A series of log examples recorded in wells drilled in North Europe illustrate the most typical logging problems likely to be encountered in the North Sea:

  1. The Cenozo?c and Mesozo?c formations mostly correspond to sands, sandstones or chalk, interbedded with shales and marls. They are normally drilled with either fresh mud or sea water.

  2. The upper Permian : the Zechstein. This is an evaporitic series with mostly salt, anhydrite, dolomite, shale and sometimes gypsum. The formation water and mud are salt saturated. The eventual objectives are the dolomites which could either be micro-crystalline, with porosities as high as 30%, or dense and fractured.

  3. The lower Permian : the Rotliegend. This is the main objective in the North Sea exploration. This horizon is a sandstone and sand series, with shale intercalations. The Rotliegend can be either clean, with high porosities up to 30%, or shaly with small effective porosities. Total sand thickness may go as high as 1000 feet. The formation water in the Rotliegend is salt saturated.

  4. The Carboniferous. This is the hard shaly sandstone and shale series. Porosities and permeabilities are low. The formation water is salt saturated.

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