Spectral gamma ray characterisation of the Dongara Sandstone, Perth Basin, W.A.: applications to high-resolution correlation

Abstract

The northern Perth Basin has been a proven hydrocarbon province since the 1960s, however, little is known about the internal stratigraphy of the primary reservoir target the late Permian Dongara Sandstone. Spectral gamma ray data has previously been used in the high - resolution correlation of sediments, but its potential in the Perth Basin has not been previously determined. The effectiveness of this technique is investigated here. Spectral gamma ray (SGR) data were acquired from cores of the Dongara Sandstone using a hand -held gamma ray spectrometer. These data were combined with the relatively sparse wireline log SGR data available for the Dongara Sandstone. A study of the ratios of K, Th and U concentrations was first undertaken to determine whether SGR analysis was applicable in high - resolution correlation in the Dongara Sandstone. Core data was proven to be adequately precise for the comparison with wireline SGR data. It was confirmed that rocktypes could be clearly differentiated from one another on the basis of their SGR signatures. It was also proven that intra- formational character of the Dongara Sandstone could be identified from the SGR signatures, and that this character may have application in high - resolution correlation. Once the effectiveness of the SGR data for correlation had been demonstrated, the data were analysed with a suite of petrophysical software, creating a series of SGR composite logs for each well. There was a good similarity between the SGR composite logs from rocks that had both wireline and core SGR measurements taken. This indicates that the core SGR can be confidently integrated with the wireline SGR data for use in interpretation. These SGR composite logs were used in high - resolution correlation of the Dongara Sandstone. Composite logs effectively display specific SGR signatures as a distinct colour, thus indicating the bulk mineralogy of the sediment and giving indications of distinct depositional facies. In particular, high- thorium intervals were interpreted as representative of beach facies due to the likely presence of the heavy mineral monazite. These facies were recognised in the composite logs, and in many cases, facies could be correlated between wells in neighbouring fields. The stratigraphic and lateral extents of these facies were defined, enabling the construction of palaeogeographic maps for three chronologically separate Late Permian sediment packages. From these maps, the paleogeographic evolution was inferred. These maps indicate that the Dongara Sandstone is interpreted as having deposited on a series of back - stepping shorelines, depositing during a sea level transgression. The sequential changes in facies distributions identified from SGR data can assist in defining the sequence stratigraphy of the Dongara Sandstone. With the internal architecture of the Dongara Sandstone defined, accurate predictions of reservoir distribution and quality can be made. This has important implications in the future exploration and exploitation of hydrocarbon accumulations within the Dongara Sandstone.