Most petroleum comes originally from sedimentary rocks- source rocks in the language of the oil business-containing plenty of organic matter usually formed in lakes or shallow seas. If these rocks were heated up to the right temperature when they were buried, the organic molecules reacted to make oil and gas. Oil reservoirs form when the hydrocarbons move away from the source rocks and accumulate in a permeable area of rock, often sandstone. In places, the petroleum oozes from the ground; elsewhere, it fills fissures and pores within the rock as what is called an impregnation.
The seepages are principally within sedimentary strata-the old Red Sandstone-but they also fill veins and fissures in metamorphic rocks. In the 19th century the local blacksmith used to quarry these veins of pitch to fire his forge. The seepage and impregnation comes principally from layers of shale rich in organic matter, within the old Red Sandstone itself. These shales were mud in lakes some 400 million years ago; the oil of at least one sea field probably came from the same source.
Elsewhere there are reports of petroleum in strange places, including impermeable igneous and metamorphic rocks that the conventional petroleum prospector would regard as barren. The hydrocarbons are far away from conventional source beds from which they could have migrated. For example, there are lumps of ozocerite, plastic, waxy paraffin oil, on the shores; bitumen impregnates granites. A few kilometers away, there was seepage of petroleum from fractured schist sufficiently abundant and pure for a local farmer to run his tractor on it. Suspecting that the farmer had found his fuel from some recent spill or a leaking tank somewhere, Customs and Excise officers tried to trace it. But even their best efforts could not find a suitable, taxable source.