Core samples of the Cenomanian glauconitic sediments from the Great Burgan oil field, Kuwait, display important mineralogical changes both under surface and subsurface conditions. Unconformities can be demarcated with confidence by studying the chemical weathering of unarmored glauconite which results in the precipitation of hydrated iron oxides and alunite. In the subsurface, close to oil reservoirs, the reduction of the hydrated iran oxide dust in glauconite pellets results in the development of black magnetic siderite. The presence of this mineral may explain the currently observed magnetic anomalies in clastic sediments close to oil reservoirs.

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