Integration of Rock Typing with Petrophysics in the Cooper Basin, Australia
Honours Degree 2002
University of Adelaide
Rock typing is a method of semi-quantitatively describing a rock’s reservoir qualities from core or cuttings, using a high powered optical microscope. Attributes such as clay content, consolidation, grain relief and sorting are visually estimated, with the end result being the placement of a sample in one of three major rock types, equivalent to specific air permeability ranges.
The Archie equation is a well known method of determining water saturation from formation resistivity logs. Its accuracy, however, is dependent on a number of factors, one of which is the cementation exponent, m. If an inaccurate m value is used, a significant error can occur in the calculated water saturation, resulting in an under or over estimation of oil and gas reserves.
Twenty-nine samples from the Tirrawarra Sandstone and the Patchawarra Formation of the Cooper Basin were rock typed, in order to determine if an accurate m value can be estimated through the rock typing process. All samples had known m and porosity values, previously determined from special core analysis. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to aid in the understanding of each sample’s pore geometry and clay content, both of which play crucial roles in the determination of m.
Relationships which should exist between rock typing estimates were found, indicating estimates of basic petrological attributes (eg. quartz cement, total porosity) are consistent. Compared with XRD determinations, an over estimation of illite often occurred when rock typing. A probable cause of this is the growth form of the clay in question. Both illite and kaolin occur as rock fragment alteration products. In this form they are fine grained and intergrown, as determined from SEM imagery. This results in the inability to distinguish between the two clays using the optical microscope alone.
All plots of textural parameters involved applying an amplifying index to the textural property in question. This was done in order to increase the sample set separation when plotted against m. If no amplifying index were applied, the sample set comprises only two families, making a trend with m impossible to observe.
It was not possible to estimate m using any one textural rock typing attribute alone. This is not only a reflection of the complexity of factors contributing towards m, but to a lesser extent, the semi-quantitative nature of visual estimation.
The plot of grain size sorting vs. m produced a complete scatter of points. A more effective way of quantifying grain size sorting was introduced using a sorting index which amplified the finer grained proportion of each framework.
Two trends were found when the sample set m values were plotted against this grain size sorting index. Rock types IA and IB were found to follow a trend of decreasing m with decreased sorting. A possible explanation for this trend involves quartz cementation, observed as overgrowths. Quartz overgrowths were shown to hinder compaction and help preserve macroporosity. One method of forming quartz cement is by releasing silica at quartz grain pressure contacts. With a decrease in sorting comes an increase in grain contacts per unit area. This would result in increased macroporosity and hence decreased m with decreased sorting, as observed.
The opposing trend found within rock types IC and ID, of increasing m with decreased sorting, can be explained by the presence of rock fragments. A decrease in sorting can be taken as an indicator of decreased textural maturity. This involves an increase in feldspar, which is usually removed from the framework in mature sandstones. An increase in rock fragments would result in an increase in subsequent replacement clay growth, with the end result being the observed relationship, of increased m with decreased sorting.
Judging from this sample set alone, the relation with the sorting index is strong enough to enable a reasonably accurate estimation of m to be made.