Reservoir distribution pattern of the hypabyssal intrusive rocks: a case study from the Miocene hypabyssal dykes of Lyttelton volcano, New Zealand

Intrusive rock reservoirs, with hydrocarbon exploration potential, are widely distributed in many basins worldwide. However, the study of reservoir patterns of dykes with primary pores has not attracted adequate attention. This manuscript takes the case study of Miocene hypabyssal dykes in the northwest of the Lyttelton volcano. The pore composition, reservoir distribution patterns and reservoir control factors of the dykes are analyzed using field outcrop survey, porosity test, image analysis, and formula calculation. The results show that the reservoirs of hypabyssal intrusive rocks belong to the pore-fracture type, and the reservoir space is dominated by vesicles, followed by shrinkage joints (columnar joints). Vesicles can be divided into directional elongated elliptical vesicles with large diameters, and discrete circular vesicles with small diameters, with the former being the main contributors to porosity. Columnar joints can be divided into regular and irregular types, with the latter having higher surface density. Most reservoir is characterized by medium porosity and medium permeability, the local parts are the reservoir with high porosity and high permeability. The morphology and regularity of cylinder cross section are the key factors affecting formation permeability. From the lower part to the upper part of the dyke, porosity increases, and columnar joints change from regular to irregular. Vesicles are connected through columnar joints, and their connectivity is controlled by the fracture spacing, aperture, and angle between the directional vesicles and the columnar joints. The hypabyssal dykes of Lyttelton volcano show that the initial connectivity of the vesicles can be as high as 35% during condensation and consolidation. In summary, the hypabyssal dykes have good reservoir performance and high initial connectivity of the vesicles, which make them favorable targets for exploration. © 2021, Science Press. All right reserved.