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, a 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.