Deep hydrocarbon reservoir: the nest Brazilian exploration frontier?
Abstract
During the last 10 years, recent discoveries of high porosities and permeabilities in deeply buried sedimentary rocks, have pointed out that deep hydrocarbon reservoirs probably will be the next Brazilian exploration frontier. According to conservative estimates, deep reservoirs contain reserves around 400 x 10 9 m 3 of natural gas, which justifies an increasing search for deep accumulations. From 4,909 exploratory wells drilled in the Brazilian basins up to May, 1989, 1,158 or 24% are deeper than 3,000 m Despite this significant percentage, the number of deep wells is still insufficient for a detailed evaluation of the hydrocarbon potential of deep reservoirs in Brazil, because there are more than 30 basins to be explored, covering an area over 5x 10 6 km 2. Also, Brazilian deep wells are poorly distributed: 53% of wells deeper than 4,000 m were drilled in 3 basins on/y (Campos, Santos, and Sergipe-Alagoas), and 38% of those deeper than 5,000 m are located at Santos Basin. Despite the relatively small amount of information avaliable, detailed studies of the deep reservoirs already Identified have pointed out some important processes and geological situations for the preservation and development of porosity at great depths in the Brazilian basins. These are: 1) late burial or little time of residence at great depths: 2) early migration of hydrocarbons, when the reservoirs are still poor/y compacted and/or cemented; 3) early migration of organic solvents into the reservoirs, improving the development of secondary porosity; 4) succession of two phases of generation of secundary porosity; 5) framework composition resistant to the mechanical (compaction) and chemical (cementation) reduction of porosity, specifically those rich in quartz grains and poor in ductile components such as mud intraclasts and fragments of state, phyllite and schist; 6) early coating of grains by clays and/or oxides, which inhibit pressure solution and cementation by quartz and feldspar overgrowths; and 7) early cementation by calcite, which preserves a relatively loose packing in the rock during burial, so that its dissolution in subsurface tends to increase significantly the reservoir porosity.
