Strata Sequence of Donghe Sandstone and Its Overlying Strata in Hudson: An Insight from Donghe Sandstone, Tarim Basin, Northwest China

The Donghe Sandstone has been generally considered as a time-transitional (several consecutive periods during Late Devonian to Early Carboniferous) set of littoral sedimentary sandstone which was deposited under a... [ view full abstract ]

The Donghe Sandstone has been generally considered as a time-transitional (several consecutive periods during Late Devonian to Early Carboniferous) set of littoral sedimentary sandstone which was deposited under a macro-background of transgression in the Tarim Basin. According to this model, sandstone was deposited in different areas during corresponding periods as the sea transgressed from west to east， making the high-quality sandstone scattered in different lithologic areas in the basin, which further resulted in different reservoir-cap assemblages and relatively scattered lithologic petroleum reservoirs. A significant argument for this model is the comprehension that both the Lower Mudstone Member and the Bioclastic Limestone Member were primitively absent above the Donghe Sandstone Member in the Hudson area on the north margin of the basin, while they were both present above the Donghe Sandstone Member in the inner side of the basin. However, further in-depth study of the stratigraphic sequence and the sedimentary history shows that the Lower Mudstone Member and Bioclastic Limestone Member were actually deposited in Hudson. Together with other evidence, it has been proved that the whole Donghe Sandstone in Tarim Basin is a set of isochronous sandstone controlled by a single third-order sequence in a relatively short time, thus overthrowing the previous recognition. In fact, the primitive sedimentary strata sequence (Standard Sequence) formed during the Donghe Sandstone and its ensuing sedimentary stages in Hudson is (from below upward): Donghe Sandstone Member-Lower Mudstone Member-Bioclastic Limestone Member-Middle Mudstone Member. However, due to the later tectonic uplift, differential denudation and mixed deposition, five types of strata sequence were formed (from below upward)：(1) Donghe Sandstone Member-Lower Mudstone Member-Bioclastic Limestone Member-Middle Mudstone Member, (2) Donghe Sandstone Member-Bioclastic Limestone Member-Breccia Member-Middle Mudstone Member, (3) Donghe Sandstone Member-Lower Mudstone Member-Middle Mudstone Member, (4) Donghe Sandstone Member-Breccia Member-Middle Mudstone Member and (5) Donghe Sandstone Member-Middle Mudstone Member. The Standard Sequence in Hudson corresponds with those in Halahatang, Donghetang and Lunnan in the North Tarim, and even in the Central Tarim and West Tarim. Therefore the whole Donghe Sandstone was actually developed during one period instead of several periods and it has a continuous and extensive distribution in the basin, which leads to a generally extensive and united reservoir-cap assemblage. According to this recognition, we can predict the distribution of the Donghe Sandstone in the basin more systematically and boldly. Favorable positive tectonic zones instead of limited lithologic zones have great exploration potentials.