From a Tight Carbonate to a Porous Potential Reservoir: Hydrothermal Dolomites in the Paleozoic of Northern Spain

Munoz, Natalia ¹; Bechstaedt, Thilo G.¹; Boni, Maria ²; Gutiérrez-Alonso, Gabriel ³
(1) GeoResources STC, Heidelberg, Germany. (2) Scienze della Terra, Univ. Napoli, Napoli, Italy. (3) Departamento de Geologia, Univ. Salamanca, Salamanca, Spain.

The Cantabrian Zone, N Spain, represents a Variscan foreland basin, subsequently affected by post-Variscan orocline formation. Pervasive porous burial dolomites, embedded by tight, mainly Carboniferous limestones, serve as outcrop analogues for dolomite reservoirs elsewhere. Objectives of the Eni-funded research are geometry and size of dolomite bodies, types of dolomite, porosity, permeability, and relation of dolomitization with geodynamic evolution.

The dolomite bodies are up to several kms long and hundreds of metres thick. Replacive and porosity-filling dolomites occur. Petrographic, geochemical and FI data are consistent with a burial, hydrothermal dolomitization, caused by hypersaline, sea water-derived brines in an almost isothermal and isochemical process.

Various dolomite types containing macro- and microporosity can be defined according to textures and internal fabrics. Macroporosity is highest in the Vuggy and Zebra types of porosity. Dolomitization strongly improved the poroperm properties of the tight limestone host rock, but local over-dolomitization reduced dramatically the porosity, just generated. This “self-cannibalization” process consists of four steps: (i) replacive dolomitization creates porosity; (ii) hydrothermal fluids are focused into the highly permeable pore network; (iii) infill of the pores with cements first stabilizes them, but (iv) subsequently occludes the pores partly/totally.

Late Variscan lithospheric delamination-induced extensional tectonics and crustal thinning provided the heat required to trigger fluid convection and dolomite precipitation. Effective fluid flow was only possible at this time of post-Variscan rifting and transtension. Fluid circulation was facilitated by the reactivation of highly permeable Variscan and post-Variscan fault zones, as well as of pre- and syn-orocline bending joints. Oversaturation and dolomite precipitation occurred, when the overpressured fluids migrated into shallower horizons, loosing temperature and pressure. Extensive bodies of hydrothermal dolomite were favoured by a large reservoir of evaporated sea-water, confined by orocline formation. The abundance of dolomitization in the western areas is associated with a high amount of fractures. Timewise, this hydrothermal pulse seems to correspond to the circulation of hydrothermal fluids in the Stephanian Coal Basins located in the area.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.