The Oil Drum

Yes, sorry I should have explained a little more on the carbonate, and will next time in more detail.  Essentially most of them are calcium carbonate, or limestone, (though there is a fair bit of magnesian limestone or dolomite around).  The rocks are all sedimentary and oil formed in the seas that were depositing the remains of the oil-forming organisms at the same time as it was laying down the skeleton remains, and fecal sediments that gave rise to limestone, or the sand being formed and deposited to make the sandstone.  A lot of cap rocks are finer grained sediments, such as shale, though the presence of the evaporated salt provides a very good cap.  Also, because salt is plastic and deforms and creates domes that are pushed up through the surrounding rock they are a good source for oil in the rocks along their edges.  Some of the oilfields in Saudi Arabia are of this type, and were formed this way.