Stratigraphic thermohistory and its implications for regional geoevolution in the Tarim Basin, NW China

Abstract

Fourteen vitrinite reflectance profiles from the Tarim Basin, NW China, show that the vitrinite reflectance profiles of individual wells follow a faulted and dislocated dual-stage pattern in the eastern section of the Tazhong Uplift and in the Tabei Uplift. Vitrinite reflectance values in these profiles change sharply at the unconformity beneath the Lower Carboniferous Bachu or Upper Devonian Donghetang Formations, where the overlying Triassic to Carboniferous strata are still in a mature phase within the “liquid oil window”. However, the underlying Lower Paleozoic reached the overmature phase beyond the “liquid oil window” towards the end of the Silurian, or in the Early Devonian at the latest. Whereas the vitrinite reflectance profiles are attributed to a continuous, single-stage pattern in the western section of the Tazhong Uplift, in which the Lower Paleozoic is also in an overmature phase, their overmaturity would have been achieved relatively late in geological time. The stratigraphic thermohistory has the following implications to regional geoevolution: (1) The overmature Lower Ordovician to Cambrian strata in the eastern section of the Tazhong Uplift and in the Tabei Uplift, as well as in the Manjiaer Depression, could not have acted as the source kitchen for normal oil (so-called black oil); (2) The dissimilarity in vitrinite reflectance profiles between the eastern and western sections of the Tazhong Uplift reveals Early Paleozoic paleotectonic features, i.e., lower at the east and higher at the west, whereas recent tectonic features formed since the Late Paleozoic are in reverse, i.e., higher in the east and lower at the west; (3) Reconstruction of the denuded thickness of sediments overlying the Lower Ordovician strata suggests reconsideration on the paleotectonic features in the Tarim Basin; (4) Based on the sustained duration of the “liquid oil window” for overmature source beds, it is predicted that the Suntuoguole Lower Uplift between the Awati and Manjiaer Depressions is a favorable potential source kitchen for Lower Paleozoic black oil.