Widespread nearshore and shallow marine deposition within the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia

Abstract In the Surat Basin of eastern Australia, the Lower Jurassic Precipice Sandstone and Evergreen Formation are a highly prospective reservoir-seal pair for notional future carbon capture and storage. However, the succession remains poorly constrained from a paleodepositional standpoint and this has impacted the capacity to construct predictive reservoir models. Here we integrate sedimentological, ichnological, and palynological data from ten cores located across a large region of the northern and central basin to produce conceptual depositional models for the interval. Our analysis shows that the Lower Jurassic Series consists of fifteen recurring sedimentary facies that are arranged into six facies associations – braidplain, lower delta plain, subaqueous delta, delta-influenced shoreface, tidally-influenced shoreline, and restricted marine shoals. The facies associations occur in the context of large scale fluvio-deltaic systems that occupied the basin. These results are supported by ichnological indications of marine and brackish-water, a coastal suite of palynomorphs, and marine palynomorphs including rare dinocysts, acritarchs, and copepod fragments. The very low abundance of marine palynomorphs are confined to the upper portion of the Evergreen Formation, and in combination with the sedimentology and ichnology suggest that marine influence increased through time as the basin was filled. The elucidation of marine-influenced deposition contravenes all but the most recent facies interpretations of the Precipice Sandstone and Evergreen Formation, and suggests that the paleogeography of the Mesozoic of eastern Australia needs to be reconsidered. Importantly, the nearshore and shallow marine depositional affinity has important implications for the size, orientation, and distribution of geobodies when building geologically-realistic static reservoir models for dynamic flow simulation.