Research on quantitative prediction method of structural fractures in metamorphic rock reservoirs

Deeply buried hill reservoirs have experienced tectonic movement during different periods, and tectonic fractures are well developed in metamorphic reservoirs, making quantitative characterization and spatial distribution prediction of fractures more difficult. Taking Block XX in the Bozhong Depression of the Bohai Bay Basin as an example, the development characteristics, formation periods and spatial distribution patterns of tectonic fractures in metamorphic buried hill reservoirs were clarified via core, thin section and image logging data. On the basis of 3D seismic and rock mechanics experimental data, geomechanical heterogeneity models of the study area in different periods were established, the 3D distributions of the tectonic stress field in different periods were clarified through finite element simulation, and a quantitative relationship between the tectonic stress and fracture linear density was established to quantitatively predict the spatial distribution characteristics of multistage tectonic fractures. The prediction results revealed that in the study area, local stress concentrations occurred at the end and inflection points of the main faults in the early Indosinian period, with strong heterogeneity; the tectonic fluctuations in the late Indosinian period were small, and the stress characteristics in the three dimensions were controlled mainly by heterogeneous mechanical parameters and faults. Under the influence of sinistral strike-slip motion during the Yanshan period, the eastern part of the study area was in the equivalent tensile stress state. Early Indosinian fractures developed mainly in the tectonic core and at the endpoints and inflection points of the trunk faults, with linear fracture densities in the range of 2‒3 m^-1. The development degree of late Indosinian fractures was controlled by faults, with the main development occurring near the trunk faults. During the Yanshanian period, the degree of fracture development was low, with a linear fracture density of only 0-1 m^-1, and the development was mainly concentrated in the western high-stress area and the intersection of the main faults. The quantitative geomechanical prediction method is an effective means for the quantitative prediction of multistage tectonic fractures in metamorphic buried hill reservoirs. This method can provide a reference for the efficient exploration and development of buried hill reservoirs in other areas.