Abstract

The Sareba Block covers much of the "Bird's Neck", ~50 km east of the Vorwata Field, at the northern end of the Lengguru Fold Belt. Potential Middle Jurassic reservoir sands were deposited there prior to rifting, which created the northeast-trending Sareba Graben. It contains (thin) Cretaceous-Tertiary pelagic sediments, in contrast to thick shelf sediments on the flanks. Mid-Miocene clastic sediments thicken to the east, sourced from the approaching Manokwari and Weyland Terranes, and correlate with Kais Limestone on the western flank of the Sareba Graben. Late Miocene collision of the Weyland Terrane constructed a relatively low-lying thin-skinned fold belt in Cenderawasih Bay, whilst the restricted Sareba Graben was filled with ~3.5 km of Klasafet Formation organic-rich marls, a potentially rich oil source. Pliocene basement thrusting created large anticlines and a major thrust front along the east side of the basin, as well as 2-3 km high mountains in Cenderawasih Bay. The Klasafet Formation was locally thickened to 8 km and it underthrust the adjacent coarse clastics deposited in a foreland basin as the Steenkool Formation. Pleistocene orogenic collapse in Cenderawasih Bay led to large normal faults cutting through the coastal portions of the block coinciding with regional subsidence. Two oil seeps indicate that Klasafet marls generated oil in the Pliocene that migrated into the large, basement-cored anticlines to the east. Jurassic reservoir rocks beneath the western Klasafet Basin and in the far eastern part of the older Klasafet basin were deeply buried. In contrast, Cretaceous and Jurassic samples from the central Klasafet Basin have vitrinite reflectance and spore colour data indicating modest depths of burial. Thus the Sareba Block contains all the essential ingredients of large thrust structures, good Jurassic reservoir and adjacent Late Miocene source rocks that matured in the Pliocene.