The Use of Gas Chimneys for Risk Mitigation in All Elements of a Petroleum System

Abstract

Gas chimneys and hydrocarbon seepage are well known from many offshore basins around the world with prolific petroleum systems. Also, there are many hydrocarbon accumulations with no observed gas chimneys. The presence of gas chimneys is often used to make inferences about source rocks and seal. The objective of this paper is to demonstrate that the application of a simple gas chimney classification scheme and critical thinking can also reveal significant information about reservoir, trap geometry, seal capacity, hydrocarbon phase and migration. Several examples from a number of basins will be used to demonstrate the concepts. Gas chimney is a poorly defined generic term that is typically used to describe areas of seismic data exhibiting a vertical diffuse, chaotic or degraded quality reflection character on 2D or 3D data. Gas chimneys can also be accompanied by velocity and amplitude anomalies or even complete wipeout zones. The perturbations on the seismic are thought to be caused by the presence of free gas in the section; although fracturing and overpressure might contribute to the deteriorated seismic response. Gas chimneys can generally be distinguished from other vertical seismic noise bands caused by near surface/seafloor effects, permafrost, hydrates or very shallow gas by the direction of propagation of the noise – top down or bottom-up. In-basin calibration organized by play-type is also an important element of this analysis and can be used to high-grade an exploration portfolio or play trend. The integration with basin modeling can further constrain interpretations and contribute to the understanding of geological risk. Characterization, classification and calibration of gas chimneys are a best practice for mitigating geological risk in an exploration basin. A gas chimney can be a positive sign for many exploration opportunities but the lack of a gas chimney cannot automatically be taken as negative evidence.