# Vesicles and Reservoirs of Basic Lava Flows in the Laobeishan and Huoshaoshan Volcano, NE China
## 5 Discussion
### Connectivity between Shrinkage Fractures and Vesicles  
A single vesicle cannot form an effective reservoir space, which needs to be connected through channels. Fractures are indispensable reference factors for evaluating the reservoir performance of volcanic rocks. For volcanic reservoirs, it is impossible for an effective reservoir to form without fracture communication [41]. The degree of fracture development directly affects reservoir connectivity. In the study of volcanic rock reservoir connectivity, in addition to the connectivity between vesicles, the connectivity between vesicles and fractures was more evident. Fractures are not only a suitable reservoir space but also an important seepage channel. Therefore, the more developed the fractures, the stronger the general connectivity. As influential factors in reservoirs, fractures are more important than vesicles. Volcanic fractures are divided into five types. The primary fractures can be divided into shrinkage fractures and explosive fractures. The shrinkage fractures are mainly distributed in lava flows, and the explosion fractures are mainly distributed in pyroastic rocks. Secondary fractures include tectonic fractures, weathered fractures and dissolution fractures. Tectonic fractures are developed in various lithologies; the weathered fractures are mainly distributed at the top part of the weathered crust and the dissolution fractures are mainly distributed in the areas communicating with the fluid channels [2]. Most fractures in the study area are primary shrinkage fractures. Researchers have divided shrinkage fractures into six categories: quench, columnar, platy, suture-like, macrotortoise shell, and microtortoise shell joints. The fractures in different facies belts have different degrees of development. However, most of them develop in the top zone of the lava flow and extend downward. In the crater-near-crater belt, fractures run longitudinally through the lava flow outcrop. According to the classification, most shrinkage fractures in the study area are macrotortoise and microtortoise shell joints. In the case of a relatively single lithology and tectonic activity, fracture development is influenced by the volcanic mechanism facies in the diagenesis process. The rocks in the crater-near-crater belt and proximal belt are brittle and often form dense massive deposits, and the fractures formed are longer and wider. However, the rocks in the proximal belt are brittle, with many fractures, but are relatively poor in development [42]. Some researchers have studied the degree of fracture development in different types of volcanic rocks, among which the fracture of overflow facies andesite is the most developed, followed by explosive facies volcanic breccia and overflow facies basalt, accounting  
Figure 10: Lava flow vesicles and facies belts relationship model of the Wudaliandi Quatemary Laobeishan. for only 19% of all fractures [43]. In this study, crisscross fractures developed in the basic lava flow of Laoheishan, which are shrinkage fractures whose formation mechanism can be explained by the temperature model [44]. During the cooling process of the rising and surface flows, the molten lava is cooled by contact with the relatively low-temperature surrounding rock, air, and water. As the molten lava is cooled during the solidification stage, it is quenched and quickly condensed, and the volume contraction decreases, resulting in fractures formed by tensile stress. The following factors are taken into account when analyzing fracture types: (1) the lava flow in Laoheishan was formed only 300 years ago, and there is almost no strong tectonic movement in the area; (2) most of the fractures exist at the top zone of the lava flow and extend downward, with irregular fracture surfaces and condensation shrinkage fracture characteristics; and (3) explore the characteristics and genesis of lava flow fractures in Laoheishan [45; 46]. There are 11 fractures in the crater-near-crater belt, of which nine are longitudinal and two are transverse.