Summary

Petroleum exploration in the Mesozoic rift system of the Lusitanian Basin of Portugal is at an early stage. Abundant oil seeps in the area attest to working petroleum systems, and have attracted industry interest in the past. Exploratory drilling has been intermittent since the mid-1950's, however, drilling density of significant exploratory tests in the northern half of the basin remains approximately 1 well per 150,000 ac.

Syn-rift, late Triassic-Hettangian salt divides the basin stratigraphy into two distinct petroleum systems. The pre- or subsalt system relies on Paleozoic and/or Triassic source rocks, matured during rift subsidence and sediment loading, to charge basal syn-rift continental clastic reservoirs of the Silves Fm. The overlying syn-rift to early post-rift Dagorda Salt regionally seals the system. Primary traps are horst blocks formed by episodic wrench-inversion of older rift half-graben.

Post-rift marine flooding of the basin established the region as a carbonate shelf during Jurassic time. Well-documented Jurassic shaley carbonate source rocks of the Brenha Fm. source the overlying (suprasalt) petroleum system related to this sequence. These rocks charged various Jurassic reservoir facies including tidal flat dolomites, bioherms, bioclastic and oolitic shoals, detrital carbonate grainstones, and nearshore and continental sandstones. Seals in this system are local interbedded shales and tight limestones, or (in the basal part of the section) tidal flat anhydrites.

Mohave Oil and Gas Corporation and partners will drill a significant pre-salt Triassic test in the fall of 1997, the results of which will be reported at this session.