Petroleum is defined as a gaseous, liquid, and semisolid naturally occurring substance that consists chiefly of hydrocarbons (chemical compounds of carbon and hydrogen). Petroleum is therefore a term that includes both oil and natural gas. Petroleum is nearly always found in marine sedimentary rocks. In the ocean, microscopic phytoplankton (tiny floating plants) and bacteria (simple, single-celled organisms) are the principal sources of organic matter that is trapped and buried in sediment. Most of the organic matter is buried in clay that is slowly converted to a fine-grained sedimentary rock known as shale. During this conversion, organic compounds are transformed to oil and natural gas.

Sampling on the continental shelves and along the base of the continental slopes has shown that fine muds beneath the seafloor contain up to 8 percent organic matter. Two additional kinds of evidence support the hypothesis that petroleum is a product of the decomposition of organic matter: oil possesses optical properties known only in hydrocarbons derived from organic matter, and oil contains nitrogen and certain compounds believed to originate only in living matter. A complex sequence of chemical reactions is involved in converting the original solid organic matter to oil and gas, and additional chemical changes may occur in the oil and gas even after they have formed.

It is now well established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is analogous to groundwater migration. When oil and gas are squeezed out of the shale in which they originated and enter a body of sandstone or limestone somewhere above, they migrate readily because sandstones (consisting of quartz grains) and limestones (consisting of carbonate minerals) are much more permeable than any shale. The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals. Hence, because oil and water do not mix, water remains fastened to the quartz or carbonate grains, while oil occupies the central parts of the larger openings in the porous sandstone or limestone. Because oil is lighter than water, it tends to glide upward past the carbonate- and quartz-held water. In this way, oil becomes segregated from the water; when it encounters a trap, it can form a pool.

Most of the petroleum that forms in sediments does not find a suitable trap and eventually makes its way, along with groundwater, to the surface of the sea. It is estimated that no more than 0.1 percent of all the organic matter originally buried in a sediment is eventually trapped in an oil pool. It is not surprising, therefore, that the highest ratio of oil and gas pools to volume of sediment is found in rock no older than 2.5 million years—young enough so that little of the petroleum has leaked away—and that nearly 60 percent of all oil and gas discovered so far has been found in strata that formed in the last 65 million years. This does not mean that older rocks produced less petroleum; it simply means that oil in older rocks has had a longer time in which to leak away.

How much oil is there in the world? This is an extremely controversial question. Many billions of barrels of oil have already been pumped out of the ground. A lot of additional oil has been located by drilling but is still waiting to be pumped out. Possibly a great deal more oil remains to be found by drilling. Unlike coal, the volume of which can be accurately estimated, the volume of undiscovered oil can only be guessed at. Guesses involve the use of accumulated experience from a century of drilling. Knowing how much oil has been found in an intensively drilled area, such as eastern Texas, experts make estimates of probable volumes in other regions where rock types and structures are similar to those in eastern Texas. Using this approach and considering all the sedimentary basins of the world, experts estimate that somewhere between 1,500 and 3,000 billion barrels of oil will eventually be discovered.

石油被定义为一种天然存在的气体、液体、和半固体物质，其主要成分是烃（碳氢化合物）。因此石油是一个包括石油和天然气的术语。石油几乎总是在海洋沉积岩中发现。在海洋中，微小的浮游植物（微小的漂浮植物）和细菌（简单的单细胞生物）是被留在或是埋在沉积物中的有机物的主要来源。大多数有机物都被埋在泥土里，泥土慢慢转化成一种细粒沉积岩，叫做页岩。在这个转化过程中，有机化合物转化为石油和天然气。

对大陆架和沿大陆斜坡底部进行采样可以发现，海底下面的细泥浆含有高达百分之八的有机物。另外还有两类证据证实了石油是一种有机物的分解产物的假说：一是石油具有一些光学特性，这些特性只有来源于有机物的碳氢化合物才有，二是石油还含有氮和某些化合物，这些化合物被认为只能来源于有生命的物质。将有机固体转化成石油和天然气涉及一系列复杂的化学反应，并且在石油和天然气形成之后可能还会发生其他的化学反应。

目前公认的说法是，石油向上迁移，穿过含水层，可能被困在蓄水层。石油的迁移与地下水的流动相似。当石油和天然气被从形成它们的页岩中挤出，进入砂岩或石灰岩时，它们就很容易流动，因为砂岩（由石英颗粒组成）和石灰岩（由碳酸盐矿物组成）比页岩的渗透性要强。石油和石英或碳酸盐矿物之间的分子引力弱于水和石英或碳酸盐矿物分子之间的引力。因此，由于石油和水不混合，水会依附在石英或碳酸盐颗粒上，而石油则占据了多孔的砂岩或石灰岩有着更大空隙的中央区域。由于石油比水轻，所以它会向上流动，越过吸附于碳酸盐和石英上的水。这样，石油就与水分离开来；当遇到圈闭时就形成一个油藏。

大多数形成于沉积物的石油都没有找到合适的圈闭，并最终通过某种方式随着地下水来到了海洋的表面。据估计，这些最初埋在沉积物中的有机物只有不到0.1%被留住最终形成了油藏。因此也就不奇怪，油藏和气藏的数目与沉积物的总量相比，最高的比例出现在年龄不超过250万年的沉积岩中——岩石还较新，所以只有很少的石油泄露——而迄今发现的近60%的石油和天然气都出现在过去650万年形成的地层中。这并不意味着年龄较大的岩石产生的石油较少，这只是意味着石油在更古老的岩石里的时间较长，而在这段时间里石油泄漏了。

世界上有多少石油？这是一个非常有争议的问题。数十亿桶石油已经被从地下抽出。还有很多石油已被钻井所发现，等待被抽出。也许还有更多的石油等待着被钻井发现。石油和煤不一样，煤的储量可以被准确估计，未探明的石油储量只能估计。这种估计依靠的是一个世纪以来的钻探经验。在一个集中钻探的区域，如东部德克萨斯州，在知道此处已经发现的石油量之后，专家们以此估计其他岩石类型和结构与其类似的区域的潜在石油储量。使用这种方法，并考虑到世界上沉积盆地的总量，专家估计未来还会有1.5万亿到3万亿桶石油被发现。