EXPLORATION FOR OIL-BEARING SAND TRENDS IN THE FLY RIVER AREA, WESTERN PAPUA

Abstract

The Magobu Island No. 1 well, drilled by Endeavour Oil Company N.L. late in 1970, was located to test a large, northwest-trending, Mesozoic anticline at the mouth of the Fly River, Western Papua. The anticlinal trend was postulated to be coincident with an intra-Mesozoic strand line where massive lenticular sand bodies may have developed.

The well penetrated 3,086 ft. of Pliocene and Miocene carbonates and 5,360 ft. of Lower Cretaceous and Jurassic elastics before bottoming in Carboniferous volcanics.

Two massive sandstone units of particular significance, one of Lower Cretaceous age at a depth range of 4,265-4,775 ft., and the other of Middle Jurassic age at a depth range of 7,065-7,345 ft. were intersected. These clean, quartzose sandstones have excellent porosity and permeability but are salt-water saturated at the Magobu location.

The electric log character, grain gradation, continuous dipmeter information, and depositional environment (determined by interpreting the lithologic sequence and faunal content of the Mesozoic) indicate that these sandstone units consist of northwest-trending barrier bar complexes. The basal member of the upper unit, in particular, is interpreted as having formed a barrier island.

The seismic data support this interpretation with respect to both sandstone units. In addition, a second and thicker bar complex of similar trend, situated some 10 mi. down-dip to the northeast under Kiwai Island, is indicated by the seismic data and can be correlated with the upper sandstone unit at Magobu.

Structural interpretations of the relationship between the upper barrier bar complexes at Magobu and Kiwai suggest that oil migrating up-dip would be trapped by the structurally lower bar build-up. This conclusion gives a lead to follow-up drilling in the search for oil-bearing sand trends.