Vertical and Horizontal Migration of Gas Through the Marine Sediments of the Gulf of Mexico

Since petroleum exploitation began activity in the Gulf of Mexico, geophysical studies have been carried out to detect geological hazards and features, which can affect the design or installation of platforms or pipelines. In some of the studies it was noted that shallow gas accumulations increase their size and number through time. This paper presents a comparison of three geophysical studies which were carried out in 1978 [1], 1997 [2] and 2002 [3]. The comparison is complemented with results obtained from geotechnical studies performed in 2002 [4–7] to evaluate the influence of the gas on the properties of the foundation zone soils. In the 1997 geophysical study, there were more detected gas accumulations than in 1978. The existing gas accumulations (originally detected in 1978) also increased in size over the nineteen-year period. The same phenomenon was also identified in the geophysical study of 2002, when the results were compared with the study performed in 1997, it appeared that the shallow gas accumulations increased both their size and number considerably over a period of only five years. The only explanation found for this phenomenon was that gas migrates vertically from the reservoir along a major geological fault that crosses the entire area, forming a chimney. Once the gas reached the shallow strata, the gas spread laterally, saturating the area around the geological fault with gas. Sand Strata A, B, C and D (identified in the geotechnical investigation) were the primary gas-charged strata, that is, the gas migrates laterally within these strata until encountering soils that can not confine the pressure developed by the gas accumulation [8] or until the gas finds a route to escape upwards. As a consequence, a sudden or gradual emanation occurs, altering the initial characteristics of the foundation zone soils and/or possibly damaging marine structures, resulting in economic loss.