Stratigraphy, sedimentary patterns, and reservoir characteristics of Jurassic carbonate successions in the Lusitanian Basin

Abstract

The Jurassic of the Lusitanian Basin (LB) is instructive for interpreting reservoir and source potential in carbonate ramp successions. Selected examples from the Lower, Middle, and lower Upper Jurassic of the LB are used here to illustrate a complete range of carbonate facies across the ramp system, from outer-ramp (Rabaçal), inner–mid-ramp (Maciço Calcário Estremenho and S. Pedro de Moel), and mixed non-marine–paralic–shallow-marine (Cabo Mondego) settings. Each example is used to demonstrate the interaction of tectonics and eustasy in controlling depositional facies, diagenesis, sedimentary patterns, and stratigraphic evolution. Emphasis is placed on the reservoir and/or source-rock potential of each major depositional system. Outcrops are visited in the regions of: i) Maciço Calcário Estremenho (limestone hills in the eastern part of the LB); ii) Rabaçal, near Coimbra, also in the eastern part of the LB; and iii) Cabo Mondego and S. Pedro de Moel, both located in the western part of the basin (shoreline). In the Maciço Calcário Estremenho region, Middle Jurassic shallow-water facies (oolite- and skeletal-dominated sandbodies, coral–algal biostromes, and lagoonal, peritidal, and calcrete sediments) are examined. At Rabaçal, a Lower Jurassic hemipelagic succession is presented, supported by an integrated and detailed stratigraphic analysis emphasizing aspects related to the evolution of the Lower Jurassic series at the basin scale. At the coastal section of Cabo Mondego, the main features of interest are the Middle–Upper Jurassic basinwide disconformity, which shows the effects of a major forced regression, and various marginal- to non-marine Oxfordian facies, including source rocks. At S. Pedro de Moel, the focus is on Sinemurian restricted-marine facies with text-book examples of stromatolites.