Oil filling history of the Ordovician oil reservoir in the major part of the Tahe Oilfield, Tarim Basin, NW China

The Ordovician oil reservoir in the Tahe Oilfield, northern Tarim Basin is so far the largest marine oil reservoir in China. Ordovician oil was sampled from 73 exploration/production wells in the oilfield. According to oil-oil correlation, all the Ordovician oil samples belong to the same oil population of mature to high-mature phase. Based on the coexisting intact n-alkane series, evident UCM and 25-norhopanes in the oil as well as the bimodal distribution pattern of homogenization temperatures (T-h), ranging from 80 to 100 degrees C and from 115 to 135 degrees C, respectively, in fluid inclusions within the reservoir rocks, it is concluded that the oil reservoir has been twice charged during its oil filling history. By means of 1D basin modeling, the measured T-h values can be related to relevant geological ages of 429415 Ma and of 8-2 Ma, respectively, i.e., the Middle to Late Silurian and the Miocene to Pliocene, representing two phases of oil filling and entrapment. Using independent molecular parameters, e.g., the 4-/1-MDBT ratio and total concentrations of pyrrolic N compounds as indicators to reconstruct the oil filling process in the reservoir, oil filling pathways can be traced and mapped. The general orientation of oil filling is from south to north in the oil reservoir. Therefore, it can be predicted that its source kitchen should be oil the south side of the oilfield, most probably at the Shuntuoguole Uplift between the Awati and Manjiaer Depressions. (C) 2008 Elsevier Ltd. All rights reserved.