The Mananda Anticline, Papua New Guinea; Two Oilfields, Four Dry Holes and Deep Potential?

Keenan, Scott E.; Hill, Kevin C.; Iwanec, Jeremy B.
Exploration, Papuan Oil Search, Sydney, NSW, Australia.

The Mananda Anticline is one of the larger structures in the Papuan Fold Belt at 40 km long, 15 km wide and up to 2000m high. It is >20 km from the nearest road and airstrip, is deeply karstified, covered in equatorial jungle and is regularly in the clouds. The stratigraphic sequence comprises 1 km of Miocene limestone, above 1 km of Cretaceous shale seal overlying earliest Cretaceous Toro and Digimu sandstone reservoirs and inferred Jurassic source rocks. The size of the structure suggests basement inversion and hence deep sub-thrust plays, but wells to date have been confined to the hangingwall where they have encountered thin-skinned structures and transported old normal faults. Ten wells have been drilled on the anticline.

The Mananda 1X well, drilled in 1971-2, encountered a repeat of Miocene limestone without testing the Mesozoic reservoirs. In 1984 the Mananda 2X well was drilled on the topographic crest with a slimhole rig, but was beset with mechanical difficulties. After 160 days of drilling, it failed at 2434mMD, only 4 m short of the Toro sandstone reservoir as subsequently proven by the Mananda 3X redrill in 1986 with a conventional rig. Mananda 3X cored porous Toro reservoir with oil shows and recovered a few barrels of oil, but overall it was wet. Mananda 4X was drilled along strike in 1990, to test deeper reservoirs, but was also water wet. In 1991 however, SE Mananda 1X discovered oil and gas, 30 years after the first exploration well and was followed by the SE Mananda 2X appraisal in 1994.

Between 2000 and 2005, six moderate-quality seismic lines were acquired across the Mananda Anticline, as well as additional field data, including Sr isotope dating of the surface limestone. This preceded drilling of SE Mananda 3/4/5 in 2005 to develop the field and lead to a re-evaluation of the whole structure. The seismic, surface dips, Sr dates and Toro dips in the Mananda 3X/4X wells all indicated that the structural crest was SE of the topographic crest. In 2010-11 the Mananda 5 well was drilled 1.5 km south of Mananda 3X and tested oil and gas in the Toro sandstone reservoirs, which were 400mTVD higher than in Mananda 3X and probably separated by an old normal fault. Dipmeter data suggest that the crest has yet to be drilled so additional seismic data is to be acquired to map the hangingwall crest and better define the footwall play.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012