Datapages, Inc.Print this page

AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Hydrothermal Dolomites along Faults/Fractures in the Aptian-Albian Platform Carbonates (Karrantza Valley, N.W Spain): Petrographic and Geochemical Results

Mumtaz M. Shah1; Fadi H. Nader1; Julie Dewit2; Rudy Swennen2; Daniel Garcia3

(1) Sedimentology-Stratigraphy, Institut Français du Pétrole (IFP), Rueil-Malmaison, France.

(2) Earth and Environmental Sciences, Katholieke Universiteit, Leuven, Belgium.

(3) GENERIC; Centre SPIN, Ecole Nationale Superieur des Mines, Saint-Etienne, France.

Aptian-Albian carbonates were deposited in the Basque-Cantabrian Basin (NW Spain) as a result of an intense rift-related subsidence. This rock succession is affected by N-S, E-W, NW-SE and NE-SW oriented normal and strike-slip faults, which acted as fluid flow conduits to selectively dolomitize the Aptian-Albian carbonates in the study area (Karrantza valley, NW Spain).

Field studies exhibit excellent beige coloured dolomite exposures along the faults surrounded by light grey coloured host limestone. Linear fault restricted dolomite and irregular dolomite bodies, indicate that lithology mechanically controls the geometry of the dolomites.

Different dolomite facies exhibit systematic variation away from the fault/fracture. Paragenesis of these dolomites indicate their development over different dolomitising events. Several phases of calcite cement pre- and post-date the dolomitisation events. Pyrite (2-phases), galena and sphalerite are also locally observed. Zebra dolomite shows the most depleted values with δ18O varying between -18.6 to -15.2 ‰V-PDB and δ13C between -0.7 to +1.6‰ V-PDB compared to non-planar and planar dolomite which show overlapping stable isotopic ratios ranging from -18.7 to -11.5‰ δ18O V-PDB and -2.6 to +1.9‰ δ13C V-PDB. The broad range and highly depleted values of δ18O are interpreted to reflect multiphase dolomitization and dolomite recrystallization. Limestones close to the dolomites show depleted δ18O values (similar to those of the dolomites), implying isotopic resetting during dolomitization and large scale fluid migration. Microprobe analysis indicate Fe-rich central part of dolomite (1.6 to 1.9 wt.%) as compared to their external zones and the dolomite cement (Fe: 0.0 to 0.6 wt.%). The Mg content in the void-filling, non-luminescent calcite is high (0.9 to 1.3 wt.%) as compared to luminescent, replacive calcite (0.1 to 0.8 wt.%). Fluid inclusion analyses show Th values range from 120 to 200°C and estimated salinities between 10 and 24 eq. wt. % NaCl for zebra and nonplanar dolomites, which confirms very hot dolomitizing fluids (hydrothermal origin)of the investigated dolomites.

Above stated studies helped in defining the geometry of dolomite bodies (reservoir heterogenity characterization), constraining the genesis of the dolomites and superposition of diagenetic events, documenting the characteristics of hot, saline dolomitizing fluids and related fluid flow history.