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An Ultimate Unconventional Play: Diagenesis Controls on Porosity in the Proterozoic Barney Creek Formation, Northern Territory, Australia


One of the biggest challenges in characterising unconventional reservoirs today lies in the poor understanding of porosity and controls affecting its distribution and connectivity. Porosity in these reservoirs is defined as an intricate network of matrix- and organic-related pores developing or occluding based on the impact of processes such as compaction, maturation and diagenesis. Thus, it is only reasonable to suggest that complete understanding of porosity in unconventional reservoirs can be achieved only when processes affecting the delicate balance of pore creation and destruction through time are identified. Current convention is that bulk of porosity is due to transformation of labile organic matter resulting in a complex organic pore system with increasing maturation. Other porosity contributors, such as matrix-related pores, are usually described as sparse diminishing their impact and potential. Despite current efforts of the scientific community to emphasise its relevance and understand the role that diagenesis and organic thermal maturation play on its development, a comprehensive model is still lacking. Our study highlights the importance of diagenesis and thermal maturation on porosity and proposes a diagenetic model for the oldest producing unconventional system in Australia, the Proterozoic Barney Creek Formation. Results show a consistent pattern of reducing matrix porosity with increasing thermal maturation from immature to oil mature, followed by an increase within the gas mature window. Such variations are attributed to an increase in the volume of precipitating pore-occluding cements in the former and grain dissolution and secondary porosity in the later. Furthermore, we provide a comprehensive explanation to the origin and timing of cements based on mineral patterns, associations and paragenesis. The sample set analyzed spans a gradient of thermal maturation from immature to wet gas mature. Analytical techniques include nitrogen low-pressure adsorption and mercury intrusion capillary pressure for analyses of pore volume, pore sizes and pore size distribution, organic petrography for estimates of vitrinite reflectance and thermal maturation trends plus scanning electron microscopy, x-ray diffraction and elemental microprobe mapping for defining mineralogy, mineral associations and capturing relationships between pore-occluding cements and detrital grains at the scale where they form (micro- and nanometer scales)