The influence of diagenetic alterations on porosity in the Triassic Narrabeen Group, Southern Sydney Basin, Australia

Abstract

Petrography of the Triassic Narrabeen Group (Southern Sydney Basin of Australia) was described by thin section, scanning electron microscope and X-ray diffraction techniques. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present. In thin section porosity is visible in all units, particularly the coarse-grained deposits. Secondary porosity is greater than the primary porosity. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Compaction occurred in all units during diagenesis. Both early and late diagenetic carbonate cement is important in the Narrabeen Group. Pore-filling carbonate cement reduced porosity whereas dissolution of carbonate resulted in secondary porosity. Authigenic clay minerals are 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. Dissolution of unstable feldspar and lithic grains provided secondary porosity. In the Narrabeen Group, shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone, Bald Hill Claystone and Newport Formation, forming lithological seals and confining layers. Minor sandstone beds occur in the Wombarra Claystone and contain more porosity. Medium- and coarse-grained sandstone is common in the Coalcliff Sandstone, Scarborough Sandstone and Bulgo Sandstone and shows low to moderate porosity. Thus, these sandstone units probably contain gas or water.