Hydrocarbon leakage interpreted on seismic data

Abstract

In an active petroleum system the amount of trapped hydrocarbons is the difference between the volumes charged and the volumes that have leaked or are otherwise destroyed. This paper focuses on the leakage processes taking place above a hydrocarbon-filled trap and how leakage is expressed on seismic data. A variety of seismic anomalies related to hydrocarbon leakage are interpreted and illustrated. A three step workflow is suggested for hydrocarbon leakage interpretation. First, all anomalies related to hydrocarbon leakage in the study area should be observed, described and mapped. The description should focus on both simple reflection amplitude and patterns or groups of anomalies. Geographical distribution and 3D shapes should also be revealed. Second, each anomaly should be interpreted individually. This paper presents several seismic examples of leakage anomalies and their interpretations are discussed. The interpreted leakage-related anomalies imaged on seismic data are subdivided into two categories: (1) permanent deformation of the primary bedding post-deposition and/or build up of new “syn-leakage” features, and (2) changes in seismic expression and/or secondary effects caused by continuous or discontinuous change in formation fluid from formation water to oil or gas. Third, genetically related leakage anomalies should be grouped into a leakage zone. The leakage zone has a root where the leakage from the reservoir initiates, a body or the zone itself where vertical movements of hydrocarbons occur and a top where the leakage terminates. Seismic data often image only parts of the leakage in the rocks and hence there may be significant differences between the real leakage zones in nature and that imaged on seismic data. The seismic observations in the leakage zone, at the top and the root may help to reveal if hydrocarbons are preserved in the underlying trap.