CHARGING OF ORDOVICIAN RESERVOIRS IN THE HALAHATANG DEPRESSION (TARIM BASIN, NW CHINA) DETERMINED BY OIL GEOCHEMISTRY

Petroleum at Halahatang oilfield in the Halahatang Depression (Tabei Uplift, Tarim Basin, NW China) occurs within calcarenites and bioclastic limestones in the Middle Ordovician Yijianfang Formation (O-2yj). The petroleum is sourced from intervals in the Upper Ordovician Lianglitage Formation (O-3l), and reservoirs are sealed by Upper Ordovician - Silurian argillaceous limestones and marls. The charging history of Halahatang oilfield is however poorly understood. For this study, the geochemical characteristics of 17 oils from Ordovician reservoir rocks at Halahatang field were investigated by GC and GC-MS. Oil-oil correlation studies indicate that all the oils belong to a single mature oil population. Based on the co-occurrence of an intact n-alkanes series together with an unresolved complex mixture (UCM) and fully-developed 25-norhopanes in the same oil samples, as well as a bimodal distribution of homogenisation temperatures of aqueous fluid inclusions, it was concluded that the reservoir experienced two separate oil charge events. 1D basin modelling indicates that the early phase of charging occurred from about 412 to 409 Ma and the later phase from 11 to 9 Ma. Using molecular parameters as tracing indicators, the general direction of oil charging was deduced to be from south to north. The source kitchen is therefore inferred to be located to the south of the Halahatang oilfield, probably in the Shuntuoguole Uplift between the Awati and Manjiaer Depressions.