Geoscience Reference
In-Depth Information
relatively large change
in impedance related to porosity
relatively small change
in impedance related to pore
geometry and fluid fill
First order trend of AI and PHI
impedance of
overlying Shale
Top Chalk = Trough
Top Chalk = Peak
Porosity
35%
Fig. 5.7
values from the Top Chalk amplitude. A similar approach can be adopted to deduce
sand porosity from seismic. However, this method assumes that the rock above the
interface (e.g. a caprock shale) does not vary laterally. If there is a risk that it does,
then the inversion methods discussed in
chapter 6
are a better way to estimate reservoir
porosity.
A completely different sort of DHI is the gas chimney. This occurs where gas has
leaked from a deeper level into the overburden, typically along a fault plane, but the
overburden is mainly shale with limited permeable zones (e.g. silts). The result is a
diffuse cloud of gas-bearing material, typically with low saturations. There may be a
few high-amplitude gas sand reflections at the top or within the body of the cloud, but
in general scattering and absorption cause amplitudes to be much reduced below and
within it, so that amplitude measurements are usually meaningless. There is often an
apparent sag in TWT of reflectors below the cloud, due to the velocity decrease in the
gas-bearing layers; this can cause great difficulty for accurate structural mapping in
depth. Shear-wave data, which are almost unaffected by the gas, may be the best way to
image the horizons below the cloud. This is often important because although the gas
saturations within the chimney are too low to be of any economic value, the presence
of the chimney points to the possible presence of a leaking trap below it.
Tuning is a complication for the study of amplitudes. As we saw in
section 4.1
, am-
plitudes from a thin bed can be greater or smaller than the value expected for a single
interface, depending on the thickness of the bed relative to the seismic wavelength. It
