UNDERSTANDING OF FLUID AND SEDIMENT SEEPAGE THROUGH DIAGENETIC STUDIES

Seafloor and subsurface studies show abundant evidence for both fluid seepage and sediment injection. These processes are not distinct but form a related continuum of mobilization features. For example, sediment injection structures are found closely related to some seafloor pockmarks, which are also regarded as fluid venting sites. It is generally assumed that sediment mobilization occurs soon, geologically, after deposition. However, age relationships between sediment source and host, lithification of host during injection, and evidence for disaggregation of lithified sand all suggest that substantial time gaps (>50 My) can occur before mobilization. Both sediment injections and fluid seepage sites exhibit marked diagenetic modification. Diagenesis particularly involves carbonate cementation. Sediment injections (sandstone dykes and sills) are ubiquitously carbonate-cemented in locations/ages worldwide, and seepages are commonly marked by carbonate crusts. Numerous isotopic studies have shown that seepage carbonates are derived from oxidation of organic carbon (chemosynthetic carbonate), but data from sediment injections is only just emerging. Diagenesis is closely related to the evolution of porosity in sandstone dykes and sills. Initially, these structures may be preferential conduits for fluid flow where they cross-cut mudrocks. After carbonate cementation their role may be reversed as they become permeability barriers. They also become more rigid than their hosts and so are prone to brittle deformation, which can involve development of fracture porosity, but also development of granulation seams, which are beneficial and detrimental to fluid flow respectively. Generation of secondary porosity at depth may lead to renewal of flow in sandstone structures. Clearly these structures have a complex history of diagenetic evolution, with fundamental consequences for fluid flow behaviour. Fluid inclusion studies are beginning to throw light on their evolution, and confirm that they can have a long history. Even in seafloor crusts, methane-bearing inclusions are providing the most direct evidence for the fluids involved in their diagenesis.