Characteristics of life-cycle stages and reservoir control in the development of extensional faults in the Dongying Sag

Abstract

Objective Faults are among the most prevalent geological structures in oil and gas basins. Because of their significant connection to oil and gas resources, they have consistently attracted the attention of experts and scholars in the field, making them a hot research topic. Although previous researchers delved tirelessly into the correlation between faults, oil, and gas, new theoretical breakthroughs have been steadily emerging and have been used to promote advancements in oil and gas exploration. Nonetheless, there continues to be a dearth of thorough investigations into the underlying links between faults and the formation and distribution of oil and gas reservoirs, as well as methods for comprehensively and quantitatively defining the connections between faults and oil and gas. Methods The formation of a fault from inception encompasses multiple stages of development, including implicit and explicit stages, and differentiating the diverse growth stages of a fault, ranging from the initial embryonic stage to the terminal stage, poses a significant challenge. To address this issue, the Dongying Sag in the Bohai Bay Basin was selected as the focal point of this study. By employing physical and numerical simulation techniques, the researchers sought to replicate the entire life cycle evolution of the Chennan Fault, a basin-controlling boundary fault, from its embryonic stage to its terminal stage while elucidating the distinct characteristics of each stage. Building upon this foundation, the relative ages of the primary faults in the Dongying Depression and the various modes of fault activity at different stages were qualitatively and quantitatively determined, leading to the establishment of a reservoir-control model. Results The research findings indicate that normal faults tend to grow in six distinct stages: the embryonic stage (0 < RA (relative age) ≤ 1), characterized by microfractures or induced fracture zones; juvenile stage (1 < RA ≤ 2), with an intermittent connection of fault geometry; mature stage (2 < RA ≤ 3), marked by the connection of plate-like fault geometry and clear fault throw; declining stage (3 < RA ≤ 4), in which induced fracture zones form on both sides of the fault core, resulting in a shovel-like fault geometry; terminal stage (4 < RA ≤ 5), ramp-flat fault geometry, which has complex derived structures; and death stage (5 < RA ≤ 6), in which fault movement stops or undergoes inversion. The activity pattern of a fault is intricately linked to the duration and intensity of its age. Stable continuous, and high-intensity fault activity promotes the evolution of faults into their terminal stage. Research on reservoir control traps indicates that faults can create reservoirs at all stages of their development. However, as faults age, their ability to control reservoir formation strengthens. The types of traps influenced by faults transition from individual, isolated structures to a variety of arrangements. Moreover, the diversity of oil and gas reservoirs evolves from singular to multifaceted, and the size of these reservoirs expands from small to large. The embryonic and juvenile stage faults primarily influence closure; the mature stage faults predominantly impact sand and reservoir; and the declining stage and terminal stage faults primarily govern the overall distribution range of source rocks, as well as the migration, accumulation, and dissipation of oil and gas. Conclusion The reservoir control potential of the Chennan Fault was assessed by considering factors such as reservoir control advantages, the degree of oil and gas enrichment, and the scale of oil and gas accumulation. The reservoir control capacity of the Chennan Fault was classified as “strong.” Reevaluation of the fault’s reservoir control potential from the perspective of its developmental and evolutionary stages significantly enhances and elevates theoretical research on fault reservoir control and also advances exploration efforts in established mature areas. [Significance] Identifying the formation age and evolutionary patterns of extensional faults has immense theoretical and practical importance for comprehending alterations in the fault’s reservoir control capabilities. Moreover, it offers crucial guidance for oil and gas exploration, particularly for enhancing the reserves in existing exploration areas.

Keywords

• tensional faults
• life-cycle evolution stages
• implicit-explicit features
• mechanisms and patterns
• sandbox experiment