Diversity of Jurassic and Lower Cretaceous Carbonate Sequences in the Lusitanian Basin of Portugal: Models for Exploration in the Mediterranean and Atlantic

P. M. Ellis, P. W. Skelton, M. P. Watkinson, R. C. L. Wilson, P. M. Ellwood, D. C. Kitson, R. R. Leinfelder, V. P. Wright

Seven distinctive carbonate associations can be examined within a 3-hr drive of Lisbon. In most cases, their facies geometries and tectonic settings are also illuminated by well and seismic data. (1) Carbonate fan, Toarcian-Middle(?) Jurassic (~300 m). A prograding sequence composed predominantly of shallow-water allochems. The carbonate factory supplying sediments developed over an uplifted Hercynian basement horst to the west of present-day Portugal. (2) Westward inclined Lower and Middle Jurassic ramp systems (50-200 m). During the Sinemurian-Hettangian and Bajocian-Callovian, onshore ramp facies with grainstone barrier systems and lagoonal and peritidal back-barrier facies developed. In the Bajocian, fine-grained turbidites, large slump scars, and slumped horizons ind cate a distally steepened offshore ramp. (3) Rimmed shelf systems, Oxfordian-Kimmeridgian (200-400 m). These developed over fault blocks on the east side of the basin. During the Oxfordian, they were moderate energy systems, but during the Kimmeridgian a high-energy reef-related bypass margin developed over one fault block. (4) Grainstone-dominated, salt structure-related sequences, Oxfordian-Kimmeridgian (up to 1,500 m) sequences were deposited on the northwest side of the basin and consist of a coarsening-upward sequence, the top of which shows mounded clinoforms on seismic sections. (5) Distally steepened, mud-dominated ramp system, Kimmeridgian-Tithonian (~1,000 m). Situated on the southwest side of the basin, this sequence consists of shales, debris flows, and turbidites overlain by a carbonate mud-dominated succession with subsidiary packstones and grainstones. The ramp/slope was probably inclined to the southeast. (6) Coralgal mound and ooid grainstone cap to siliciclastic slope system, upper Kimmeridgian (up to 8 m). Facies occurring within this sequence have strong similarities with the Smackover Formation of the Gulf of Mexico. (7) Shelf carbonates, Lower Cretaceous (up to 50 m). Three carbonate intercalations in a predominantly siliciclastic sequence contain coralgal bindstone and rudist bafflestone barrier zone facies and grainstone spillover units.

All but number 5 illustrate potential hydrocarbon plays, and most are associated with nearby source rocks. Numbers 1 and 2 grade laterally or are underlain by deeper water facies containing bituminous shales. Numbers 3 and 4 are underlain by lacustrine carbonates with a high (>2%) organic content. Oil shows occur in 1-4, but reservoirs were fractured by late Miocene tectonism. Numbers 1-3 occur in Mediterranean and Atlantic settings, but 4 and 5 may be confined to the latter. Though thin, 7 contains facies associations typical of Middle Eastern reservoirs of the same age.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.