GEOCHEMICAL ANO HYDROCARBON MIGRATION MODELS FOR THE RIO DO BU FIELD-INTEGRATION WITH DE NORTHEASTERN SECTOR OF THE RECÔNCAVO BASIN, STATE OF BAHIA

Abstract

Focusing on the Rio do Bu Field in the northeastern portion of Brazil's Recôncavo Basin, this studv sought to integrate organic geochemical data with the area's stratigraphic and structural framework. The Candeias Formation's Gomo and Tauá member shales are the main source rocks, with total organic carbon values ranging from 0.7 to 1.0% and hydrocarbon source potential, from 3.2 to 6.0 kg HC/ton rock. Although type II organic matter predominares, type I is also present. Liquid and gas chromatographic analyses of Rio do Bu Field oils show that these are paraffinic, w1th a pristanelphvtane ratio of around two. Carbon is a topic ratios varv from -28.9°loo to -29.9°loo. These data, in combination with biological marker results, suggest that the oils originated from organic matter deposited in a fresh water continental environment and that thev can be classified in one sole group. Vitrinite reflectance was used to estimate the level of thermal evolution of the organic matter and the Lopatin technique applied in modeling this level. Simulations showed that the factor which corresponds to the eroded thickness of sediments has a substantial effect on the calculated value of the time-temperature index. The simulation yielding the best adjustment between calculated and observed vitrinite reflectance values revealed an eroded thickness of 1,750 m. Hydrocarbon generation began at least 115 million vears ago in the basal portion of the Gomo and Tauá members. The Gomo Member, below Marker 54, lies within the hydrocarbon generation window and does not reach the senile zone. Two models are proposed for secondarv hydrocarbon migration and accumulation models in the Rio do Bu Field: direct contact between source rocks and reservoirs, and migration through fault and fracture planes.