Henderson  Petrophysics

Wireline Log Response in a Heterogeneous Environment
An Example Using a Model of Brecciated Basement

Introduction:

Conventional log analysis methods are rarely useful in fractured and/or highly heterogenous formations.

An EXCEL spreadsheet was used to develop a simple model of high resistivity basement rock intersected by low resistivity brecciated and diagenetically altered zones.  This model was used to investigate features resulting from logging the section with tools having very different volumetric response functions.  Water saturation (Sw) was calculated using the model log curves and conventional log analysis equations.

The Model:

The model in the left portion of the accompanying figures is a highly simplified 2-dimensional depiction of a fractured/brecciated basement rock. The host rock (blue shading) is modelled as having very high resistivity. The fractured zones (red shading) have low resistivity resulting from both tectonic induced fracture porosity development and to diagenesis of the brecciated zone.

The model does not contain any hydrocarbons.

The deep resistivity curve in the model has poor volumetric resolution and response is influenced by a large rock volume (large shaded area in the figures).  This curve is analagous to conventional Induction or Laterolog tools.

The model also contains a "log" that responds to rock properties in a relatively small volume (small shaded area in the figures). This curve is analagous to a conventional density or sonic logging tool used to estimate porosity.

The model does not contains hydrocarbons so the "porosity" log response can be converted to an apparent wet resistivity curve (Ro).  This is analagous to conventional log analysis methods where a porosity curve is used to determine the expected Ro.  The Ro curve has the volumetric resolution of the porosity curve.

The model therefore contains the equivalent of two resistivity logs with each having very different volumetric response functions.

Water saturation (Sw) in the model was calculated using the Archie equation (the square root of the ratio Ro to deep resistivity). The right portion of Figures 1 and 2 shows the calculated Sw curves.

Discussion:

In a heterogeneous environment, the log analysis results are often controlled by lithology not seen close to the borehole.  In our fractured basement model volumetrically dominant high resistivity matrix or low resitivity fractures may not be "seen" by the high resolution logging tools.

Since the volume fractions of the two rock components away from the wellbore cannot be determined, it is not possible to filter the high resolution apparent resistivity to match the volumetric response of the lower resolution measurement.

The highest apparent hydrocarbon saturation (1 - Sw) occurs where the low resistivity zones are intersected by, or close to, the wellbore.

The log response for the well in Figure 1 results in most of the interval having calculated Sw of more than 100 percent.  This is not the result of poor parameter selection but from the combination of log measurements with different volumetric response functions and the influence of litholgy not seen in the wellbore.  The high resolution log "sees" a high resistivity environment wheras the low resolution tool is influenced by low resistivity rock not observed near the wellbore.  Since expected wet resistivity (Ro) is higher than Rt calculated Sw is more than 100 percent.

Calculated Sw in Figure 2 is mostly less than 100 percent because the high resolution log "sees" a high porosity/low resistivity environment wheras the low resolution log investigates a more highly resistive environment.

In heterogeneous rock, log analysis results will have poor coherence - That is they do not readily match the responses we expect from conventional reservoirs having good horizontal continuity of rock properties.  For example, in conventional hydrocarbon-bearing reservoirs we expect to have an increase in resistivity associated with an increase in porosity.  In Figure 1 "deep" resistivity actually drops across the apparent hydrocarbon zone.  Likewise, the apparent hydrocarbon zone near the top of Figure 2 shows no increase in resistivity.

In highly heterogeneous environments it is often very difficult or impossible to normalise log analysis results.  Normalisation is the process of adjusting log analysis parameters to calculate 100 percent Sw in known water zones.  In heterogenoeus environments, the log response is influenced by rock properties not seen at, or close to, the wellbore.

	Blue shading: High resistivity host rock Red shading: Low resistivity fractured/brecciated zones Red curve: Low resolution resistivity Black curve: High resolution porosity converted to resistivity  Blue curve: Calculated Archie Sw  Note: Calculated Sw is mostly higher than 100 percent.
Figure 1: Model well intersects few fractured/brecciated zones

 

	Blue shading: High resistivity host rock Red shading: Low resistivity fractured/brecciated zones Red curve: Low resolution resistivity Black curve: High resolution porosity converted to resistivity Blue curve: Calculated Archie Sw Note: Calculated Sw is mostly less than 100 percent in fracture zones.
Figure 2: Model well intersects fractured/brecciated zones