Triassic naturally fractured carbonates have proved to be very prolific in British Columbia. The fractures are of tectonic origin and were probably generated during the Laramide orogeny which created a series of compression type structures.
The formation evaluation was conducted with a dual-porosity model which allowed estimates of matrix and fracture porosities from logs. Water saturation in matrix and fractures were calculated with the use of a statistical procedure without previous knowledge of water resistivity.
In addition, the interpretation led to estimates of bitumen content, and percentages of siltstone, limestone and dolomite. The determination of the bitumen content is very important because bitumen can reduce effective permeability to gas quite significantly.
Well log analysis of Pardonet-Baldonne formations was carried out with the use of a dual-porosity model which takes into account the presence of natural fractures and matrix porosity1.
In this process, Archie's basic formation evaluation
Equations: Equation(1) (available in full paper)
And Equation(2) (available in full paper)
are combined as proposed by Pickett2,
Equation(3) (available in full paper)
To obtain: Equation(4) (available in full paper)
Or in the case of shaly formations:3.
Equation(5) (available in full paper)
In the above equations Ø stands for total porosity (fraction), m is the porosity exponent of the composite system of matrix and fractures, I is the resistivity index, R1 is the true resistivity, A is a function of shaliness, F is the formation factor, Rn is water resistivity at reservoir temperature, and "a" is a constant.
Analysis of equations 4 and 5 indicates that (I) a crossplot of porosity or the response of a porosity log vs R1 or (2) a crossplot of porosity vs R1/An, on log coordinate paper should result in a straight line with a slope of -m for zones with constant aRn and constant I (Figures I-A and I-B). For reservoirs with natural fractures, the value of m is smaller than the cementation exponent mb, determined from a matrix plug in the laboratory at simulated reservoir overburden conditions. This is reasonable because open unhealed fractures produce a reduction in tortuosity and cementation.
In many cases, it is possible to carry out the evaluation by crossplotting (Δt - Δtm) vs R1 or (px - Pb) vs R1 on log-log paper. In these crossplots At is sonic transit time as determined from the sonic log,Δtm, is sonic transit time at zero porosity, Ps is graindensity and Pb is bulk density.
When working with sonic logs in fractured carbonates the usual values of Δtm and Δtf no longer correspond to well defined physical parameters as they do in the case of the well known Wyllie formula. Rather, these parameters have to be determined specifically for each case. We have found through experience that the sonic log is very useful in enlarged boreholes.
The value of m is determined by calculating the slope of the line drawn through the lower points in the log-log plot. With the value of m determined from the crossplot and mb determined in the laboratory it is possible to complete the evaluation with respect to porosities.
