The paper reviews tectonic influence on pore pressure distribution in overpressured sealing zones, and their recognition on logs. Based on materials from West Siberia (Russia) and Gulf-of-Mexico (Texas and Louisiana) a study of tectonic and diagenetic impact on transitional zones forming and associated drilling problems is performed.

Faults play an important role in overpressure forming, but they can also cause partial or total damage of the seal. A fault can enhance sealing properties of shale cap or partially destroy them, allow «escape» of formation fluids and pressures from down-dip formations, forming a shallow overpressure zone (Matagorda Island 622/623 and East Cameroon). Various stages of shale diagenesis are related to water expulsion and can aid to abnormally height formation pressure forming. A nature of Bazenov abnormal pressure is related to TOC (total organic carbon) alteration, specifically to hydrocarbon generation. This overpressure was "translated" into Abalak shaly formation with complex fractured-cavernous reservoirs (Severo-Demyanskoe, Em-Egovskoe fields). Both events are causing drilling problems and could be recognized on NMR (nuclear-magnetic resonance) and conventional (resistivity, gamma-ray) log diagrams. By incorporating such information into the drilling model one can characterize complex formations and avoid them when possible from penetrating.

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