Geological structure changes, including deformations and ruptures, developed in shallow marine sediments are well recognized but were not systematically reviewed in previous studies. These structures, generally developed at a depth less than 1000 m below seafloor, are considered to play a significant role in the migration, accumulation, and emission of hydrocarbon gases and fluids, and the formation of gas hydrates, and they are also taken as critical factors affecting carbon balance in the marine environment. In this review, these structures in shallow marine sediments are classified into overpressure-associated structures, diapir structures and sediment ruptures based on their geometric characteristics and formation mechanisms. Seepages, pockmarks and gas pipes are the structures associated with overpressure, which are generally induced by gas/fluid pressure changes related to gas and/or fluid accumulation, migration and emission. The mud diapir and salt diapir are diapir structures driven by gravity slides, gravity spread and differential compaction. Landslides, polygonal faults and tectonic faults are sediment ruptures, which are developed by gravity, compaction forces and tectonic forces, respectively. Their formation mechanisms can be attributed to sediment diagenesis, compaction and tectonic activities. The relationships between the different structures, between structures and gas hydrates and between structures and authigenic carbonate are also discussed.

It is important to predict the fracture distribution in the tight reservoirs of the Ordos Basin because fracturing is very crucial for the reconstruction of the low-permeability reservoirs. Three-dimensional finite element models are used to predict the fracture orientation and distribution of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on the distribution of sand bodies in the Chang 7₁ and 7₂ members, and the different forces that have been exerted along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Cenozoic are NW–EW and NNE–ENE, respectively. In this paper, the two-factor method is applied to analyze the distribution of fracture density. The distribution maps of predicted fracture density in the Chang 7₁ and 7₂ members are obtained, indicating that the tectonic movement in the Late Mesozoic has a greater influence on the fracture development than that in the Cenozoic. The average fracture densities in the Chang 7₁ and 7₂ members are similar, but there are differences in their distributions. Compared with other geological elements, the lithology and the layer thickness are the primary factors that control the stress distribution in the study area, which further determine the fracture distribution in the stable Ordos Basin. The predicted fracture density and the two-factor method can be utilized to guide future exploration in the tight-sand reservoirs.