The Kawau sandstone of the Great South basin

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Jillett, David N. (David Nicholas), 1968-

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The Kawau Sandstone is defined as a member of the Wangaloa Formation. The two well penetrations of this unit show it to be a moderately to well sorted, subangular, medium sand, subfeldsarenite to lithic feldsarenite in the .HoiholC well, and a moderately to poorly sorted, subangular, medium to coarse sand, sublitharenite to litharenite in the Kawau-lA well.
The main compositional differences are diagenetic in origin and include the absence of feldspar and higher clay content in the Kawau-lA well. The examination of detrital mode analysis and statistical quartz provenancediscrimination has found that caution is required in their use. Where detrital grains have undergone significant alteration, detrital mode analysis should be avoided. Any provenance determinations based on the polycrystallinity and undulatory nature of detrital quartz should be treated with caution. Other provenance indicators including inclusions in quartz, quartz grain size, lithic types, potassium feldspar compositions, the presence of twinned plagioclase and muscovite compositions suggest that the Kawau Sandstone is derived primarily from a granitic source with additional minor input from local sediments and metasediments. The most probable granitic source is also the Great South Basin basement rock.
The diagenetic sequence of the Kawau Sandstone in the Hoiho-lC well is inferred to be: framboidal pyrite - feldspar dissolution - partial alteration of muscovite to illite/kaolinite intergrowths - kaolinite precipitation, with a maximum temperature of 70°C at present. The diagenetic sequence in KawaulA is inferred to be: framboidal pyrite - siderite precipitation - feldspar and siderite dissolution· :. partial alteration of muscovite to illite/kaolinite intergrowths - kaolinite precipitation-hydrocarbon emplacement - kaolinite precipitation - authigenic quartz precipitation - illitization of kaolinite - euhedral pyrite, with a maximum temperature of 120°C at present.
The large number of substituted ions in the composition of early diagenetic siderite in the Kawau-lA well suggests that the Kawau Sandstone here is marine in origin.
The presence of a significant gas accumulation in the generally low porosity Kawau Sandstone of the Kawau-lA well suggests that reservoirs of apparently poor quality may still represent good hydrocarbon prospects.

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116 leaves : ill. (some col.), maps ; 30 cm.

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1993Jillett

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Jillett, David N. (David Nicholas), 1968-, “The Kawau sandstone of the Great South basin ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/281.

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