Gas chimneys and their impact on gas migration, accumulation and de-risking of prospects, offshore Nile Delta - Central Sub Basin.

Abstract

Abstract Baltim Concession is a large exploration/exploitation license located in the offshore Nile Delta, Egypt. The concession covers an area of 430 Km2 of the central portion of the present day Nile delta cone. High quality three dimensional (3D) seismic data, coupled with data from some wells drilled in the area, have highlighted the presence of some gas chimneys well recognizable started from pre-Messinian until the Plio-Pleistocene slope succession. The geometry and architecture of the gas chimneys in Baltim area have been imaged by 3D seismic techniques, time section and a variety of attribute extractions, providing us with a high resolution definition of these features. The main indicator for gas chimneys in seismic is an almost cylindrical shaped chaotic behavior of seismic signal, due to scattering of seismic energy by diffused gas through the cap rocks above the leaked reservoirs. The origin of gas chimneys has to be related to hydraulic fracturing by gas leaking through faults from deep accumulation where overpressure conditions have been generated by fast burial during the Plio-Pleistocene mega sequence deposition. The petroleum system in Post-Messinian (Plio-Pleistocene) succession generated biogenic gas only, whereas the Pre-Messinian system proved to generate thermogenic gas and oil. The role of gas chimney and associated structures as hydrocarbon migration pathways from the pre-Messinian kitchen section to the Post-Messinian reservoirs is testified by many DHI's within the Pliocene – Pleistocene reservoirs that are in contact with the boundaries of the gas chimneys. In addition, geochemical analysis on gas samples from targets drilled in the Pliocene – Pleistocene indicates the presence of thermogenic gas clearly generated from pre-Messinian units. From isotopic data, PVT samples and production data from gas fields discovered in Plio-Pleistocene succession evidenced that in many cases gas migrated from the pre-Messinian through high fracture zones related to gas chimneys activities in the studied area.