A volume-based approach to calculation of ancient carbonate accumulations

Volumes of carbonate rocks are computed for Mesozoic platforms over several orders of length scales (reservoir to basin scale) for millions of years time intervals. The calculation is based on the restoration of depositional geometries by correlating genetic sequences and on the estimation of carbonate concentrations from core and well-log data. The interpolation yields the total amount of carbonate buried in a closed system and represents the net budget of production/preservation of sedimentary carbonate for a given time interval, over a defined area covering several depositional environments. A three-dimensional (3-D) accumulation rate is obtained by dividing the total volume of carbonate rocks by the total surface over a time interval derived from biostratigraphic data. Estimates of such a carbonate accumulation rate for various Jurassic and Cretaceous platforms range from 8 to 67 m/m. yr. Errors in computing carbonate volumes are limited, whereas an uncertainty is associated with the calculation of depositional rates because the duration of stratigraphic intervals is moderately controlled. Application to the Middle Jurassic carbonate platform of the Paris Basin reveals that the accumulation rates match the stratal geometry, i.e., progradation, vertical stacking, and retrogradation. The procedure also makes it possible to map the carbonate accumulation, to describe the volumetric partitioning of carbonate products between different depositional environments, and to calculate the inshore/offshore depositional ratio. The accumulation rates obtained are relevant to problems as diverse as change of depositional profile and numerical modeling of carbonate systems, growth and drowning of carbonate platforms, and surface chemical cycles through the inorganic carbon burial in ancient cratonic domains.