Carbonate coquina reservoirs: New insights for petrophysical reservoir characterisation

With the recent discoveries of Pre-Salt oil fields in lacustrine carbonate reservoirs of Barremian-Aptian age, on both African and Brazilian Margins of the South Atlantic, interest has increased in the petrophysical aspects of these complex reservoirs. Some of these reservoirs are composed of coquinas which are limestones composed of shells or shell fragments, in this case bivalves. The petrophysical challenges in coquina pore systems result from wide range of textures, fabrics and diagenetic processes showing; a. Primary shells preserved in fine-grained carbonate and/or mixed carbonate-clastic matrix b. Dissolved or partially dissolved and corroded shells in a fine-grained matrix c. Recrystallised shells and matrix d. Dissolution of fine-grained matrix and preferential preservation of shells e. Gradations between, and combinations of, all of the above We have investigated methods to quantify porosity, measure geometry, scale and distribution of pore space and its connectivity in plugs from coquina samples from the Sergipe-Alagoas Basin (onshore NE Brazil). These are the results of our initial investigation, where we could relate m to the type of porosity, connectivity of pores, pore throat size distribution and permeability. In general, the increase of porosity in these samples correlates with an increase in m.