Semi- quantitative evaluation of mixed oil in northern uplift of the Tarim basin

Abstract

Various types of oil have been discovered in northern uplift of the Tarim basin. Multiple charges and within reservoir mixing are very common in this region. The first charged oils have been biodegraded severely and the residual oils are extremely heavy. Hydrocarbon contents and compositions are distorted severely. Biodegradation results in completely depletion of normal alkanes and isoprenoids, destroys most steranes and hopanes, forms large amount of 25 norhopanes and enriches triaromatic steroids in residual oils. The late charged oils are light oils with full series of normal alkanes and isoprenoids, almost no 25 norhopanes and very low amount of triaromatic steroids. Once mixed in reservoirs, the oil properties and molecular compositions are determined by the proportion of two charged oils. This proportion can be semi quantitatively calculated on the basis of absolute concentration of normal alkanes and 25 norhopanes and relative amount of triaromatic steroids in aromatic fraction. The results indicate that roughly equal amount of two charged oils occurs in the No.4 structure of the Tahe oilfield, while in the No.3 structure almost all oil from the late charges occurs except Well Sha47, which contains about 20% of the first charges. In the Hadexun oilfield contribution from the first charges is 5% ～ 15% , suggesting late oil accumulation in this region. About one fourth amount of oil from Well Xiang3 is the first charged residual and the oil from Well YW2 is dominated by the late charges.P