Localization of shallow gas deposits and uncontrolled gas flows in young and unconsolidated sediments by geophysical methods

The great mass of Neogene sediments in the Hungarian basin, where several hydrocarbon accumulations are known, is affected by Pliocene strike-slip movements, resulting in many [open quotes]flower structures.[close quotes] The gas may migrate from the reservoirs upward to the surface along the faults. Thus, shallow gas deposits can be located in the young, unconsolidated sands. There are also several shallow gas deposits derived from uncontrolled gas flows. In Hungary, the shallow gas reservoirs, which are small but increasingly important, have not yet been explored properly. However, the depleting gas may pollute the water in the soil as well as cause explosions. Our purpose is to develop inexpensive, complete, and highly sophisticated field- and data-processing techniques and an integrated complex of geophysical methods in order to define the limits of shallow gas deposits. To avoid anomalous behavior on seismic sections of the depleting gas, we started from uncontrolled gas flows which require special velocity and amplitude vs. offset analyses. In addition, natural and controlled source electromagnetic/electric surveys with various parameters were applied. An industrial-scale seismic section over an uncontrolled gas flow, special sections over flower structures and geoelectric sections, and a magnetic map are presented. The integrated complex of geophysical methodsmore » outlined above is being developed in order to establish the conditions for the exploration of gas reservoirs which have been used close to their locality and which could be recovered inexpensively.« less