Fractures, fluids and hypogenic karsts: key features in carbonate reservoirs and aquifers

Karsts have been documented in a large number of hydrocarbon reservoirs but their origin has generally been attributed to meteoric processes. In the last years, however, it has been increasingly recognized that many of them have a hypogenic origin which implies that the CO2 and/or H2S responsible for carbonate dissolution come from deep sources transport being favoured by upward flow of hot fluids. These cavities can be meter to few tens of meters large, therefore being below the resolution power of seismic studies; they are often associated with large dolomitized bodies and the occurrence of silicified rocks, as well as the presence of exotic minerals such as barite. Similar features are being identified in carbonate successions targeted for geothermal heat extraction. Tools are needed to predict the distribution and geometry of these karsts and, eventually to implement them in reservoir simulators. In this contribution we report on the extensive work we have performed in caves developed in Neoproterozoic, generally tight carbonates of the Salitre Formation (Irece basin, Bahia state, Brazil). Integrating portable LiDAR modelling with structural and petrographic study we we propose a model for the development of these large-scale dissolution features. Fluids responsible for the formation of the hypogenic caves were expelled from the base of the Braziliano orogeny and/or during subsequent exhumation and experienced long-range migration towards the foreland along regional aquifers. Once flow encountered steep features such as strike-slip or thrust faults, hot aggressive fluids started moving upward along the fault zone and/or related fracture corridors. If upward flow became prevented by the presence of an impermeable layers, fluids were compelled to flow laterally along high permeability layers. Permeability changes in the carbonates we have analysed are essentially related to changes in fracture density. The lateral flow and associated cooling of chemically aggressive fluids caused widespread speleogenesis and, roughly at the same time, deposition of silica. Geometry and dimensions of cave passages are controlled by a complex interplay between the ability of flow to bring acids to the carbonate wall and retrieve the reaction products and the reaction kinetics. The direction of passage was influenced by fracture sets, but their spacing is related to intrinsic flow processes rather than to the spacing of the fractures themselves. Our model has a general value and is applicable not only to foredeep settings but also in rifted continental margin were regional flow takes place and is often intercepted by steep faults.