Mixed wetted carbonate reservoir: origins of mixed wettability and affecting reservoir properties

The greatest Polish oil fields are built of the Main Dolomite. They are mainly created as a single porosity - double permeability reservoir rocks showing mixed wettability. Mixed wettability is a result of differentiated diagenetic processes connected with basin evolution. The Main Dolomite sediments were sealed very quickly between salt and anhydrite sediments within the whole basin. Hence, reservoir rocks were also source ones when carbonate grains were mixed with organic matter. This caused that residual organic matter is still present in the Main Dolomite rocks. Presence of organic matter and way of mixing are factors affecting wettability of rocks. There is no evident correlation between the content of residual organic matter and wettability index. The main role is played by the process of mixing carbonate grains and organic matter. Rock Eval analyses were applied to indicate the type of oil wet parts within pore space. It was shown that oil wet pores occupy the range from the smallest to the greatest pores. Oil and water paths of fluid migration are practically independent. A large part of samples (70) are predominantly oil wet, 17 show predominant water wetting. There was no pure oil or water wet rocks. Such type of wettability produces irreducible oil and water in all samples.