Subsea petroleum is one of the mineral resources buried in sedimentary rocks and bedrock beneath the ocean floor.

Subsea petroleum and natural gas are a pair of "twin brothers," often residing beneath the "continental shelf" and "continental slope" in the ocean.

Tens of millions to hundreds of millions of years ago, the climate was warmer and more humid than it is today. In coastal areas, seawater was rich in oxygen and sunlight, and rivers brought in large amounts of nutrients and organic matter, providing abundant "food" for the growth and reproduction of marine life, leading to the rapid and abundant reproduction of many marine organisms.

When these marine organisms died, their bodies could generate over 60 billion tons of organic carbon per year. This organic carbon is the "raw material" for generating natural gas and subsea petroleum.

However, merely having these biological remains is not enough to form oil and gas; certain conditions and processes are also required. The ocean receives over 160 billion tons of sediment every year, with more sediment being transported into the ocean annually than in other areas. As a result, a large number of biological remains are buried layer by layer.

The accumulated sediment and buried biological remains become thicker over time. Isolated from the air, these buried biological remains undergo slow decomposition in oxygen-deficient environments, with increasing temperatures and the action of bacteria. Over geological periods, these biological remains gradually turn into natural gas and petroleum.

The generated oil and gas also need reservoir rocks to store them and cap rocks to prevent them from escaping. Due to the pressure from the overlying rocks, dispersed oil droplets are squeezed into porous rock layers around them. These oil-bearing rock layers become reservoir rocks. Some rock layers have very small pores, preventing oil from accumulating within them.

However, their small pores act as a "protective shell," preventing oil from escaping. If such rock layers are located at the top and bottom of the reservoir rock, they will enclose the oil inside, becoming cap rocks that protect the oil.

Oil dispersed in sandstone has no exploitable value; only areas where oil and gas are concentrated are economically viable. The geological formations of shallow seas often consist of sedimentary rocks such as sandstone, shale, and limestone. Originally, sedimentary rocks should be horizontally distributed on the seabed, but due to crustal movements, they become curved, inclined, or fractured.

Curved upward structures are called anticlines, while downward ones are called synclines. Some raised structures, resembling steamed buns, are called anticlinal domes. Some sedimentary rock layers containing oil and gas deform under enormous pressure, causing the oil to migrate into anticlines, forming accumulation zones.

Therefore, anticlinal structures often serve as "warehouses" for storing oil, known as "oil-bearing structures" in petroleum geology. Typically, because natural gas has the lowest density, it is found at the top of anticlines, while oil is in the middle, and water is at the bottom. Searching for oil and gas resources involves primarily locating such areas.

Hence, the saying "oil in anticlines, water in synclines" is derived from this principle.