Using Analogue Hypogene Karst Systems to Understand the Pre-Sal Carbonate Reservoirs Offshore Brazil

Abstract

Abstract

It became dogmatic that carbonate reservoirs affected by karst processes were subjected to subaerial exposure. However, there is a growing recognition that hypogene karstification, formed under deep-seated, confined conditions by rising fluid flow, can generate macro and mega (meter-scale) porosity. Although this kind of karst process has the potential to create extensive caves, it could also form features below seismic resolution, thus justifying the use of analogue studies. We investigated the major cave system in South America to unravel the geometry and origin of the hypogene system and use it as a predictive tool to understand the poro-permeability properties of pre-salt carbonate reservoirs offshore Brazil. The caves were developed in Neoproterozoic carbonates of the Salitre Formation in the São Francisco Craton, NE Brazil. We mapped both sedimentary facies and deformation patterns in the caves. In addition, we described in detail the carbonate and siliclastic facies in thin section and carried out a petrophysical analysis that included X-ray microtomography and the determination of porosity, permeability and grain density. We identified five lithostratigraphic units, from bottom to top: (1) grainstone with cross-bedding stratification, (2) fine grainstone with chert nodules, (3) microbial carbonate, (4) interbedded fine siliciclastics and marls, (5) crystalline carbonate that interfingers with chert layers. Field and laboratory data indicate that units 4 and 5 formed a kind of seal for fluid flow, below which flow occurred laterally causing dissolution of units 1, 2, and 3. These five units are identified throughout the Salitre Formation, which makes the stratigraphic control predictable. In addition, the hypogene karst system occurs immediately in the gently folded hangingwall block of a thrust fault, which served as conduit for deep-seated fluids. Dissolution mainly enlarged subvertical fractures developed mostly along the anticline hinges. These hinges follow the general trend of regional deformation, which is also predictable. This study highlight the importance of determination of the origin of karst systems for understanding highly heterogeneous fractured and karstified carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016