their
lateral facies relationships. Either lagoons or continental coastal plains lie immedi-
ately updip and the shallow subtidal zone of the open sea lies immediately
downdip.
Fundamental rock properties are intrinsic characteristics. Beach – dune succes-
sions also have extrinsic or external characteristics that help distinguish them from
other carbonate successions. For example, beach – dune successions are elongate
parallel to depositional strike and may extend for many miles. Barrier beaches may
be dissected along strike by tidal inlets and their accompanying tidal deltas. Beach
sequences formed during sea - level stillstands are only a few meters thick and may
extend seaward from the foredune or berm area 2 – 3 km to the toe of the lower