Oil depletion is the decline in petroleum production of a well, oil field, or geographic area.
Resource availability
Earth's natural oil supply is effectively fixed because petroleum is naturally formed far too slowly to be replaced at the rate at which it is being extracted. Over many millions of years, plankton, bacteria, and other plant and animal matter became buried in sediments on the ocean floor. When conditions were right—a lack of oxygen for decomposition, and sufficient depth and temperature of burial—these organic remains were converted into petroleum compounds, while the sediment accompanying them was converted into sandstone, siltstone, and other porous sedimentary rock. When capped by impermeable rocks such as shale, salt, or igneous intrusions, they formed the petroleum reservoirs which are exploited today.
Production decline models The Hubbert peak theory makes predictions of production rates based on prior discovery rates and anticipated production rates. Hubbert curves predict that the production curves of non-renewing resources approximate a bell curve. Thus, according to this theory, when the peak of production is passed, production rates enter an irreversible decline. For the short and medium-term, oil production decline occurs in a predictable manner based on geological circumstances, governmental policies, and engineering practices. The shape of the decline curve varies depending upon whether one considers a well, a field, or a set of fields. In the longer term, technological developments have defied some of the predictions.
Oil well production decline
An individual oil well usually produces at its maximum rate at the start of its life; the production rate eventually declines to a point at which it no longer produces profitable amounts. The shape of the decline curve depends on the oil reservoir and the reservoir drive mechanism. Wells in water-drive and gas-cap drive reservoirs often produce at a near constant rate until the encroaching water or expanding gas cap reaches the well, causing a sudden decline in oil production. Wells in gas solution drive and oil expansion drive reservoirs have exponential or hyperbolic declines: rapid declines at first, then leveling off. The shape of production curve of an oil well can also be affected by a number of nongeologic factors:
Well may be restricted by choice by lack of market demand or government regulation. This decreases the rate of decline, but will not change the well's total production significantly. Hydraulic fracturing (fracking) or acidizing may be used to cause a sharp spike in production, and may increase the recoverable reserves of a given well. The field may undergo a secondary or tertiary recovery project, discussed in the next section.
Oil field production decline Individual oil wells are typically within multi-well oil fields. As with individual wells, the production curves for oil fields vary depending on geology and how they are developed and produced. Some fields have symmetric bell-shaped production profiles, but it is more common that the period of inclining production is briefer and steeper than the subsequent decline. More than half the production usually occurs after a field has reached a peak or plateau. Production profiles of many fields show distinct peaks, but for giant oil fields, it is more common for production to reach and maintain a plateau before declining. Once a field declines, it usually follows an exponential decline. As this decline levels off, production can continue at relatively low rates. A number of oil fields in the U.S. have been producing for over 100 years. Oil field production curves can be modified by a number of factors:
Production may be restricted by market conditions or government regulation. A secondary recovery project, such as water or gas injection, can repressurize the field and increase the total recovery. the field may undergo an enhanced oil recovery project, such as drilling of wells for injection of solvents, carbon dioxide, or steam. This allows more oil to be coaxed out of the rock, increasing the ultimate production of the field.
Multi-field production decline
Most oil is found in a small number of very large oil fields. According to Hubbert peak theory, production starts off slowly, rises faster and faster, then slows down and flattens until it reaches a peak, after which production declines. In the late stage, production often enters a period of exponential decline in which the decline becomes less and less steep. Oil production may never actually reach zero, but eventually becomes very low. Factors which can modify this curve include:
Inadequate demand for oil, which reduces steepness of the curve and pushes its peak into the future. Sharp price increases when the production peak is reached, as production fails to meet demand. If price increases cause a sharp drop in demand, a dip in the top of the curve may occur. Development of new drilling technology or marketing of unconventional oil can reduce the steepness of the decline as more oil is produced than initially anticipated.
United States production Oil production in the United States, provided one excludes Alaska, began by following the theoretical Hubbert curve for a few decades but is now deviating strongly from it. U.S. conventional oil extraction peaked in 1970; by the mid-2000s, it had fallen to 1940s levels. In 1950, the United States produced over half the world's oil, but by 2005, that proportion had dropped to about 8%. In 2005, U.S. crude oil imports peaked at twice as high as domestic production; since then, U.S. oil production has increased, and imports have fallen 41%. The conventional peak of oil extraction in 1970 was predicted by one of the two projections proposed by Hubbert in 1956. By 1972, all import quotas and controls on U.S. domestic production had been removed. Despite this, and the quadrupling of prices during the 1973 oil crisis, the production decline was not reversed in the lower 48 states until 2009. Crude oil production has since risen sharply from 2009 through 2014, so the rate of US oil production in October 2014 was 81% higher than the average rate in 2008. The actual U.S. production curve deviates from Hubbert's 1956 curve in significant ways:
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