Geology Reference
In-Depth Information
mary porosity and early generated secondary anhydrite
cementation. Reservoir potential may be restored by
karstification and fracturing.
(4)
Non-evaporitic carbonate sequences associated
with topographic high/unconformity, platform-margin
buildups, or faults and fractures.
Dolomites are the
results of the reflux of slightly evaporated seawater
(high microintercrystalline porosity resulting from early
diagenetic dolomitization of lime-mud-rich sediments)
or burial-derived dolomitizing fluids (leading to
coarsely crystalline sucrosic dolomite; solution and
intercrystalline porosity is common because of an ex-
cess of solution over precipitation during the replace-
ment of limestone by dolomite). This category also in-
cludes
hydrothermal dolomites
that are the focus of
current reservoir research.
17. 1.4 Methods
Hydrocarbon exploration uses seismic data, log re-
sponse, the investigation of cores and cuttings and out-
crop analog studies. A subsurface depositional model
is constructed in three dimensions, using rock type el-
ements, depositional sequence, geometry and bound-
ing lithologies (Cant 1992). The recognition of se-
quences in seismic data leads to an understanding of
facies distribution, which in turn leads to more accu-
rate basin models, making the prediction of criteria re-
quired for hydrocarbon entrapment, such as reservoir,
seal, source and migration pathway, more realistic and
dependable.
17.1.4.1 Seismic Interpretation
•
Fractured reservoirs.
Many carbonate reservoirs are
hosts to extensive fracture networks characterized by
sub-meter scale fracture zones that may penetrate ver-
tically across lithologic and facies boundaries. Con-
nected laterally, these fractures give rise to good reser-
voir permeability. Fracture and breccia porosity pro-
vide the possibility of extracting hydrocarbons for oth-
erwise poor-quality reservoir rocks with low porosi-
ties (Nelson et al. 2000). Natural fracture patterns con-
trol producibility and permeability systems. Permeabil-
ity is provided by joints and faults crossed by voids
and microcracks (Fig. 17.4). Analysis of natural frac-
tures must regard fracture cementation, size, fracture
continuity, orientation and fracture density. The origin
of fractures in carbonate reservoirs is related to fold-
ing, desiccation or unloading during subaerial expo-
sure. Fracture intensity is higher in dolomites than in
limestones and also appears to be related to the crystal
size of the matrix. Fine-grained dolomites often are
more intensively fractured than coarse-grained dolo-
mites. Examples are the Ordovician dolomite reservoir
in Michigan, the Late Cretaceous La Paz field in Ven-
ezuela, where strike-slip tectonics associated with faults
and fracturing create the producing system, the frac-
tured chalk of the North Sea basin, and the Oligocene
Asmari limestone in Iran.
Interpretation of 3D seismic data allows analyzing the
depositional setting of carbonate reservoirs. In case of
shallow reservoirs and/or high-fold seismic data acquis-
ition, depositional geometries and a low-resolution se-
quence stratigraphic analysis can be performed. Com-
parison of seismic geometries with analog examples is
greatly supporting reservoir characterization.
17.1.4.2 Log Response
Log responses are used to infer penetrated lithol-
ogy, properties and the depositional character of rocks.
Log character allows correlations and the construction
of models showing the vertical trends in stratal thick-
ness (fining or thickening upward) and geometry.
Wireline logs provide the most powerful tool for pre-
dicting downhole lithology from physical properties.
Petrophysical logs are measurements of various elec-
trical, nuclear and acoustical properties, recorded as a
function of depth.
Commonly used logs are:
•
Sonic log.
Measures the speed at which sound trav-
els in rock units. It is related to both the porosity
and lithology of the rock being measured. If the li-
thology is known, the log can be used in determin-
ing porosity.
•
Hydrocarbon migration.
Many carbonate reservoirs
are influenced by diagenesis which is contemporane-
ous with hydrocarbon migration (e.g. Kharaib Forma-
tion, Middle East; Bab Asab, Sdahail Field, UAE). This
is demonstrated by gradual deterioration of reservoir
properties from crest to flank across facies boundaries
(Grötsch et al. 1998). Diagenetic fluids also cause the
formation of bedding-parallel dense diagenetic bodies
with abundant stylolites at small-scale sedimentary
cycle boundaries.
•
Neutron log.
Measures the porosity of a rock, indi-
cating its response to the quantity of hydrogen
present in the rock unit.
•
Gamma ray log.
Records the radioactivity of a for-
mation and shaliness, clay content and presence of
organic matter.
The most common logs used in carbonate reservoirs
are the gamma-ray-neutron log and the compensated
neutron-density log that distinguish limestone, dolo-
