*4.2. Volcanic Lithofacies*

Another important factor that controls oil and gas reservoirs is volcanic lithofacies. Reservoirs with di fferent lithofacies have di fferent pore fracture structures and reservoir space combinations. In volcanic breccias, intragranular dissolved pores, intergranular pores, and matrix pores are well developed and the storage performance is best. Basalt pores are well developed but most of them are filled pores, therefore, their storage performance is not ideal. Tu ff has many micropores and the reservoir properties are worst, therefore, tu ff can act as a local cap rock.

The Carboniferous volcanic rocks in the Hong-Che Fault Zone developed various types of reservoir-caprock assemblages, which mainly include the following four types: (1) Eruptive facies form the reservoirs and the volcanic sedimentary facies tu ffs form the caprocks. (2) Eruptive facies function as reservoirs and the dense basalts in the overflow facies function as cap rocks, such as the oil and gas reservoir in the Che 47 well. The Che 47 well is located in the middle section of the Hong-Che Fault Zone. The Carboniferous volcanic reservoir lithology in this well mainly consists of eruptive facies basaltic volcanic breccia, gray tu ffaceous volcanic breccia, and variegated andesitic basaltic volcanic breccia. The cap rock is a dense basalt in the overflow facies. (3) The transitional facies between the volcanic and sedimentary rock acts as the reservoir and the volcanic sedimentary facies tu ff is the caprock. (4) The overflow facies function as reservoirs and the volcanic sedimentary facies tu ffs are cap rocks [21].

On the whole, the physical reservoir properties of the eruptive facies in the Hong-Che Fault Zone are most favorable and are followed by overflow facies, while the volcanic sedimentary facies are least favorable. In addition, there are also tu ffaceous sandstone, sandy tu ff, and mudstone in the transitional facies zone. Under favorable conditions of oil and gas sources and plugging conditions, tu ffaceous sandstone can also migrate and accumulate into reservoirs.
