DHI interpretation and risking

摘要

Seismic amplitude anomalies, called direct hydrocarbon indicators (DHIs), are often used to assess the risk (the probability of geological and commercial success and the resource scope) for exploration prospects. The beginning of quantitative DHI analysis occurred in 1970 with "bright spot" technology, and it expanded during the 1980s up to the present with amplitude vs. offset (AVO) studies. Seismic technology can be used to interpret and risk seismic amplitude anomalies that may be related to the presence of hydrocarbons in low impedance ("soft") and high impedance ("hard") rocks in the form of bright spots, phase changes and dim spots. DHI technology is based on the premise that when gas or oil (with gas in solution) replaces brine in a reservoir (sand, in this article), the acoustic impedance (velocity times density) of the interval decreases. Also, the resulting impedance contrast with the adjacent rocks (usually shales) will generate a reflection amplitude that changes with increasing offset distance. This AVO effect is measured on a seismic gather which is a display of common mid-point seismic traces (usually 30 to 40 recordings) across an offset distance range (seismic source to surface detector) up to 20,000 ft (6,100 m) or more that are combined to make a stack seismic trace.