Spatial variations in the porosity development of carbonate sediments and rocks

Abstract

Diagenetic sequences of many stages are know from various carbonate rocks, but the diagenetic spatial variations within a given facies, outcrop, hand specimen or thin section are only gradually rediscovered and taken into account in porosity studies. The variations occur in various carbonate facies such as reefs, beachrocks, shallow-water deposits, hardgrounds, and concretions. Multiple, parallel diagenetic pathways, branching in time, are recognized in such cases. Diagenetic spatial variations are caused by individual characteristics of particles and cements (mineral and chemical compositions, ultra- and microstructure, surface properties); collective characteristics of sediments and sedimentary rocks (component mix due to ecological and transport selection, fabrics as determined by primary sedimentary and skeletal structures); synsedimentary to early diagenetic processes (internal sedimentation, bioturbation, bioerosion); early to late diagenetic processes (cementation, dissolution, fracturing, decomposition of organic matter); hydrology (wave and tide ranges, sea and ground water levels, mixing zones), as well as substrate mobility and solubility. Many of the causal factors are determined, directly or indirectly, by biologic, meteorologic, or oceanographic factors. Causal factors may combine and thus, as a given diagenetic stage, provide for considerable diagenetic complexity. In the course of diagenesis, i.e. in time, the spatial variations increase: The spatial heterogeneity of one stage provides the scene for cementation, dissolution and/or alteration to act in their respective ‘local’ or ‘micro-local’ environments, and thus to increase the heterogeneity for the next scene or diagenetic stage. Superposition of respective variations gradually increases the diagenetic diversity. Porosity development therefore progresses not only in time, but also in space, and thus is four-dimensional. The study of diagenetic spatial variations and the respective detailed analyses lead to a comprehensive understanding of porosity development and of the highly variable and complex diagenetic history of carbonate rocks. The complexity provides problems for the application of diagenetic information in cement stratigraphy and porosity assessment. Cement-stratigraphic correlation, when taking into account spatial variations, uses selected pore types and pore fillings in a defined space/time framework. Examining pores and pore fillings of different diagenetic stages, the respective paleo-hydrologic gradients, the sources of solutes, and thus the hydrologic history of depositional and diagenetic environments can be established. Porosity assessment, when coping with diagenetic complexities, either is based on quantification of diagenetic pathways/products encountered, or on selection of indicative pores or pore fillings. Differentiation according to diagenetic stages permits assessing the porosities at successive stages. Realization of spatial variations in porosity development spoils fast and simple correlations, assessments, or predictions. The synthesis of the detailed analytical information, however, provides a route toward application of diagenetic complexities.