Page 152 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 133

Scholle (1977) illustrated the difference between porosity in the Austin Chalk and
that in a typical North Sea chalk. Although the Austin Chalk has low matrix porosity,
it is a prolific producer from fractured reservoirs that extend across much of south

central Texas. Fractured reservoirs are important in North Sea chalks that have
diagenetically reduced matrix porosity. In some chalks with extensive flint beds, the

more brittle flint exhibits more intense fracturing than the chalk.

5.2.13 Ideal Depositional Successions Illustrated
The ideal depositional successions and their typical locations on ramps, rimmed

shelves, and open shelves are illustrated in Figures 5.8 – 5.14 . All of the ideal succes-
sions are drawn to an arbitrary thickness scale of about 10 m, the length of a standard
core barrel. Some additional comments about the drawings may be helpful (see
Figure 5.15 ). Dunes are normally part of the beach succession, but in cases where
sediment supply is limited dunes may not be present. The ideal depositional succes-
sion for dunes is therefore not always present. Attached beaches extend seaward
from the nonmarine mainland shore but barrier island beaches extend seaward from
an island shore with a lagoon separating the island from the mainland. Lagoonal
successions may or may not contain storm washover deposits of poorly sorted,

May/may
not be
present
Dune
Upper
Shoreface

Middle
Shoreface

Lower
Shoreface

Shallow
Subtidal

Figure 5.8 Ideal depositional succession of a beach – dune or barrier island environment.

Barrier islands are simply beach – dune successions separated from the mainland shore by a
lagoon. Each of the following successions is drawn in the form of an idealized borehole core
that exhibits easily recognizable vertical sequences of sedimentary structures. Note that beach
successions can be divided into lower, middle, and upper shoreface segments. Dunes may or
may not be present at the top of the succession. If present, they exhibit meter - scale inclined
or spillover beds that pass upward to smaller trough crossbeds and ripples. Plant roots
(rhizocretions) or land animal tracks, trails, and burrows may mark the tops of dunes.