*4.3. Unconformity*

In the hanging wall of the Hong-Che Fault Zone, the volcanic rocks at the top of Carboniferous are in direct contact with Permian, Jurassic, and Cretaceous strata and form a large-scale unconformity. From east to west, the overlying strata of the Carboniferous change from older to younger. Unconformity surfaces act as migration channels for oil and gas. The Permian oil and gas from the Shawan Depression first migrated along faults and then migrated upward (westward) along an unconformity to e ffective volcanic traps and then formed hydrocarbon reservoirs [47]. The unconformity surface was weathered and denuded into a weathering crust, which can seal oil and gas to form caprock.

The Carboniferous rocks in the fault terrace zone of the northwestern margin of the Junggar Basin su ffered from long-term denudation and weathering leaching and a weathering crust developed. The Carboniferous volcanic rocks and clastic rocks have a weathering crust, which was modeled as five layers, namely, a soil layer, hydrolysis zone, corrosion zone, disintegration zone, and parent rock [48,49]. According to the statistics, the distances between the soil layer, hydrolysis zone, corrosion zone, and disintegration zone to the top boundary of the unconformity surface of the Carboniferous are 15–50, 50–150, 150–250, and 250–450 m, respectively. The thickness of the weathering crust in the fault terrace zone of the northwestern margin is generally approximately 450 m, and the thickness of the weathering crust in the fault development area can be greater than 600 m.

The physical properties of di fferent structures in the weathering crust are quite di fferent. The average porosities of the soil layer, hydrolysis zone, corrosion zone, and disintegration zone are 2.6%, 6.4%, 15.8%, and 12.7%, respectively. Reservoirs in the corrosion zone have the best physical properties and belong to type I reservoirs. The second most favorable physical properties are in the disintegration zone, which belong to type II–III reservoirs. The soil layer and hydrolytic zone are mainly distributed in the lower part and slope area of the paleogeomorphology and the high part is mostly missing. The soil layer is a nonreservoir layer and the hydrolysis zone is a type IV reservoir with visible oil and gas displays but no productivity [48]. The physical properties of the weathering crust reservoir are controlled by the lithology of the parent rock and degree of weathering leaching.

The hydrocarbon accumulations in the Carboniferous in the fault terrace zones of the northwestern margin are characterized as oil-bearing throughout the entire zone and exhibit local enrichment [50,51]. The controlling factors for local enrichment of oil and gas are mainly e ffective sealing in an upward direction of the oil and gas migration, physical properties of weathering crust reservoirs, and preservation conditions. Among them, the physical properties of weathering crust reservoirs determine the output of the reservoir and the preservation conditions determine the viscosity of the reservoir. The quality of weathering crust reservoirs and their spatial distribution scales are the primary controlling factors for hydrocarbon accumulation.
