Pangea Breakup and Petroleum Reserves
Geologic Setting
Restricted seaways formed as Pangea began separation, initially in the central Atlantic between North America and Africa in the Jurassic. The climate was arid yielding high evaporation. This resulted in high salinity which caused precipitation of salts (sodium chloride, calcium sulfate, and calcium carbonate) on the seafloor [evaporite deposits]. Jurassic salt deposits are overlain by Cretaceous and Tertiary marine sediments on the continental shelf from the Gulf Coast and portions of the eastern seaboard of North America as well as on the opposite side of the Atlantic.
Extensive carbonate reefs also formed in the warm shallow waters of the continental shelves during the latter half of the Mesozoic.
As the Atlantic widened during the Cretaceous, there was a period of extensive black shales deposited in the oceans. The shales are black due to organic matter that did not decay. This indicates that the deep ocean was depleted in oxygen which in turn indicates that the ocean conveyor belt, which normally brings oxygenated waters from the surface into the ocean depths, was not working. The formation of bottom water currents, like the present day North Atlantic Deep Water and the Antarctic Bottom Water, was inhibited.
Turning it into Exploitable Petroleum Reserves
In burial the sedimentary rock becomes heated up (the Earth's interior is hot). Within a certain temperature range organic matter in organic rich sediments like shale and limestone cooks off volatile hydrocarbons, petroleum and natural gas. These fluids will percolate upward through permeable layers (sandstone and limestone) but be halted by impermeable strata (shale).
Because salt is much less dense than rock, salt diapirs (like magma diapirs) have formed and risen up through the overlying sedimentary strata, shouldering them aside and bowing them up. Salt diapirs form structural domes in sedimentary strata. These salt domes make excellent petroleum traps if one of the domed up layers is shale.
It is a relatively straightforward task to identify dome structures in the subsurface via seismic imaging techniques (bouncing sound waves off the rock layers). Also, stratigraphic studies are done to decide if there is a good likelihood that underlying strata would have generated significant petroleum Then oil companies drill the most promising domes to see if petroleum has collected.