Revealing the Dynamics of Cementation in Carbonate Reservoirs

Oil reservoirs have a greater porosity than their depth would indicate because they contain a lower volume of cement. The occlusion of porosity can be understood by undertaking a detailed analysis of the pore occluding cements. Cement phases contain a number of Cathode Luminescence (CL) zones, these CL zones can be used to understand the occlusion of porosity. The in-situ δ18O signature for each of these CL zones was obtained via SIMS. This assessment showed that the δ18O concentration became progressively more negative in the younger CL zones; this is likely related to increasing temperatures through progressive burial. The δ18O signal can therefore be used to help understand the dynamics of cementation. This study shows that the effective porosity in the water leg of the field is fully occluded by early burial cement phases; whereas, effective porosity is still present in the oil leg. Oil charge retards cementation in the reservoir, preserving the porosity in the oil leg. The field was progressively filled leading to lower volumes of cement being present in the crest of the field and larger volumes in the flanks, this is related to the progressive filling of the field with oil.