Implications of the depositional and diagenetic attributes on the reservoir properties of the siliciclastic mangahewa formation, Taranaki Basin, New Zealand: Integrated petrographical and petrophysical studies

Abstract

The siliciclastic Mangahewa reservoir in the Pohokura Gas Field presents a formidable challenge for gas exploration due to its diverse petrophysical properties. To address this challenge, our study employs a comprehensive approach integrating petrography and petrophysical analyses for core and well logging data. In the present study, we uncover an association of sandstone microfacies, in the key wells like Pohokura South-1, Pohokura-1, and Pohokura-2 wells, which change from fine to medium-grained subfeldsarenites to lithic feldsarenites microfacies. The petrographic investigation pinpoints the critical factors influencing the reservoir quality, including facies types, grain size, pore types, sedimentary structure, detrital clay content, and mineral composition. Both the primary intergranular and secondary dissolution pore spaces are responsible for the influential large intergranular pore volumes causing a superior reservoir quality. Mineral composition, influenced by the source and lithofacies, significantly impacts the reservoir properties. Diagenetic processes further control the reservoir quality, where the abundant silica cement, the illitic clays, and the rare pervasive carbonate cementation render the low reservoir quality. Compaction significantly reduces porosity during diagenesis. High-quality reservoir sandstones are characterized by coarse grain size, minimal detrital clay presence, predominantly quartz composition, limited carbonate and moderate silica cementation, low authigenic illite levels, and minor compaction impact. Based on the petrophysical data, the Mangahewa reservoir is composed of five RRTs (reservoir rock types) with the RRT1 samples having the best reservoir characteristics due to their fair porosity, very good permeability, and macro pore sizes. On the other side, the RRT5 samples are characterized by the lowest reservoir quality (poor porosity and permeability and micropore sizes) due to cementation and compaction. Our study provides valuable insights into the intricate petrophysical properties of the Mangahewa reservoir, enhancing the predictability of the reservoir behavior and optimizing hydrocarbon production in this challenging geological setting.