Abstract
Key petrophysical properties of reservoir sequences are determined by their individual
mineral compositions, and are routinely evaluated through the analysis of cores and
geophysical well logs. However, mineralogical studies are seldom incorporated in reservoir assessment. The objectives of the study were to investigate the influence of mineralogy on petrophysical properties of petroleum reservoir beds and the application of mineralogical studies in reservoir evaluation.
Mineralogical analyses were performed on core samples from the Plover Formation, the
principal reservoir sequence in the Northwest Shelf area of Australia, intersected in two
separate wells in the Laminaria petroleum field. The techniques used included X-ray powder
and oriented-aggregate analysis, optical microscopy and whole rock geochemistry.
Quantification of each mineral phase based on whole-rock powder data was performed using
the Rietveld-based Siroquant technique. Results from the Siroquant assay were used as an
indicator of mineralogy for the individual samples and were compared with core plug and
geophysical log data. X-ray micro-tomography analysis of selected samples was also
performed.
The reservoir sequences in both wells were sand-dominated, consisted mostly of quartz, clay
mineral matrix and cement of silica, pyrite or calcite. The abundance of clay minerals
increased in the shale and shaly sandstone intervals. Comparison of mineralogical and core
plug analyses of samples from the same depths showed that the down-hole variations in
porosity, permeability, grain density and radioactivity were accompanied by changes in
mineralogy.
Higher proportion of clay minerals in shales was indicated by higher gamma log signals. The
gamma log may be taken as an indicator of shaliness only in intervals where kaolinite is
proportional to the quantity of illitic clays. Sonic log and neutron log porosity values are
comparable with core plug porosity data in sandstone intervals. However, clay minerals
increase the sonic log response, thereby increasing porosity in shaly intervals. Clay minerals
tend to decrease the neutron log response causing higher porosity indication in shales,
similar to that expected in sandstones. Routine density log analysis underestimated porosity
values because of the contribution of dense minerals to the bulk density of the formation.
Use of laboratory determined grain and fluid densities resulted in improved density log
porosity compared to core porosity. X-ray tomography analysis revealed an overall positive
correlation between mineralogy and porosity data.
Routine geophysical log evaluation revealed inconsistent results when compared to core
analysis data because of the influence of minerals on various logs. It is essential that
mineralogical studies be included in reservoir assessment. X-ray tomography may provide an
alternative approach in evaluating porosity and mineralogy.