Diagenetic controls on evolution of porosity in the Permian Illawarra Coal Measures, Southern Sydney Basin, Australia

Abstract

Petrography of the Illawarra Coal Measures was described by thin section, scanning electron microscope and X-ray diffraction techniques. Sandstone composition of the Illawarra Coal Measures includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals. The Illawarra Coal Measures consists of litharenite and rarely sublitharenite. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present in the Illawarra Coal Measures. Thin-section porosity occurs in all units, particularly the coarse-grained deposits. Secondary porosity is more common than primary porosity in the Illawarra Coal Measures. Thin sections and scanning electron microscopy were used to describe diagenetic alterations and their influence on porosity in Illawarra Coal Measures. These diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. In the Illawarra Coal Measures, shale, siltstone and fine-grained sandstone are common in the Wilton Formation, Bargo Claystone, Darkes Forest Sandstone, Allans Creek Formation, Unnamed Member Three and Unnamed Member Two. These units have low porosity and form lithological seals and confining layers. Medium- and coarse-grained sandstone is common in the Kembla Sandstone, Lawrence Sandstone Member and Loddon Sandstone and contains low porosity. Thus, gas or water may be present in these formations. The Tongarra Coal, Allans Creek Coal Member, Wongawilli Coal, Hargrave Coal Member, Cape Horn Coal Member, Balgownie Coal Member and Bulli Coal are the main sources for gas in the Illawarra Coal Measures.