Core samples from two different depths of a Carbonate oil field have been investigated using optical and electron microscopy, elemental mapping and atomic force microscopy. The sample from the greater depth displays a striking magnetic response, greater compaction and cementation of coccoliths, and an oily film on all surfaces, whereas the sample from higher up shows no magnetic response, no oily film and less well-developed coccoliths. Our results reveal the link between oil formation and the presence of iron compounds.