--> Characterization of Dolomite Bodies in Carboniferous Host Rock, Picos de Europa, Northern Spain

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Characterization of Dolomite Bodies in Carboniferous Host Rock, Picos de Europa, Northern Spain

Abstract

Dolomitization is a diagenetic process that can significantly alter the rock fabric and affect carbonate reservoir quality. Modeling dolomitized reservoir is a complex process as the dimensions and extent of dolomitization cannot be quantified from subsurface data alone. Hence, outcrop analogues are used to provide data in order to constrain subsurface reservoir models. This study focuses on late diagenetic dolomite geobodies in the Carboniferous host rock in Picos de Europa Province, northwest Spain. The outcrop study allows the characterization of 53 dolomite geobodies and an evaluation of its link with sedimentary lithologies and structures. An integration of datasets including (1) photo panoramas from the field, (2) dolomite and limestone systematic sampling, (3) fracture analysis, (4) satellite aerial images, (5) and mapping of dolomite geobodies with GPS are utilized to characterize the geobodies in three dimensions. Outcrop observations have shown that dolomite bodies of varying sizes are present, with an average length of 80 m, width of 51 m and height of 37 m. The dimensions of the geobodies are also significantly different between two host limestone of the studied formations. We suggest that larger dolomite bodies are favoured in zones with large fluid input and accumulation to allow for extensive chemical reaction. The dolomite geobodies are generally elongate in geometry, with the longest dimension along main fault planes, with length/width ratio of about 2. In addition, porosity measurements undertaken in this study demonstrate a higher porosity in the dolomite geobodies than in the surrounding limestone host rock, where porosity values less than 1.6% characterize the limestone host rock and porosity values for the dolomite range from 0.9 to 5.8%. Field observations and petrographic analysis of rock samples have shown evidence of structurally controlled dolomitization, generally interpreted as hydrothermal dolomite (HTD). Observations that support this interpretation include the presence of saddle dolomite, zebra fabrics as well as dolomite infilled fractures. Fracture analysis was also conducted to assist in understanding the tectonic history of the dolomitized host rock. This study has shown that the geometry, dimension and distribution of the dolomite bodies are inherently linked to a combination of factors including the original lithology of the host limestone, predolomitization diagenetic history, and structural elements. This study has also provided an understanding of the diagenetic history in the study area, as well as a dataset of dolomite body dimensions, relevant in modeling analogous subsurface reservoirs.