Page 27 - Geology of Carbonate Reservoirs

8 INTRODUCTION

distribution, pore throat characteristics, and height of the hydrocarbon column
above free water in a reservoir. Capillary pressure data are also used in estimating

connectivity, fluid recovery efficiency, reservoir quality, thickness of the hydrocarbon
column in reservoirs, and in evaluating seal capacity.
Data from indirect measurements made with wireline logs are the “ bread and
butter ” of everyday work in subsurface geology. Some logs provide much more than
others, however. Laboratory measurements of nuclear magnetic resonance (NMR)
responses to different pore characteristics in carbonates are helping us develop
better methods for interpreting reservoir quality from new - generation NMR log
data. Of course, the traditional wireline log data are analyzed to extract useful
information about reservoirs, traps, and seals. Of these logs, gamma ray, acoustic,
resistivity, photoelectric effect, neutron, and density logs are typically included in
modern logging runs. Even ordinary mud logs with drilling times, mud characteris-
tics, and basic “ lith log ” descriptions of borehole cuttings can offer valuable informa-
tion to the well analyst. Other indirect measurements of reservoir characteristics
include reflection seismic traces that can be interpreted to reveal trap geometry and,

in some cases, reservoir rock attributes. Borehole testing such as pressure transient
tests helps confirm the presence or absence of fracture permeability. When used

with cores or imaging logs, fractured reservoirs can be classified according to the
relative contribution of fractures to overall reservoir performance.
Gravity and magnetic measurements are less commonly used in exploring for
carbonate reservoirs than for terrigenous sandstone reservoirs, probably because
gravity anomalies associated with shallow salt domes are comparatively easy to
identify and are relatively common in sand – shale geological provinces such as the
Gulf of Mexico onshore areas. Magnetic anomalies may sometimes be useful in
identifying paleostructural trends associated with depositional carbonate reservoir
trends that overlie magnetically susceptible basement rocks.
Reflection seismology is a widely used and fundamental tool in exploration for

carbonate reservoirs. In some cases, particularly with today ’ s high - resolution, three -
dimensional (3D) seismic data, seismic attribute analyses might be used in fi eld
development where the analyses can help discriminate between reservoir and non-
reservoir zones. High porosity zones, high fracture intensity, or strong contrasts in

fluid content might be detectable as a seismic wave characteristic, or attribute.
Seismic data usually provide the essential information to identify structural anoma-
lies and can sometimes pinpoint stratigraphic anomalies if the anomalies are large
enough. A rule of thumb is that the target reservoir zone must be thicker than one -
quarter of the seismic wavelength. Also, the impedance contrast between reservoir
and nonreservoir horizons must be strong enough for the difference between them
to be detected. Seismic records are commonly helpful in detecting fractured reser-
voirs and a substantial literature exists on this subject. Suggestions for additional
reading are presented at the end of each chapter of this book.
Analyses of these different kinds of data help to determine the size and shape
of the reservoir body, the spatial distribution of the pore types within it, and how
the pore system interacts with reservoir fluids. Evaluation of depositional charac-

teristics draws from carbonate sedimentology to utilize depositional sequences and
lithofacies in establishing links between depositional setting, sedimentary processes,

and pore types. Paleostructure can have a strong influence on depositional facies, as
is easily seen by examining depositional facies maps overlain on interval isopach