BARBOZA, GUILLERMO, and JAIME BARRIENTOS, RECOPE S.A., San Jose, Costa Rica

ABSTRACT: Tectonic Evolution, Seismic Stratigraphy and Oil Potential of the Central Pacific Margin of Costa Rica

The Nicoya and Parrita Basins are two elongated depressions that lay in the Central Pacific of Costa Rica and are filled with 3.5 km and more than 4.5 km of Cretaceous-Tertiary sediments respectively. Both basins are limited on the south by a shallow basement swell that represents an extension of the outer arc over the shelf. The northern limits extend into the onshore side of the Nicoya Gulf and towards the Parrita plains where basement outcrops.

In the Cretaceous, the area now occupied by the Nicoya and Parrita basins was part of an ample NW-SE forearc system extending along the Pacific margin and encompassing, the nearby Tempisque and Terraba forearc basins. The development of both basins initiated on the middle Eocene just after the Chorotega and Chortis exotic terranes became coupled. Further shear stresses caused by clockwise rotation of the southern Costa Rican crustal block along a major left lateral transcurrent fault zone resulted in the half-graben configuration and the extensive strike-slip fault system of the Nicoya and Parrita transtensional depocenters.

The overall Cretaceous-Tertiary basin infill represents a shallowing upward megasequence whose units grade from pelagics to turbidites to shallow marine to continental deposits.

Several unconformities are recognized on seismic data as a combination of accretion, uplifting through basin history, and eustatic processes.

Late Cretaceous pelagic oil shales containing thermally mature, type I organic matter are the main source rock in the region. Friable tidal Neogene deposits outcropping in the onland basin margin provide good reservoir conditions. Potential traps are identified on seismic data.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.