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Hydrothermal dolomitization of Paleozoic successions in Northern Spain: petrophysical properties and structural control

Muñoz Quijano, Ingrid Natalia

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The Cantabrian Zone in NW Spain represents the foreland fold and thrust belt of the Variscan orogen, formed by Precambrian basement covered by an almost complete succession of Palaeozoic sediments, mostly deposited on a passive margin. Most of the succession underwent deformation with almost no internal strain (Pastor-Galán et al., 2009), and in most areas with only a diagenetic overprint. This includes, however, several thermal events (only locally up to the epizone) and different episodes of fluid flow. One of these epigenetic fluid flow events produced pervasive dolomitization on a large scale covering over 1000 km2 in todays outcrops. The present research focused on this post-orogenic hydrothermal dolomitization, which strongly increased porosity and permeability in large volumes of rocks. It affected mainly Carboniferous but to a smaller degree also older carbonate successions. Earlier research (Gasparrini et al., 2006; Lapponi et al., 2007) concentrated on the type of dolomite and its geochemical and isotopic characteristics, as well as the fluids generating this dolomite (very saline, originally evaporitic brines with elevated temperature). Of special interest was the timing of dolomitization (latest Carboniferous/earliest Permian) and the relation with the geodynamic setting: dolomitization was associated, at least time wise, with the bending of the Cantabrian Arch (see Gutiérrez-Alonso et al., 2004; Weil et al., 2013) related with a lithospheric delamination-induced thermal event. These main results were confirmed by this research.

The investigated area was distinctly extended compared with the mentioned earlier research. A study of petrophysical properties, geochemistry and fluid inclusions was carried out, in relation to the characterization of the dolomite bodies and the structural setting. Of special interest was the amount, types and regional distribution of porosity and permeability. Various parameters such as crystal size, orientation of the porosity in relation to dolomite textures and fabrics, and eventual rhythmicity of pore space are used to identify regional dolomitization patterns The data from the investigated outcrops within the large area of research serve to outline areas of increased porosity and permeability. The latter favourable factors might allow to predict locations of dolomite reservoirs with enhanced porosity elsewhere.

Large bodies of hydrothermal dolomite were favoured by: (i) a restricted and large reservoir of highly evaporated sea water; (ii) intense fracturing acting as fluid flow pathways; and (iii) a high thermal gradient enhancing convectional fluid flow. Positive petrophysical properties are affected by specific dolomite fabrics (Zebra, Vuggy, and Non Macro Porous), over-dolomitization reduced dramatically porosity.

The outermost western region of the Cantabrian Orocline provides a combination of the most positive factors for potential dolomite hydrocarbon reservoirs. Here the dolomite bodies are large, petrophysical properties are favourable, fracturation intensity is high and over-dolomitization is low.

Item Type: Dissertation
Supervisor: Bechstädt, Prof. Dr. Thilo
Date of thesis defense: 13 February 2015
Date Deposited: 09 Mar 2015 11:05
Date: 2015
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 550 Earth sciences
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