Page 136 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 117

shoreface, depending on the slope angle. Most barrier islands consist of repeated
depositional episodes stacked obliquely in the direction of sea - level advance or
retreat. Regressive beach and barrier complexes are normally thicker than trans-
gressive beaches because the rate of sedimentation is greater than the rate of
erosion during shoreline progradation. Stacked beach sequences may be tens of
meters thick in the aggregate.
Three offset barrier island sequences were described from the Jurassic Smack-
over Formation at Oaks Field, North Louisiana (Erwin et al., 1979 ). Each is about

0.8 km wide and extends about 3 km along depositional strike. These regressive bar-
riers range in thickness from about 10 to 15 m, they are laterally offset, and they are
enclosed in tight, siliciclastic facies that seal this stratigraphic trap. The barrier
islands consist of well - sorted oolitic grainstones that exhibit vadose, meteoric phre-
atic, and mixing - zone diagenesis — mainly cementation. The reservoir produces from
cement - reduced, intergranular porosity that ranges from about 1% to 15% (Erwin
et al. 1979 ). Thirteen of the 18 wells drilled in this trend were productive because
the explorationist projected the barrier island facies along depositional strike, mini-
mizing the number of dry holes.

5.2.3 Tidal-Flat and Lagoon Environments

Tidal flats are mud - dominated environments because they are protected from open
ocean waves and currents. They are sinks for lime mud transported to, or formed
in, sheltered environments behind barrier islands or wave - fi ltering rims on rimmed

shelves. Widely cited modern examples of tidal flats occur behind barrier islands in
the Persian Gulf (Figure 5.3 ) and on the leeward side of Andros Island in the
Bahamas (Figure 5.4 ). Ancient examples are present on the shallow interior of the
restricted platform that existed during deposition of the Permian San Andres For-
mation of Texas and New Mexico and over much of the tidally influenced or peritidal

setting that covered vast areas on the ramp - like North American Craton during

Cambro - Ordovician times. Tidal flats offer limited potential for the formation of

depositional porosity because they are mud - dominated systems. Tidal flats in arid
climates like the Persian Gulf usually include evaporite deposits; those in wet cli-

mates like Andros Island in the Bahamas do not. These low - energy flats lack waves
and strong currents that concentrate grainy sediments and they are inhospitable
places for reef organisms to grow; consequently, depositional porosity is limited to
small areas of grainy sediment accumulations in tidal channels and to zones where
fenestral porosity is common. Fenestral porosity is formed mainly by desiccation of
lime mud and, in part, by decomposition of algal mats. Tidal - fl at environments can
be subdivided into three zones: (1) lagoonal or open ocean environments at their
downdip margins — the always - wet, subtidal zone; (2) channeled areas between open
water and dry land — the intermittently wet, intertidal zone; and (3) the area above
normal high tide — the mostly dry, supratidal zone.
The subtidal or always - wet zone may be a lagoon ponded behind a barrier or a
shallow reach of the open ocean depending on the physiography of the tidal fl at.
The character of the always - wet zone sediments depends largely on climate and on
the volume of water exchanged between the lagoon and the open sea. Restricted
circulation and dry climate result in deposition of evaporites and relatively low rates
of lime - mud accumulation. Open circulation and wet climate limit the amount of