--> Origins of Kaolinite Controlling Porosity and Permeability in the Mungaroo Formation, Carnarvon Basin, Australia

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Origins of Kaolinite Controlling Porosity and Permeability in the Mungaroo Formation, Carnarvon Basin, Australia

Abstract

The Triassic fluvio-deltaic Mungaroo Formation is the main reservoir in the multi-TCF gas plays offshore Northern Carnarvon Basin, Western Australia. The Mungaroo Formation reservoir quality data were compiled from sedimentological descriptions, petrographic analytical data and routine core analysis data from various fields in the basin. Mungaroo Formation sandstones are subarkose and quartzarenite with sublitharenite, but show a wide range of compositions. Sands are typically fine-to coarse-grained and poorly-to moderately well-sorted. Framework grains are dominated by mono crystalline quartz, with minor polycrystalline quartz, feldspar and lithic fragments, mica, heavy minerals, organic matter and detrital clay are rare. Average composition is Q82F7L11. Jurassic uplift exposed the Mungaroo Formation to meteoric water resulting in feldspar dissolution and kaolinitic alteration. Petrographic evidence of grain (feldspar, mica or rock fragments) dissolution is common. Feldspars and micas are altered to kaolinite which occurs as pore fillings (cement) and as grain replacements. Microporosity is associated with the kaolinite both in primary pores and in the grain dissolution pores. Kaolinite exhibits at least four morphological textures, distinguished by dominant crystal size, ranging from fine-grained (<1 μm), medium-grained (1–10 μm) vermicules, to coarse-grained vermicules with individual diameters of up to 20 μm (10–20 μm) and coarse compact blocky crystals (>20 μm). Vermicular kaolinite has visible porosity of 20–60% within the kaolinite cemented zones, while finer grained (<1 μm) seem to have the high microporosity association. The porosity associated with kaolinite constitutes 10% to 80% of the total porosity. Principal porosity reducing cements include authigenic quartz. However, kaolinite development inhibits quartz cementation and thereby helps to preserve primary porosity. The absence of Potassium-feldspar in the sandstone, due to kaolinisation, prevents illitisation and thus helps retain permeability. Kaolinisation becomes more significant for total porosity maintenance at deeper depth because of the increase in quartz cementation at higher thermal exposure.