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

Abstract

The in flu ence of diagenetic al ter ations on po ros ity in the Tri as sic Narra been Group, South ern Syd ney Ba sin, Aus tra lia Pe trog ra phy of the Tri as sic Narra been Group (South ern Syd ney Ba sin of Aus tra lia) was de scribed by thin sec tion, scan ning elec tron mi cro scope and X-ray dif frac tion tech niques. Sand stone com po si tion in cludes mostly lithic grains with mi nor quartz, feld spar, mica and heavy min er als; litharenites and sublitharenites are com mon and quartzarenites are rare. Quartz in cludes monocrystalline and polycrystalline grains while the feld spar in cludes both K-feld spar and plagioclase. Vol ca nic, sed i mentary and chert rock frag ments are pres ent. In thin sec tion po ros ity is vis i ble in all units, par tic u larly the coarse-grained de posits. Sec ond ary po ros ity is greater than the pri mary po ros ity. The diagenetic al ter ations in clude com pac tion, quartz overgrowths, authigenic clay min er als, car bon ate ce ment and authigenic feld spar. Com pac tion oc curred in all units dur ing diagenesis. Both early and late diagenetic car bon ate ce ment is im por tant in the Narra been Group. Pore-fill ing car bon ate cement re duced po ros ity whereas dis so lu tion of car bon ate re sulted in sec ond ary po ros ity. Authigenic clay min er als are widespread in all units fill ing pores and oc cur ring as grain-coat ings on de tri tal and authigenic grains. Where they coat quartz grains they pre serve po ros ity by pre vent ing growth of quartz overgrowths. Dis so lu tion of un sta ble feld spar and lithic grains pro vided sec ond ary po ros ity. In the Narra been Group, shale and siltstone are abun dant in the Wom barra Claystone, Stanwell Park Claystone, Bald Hill Claystone and New port For ma tion, form ing lithological seals and con fin ing lay ers. Mi nor sandstone beds oc cur in the Wom barra Claystone and con tain more po ros ity. Me dium-and coarse-grained sand stone is com mon in the Coal cliff Sand stone, Scarborough Sand stone and Bulgo Sand stone and shows low to mod er ate po ros ity. Thus, these sand stone units prob a bly con tain gas or wa ter.Key words: pe trog ra phy, diagenesis, pri mary po ros ity, sec ond ary po ros ity, quartz over growth, car bon ate ce ment. INTRODUCTIONIn the Syd ney Ba sin (Aus tra lia), the Narra been Group is the low er most of three Tri as sic rock units and is lo cated be tween the Up per Perm ian Illa warra Coal Mea sures and the Hawkesbury Sand stone (Ward, 1971a). Fine-grained sed i ment in the Narra been Group was pre vi ously stud ied by sev eral authors (e.g., Baker, 1956;Loughnan et al., 1964Loughnan et al., , 1974Goldbery and Hol land, 1973;Retallack, 1977). Ward (1972) noted quartzose, quartz-lithic and vol ca nic sand stones as the three sed i ment suites in the Narra been Group. Prov e nance mark ers of fore land ba sin-fill sed i ments were de ter mined by Cow an (1993). The com po si tion and tex ture of the Narra been Group were de scribed by McElroy (1954). De scrip tions of the pe trology in the N...

Direct stable isotope porewater equilibration and identification of groundwater processes in heterogeneous sedimentary rock

“…The correlation between stable isotope and Cl concentration is not significant. The change in Cl is likely to be reduced by sorption onto Al and Mg-illite which is present in intragranular pore spaces (Al Gahtani, 2013). Towards the bottom of this unit, density decreases, porosity increases and gamma is higher, which indicates tighter material packing and higher clay content.…”

“…Within BGSS the porewater concentrations in Zone 3 are around 100 mg/L with an increase with depth in Zone 4 to about 600 mg/L. A possible explanation for the relatively low Cl concentrations in Zone 3 might be that recharge occurs laterally where BGSS outcrops at a distance several kilometres from site and the presence of higher permeability thin interlayers and dissolved lithic grains (Al Gahtani, 2013), as determined by ITRAX XRF scanning and density log, enable preferential groundwater flow and advective mass transport.…”

“…Where sandstone pores are filled with illite, kaolinite, chlorite, clay-limonite (Ward, 1970(Ward, , 1980Bowman, 1974) and iron oxides the porosity will be significantly reduced. The exception is the iron oxide coating of siderite, or clay coating of quartz that inhibits its growth and therefore preserves the primary porosity (Al Gahtani, 2013). An increase in porosity can result from dissolution of carbonate cement within pores.…”

Sedimentología, trazas fósiles y evolución diagenética de la Formación Calafate, Cretácico Superior de la Cuenca Austral, Provincia de Santa Cruz, Argentina

“…Por todo lo anterior se concluye que la diagénesis es determinante en la evolución de la porosidad de los reservorios clásticos o convencionales durante la etapa de soterramiento (Schmid et al, 2004;Al Gahtani, 2013;Nguyen et al, 2013;Wilson et al, 2013;Henares et al, 2014;Wang et al, 2017), asimismo, la sedimentación y la naturaleza heterogénea de las areniscas pueden contribuir considerablemente en la variación y distribución de la porosidad y permeabilidad en las sedimentitas (Bloch, 1991;Dutton y Diggs, 1992).…”