Meteoric Calcite Cementation: Diagenetic Response to Relative Sea-level Fall and Effect on Porosity and Permeability, Upper Miocene, Southeast Spain

Li, Zhaoqi; Goldstein, Robert H.; Franseen, Evan

Meteoric calcite cementation commonly reduces porosity and permeability. Quantifying its effect on porosity and permeability, and predicting its distribution, is essential for oil and gas exploration and production in carbonate reservoirs. This study investigates calcite cementation in an Upper Miocene carbonate platform in southeast Spain. The project uses field data, transmitted light and cathodoluminescence petrography, core plug data, petrographic image analysis, carbon and oxygen isotope analysis, and fluid inclusions. The succession consists of lower depositional sequences composed of heterozoan carbonates, middle sequences of reefal platform, and upper sequences of microbialite and oolite. Although sequence boundaries are also surfaces of subaerial exposure, those exposure events have had little impact on porosity and permeability. The subaerial exposure that caps the entire succession has had the most profound effect, producing an upper and lower zone of extensive calcite cementation in which each zone boundary cuts across stratigraphy. Cement textures suggest phreatic conditions. Cements in each of the two zones have a unique cathodoluminescence signature. Isotopic values for cement in the upper zone yield a meteoric calcite line with δ18O at -5.0 ‰ VPDB, whereas the lower zone is at -6.3‰ VPDB. δ13C for both are predominantly negative values, ranging from -10 to +2 ‰ PDB, suggestive of soil gas. Measurements of Tm ice in primary fluid inclusions yield a mode of 0.0 °C for both zones, indicating calcite cementation from fresh water. The upper zone pre-dates the lower zone on the basis of known relative sea-level history. These two zones are associated with the positions of two different paleo-water tables of different ages that formed during relative sea-level fall and erosional downcutting of the landscape during the Plio-Pleistocene. Thus, distribution of cements may be predictable on the basis of known sea-level history. Meteoric calcite cementation contributed to porosity and permeability reduction, but it was variable, as indicated by petrographic data and measured porosity/permeability values, which show a different correlation between percent calcite cement and each lithofacies. The cementation can be incorporated into subsurface geomodels by defining surfaces that separate cemented zones from uncemented zones, and applying lithofacies-specific relationships between cementation, porosity, and permeability.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013