The Tazhong hybrid petroleum system, Tarim Basin, China

Abstract

The Tazhong petroleum system is geographically located in the central Tarim Basin, including the Tazhong Uplift, the North Slope and the southern area of the Manjiaer Depression. The total thickness of early Palaeozoic to Cenozoic sediments reaches 6500–9500 m. The Manjiaer Depression and the North Slope are the two major petroleum source areas with three sets of source rocks: Cambrian black shales and Ordovician dark carbonates of the Manjiaer Depression and Ordovician argillaceous limestones of the North Slope. There are three sets of reservoir-cap rock combinations in this petroleum system. The main periods for oil generation from these source rocks occurred during the middle Silurian to Early Devonian, late Carboniferous to Early Triassic, and Tertiary. There were three corresponding phases of petroleum migration. Well developed thrust faults and unconformity surfaces in this area provided the main pathways for petroleum migration. As some of the faults cut through all the lower Palaeozoic strata, the oil in any given reservoir rock may be actually a mixture of oils from different sources, complicating the origin and properties of the hydrocarbons in the reservoir, and forming a hybrid petroleum system. The formation of a typical oil and gas pool in this petroleum system may be attributed to one or all of three petroleum events which were closely associated with the regional tectonics. The first event occurred in the early Palaeozoic tectonic cycle and the generated oil accumulated in lower Silurian sandstone. The second event occurred in the late Palaeozoic tectonic cycle and formed light oil pools mainly in lower Silurian and Ordovician reservoirs. The third event took place during the Cenozoic tectonic cycle and formed normal crude oils. The oil and gas generated from different sources in different phases may co-exist with one another in a reservoir rock. However, the commercial oil and gas pools are related to the Cenozoic and late Palaeozoic events.