THE HUTTON SANDSTONE-BIRKHEAD FORMATION TRANSITION, ATP 269P(1), EROMANGA BASIN

The Hutton Sandstone and Birkhead Formation are the reservoir and seal, respectively, for most ofthe oil discovered in the Eromanga Basin. The traps are generally simple anticlinal closures; however stratigraphic complexities are common. Development well failures in ATP 269P(1) are commonly caused by stratigraphic variations at the Hutton-Birkhead transition. In ATP 269P(1) two distinct sandstone compositions are present over the Hutton-Birkhead transition, and are end-members of a spectrum of compositions. The first end-member consists of porous and permeable quartzose sandstone which is typical of the Hutton Sandstone. The second end-member is a lithic labile sandstone which is generally not of reservoir quality and is typical of the Birkhead Formation. Mixed and interbedded varieties are common, and are included in the Birkhead. The sandstone composition indicates that the quartzose 'Hutton' variety was derived from a stable, plutonic-metamorphic craton, whereas the lithic 'Birkhead' variety was sourced from a tectonically mobile belt, consisting of acid to intermediate volcanics, regional metamorphics, and metasediments. The Bodalla South Field displays a gradational change from quartzose sandstone in the Hutton to mixed and interbedded varieties in the Birkhead. Substantial range in reservoir quality is seen in the field and is related to sandstone composition and to a fluviodeltaic environment of deposition. In the Kenmore Field lithic sandstone, deposited in channels eroded into the quartzose sediments, provides stratigraphic enhancement of the structural closure. Similar relationships are seen in the Jackson-Naccowlah area and the Surat Basin and are linked to a change from a single provenance during deposition of the Hutton to a dual provenance during deposition of the Birkhead. This change was caused by increased activity in a mid-Jurassic arc system off the present Queensland coast and has important implications for petroleum exploration in the Eromanga and Surat Basins.