Page 25 - Geology of Carbonate Reservoirs

6 INTRODUCTION

around porous and permeable reservoir rocks. Traps are created by structural, strati-
graphic, hydrodynamic, or diagenetic processes. It is important to recognize that the
geometry of the reservoir – trap system may or may not correspond with present - day

structural configurations. Subsurface structures may form and then be deformed by
later episodes of tectonism. Ancient or paleo - highs may become present - day lows
or saddles. Likewise, paleo - lows may be tectonically elevated to exhibit present - day
structural closure and be “ high and dry. ” This is called structural inversion and it is
especially characteristic of basins with mobile salt or shale in the subsurface and in
some structural settings where multiple episodes of tectonism have changed older
structures.
Seals are the physical mechanisms that restrict fluids from flow out of the trap

and are usually described in terms of capillary pressures. Seals may extend along
the top, side, or bottom of the trap. Later we will define seals on the basis of the

high capillary pressure exhibited by the seal rock as compared to the reservoir rock.
These differences usually correspond to changes in rock type such as a change from
sandstone to siltstone or shale in the case of siliciclastics, or porous grainstone to
mudstone in carbonates. Most seals are not completely impermeable and will allow
some leakage of hydrocarbons. Less commonly, seals may consist of totally imper-

meable barriers to flow such as evaporite deposits.
Source rocks are rich in kerogen, the parent organic matter that produces petro-
leum hydrocarbons when it reaches a threshold temperature during burial and
thermal maturation. Source rocks usually consist of shales or lime mudstones that

were deposited in oxygen - deficient environments where lipid - rich organic matter
was preserved and converted to kerogen on further burial.
An integrated petroleum exploration program includes geophysical and geologi-
cal studies of basin stratigraphy and structure to isolate the regions where reservoir
rocks are most likely to be found, where structural, stratigraphic, or diagenetic pro-
cesses have formed traps and seals, and where the basin contains an ample thickness
of source rocks buried to a depth at which the temperature would have been high
enough to liberate hydrocarbons from kerogen. In the initial phases of exploration,
knowledge of how and where reservoir rocks form is critical; however, until a well
is drilled the reservoir remains a hypothetical entity. Trap configurations may be

identified as structural and stratigraphic anomalies, but without a hydrocarbon - fi lled
reservoir, they only beckon explorationists to drill dry holes. After a successful well
is drilled, the discovery is evaluated to predict the size and shape of the reservoir,
to estimate its economic value, and to formulate a development program. At this
stage, knowledge of reservoir characteristics is obviously the most important
consideration.

1.2 FINDING AND DEVELOPING CARBONATE RESERVOIRS

The main reasons to study carbonate reservoirs and aquifers are to learn more about
how to find, extract, and manage the oil, gas, usable water, or other resources they

contain. Carbonates hold about half of the world ’ s oil and gas, much of its ground-
water, and extensive deposits of metallic ores, yet of the relatively few texts on res-
ervoir geology, only a handful deal with carbonates. Carbonate reservoirs occur in
the subsurface so most of the data used to study them comes from borehole cores,