Study of the genesis and variation in geochemical and reservoir characteristics of hydrothermal dolomite bodies in the Cantabrian Zone (Northern Spain).

The Cantabrian Zone in Northern-Spain represents the foreland of the Variscan Orogeny in the Iberian Peninsula. Post-orogenic curving resulted in the creation of an orocline, with extension and crustal thinning in the outer part of the bend and compression and crustal thickening in the inner part (Weil et al., 2000; Gutiérrez-Alonso et al., 2004). The Bodón Unit in the southwestern Cantabrian Zone consists of three thrust nappes (Forcada, Bodón and Gayo; Marcos, 1968) with stratigraphic successions ranging from Cambrian to Carboniferous in age. Several impressive dolomite bodies can be found in the formations of the Bodón Unit.

The dolomite bodies are very well studied from petrographic and geochemical point of view by Gasparrini et al. (2006a & 2006b). The proposed dolomitization model implies the circulation of hypersaline and hydrothermal marine-derived brines by thermal convection. However, the influence of dewatering shales and clay mineral transitions is most likely underestimated in this model (M. Gasparrini/R. Swennen, pers. com.).

The first goal of this research project is to fine-tune the model postulated by Gasparrini et al. (2006a & 2006b) by using different methodological approaches. A petrographical and geochemical study of the San Emiliano Formation (the siliciclastic formation suspected to have influenced the dolomitization) will be conducted. Also, a volumetric study of this formation and of the dolomite bodies will be performed as to estimate their relative volumes. The application of Mg isotope geochemistry will be studied. A proven influence of the San Emiliano Formation can lead to a conceptual model on the influence of dewatering shales in dolomitization, a model that could be applied to other dolomite occurrences worldwide.

As a second goal, the fine-tuned dolomitization model will be used to interpret and explain the variation of geochemical and petrophysical characteristics in the structurally controlled hydrothermal dolomite bodies. Most carbonate sedimentologists assume a certain variation of characteristics in these bodies (e.g. Warren, 2000), however, a study on this topic is seldom included. The main aim of this study is thus to map these variations and try to understand the parameters that result in the variations. There will be a strong focus on mechanical stratigraphy (as to include both matrix and fracture porosity) and overdolomitization. Well-exposed outcrops of structurally controlled dolomite bodies in the Bodón Unit will be chosen to perform detailed and systematic geochemical and petrophysical sampling. In the scope of this second goal, carbonate clumped isotope thermometry will be applied. The predictability of porosity and permeability variations within dolomite bodies can lead to optimized exploration methods, not only for the gas and oil industry but also for geothermal energy and CCS purposes.