DIFFERENTIAL ENTRAPMENT OF CHARGED HYDROCARBON IN ALAN FIELD, NORTHERN IRAQ
The Alan oilfield consists of three domes western, middle and eastern. Well Alan-1 and Well Alan-2 are drilled in the middle and western domes respectively. Oil fills the lower part of the Kurra Chine Formation (Late Triassic) in the middle dome while it occupies the upper part of the formation from the western dome; also, the gas and condensate fill the lower part of the formation in the western dome. Alan field consists of the lower and upper reservoirs of the Kurra Chine Formation separate by 220 m of anhydrite cap rocks. Alan structure develops at the end of Pliocene at the northwest ridge of the Mosul High. Source rocks of Triassic sequence, especially northwest Iraq, are matured and generate hydrocarbon that migrates laterally up-dip toward Alan structure. The folding plays an important role to trap the hydrocarbon in the lower reservoir. Western dome fills by gas and condensate completely until gas-oil contact reaches to the spill point between the western and middle domes. The oil is pushed toward the middle dome because it beyond and higher than the western dome. Another factor affected the hydrocarbon distribution in the lower reservoir was the erosion of about 800 m from overburden of Miocene sequence upward which was led to increase the void volume of porosity for the reservoir carbonate rocks. This increase allows to release of gas and push the oil to the middle dome. Two scenarios of hydrocarbon entrapment in the lower reservoir, the first one depends on the concepts of Gussow′s fill and spill, in which the source rock enters maturity levels after or within the formation of the structure. The second scenario is in which the development of structure after charge of hydrocarbons. The latter is the most widely accepted in the Alan field case.