Impacts of dolomitization on the petrophysical properties of El-Halal Formation, North Sinai, Egypt

The Cenomanian marine sequence in north Sinai is mostly represented by El-Halal Formation. Petrographically, it is composed of four microfacies, namely: (1) dolomitic micrite, (2) dolomicrite, (3) dolowackestone and (4) dolosparite. The assigned pore volume (10-25%) could be differentiated into: micro intercrystalline pore spaces, micro to meso vugs and micro pore channels. The present pore spaces are frequently reduced by drusy and/or xenotopic dolosparite and micro sparry calcite. El-Halal Formation has been deposited in a restricted marine platform (SMF-23, FZ-8) with a progressive shallowing sea level upward. The diagenetic history has been controlled by cementation, dolomitization, aggrading neomorphism and creation of authigenic illite. Petrophysically, the studied samples could be grouped into three petrophysical facies: (1) dolomitic micrite/dolomicrite facies, (2) dolowackestone and (3) dolosparite. The permeability (ave. 3.54, 12.9 and 0.49 md, respectively) is dependent on the pore channel radius (ave. 1.03, 1.92 and 0.19 mu m, respectively) and porosity (ave. 22.1%, 25.8% and 11.4%, respectively); it could be related to the electrical tortuosity as well (ave. 3.28, 1.40 and 5.06, respectively). The apparent formation resistivity factor was measured at five consequent saline concentrations of 6, 30, 60, 90 and 120 kppm and the true one has been calculated. It is controlled by the effective porosity and electrical tortuosity. Though dolomitization has an enhancing effect on the studied petrophysical features, it had a reducing effect in its first stages of invasion through filling the pore spaces and in the last stages through the aggrading neomorphism.