Page 133 - Geology of Carbonate Reservoirs

114 DEPOSITIONAL CARBONATE RESERVOIRS

NORTH
7 6 5 4 3 2 1
150’

Figure 5.1 Sketch of a Pleistocene dune on New Providence Island, Bahamas showing large -

scale spillover and foreset beds in the lower part of the dune passing upward to smaller scale
trough crossbeds. Note the change in character from large, convex - upward and inclined
bedding at the bottom to smaller scale, concave - upward (troughs) toward the top of the dune.
Imagine what one might see in a 15 - cm diameter × 10 - meter long borehole core through such
a large feature and if the entire body of the dune were above the oil – water contact, estimate
the OOIP volume in only the one dune. (Adapted from an illustration by Ahlbrandt in
McKee and Ward (1983) .)

near the base, intermediate to smaller scale, trough crossbeds in midsections, and
small scale ripple marks in upper sections is useful in developing model successions
for carbonate dunes. Ideal dune sequences are divisible into three components: (1)
large - scale tabular and spillover crossbeds at the base, (2) intermediate scale, trough
crossbeds in the middle, and (3) small - scale trough crossbeds with asymmetrical
ripples at the crest (Figure 5.1 ).
Although they are not mechanically formed sedimentary structures, diagenetic
characteristics can be very useful to distinguish dunes from other crossbedded
grainstone successions. Dune deposits are sometimes distinguishable in thin section
by the presence of vadose diagenetic features. Chapter 6 deals exclusively with dia-
genetic reservoirs, but vadose diagenesis is discussed briefly here because this dis-

tinctive attribute is a tool to aid in recognizing eolianites. Vadose diagenesis includes
the chemical changes that take place in rocks above the water table. Dunes may rise
many meters above the fresh or marine water table on the coastline so that the only
time water percolates downward through the dune pores is during and after a rain.
Usually the pore walls and pore throats in dune rocks are dry. As rain falls and

passes through the pores, they become temporarily filled with a mixture of water
and air. The surface tension of water causes it to migrate as a film along grain sur-

faces until the film reaches a point of contact between grains. There, a meniscus

forms as the water film bridges from one grain to another. Over time, as the water
is saturated with CaCO 3 , it precipitates cement in the form of the meniscus at grain
contacts (Figure 5.2 ). In some instances, pendulous cements form where droplets of
water clung to the lower surface of grains. These meniscus and pendulous cements
may be recognizable in ancient dunes and provide another intrinsic property of
eolian facies to simplify the task of distinguishing dunes from other carbonate
grainstone buildups.
Beach and dune biota vary depending on climate and environment. Tropical
environments produce aragonitic constituents and phototrophs. Temperate environ-