Meteoric diagenesis in carbonates below karst unconformities: heterogeneity and control factors

Abstract

AbstractThe combination of sedimentological and diagenetic data is important for the characterization of carbonate pore systems. This is particularly true for carbonates that were affected by meteoric diagenesis during sub-aerial exposure, for instance at sea-level lowstands. This diagenetic environment is commonly believed to be associated with increases in porosity, permeability and pore-throat diameters. Using data from three localities, improvement or deterioration of reservoir parameters below karst unconformities were analysed with a three-fold approach. In the first step, meteoric dissolution was characterized and early to late diagenetic products were described. In the second step, sedimentological and diagenetic data were converted to petrophysical data. In the third step, modelled climate data, in particular the occurrence of monsoon cells, in conjunction with other control mechanisms, were considered to understand the processes that controlled meteoric dissolution and later pore infill.Three case studies were analysed: (1) Lower Triassic oolites (sedimentary rocks dominated by ooids) and microbialites of the Calvörde Formation (Buntsandstein Group, Germany); (2) stacked shallowing-upward cycles of carbonate platform deposits in the Middle–Upper Triassic Mahil Formation (Arabian plate, Oman), capped by palaeosols; and (3) an Upper Triassic coral patch reef and overlying strata (Adnet, Salzburg region, Austria). Data integration allowed the establishment of three scenarios of significantly different processes related to meteoric diagenesis below unconformities: (1) increase of porosity and permeability and their preservation through time; (2) increase of porosity and permeability and subsequent pore system occlusion; and (3) decrease of porosity and permeability and creation of a barrier for pore fluids. Knowledge of the time span involved in meteoric diagenesis and the nature of the climatic regime helped to explain the origins and control mechanisms of the meteoric pore systems. The study provided evidence that a well-connected, large karst system, typical of a humid climate, is likely to be sealed subsequently by sediment and cement. Under arid climatic conditions, tight palaeosols developed at the unconformity and small karst pore systems developed which had the potential to remain open during basin evolution. Depending on the aforementioned parameters, carbonates affected by meteoric diagenesis may either become tight rocks or reservoirs.