Deep carbonate reservoir of the Eo/Mesoalbian, Santos Basin

Abstract

The Tubarão Field, discovered in 1988 by wildcat well 1-PRS·4, was the first commercial hydrocarbon accumulation lo be found in the Eo/Mesoalbian limestones of the Santos Basin. Belonging to the lower Guarujá Formation, these limestones represent the first marine sedimentary record of the basin. Faciological detailing of the Tubarão has made it possible to identify various lithological types, including oolitic and oollitic/oncolitic grainstones and packstones as well as peloidal packstones and wackestones. These faciological types occur in different arrangements, m shoaling-upward cycles. The facies displaying a grainstone texture represent the upper unit of one cycle, while those with a wackestone texture represent the base unit. The Tubarão Field, studied here in production zones B1 and B2, has two notable characteristics. One is the faciological control of reservoir quality, with preservation of porosity occurring only in facies of high depositional energy (grainstones). The second is related to diagenesis. The intense meteoric activity to which zone B1 reservoirs were submitted partially obliterated the original porosity of the grainstones, producing a domain of microporosity that /s reflected in low permeability values. Zone B2 reservoirs wentstraight from marine to subsurface diagenesis/s, producing a domain of macroporosity and, consequently, high permeability values. What is most worthy of note is that the two reservoirs currently display nearly identical porosity values. Accumulation was thus controlled by the occurrence of high-energy facies and by the action of diagenesis, which, together with the early migration of hydrocarbons to the structure, fostered the preservation of interparticle primary porosity at depths of over 4,500 m.