Seismic characteristics of fluid escape pipes in sedimentary basins: Implications for pipe genesis

Fluid escape pipes were first documented from 3D seismic data over a decade ago, and have subsequently been identified in many petroliferous basins worldwide. They are characterized on seismic data by vertical to sub-vertical zones of reduced reflection continuity that have a columnar geometry in three-dimensions. The upper terminations of these pipes commonly coincide with pockmarks or palaeo-pockmarks, signifying a close connection of pipe formation with a high flux fluid expulsion process. Dimensions range from tens to hundreds of metres in diameter, and hundreds to over a thousand metres in height, and the slenderness ratio, defined as height/diameter (Ω), ranges from 0.8 to over 20. Pipes are frequently associated with sub-vertical clustering of amplitude anomalies on seismic data, related either to the presence of free gas, or to cementation linked to the passage of hydrocarbons.Three mechanisms have been suggested to explain pipe genesis: (1) hydraulic fracturing, (2) erosional fluidisation, and (3) capillary invasion. We suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation. We review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts. Fluid escape pipes may be far more widespread that currently appreciated, and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals.