Sequence stratigraphic analysis of the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the architecture of reservoirs and seals for CO2 storage

The Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia are a highly prospective interval for carbon capture and storage (CCS). However, their stratigraphic nomenclature is inconsistently applied, and a basin-wide stratigraphic framework that is ubiquitously agreed upon remains elusive. In particular, a sandy transitional sandstone interval that occurs above the main Precipice Sandstone reservoir unit has caused confusion about how the two units should be subdivided. To address this issue, we integrate core observations with seismic interpretation and wireline log correlation to construct a sequence stratigraphic framework for the Lower Jurassic System that goes beyond lithostratigraphy. The main findings of our work include: (1) the identification of three 3rd-order sequences, SQ1-SQ3, from base to top. Two main sandstone intervals, the Precipice Sandstone and the Boxvale Sandstone Member (Evergreen Formation), represent the lowstand systems tracts (LSTs) of SQ1 and SQ3, respectively. The Westgrove Ironstone Member in the upper Evergreen Formation - a distinctive oolitic ironstone unit represents the transgressive system tract (TST) of SQ3; (2) the transitional interval above the main Precipice Sandstone reservoir corresponds to the lower TST in SQl (lower Evergreen Formation). It is manifest as an upward-fining succession with a retrogradational facies stacking pattern, and is significantly different from the main Precipice Sandstone reservoir which shows an aggradational stacking pattern; (3) the Precipice Sandstone and lower Evergreen Formation progressively onlap the western and the eastern margins of the Surat Basin, and this confines the Precipice Sandstone to the central portion of the basin; (4) the three sequences (SQ1-SQ3) approximately coincide with global sea-level cycles documented from the early Jurassic. Two episodes of accelerated tectonic subsidence were responsible for the thinning and onlap of SQl and SQ3 onto the basin margins. In contrast, weaker tectonic subsidence was responsible for the overall finer grained sediment, the uniformity in thickness, and widespread distribution of SQ2. The results of our stratigraphic analysis indicate that the architecture of the reservoir and seal significantly differs from previous thinking, such that: (1) the Precipice Sandstone, the main reservoir unit, has a distribution that is limited to the basin centre; (2) the transitional interval above the main Precipice Sandstone reservoir forms poorly-connected, localized reservoirs that onlap and backstep towards the basin margins; and, (3) SQ2 and SQ3 are sealing units because they are very widespread and mud-dominated. Viewing the Precipice Sandstone and Evergreen Formation from a sequence stratigraphic standpoint provides important information about how to subdivide them and helps clarify the stratal relationships that were previously unclear from a mixture of terminology. Furthermore, this study gives insights into the spatial-temporal evolution of this reservoir and sealing interval that is an important CCS target in Australia.