SEQUENCE STRATIGRAPHY AND THE DEPOSITIONAL HISTORY OF THE MURTA MEMBER (UPPER HOORAY SANDSTONE), SOUTHEASTERN EROMANGA BASIN, AUSTRALIA: IMPLICATIONS FOR THE DEVELOPMENT OF SOURCE AND RESERVOIR FACIES

Abstract

The Hooray Sandstone (Namur Sandstone Member and overlying Murta Member) was deposited during the Early Cretaceous in southwestern Queensland and northeastern South Australia. A sequence stratigraphie model for the Hooray is proposed. The basal beds of the Hooray Sandstone are the braided fluvial deposits of the Namur Sandstone Member, deposited during the late Berriasian and earliest Valanginian on a Type 1 sequence boundary developed during the 131.5 Ma sea level lowstand. Two sequences are recognised within the Namur (Namur Nl and N2) with a Type 1 sequence boundary interpreted within the member (128.5 Ma sea level fall). The upper Namur is gradational with the lower Murta Member M6 which forms a transgressive systems tract. The M5 and M4 are highstand systems tract sediments deposited over the maximum flooding surface at the base of the M5 (127.5 Ma condensed section). Low sea levels led to incision of channels basinwards of exposed basement rocks at the end of M4 time (about 126 Ma). Sea level rise led to the gradual Willing of these channels until the southeastern highlands were drowned but islands in the southeast contributed vitrinitic material during mid M3 time, which formed the source for the oil in the Nockatunga area. Reservoir sandstones are developed within the basal M3 incised valley fill sands and in the transgressive microtidal barrier facies sands at the top of the M3. The former can produce stratigraphic traps, whereas the latter form structural traps. Reservoir quality is probably best in the thicker mid 'estuarine' sands due to tidal reworking, and generally poorer in the barrier facies due to little winnowing of fines because of the low tidal range, weak wave activity, and gene-contemporaneous carbonate cementation. During later M3 time, transgression led to the landward movement of barrier bars, forming a generally sheet-like sand body. The top M2 and M3 barriers probably formed in a microtidal environment under a seasonal climate with periodic storm conditions. Following inundation of the islands during M2 and Ml times, organic matter supplied to the Nockatunga area was dominantly finely comminuted, bacterially-altered plant matter and algal material that forms a potentially rich but immature oil source. The 'hot gamma' shales of the Ml are interpreted as the 123.5 Ma condensed section.