Pennsylvanian rocks in the Paradox Basin act as the source as well as the reservoir for oil and gas. It is difficult to look at the Pennsylvanian rocks of the Paradox Basin at the surface of the Earth because in most places they are buried. Cores taken by oil companies provide some rock, and electric logs give information about areas where no core is available. A prominent characteristic of the Pennsylvanian rocks is their cyclicity. Salt, carbonates, and black shales alternate again and again. Some geologists think that these cycles reflect changes in sea level and/or changes in climate. Because the southern continents (Gondwana) were glaciated during the Pennsylvanian, repeated formation and melting of glaciers could be a mechanism that produced the Pennsylvanian cycles.
