Abstract: Deep Reservoirs in Santos Basin, Brazil

Ramos, Ramiro; Edson Duraes; Luiz Fochesatto; Luiz Gamboa - Petrobras/ Cenpes

The Santos Basin is located in the southern portion of Brazilian continental margin. Extending in an area of 206,000 Km², it is geologically limited to the north by the Cabo Frio High and to the south by the Florianopolis Platform. From one hundred wells drilled in the Santos Basin, some discovered, in shallow waters, light oil and gas from Albian limestones and Upper Cretaceous sandstone (Fig. 1). These reservoirs range in depth from 4,800 to 5,200 meters. In spite of its high depth, these reservoir rocks still preserves good permo-porosity characteristics.

The Merluza field produces gas and condensate from siliciclastic rocks of Jureia and Itajai formations. These deep reservoirs (around 4,500 meters) present thickness of 70 meters, porosity ranging from 12% to 16% and average permeability of 14 mD for Jureia and 4 mD for Itajai reservoir. The cumulative production reached 88 BCF of gas and 7 million BBL of condensate, with remaining reserves of 194 BCF and 10 million BBL, respectively.

Five discovered fields from carbonate rocks, Tubarao, Estrela do Mar, Coral, Caravela and Caravela Sul, hold reserves of 79 million BBL of light oil (40° to 45° API) and 160 BCF of gas. These reservoirs have an excellent lateral continuity and a high vertical compartmentation, comprising four main zones ranging in depth from -4900 to -5200 meters. The diagenetic history is believed to be responsible for the distinct reservoir quality of each zone. The upper zone (B1) was submitted to intense meteoric activity with resulting microporosities and low permeability (< 0.5 mD). The zone just below (B2), although having the same average porosity (around 12%) was better preserved, showing macroporosities and, consequently, high permeability values (>300 mD). The deeper two zones (B3 and B4) have intermediate characteristics. The Tubarao field has gas and condensate in the B2 zone. The original pressure and temperature for all reservoirs are around 8,500 psia and 150° C at -4900 meters. Contents of H₂S ranging from 5 to 40 ppm, but leveling up to 10,000 ppm in the Tubarao field, plus a CO₂ content around 0.5% and 1.5%, demand special subsurface equipment to resist the severe corrosive environment. The Caravela field produces since December 1993 from two wells with special steel string. This field has already produced 13 million BBL of oil. An integrated development project is planned for the others fields to optimize the production facilities. The gas from Tubarao, Estrela do Mar, Coral and Caravela fields will be integrated to the already running Merluza exportation system, through a new gas pipeline.

There are also possibilities of new findings along the carbonate-producing trend. One of these structures was already drilled by BSS-70 well and encountered condensate and gas in high pressure (>14,000 psi) with H₂S content around 500 ppm. Due to equipment capacity, it was not possible to complete the reservoir evaluation. This accumulation will be better tested by a new and special well soon to be drilled.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil