Petroleum engineering refers to the subsurface engineering activities related to the production of hydrocarbons, which can be either crude oil or gas. These activities are deemed to fall within the upstream sector of the oil and gas industry which are the activities of finding and producing hydrocarbons prior to the point of refining or distribution to a market, which is referred to as the downstream sector. Petroleum Geology and Petroleum Engineering are the two main subsurface disciplines within the oil and gas industry, which focus on maximising economic recovery of hydrocarbons from subsurface reservoirs. Petroleum Geology focuses on provision of a static description of the hydrocarbon reservoir rock, whilst Petroleum Engineering focuses on estimation of the recoverable volume of this resource using a detailed understanding of the physical behavior of oil, water and gas within porous rock at very high pressure. The combined efforts of Geologists and Petroleum Engineers throughout the life of a hydrocarbon accumulation determine the way in which a reservoir is developed and depleted, and usually they have the highest impact on field economics. Petroleum Engineering requires a good knowledge of many other related disciplines, such as Geophysics, Petroleum Geology, Formation Evaluation (well logging), Drilling, Economics, Reservoir simulation, Well Engineering, Artificial Lift Systems, and Oil & Gas Facilities Engineering.