Secondary porosity as a play concept in the Cooper Basin, Australia: Examples from the Swan Lake and Crowsnest Fields
Warrington, Monique A.
Honours Degree, 2004
University of Adelaide
Strategies and research programs within the Cooper Basin are oriented to improve play definition and exploration opportunities. Diagenetic sweet spots, associated with secondary porosity, provide a new and innovative play type for the Cooper Basin. Anomalous reservoir porosity (20%) was previously discovered in the Swan Lake 5 production well (Patchawarra Formation), in an area where 12% porosity is expected. Such sweet spots represent areas where high performance reservoirs can exist at depths greater than currently thought to be viable.
Optical microscopy, scanning electron microscopy (SEM) and cathodoluminescence (CL) techniques were used to determine the presence and associated features of secondary porosity in the Swan Lake, Crowsnest and Dullingari Fields. Thirty-five representative core and drill cuttings samples were analysed from within the Patchawarra and Tirrawarra Formations. A technique utilising drill cuttings was established to accurately determine grain surface textures and diagenetic history. This provides a cost-effective and reliable analytical method for assessing secondary porosity.
Framework grains dominantly comprise detrital and authigenic quartz (51-87%), kaolin (<13%) and matrix (<10%), with minor amounts of carbonate, chert, rock fragments and mica. No remnant feldspar was observed but its original presence is inferred from kaolin pseduomorphs of framework grains. Three types of diagenetic alteration were recognised in the rocks of the Swan Lake and Crowsnest Fields — mechanical and chemical compaction, cementation and grain alteration. Three phases of quartz cement were apparent, often encompassing late stage etched pits. Carbonate and kaolin precipitation occurred intermittently with quartz cementation, succeeded by illite replacement.
Microscopic and cathodoluminescence analyses indicate extensive secondary porosity (<13%) in the Swan Lake and Crowsnest Fields. The prominent secondary porosity features observed within the sediment are oversized pores and partially dissolved grains and cements.
The development of secondary porosity in the Swan Lake and Crowsnest Fields is the result of carbonate and/or aluminosilicate dissolution. Precipitation of carbonate minerals onto the boundaries of quartz grains causes pitting and an impediment to co-genetic quartz overgrowth. The subsequent removal of the carbonate creates voids and embayments on the quartz grains. Secondary porosity occurs on potential migration pathways away from deeper and hotter areas of the Nappamerri Trough. Kerogen maturation, associated with a major increase in geothermal gradient during the Cretaceous (90 Ma), generated significant amounts of carboxylic acids which triggered the dissolution event.