The Aksitero Formation of central Luzon is an upper Eocene to lower Oligocene sequence of evenly bedded hemipelagic limestones with a few thin interlayers of tuffaceous turbidites. The limestones consist chiefly of planktonic Foraminifera and calcareous nannofossils, but include up to 30% noncarbonate components which are mainly volcaniclastic debris. The tuff layers are graded beds comprising glass shards, pumice fragments, crystals, and fine-grained volcanic rock fragments. Hydrocarbons migrated into the pores of the tuffaceous layers at a relatively early stage during diagenesis. However, subsequent flushing has left only a bitumen residue, chiefly as thin coatings on grains and within pumice vesicles. During later stages of burial diagenesis, zeolites (mordenite and c inoptilolite), and secondary calcite preferentially replaced glass shards and pumice fragments. The zeolite assemblage suggests maximum burial temperatures of 55 to 90°C.

Deposition of the Aksitero Formation apparently occurred at water depths of at least 1,000 m in a subsiding back-arc basin lying west of an east-facing early Cenozoic island-arc system. Pelagic carbonate skeletal material was the main sediment source, but submarine ash eruptions of silicic composition generated volcaniclastic turbidity currents whose distal edges occasionally reached the basin floor. The thicker and coarser, more proximal facies of these volcaniclastic deposits may be prospective for hydrocarbons.

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