So called 'flatspots' rarely have the appearance of flatness, even at proven gas and oil fields and on seismic data that has been converted to depth using best-practice methods. The appearance of fluid contacts on seismic reflection data is influenced by several controls, most notably, fluid type; burial depth; saturation; and the size of the 'flatspot' anomaly compared to the imaging velocity resolution. Many cases of non-flatness are caused by lateral velocity variation in the overburden. Often the structural form of the trap itself is the cause. Non-flat seismic hydrocarbon contacts are seen at shallow gas hazards in Sakhalin, Russia; at oil fields in the Santos Basin, Brazil; and at 'gas on oil' accumulations in the Browse Basin, Australia. So, it appears that non-flat 'flatspots' are ubiquitous, suggesting an urgent name change is needed for this form of DHI. The interpretation of DHIs is aided by a new 2D synthetic modelling method that characterizes the time-depth behaviour of a field or prospect simultaneously with its amplitude response. The software interactively models seismic cross-sections using rock physics or seismic velocities to compute AvO synthetics at significant layer boundaries. Hydrocarbon porefill effects can be modelled using Gassmann fluid substitution to modify the elastic properties within the proposed trap. DHIs are thus characterized at (1) the top of the trap; (2) the base of the trap (if base sealed); and (3) at the contacts between different fluid types, such as a gas/water contact; oil/water contact; or gas/oil contact. We propose a more representative terminology, referring to DHIs as either top reservoir indicators; base reservoir indicators or contact indicators, thus removing the rarely met implication that 'flatspots' ought to appear flat on a depth converted seismic section.

Open-Access Online Publication: May 22, 2023