Preliminary study of hydrogen sulfide occurrence in the Potiguar Basin - Northeast Brazil

Abstract

The drilling, complementation and production of oil wells in geologic areas containing hydrogen sulfide gas is problematic and expensive, constituting a great challenge of the oil industry. Among the problems caused by H2S the most important are its high power of corrosion in the equipment, and its high toxicity.In the Potiguar Basin, Northeast Brazil, this acid gas is known to occur in its submerged portion of the basin (Pescada area - with grades of up to 50 ppm), and in its emerged portion (fields of Canto do Amaro, Macau, Fazenda Pocinho/Palmeira, Salina Crista/Conceição, Estreito/Rio Panon, Monte Alegre e Guamaré - with grades of up to 6,000 ppm). Analysis of the available data showed that the processes responsible for generating this hydrogen sulfide gas are different at Campo de Pescada from the other fields in the basin. At Campo de Pescada tne sulfur compounds present in the organic matter of the source shales of the Pendência Formation were subjected to thermal maturation (temperature above 170°F), and this is possibly the main reason for the H2S associated with the oil in the sandy reservoirs of this stratigraphic unit. In the onshore portion ofthe besin, tne presence otbiodegradable and altered oils in the sandy reservoirs of tne Açu Formation - known as an excellent aquifer in terms of its hydrodynamics - were subjected to an average temperature of 115°F in the presence of organic and inorganic nutrients, this may suggest that the H2S was generated by the action of reducing sulfate bacteria after the oil accumulated in these reservoirs.