LAGOA FEIA FORMATION (LOWER CRETACEOUS), CAMPOS BASIN, OFFSHORE BRAZIL: RIFT VALLEY STAGELACUSTRINE CARBONATERESERVOIRS ‐ I

Abstract

The Campos Basin is one of the coastal basins of eastern Brazil formed during the separation of the African and South American continental blocks. The Lagoa Feia Formation consists of the sediments deposited during the rift valley stage of the Campos Basin, and varies from 200 m to more than I, 500m in thickness. 17 microfacies were recognized, and are grouped into four main sequences, dominated respectively by terrigenous supplies, ostracods, pelecypods and basic volcaniclastics.The vertical sequence of microfacies and the associations of syndepositional diagenetic minerals were used to reconstruct the general environment of deposition. It consisted of saline alkaline lakes, oscillating between a playa lake stage with ostracod microfacies and a pluvial lake stage with pelecypod microfacies.Five diagenetic stages were recognized: syndepositional alteration of lithics to trioctahedral smectites, early dolomitization, early silicification, early cementation by bladed rim calcite and zeolites; fresh water vadose, consisting mainly of dissolution of bioclasts and lithics; fresh water phreatic, with sparite cementation and neomorphism; mixed saline‐fresh water, represented mainly by silicification; and burial with compaction, late dolomitization and partial conversion of smectites to illite.Pelecypod‐rich limestones with primary interparticle, secondary intraparticle, moldic and moldic‐enlarged porosities constitute the main potential reservoirs. The ideal conditions for the generation and preservation of such reservoirs consisted of periods of exposure followed by rapid lake expansion and burial. In this manner, the secondary porosity formed in the vadose fresh water environment was preserved or only partially occluded by calcite cementation during fresh water phreatic conditions.