Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stylolites; Their Origin and Impact on Reservoir Quality
(1) Ammtec Ltd, Perth, WA, Australia.
(2) Applied Geology, Curtin University, Perth, WA, Australia.
Micro-structural (SEM-CL and Electron Backscatter Diffraction (EBSD)) and detailed mineralogical analyses (optical microscopy and automated mineral analysis) of a number of stylolites have been undertaken in order to better understand the relationship to other deformation features and associated fluid flow processes that influence the development of reservoir quality. Stylolites examined from the Browse and Perth Basins, Australia, are all characterised by suturing of quartz grains, presence of neoformed muscovite or illite and concentrations of organic carbon, detrital zircon and rutile and authigenic anatase. The presence of the detrital insoluble material (zircon, rutile, organic carbon) at orders of magnitude more volumes than in adjacent sandstone suggests that it is residual from the removal of large volumes of quartz by pressure solution. The authigenic anatase is confined in occurrence to the stylolites. A likely source of Titanium for the anatase growth is rutile needles frequently observed in adjacent detrital quartz grains. This also implies the removal of large volumes of quartz by pressure solution. The resultant silica in solution is often invoked as a major source for quartz overgrowths but requires a fluid flow pathway between source and sink. Some stylolites are characterised by crystallisation of pyrite that geochemically seems to be of hydrothermal rather than sedimentary/diagenetic origin. The stylolite surfaces must thus have experienced some fluid flow history.
In cored intervals the stylolites are observed to occur in clusters that are closely paralleled with clusters of other deformation features (fractures, mineralised fractures and faults). Some of the fractures and faults have mineralogical features in common with the stylolites, including the pyrite mineralisation and the presence of heavy minerals and organic carbon. The latter appear to have been entrained from intersections with stylolites. The pyrite mineralisation indicates that there are three dimensional fluid flow pathways connecting the bedding parallel stylolites with the near orthogonal brittle deformation features enabling the source to sink pathway for quartz diagenesis.