Abstract About 16,000 km of high-resolution seismic reflection lines have been shot along the Chilean Pacific offshore for oil exploration since 1971. Many oceanographic survey groups from scientific institutions have also worked here.

The northern zone, from Arica to Valparaiso, rarely has more than 5 km of shelf, generally developed as abrasion terraces in hard rocks. The slope is abrupt, but some sections show deep hanging basins filled with sediments in water depths of 1,000 to 2,000 m. A rather important sedimentary basin of this type was detected between Arica and Iquique in about 1,200 m of water by the Kana-Keoki survey in 1974 and confirmed by the Western Shoal survey in 1977.

South of Valparaiso, the shelf widens from a minimum of 10 km to a maximum of 70 km and several sedimentary basins have developed on the shelf. The oldest sediments known range from Late Cretaceous in the north to Miocene in the Valdivia basin. Over 3,000 m of sedimentary rocks have been drilled in the shelf northwest from Concepcion and over 4 seconds (two-way reflection time) have been recorded offshore from Chiloe Island.

The shelf is affected by a complex tectonic system, with little evidence of compressional folding. Two unconformities are present in the sedimentary sequence: a Pliocene-Miocene angular unconformity is seen in the seismic records, and a Miocene-Eocene discontinuity with a very strong velocity break is normally detected.

The sedimentary section found in six exploration wells drilled offshore in 1972 is entirely fine-grained, clastic marine beds. Two prominent sand bodies have been detected--a Miocene gas-bearing sandstone in the Valdivia basin and a thick basal sandstone of Late Cretaceous age in the Chanco basin. Mica schist basement was found in the six wells.