DA SILVA, HERCULES T. F., Petrobras S.A., Austin, TX

ABSTRACT: The Lower Cretaceous Tectonosequence of the Reconcavo Basin, Northeast Brazil-Sediment Accumulation Rates and Depositional History of the Most Prolific Hydrocarbon-Producing Rift Basin in Brazil

The Lower Cretaceous tectonosequence represents a major chronostratigraphic element defined for the Reconcavo Basin. In encompasses a time span of ca. of 24 m.y. (Berrasian to Barremian) and its associated sedimentary record reaches thickness over 5000 m. Hydrocarbon production within the tectonosequence is mainly associated with synrift lacustrine turbidite and fluvio-deltaic reservoirs. Biostratigraphy is based on nonmarine ostracods, which permits the subdivision of the tectonosequence into eight time intervals (biozones and subzones), i.e., from NRT 002.2 to NRT 009. Estimates on sediment accumulation rates were based on the analysis of 93 wells. During initial rifting, accumulation rates were low (0.2 to 0.3 m/l,000 yr) due to the relative low sedimentary input to the basin that ight be related to an immature drainage system. Lacustrine sedimentation prevailed during this period (NRT 002.2 through NRT 004). During middle rifting, accumulation rate values are similar to slightly higher than those related to early rifting. These values may reflect an increase of the depositional area and changes in the hydrological characteristics of the lake, oscillating from close to open conditions. During middle rift, fluvio-deltaic and lacustrine deposition prevailed. Late rift sedimentation is characterized by the highest accumulation rate values observed for the tectonosequence, reaching up to 1.4

m/1000 yr. These high values are associated with fluvial sedimentation and suggest: (1) an efficient drainage system feeding into the basin, (2) removal of structural barriers that might have hampered fluvial systems to reach the basin, and (3) tectonism renewal, which increased topographic differences between source area and depositional basin. Analysis of the sediment types found within the tectonosequence suggests hot, wet climate during early rifting, and dry, arid climatic conditions during late rifting.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.