Global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources. Unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps. The characteristic features include the following: a hydrocarbon source and reservoir coexist; porosity and permeability are ultra-low; nano-pore throats are widely distributed; hydrocarbon-bearing reservoir bodies are continuously distributed; there is no obvious trap boundary; buoyancy and hydrodynamics have only a minor effect, and Darcy's law does not apply; phase separation is poor; there is no uniform oil-gas-water interface or pressure system; and oil or gas saturation varies. Examples of unconventional hydrocarbon accumulations are the Mesozoic tight sandstone oil province and the Upper Paleozoic tight sandstone gas province in the Ordos Basin, north-central China. Generally, continuous hydrocarbon accumulation over a large A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 2 area is a distinguishing characteristic of unconventional hydrocarbon sources.Because of the great potential of unconventional petroleum resources, it is believed that research on such resources will be at the forefront of the future development of petroleum geology.

The Upper Carboniferous in northern Xinjiang, China was formed in a post-collisional depression and collapsed structural setting. Within the Upper Carboniferous, volcanic rocks and source rocks alternate over a wide region. At the end of the Carboniferous, these layers were uplifted by plate collisions and subsequently weathered and leached. Volcanic weathering and leaching led to the establishment of weathered crusts that can be divided into five layers. Corrosion and crumble zones in these layers form favorable reservoirs. Volcanic weathering crust formed in sub-aerially exposed paleogeomorphic areas; the five relatively continuous layers are generally preserved in paleogeomorphic lowland and slope regions, but the upper soil layer is usually absent in structurally higher parts of the rock record. The thickness of the weathered layer has a positive nonlinear exponential relationship to the duration of weathering and leaching, and the dynamic equilibrium time of weathered crust is about 36.3 Ma. The thickest weathered layer (~450 m) is located in fracture zones. Weathered crusts are possible from a range of volcanic rocks with different lithologies, given sufficient time for weathering and leaching. The combination of volcanic weathered crust and source rocks results in three types of hydrocarbon accumulation models: (1) sequences of volcanic weathered crust interbedded with source rocks, (2) a quasi-layered weathered volcanic core located above source rocks, and (3) volcanic rocks associated with pectinate unconformities adjacent to source rocks. Each of these three types has the potential to form a giant stratigraphic reservoir of volcanic weathered crust. This knowledge has changed the traditional exploration model of searching for favorable lithologic and lithofacies zones in volcanic rocks, and has changed the viewpoint that the Carboniferous does not have the genetic potential to be the basement of the basin in northern Xinjiang. The concepts developed here are of great scientific significance and application for focusing oil and gas exploration on volcanic weathered crust. As such, the Paleozoic volcanic weathered crust in the midwestern part of China may possibly contain large-scale stratigraphic reservoirs and thus could be a new oil and gas exploration target in the future. Carboniferous in northern Xinjiang, volcanic weathering crust, stratigraphic reservoir, accumulation mechanism and model Citation:Zou C N, Hou L H, Tao S Z, et al. Hydrocarbon accumulation mechanism and structure of large-scale volcanic weathering crust of the Carboniferous in

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