Study on Hercynian and Middle Caledonian Karstification of Ordovician in the Tahe Oilfield, Tarim Basin

The Tahe oilfield is located in the Akekule uplift in the northern part of the Tarim basin. It is the first and biggest marine oilfield up to now in China. Ordovician reservoir, as the main reservoir bed in the Tahe oilfield, accounts for 75% of total production. Hercynian and Middle Caledonian karstification had a great control on the Ordovician carbonate reservoirs of the Tahe oilfield. Hercynian or middle Caledonian karstification can be distinguished by plane distribution of karst reservoirs, fillings of fracture and cavity, paleontology, strontium isotope composition, carbon—oxygen stable isotope composition, rare earth elements content, homogenization temperature and fluorescence of fluid inclusion, etc. Tectonic movements, unconformity, ancient physiognomy and lithology are the controlling factors of Hercynian karst, among which tectonic movements is the most important factor. Tectonic movements, unconformity and lithology are the controlling factors of Middle Caledonian karst. Hercynian ancient physiognomy varied greatly and played an important role in the development of Ordovician carbonate reservoirs, while middle Caledonian ancient physiognomy varied slightly and did not control Ordovician carbonate reservoirs obviously. Hercynian and Middle Caledonian Ordovician reservoirs are evaluated respectly based on the controlling factors of karst reservoirs.