The stratigraphic significance of paralic deposits in the Precipice– Evergreen succession, Surat Basin, Queensland

Abstract

The Precipice Sandstone and Evergreen Formation in the Surat Basin, Queensland, are being examined as a reservoir-seal option for future geosequestration of CO2. Effective reservoir modelling, and prediction of dynamic storage capacity, however, depends upon accurate depositional interpretations relating to an understanding of the stratigraphic architecture. Throughout most of the basin, the Precipice Sandstone is generally considered to have good reservoir properties and lateral continuity. Refined depositional models and a widely-applied sequence stratigraphic framework will enhance prediction of the most prospective play segments for CO2 injection. We utilize integrated ichnological-sedimentological facies analysis from core to interpret the Precipice Sandstone as a braided fluvial to braid-delta succession, overlain by lower delta plain to subaqueous delta and estuarine embayment deposits of the Evergreen Formation. Facies successions and core-calibrated wireline logs show brackish-water influenced deposits at several stratigraphic intervals. Brackish-water influenced deposits overlie upper delta plain or braid-plain sediments. They occur laterally adjacent to subaerial lower delta plain strata, and generally cap parasequence-sets. Seismic surveys resolve lower-order cyclicity, showing parasequence-sets within the Precipice succession that retrograde or aggrade and onlap the basal-Surat unconformity. This stratal arrangement reflects the lowstand and early transgressive systems tracts of a 2rd order depositional sequence or a distinct 3th order sequence. Late transgressive and early highstand systems tracts are more abundant within the lower Evergreen Formation and are interpreted to consist of restricted or estuarine central basin deposits; but these may also represent a 3th order sequence. Additional chronometric data is needed to differentiate between these interpretations. Depositional and sequence stratigraphic interpretations suggest the Precipice Sandstone has a higher degree of heterogeneity than previously appreciated. Moreover, we show that the Evergreen Formation is not a simple basin-wide sealing unit due to the presence of sandstone geobodies that complexly cross-cut each other (i.e., the ‘Boxvale Sandstone Member’) that may act as vertical fluid conduits. The sequence stratigraphic characteristics of the reservoir-seal pair should be carefully considered when selecting locations for CO2 sequestration.