Evidence of High Temperature Hydrothermal Regimes in the Pre-Salt Series, Kwanza Basin, Offshore Angola

Abstract

The Lower Cretaceous Pre-salt carbonates in the Kwanza basin constitute significant potential targets for oil and gas exploration, and have been under much scrutiny over the past few years. These synrift and sag carbonates are predominantly composed of continental deposits of Barremian and Aptian age. They often exhibit very complex diagenetic transformations, which had a profound impact on reservoir properties. One particular difficulty in the understanding of the diagenetic processes at stake is the reconstruction of paleo-thermal regimes and their evolution through time. Indeed, it is well established that rift systems can be the locus of elevated geothermal gradients, which promote elevated temperatures even at shallow burial depth. In addition the common development of intrusive/eruptive volcanic activity may also induce widespread hot hydrothermalism. These two factors have played a significant role in the studied presalt carbonates of offshore Angola. Fluid inclusion and isotope (δ18O, δ13C, Sr87/86) data gathered from a reference well located in the South-Western part of the Kwanza basin, about 100 km from the Angola coast, provide valuable information about the paleo-thermal regimes and the paleo-waters involved in the diagenesis of the reservoir. Fluid inclusion measurements were done in a variety of intergranular and fracture-fill cements (mainly dolomite, calcite, quartz). The results indicate that paleo temperatures in the presalt series largely exceeded present-day temperature (90-110°C), likely approaching 180-200°C. The documented temperature record attests of a complex non-linearly-increasing thermal history, suggesting variation in heat flow as a result of margin evolution and paleo geothermal gradients well in excess of 35°C/km in the early stage. In addition, the ubiquitous occurrence and abundance of primary/secondary CO2 gas inclusions records the presence of CO2 in the pore fluid during much of the post-depositional history of the presalt series. Variation in the composition of CO2 gas inclusions suggests multiple episodes of CO2 influx and thermodynamic modeling of fluid inclusion data indicates that higher-than-hydrostatic pressures may have existed, likely reflecting local transient effects in fractures. The data further indicate that the waters involved in the diagenesis of the studied reservoirs were 18O-enriched, highly saline brines (>200 g/l), interpreted as being primarily of hydrothermal origin.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017