Abstract: Underthrusting Indenter Interpretation of Tectonic Setting for an Unusual Petroleum System in the Azua Basin, Southwestern Dominican Republic

Jens Munthe, Lonnie Smith III, J. Scott Hornafius

The regional tectonics of the North American-Caribbean plate boundary zone in Hispaniola are well known, yet the major tectonic elements controlling the 1,600 sq. km. Tertiary Azua basin are poorly understood. A 1991 Mobil seismic survey shows that the northeast-trending Barahona coastline is controlled by a sinistral wrench system. Several horsetail-splays of the strike-slip system lie beneath Ocoa Bay; it is cut off on the north by thrusts in front of the Cordillera Central. Three north-south, west-verging thrust faults are imaged beneath and east of Ocoa Bay. These thrusts appear to be a continuation of the Muertos Trough subduction zone to the southeast and frontal thrusts of the Cordillera Central to the northwest. Ocoa Bay is bounded on the west by the wrench system and on the e st by the thrusts. Thus, the eastern Azua basin is an "underthrusting indenter" which is being subducted beneath the eastern Hispaniola microplate as the Beata Ridge moves northeastward. This new model provides a geometric resolution for plate motions in a structurally complex corner of the Caribbean.

An unusual petroleum system is currently active along the north edge of the underthrusting indenter. Geochemical studies of the oils and gases from seeps and small oilfields in this area indicate derivation from a young source rock associated with carbonate or evaporite strata. A marly facies of the Middle Miocene Sombrerito Fm. is the only local unit which meets these criteria. Extremely variable thermal maturity indications, together with abundant CO₂ in some samples, suggest a great range of burial depth for the source rock. Burial history and thermal maturation modeling indicates that all geochemical results can be explained by subduction of the Sombrerito into a hydrocarbon kitchen along the east and north edges of the indenter. These results have been combined with in ications of potential reservoirs, traps and seals to develop a viable hydrocarbon play. The play concept is unique to the Caribbean, but all its elements are consistent with the unusual tectonic setting.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994