DEPOSITIONAL ENVIRONMENTS AND HYDROCARBON POTENTIAL OF THE EVERGREEN FORMATION, ATP 145P, SURAT BASIN, QUEENSLAND

Abstract

Hydrocarbon discoveries in the Jurassic Evergreen Formation along the western shelf of the Surat Basin lie in a northwest-trending structurally controlled belt adjacent to Bridge-operated ATP 145P. Shows and commercial accumulations are generally associated with specific sand/shale ratios, which influence source, reservoir, and seal effectiveness. Proximity to basement and early faults are other favourable factors.

Four distinct log facies were calibrated against cuttings and core and interpreted genetically. Small, stream-dominated alluvial-fan systems extended from low escarpments in the west, but were reworked along their basinward margins prior to being overstepped by aggradational, onlapping distal sediments. Prominent east-northeast-trending belts occupied by mixed-load fluvial channel systems terminated along a temporary shoreline of coalescing wave-dominated deltas. This deltaic-shore-zone sand belt was truncated by fluvial-distributary channels during the ensuing regression to another still-stand some 25 km farther east. Shoreline positions fluctuated locally in response to competition between rate of sediment influx and the gradually rising base level; but the net effect was regional transgression, with reworked shorezone/nearshore sheetsands grading up into silty shales of the shallow epeiric platform.

On the basis of a petrographic study the highest source potential for the Evergreen Formation in ATP 145P is around Spring Grove 1 well. A band of high source potential lies parallel to the deltaic shoreline, in a generally north-south direction. The highest source potential of the Triassic Moolayember Formation lies around Spring Grove 1 and in a north-south trending arc through Roswin 1 and Sirrah 4. The source potential of the combined Triassic Snake Creek Member, Showgrounds and (Permian) Kianga formations is highest around Roswin 1. In the area of Roswin 1 all formations studied show a high potential as source rocks for hydrocarbons. Based on the available data, the Triassic Moolayember Formation appears to be the only formation common to all producing wells with a high source potential in each well.

Zones of thick, porous sands are not very prospective for hydrocarbons in the study area, possibly because of ineffective seal. True shales are rare in the Evergreen Formation. The dominant fine-grained lithology is silt-stone, which by itself provides a leaky or dynamic seal at best. Thin, reworked delta mouth bar and splay sands with thick seals may offer the best reservoir objectives. Known hydrocarbons generally occur in structural-strati-graphic traps with the structural component caused by compaction and subsequent draping over basement topography, and the stratigraphic component arising from the lenticularity of the fluvio-deltaic sand bodies.