Diagenetic zonation and porosity development
Although a regional synthesis of porosity styles, and clay and cement diagenesis was not carried out as part of the current study, the summary of porosity styles within each diagenetic zone (Table 5) is based on an initial review of open-file petrographic reports and work by Rasmussen (1992), Tupper et al. (1994) and Laker (2000). Currently, only the ‘Kingia’ – High Cliff reservoirs are known to contain significant hydrocarbons in the deep illite zone (West Erregulla, Waitsia, Beharra Springs Deep). In these fields, grain-rimming chlorite and illite have helped preserve primary porosity from occlusion by quartz overgrowths. However, there is potential porosity development at shallower depths within the kaolinite–illite zone, where secondary dissolution porosity could be present, similar to the porosity style in the Dongara–Wagina reservoir of Senecio 3 and Irwin 1. This secondary porosity style is largely absent at greater depths due to occlusion by quartz overgrowths and clays. If chlorite is also present in the kaolinite–illite zone, then porosity development may be similar to the Dongara–Wagina reservoir at Beharra Springs, that is, grain-rimming chlorite and illite and secondary dissolution.
Porosity evolution in the Dongara–Wagina reservoir is primarily caused by compaction, cementation and dissolution events during burial diagenesis. These processes operated concurrently during burial and heating, alongside progressive illitization that created secondary porosity through grain dissolution, and with associated porosity occlusion by quartz overgrowths and carbonate cements. Consequently, at the time of hydrocarbon charge (Late Jurassic to mid-Cretaceous) reservoir position relative to the illitization window is expected to strongly influence the extent and style of porosity development and the authigenic mineral assemblage. For example, dependent on the burial history of an area during charge, a reservoir may have been either within the kaolinite zone, the kaolinite–illite zone or the illite zone. Also, during this time in the same area, the shallower Dongara–Wagina reservoir and the deeper ‘Kingia’ – High Cliff may have been in different diagenetic zones.
In those areas where reservoirs continued to be buried and heated (e.g. Dandaragan Trough), further illitization would have progressively modified porosity styles and clay and cement petrogenesis (e.g. illite zone in Irwin 1). Such changes, however, might not be recorded in shallower, cooler diagenetic zones where illitization had not commenced or was at an early stage (e.g. kaolinite zone in the Dongara field). Consequently, diagenetic sequences interpreted for wells will not be entirely comparable across the basin — some phases of grain-dissolution secondary porosity or porosity-occluding quartz overgrowths, for example, might not have an equivalent everywhere in the basin. As reservoirs transgressed the illitization window at different times during burial, the relative timing of available secondary porosity and hydrocarbon charge also could vary within the Dandaragan Trough. Further work is required to test these hypotheses, integrating all available diagenetic sequences against burialhistory models from across the basin. An illitization model for the northern Perth Basin therefore provides a framework to help interpret the diagenesis and porosity development of the of the Dongara–Wagina and ‘Kingia’ – High Cliff reservoirs. Although the formation of illite appears to have had minimal direct effect on influencing porosity (r = –0.08 and 0.04, respectively; Table 3), it helped reduce permeability (r = –0.13 and –0.62, respectively; Table 3) by clogging pore throats. Illitization created secondary porosity through grain dissolution, but this was progressively occluded by quartz overgrowths as a byproduct of the same process, alongside compactiondriven porosity loss. To date, only grain-rimming chlorite and hydrocarbon charge is known to have significantly helped preserve porosity during illitization.
HyLogger data for the onshore northern Perth Basin have important applications for ongoing diagenetic research and provide a fundamental baseline dataset for basin analysis. In particular, acquiring HyLogger datasets for entire wells using cuttings (e.g. Depot Hill 1) provides a novel technique for testing hypotheses about basin-scale diagenetic processes, such as open- vs closed-system diagenesis and thermal effects that drive localized kaolinization, illitization and carbonate cementation processes. Further petrographic analyses, and integration of clay and cement diagenesis, are required to develop porosity models for different diagenetic zones during hydrocarbon charge.
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