Parameters controlling porosity and permeability in clastic reservoirs of the Merluza deep field, Santos Basin, Brazil

Abstract

The Cretaceous (Turonian) turbidite sandstones of the Itajaí Formation/Ilhabela Member display an average porosity 0121% at a depth 014,700 m in well1-SPS-20 (Merluza gas field, Santos Basin). In adjacent well 1-SPS-25 these sandstones are also found saturated with gas and display an average porosity 01 16% at a depth of 4,900 m. These porosities are much greater than the average porosities of most Brazilian reservoirs located at such depths. Almost wholly intergranular, the porosity is believed to be predominantly primary. The preservation of porosity was due to the development of thick chlorite fringes al an early stage of diagenesis, which inhibited both pressure solution as well as the precipitation of cements. On the other hand, the sandstones of the base of the Juréia Formation, al a depth of 4,450 m in well 1-SPS-25, display an average porosity 01 only 12%. They constitute shallow platform deposits, possibly originating from a comprising barrier bar deposits. The absence of any chlorite fringes in these sandstones prompted more active chemical compaction and silica cementation, which resulted in greater destruction of porosity. The detrital composition of the structural framework and the depositional environment were the main factors that controlled both diagenesis and the preservation of porosity in the sandstones of the Ilha Bela Member and the base of the Juréia Formation.