Direct Hydrocarbon Indicators (DHI)

Abstract

Presence of gas, particularly in young porous sands significantly lowers the bulk modulus and typically creates high amplitude seismic anomalies known as ‘bright spots’, usually considered direct hydrocarbon indicators (DHI). Though high amplitudes are more commonly the characteristic of gas, light and normal grade oil can also manifest high seismic amplitude responses. However, all bright anomalies are not caused due to hydrocarbons and therefore need proper validation before drilling. Other amplitude anomalies such as ‘dim’ and ‘flat spots’ also indicate oil/gas reservoirs and may be considered as DHI anomalies. The genesis of DHI anomalies is illustrated with graphics and seismic images. Evidence of interval velocity, reflection polarity, and phenomena such as reflection ‘shadow zone’ and time ‘sag’ below the, oil/gas reservoirs manifested in seismic corroborate hydrocarbon bearing DHI anomalies and are included in discussion.

Validation of DHI anomalies are commonly done through rigorous AVO analysis on seismic 3D data (Chapter “Shear Wave Seismic, AVO and Vp/Vs Analysis”). However, an alternate simple and straight forward way to validate DHI amplitude anomalies can be by analysing angle- stack amplitudes. Reflection amplitude depends on angle of incidence and the property of angle dependent P-reflectivity at near and far-angle is conveniently utilized to indicate reservoir type and fluid content. The near-angle reflectivity shows the type of reservoir matrix whereas, the far angle the fluid. The quick-look method is useful to delineate and characterize much faster and is expounded with help of case examples of offshore hydrocarbon sands. DHI anomaly related shortcomings are also outlined.