Depositional, diagenetic and mineralogical controls on porosity development in the Ungani Field, Canning Basin

Abstract

A 75 m-thick section of Early Carboniferous Tournaisian Ungani Dolomite reservoir was continuously cored in Ungani Far West 1, an appraisal well 3 km west of the Ungani field located on the southern flank of the Fitzroy Tough in the Canning Basin. The objective of this coring program was to better understand the pore systems, mineralogy, and diagenetic history of the reservoir to allow calibration and extrapolation of the petrophysical evaluation of the reservoir at the Ungani field. Petrography, stable isotopic, XRF, hyperspectral logging, grain density and CT scan studies were conducted on the core. The core consists of a 12m overlying sealing shale and 63 m of vuggy, fractured and dolomitised reservoir. The reservoir is commonly bioclastic-rich but pervasive dolomitisation hinders recognition of earlier depositional features. The upper carbonate facies is interpreted as shallow to moderate depth marine ramp-type deposits while the lower carbonate facies is suggestive of shallow platform top settings with ‘reefal’ constructing organisms. Bio-mouldic, vuggy, fracture, cavern and inter-crystalline porosity resulting from multistage brecciation, fracturing, dolomitisation and dissolution events are all present and critical to reservoir development. Based on hyperspectral logging and thin section petrography, the reservoir is predominantly dolomite with late phase cements comprising of quartz, calcite, gypsum, anhydrite, chalcedony, dickite and pyrite. Variable grain densities that correspond with porosity have been noted throughout the core but no obvious relationship between the mineralogy and porosity can be established. Shallow to moderate burial and marine or evaporative reflux fluids are likely responsible for the pervasive dolomitisation. Subsequent leaching of calcite is also key to reservoir development.