This is gas often produced with a designated oil well (different than a natural gas reservoir). This could be due to:
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Crude oil with gas cap
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Natural gas exists above the crude oil. Its lighter density causes it to rise and take high points of the reservoir
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Undersaturated crude oil
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When oil is brought to surface, the solution gas vaporizes and is separated from the oil. Undersaturated reservoirs only remain until reservoir pressure falls below bubble point.
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