Year

2012

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Petrography of the Illawarra Coal Measures, Narrabeen Group and Hawkesbury Sandstone was described by thin section, scanning electron microscope and X-ray diffraction techniques. Sandstone composition of the Illawarra Coal Measures and Narrabeen Group include mostly lithic grains with minor quartz, feldspar, mica and heavy minerals. Quartz is dominant in the Hawkesbury Sandstone which contains very minor feldspar, lithic grains, mica and heavy minerals. The Illawarra Coal Measures consists of litharenite and rarely sublitharenite, whereas the Narrabeen Group is litharenite and sublitharenite with rare quartzarenite and the Hawkesbury Sandstone is quartzarenite to sublitharenite. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present, particularly in the Illawarra Coal Measures and Narrabeen Group.

Thin section porosity occurs in all units, particularly the coarse-grained deposits. The Hawkesbury Sandstone has more visible porosity than the Narrabeen Group or Illawarra Coal Measures. Primary porosity is more common than secondary porosity in the Hawkesbury Sandstone whereas secondary porosity is greater in the Narrabeen Group and Illawarra Coal Measures.

Thin sections and scanning electron microscopy were used to describe diagenetic alterations and their influence on porosity in Illawarra Coal Measures, Narrabeen Group and Hawkesbury Sandstone. These diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement, authigenic feldspar, authigenic pyrite and iron oxide cement. Compaction occurred in all units during throughout diagenesis. Mechanical compaction reduced thin section porosity in the Illawarra Coal Measures and Narrabeen Group whereas the influence of chemical compaction on thin section porosity was greater in the Hawkesbury Sandstone. Both early and late diagenetic carbonate cement is importantant in the Illawarra Coal Measures and Narrabeen Group. Pore-filling carbonate cement reduced porosity whereas dissolution of carbonate resulted in secondary porosity. Authigenic clay minerals are the widespread in all units filling pores and occurring as grain-coatings on detrital and authigenic grains. Where they coat quartz grains they preserve porosity by preventing growth of quartz overgrowths. Quartz overgrowths are only common in the Hawkesbury Sandstone where they have a strong influence on porosity. Dissolution of unstable feldspar and lithic grains provides secondary porosity in the Illawarra Coal Measures and Narrabeen Group. This process is absent in the Hawkesbury Sandstone.

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 permeability and form lithological seals and confining layers. Medium- and coarse-grained sandstone is common in the 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.

In the Narrabeen Group, shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone, Bald Hill Claystone and Newport Formation and form lithological seals and confining layers. Minor sandstone beds occur in the Wombarra Claystone and contain more porosity and permeability. Medium- and coarse-grained sandstone is common in the Coalcliff Sandstone, Scarborough Sandstone and Bulgo Sandstone and shows low to moderate thin section porosity. Thus, these sandstone unis probably contain gas or water.

The Hawkesbury Sandstone is characterized by primary and secondary porosity, thus it has good groundwater storage and flow potential. Medium- and coarse-grained sandstone beds are common in the Hawkesbury Sandstone but it shows vertical variations in porosity. A few impermeable shale and siltstone units occur in the Hawkesbury Sandstone forming local confining layers.

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