THE FEATURES AND IMPORTANCE OF THE UNCONFORMITIES FOR TAHE OILFIELD,TALIM BASIN OF CHINA

Tahe oilfield is very large in size. It locates on Arke-kule uplift of Tabei uplift belt, Talim basin, northwestern China. The strata of Ordovician system in Talim basin, are composed of Panglai ba formation (O_1P, 250-400m in thickness), Yingshan formation (O_1-2y, 600-900m), Yijianfang formation (O_2yj, 50-100m), Qiarbak formation(O_3q, 30-60m) , Lianglitak formation(O_3l, 45-102m), Sangtamu formation(O_3s), and the lower part of Kepingtak formation ((O_3-S₁) k). There appear two unconformities in Ordovician system of Talim basin. One corresponds to the bottom of Qiarbak formation (O_3q). The other lies on the base of Kepingtak formation ((O₃-S₁) k). One of the unusual features of the Ordovician system in Tahe oilfield is that Yingshan formation belongs to lower Ordovician series, but not to lower-middle Ordovician series, and there is not Yijianfang formation. One of the unconformities within Ordovician system overlaps directly on the top of Yingshan formation, which implies a longer period of erosion. Another very important feature is that the strata of from lower Silurian series to middle Devonian series are absent in Tahe oilfield. The sandstone rocks of Donghetang formation (D_3d) contact with the strata of Ordovician system, which means that the unconformity experienced a much longer period of erosion. Donghetang sandstones (D_3d) distribute in a very small area. The strata of Ordovician system are overlapped directly by the strata of Ordovician system in large area. So there exist three important unconformities which are all formed in very long period of time. These unconformities make so good conditions for karst developing of carbonate rocks. The unconformities of Tahe oilfield are of different types. The strata of Ordovician system are of stratigraphic unconformities. The Donghetang sandstones (D₃) appear as onlap unconformity, and the strata of Carboniferous system act as overlap unconformities. The study on the unconformity is necessary for understanding karst development and distribution. Beside Paleokarst, erosion or exposion to the air is the most significant factor controlling karst formation, which implies the importance of unconformity for the reservoir of carbonate rocks.