The interpretation of high resolution seismic plays a crucial role in shallow gas operating procedures The results and reliability of the shallow gas investigation however rely on the input to the seismic interpretation in practice, this input varies, depending on the amount of additional seismic and geological information which is consulted to support the interpretation, and depending on the method of computer-based techniques which are employed to enhance quantitative aspects of the seismic data analysis
The standard approach in shallow gas data interpretation is considered to include the use of exploration seismic and well data, and the use of interpretation programs that offer the facilities to interactively digitise, depth-convert and contour seismic horizon data The standard approach should be complemented by employing more advanced interpretation tools A primary requirement are interactive workstations which thereby are substantially enhancing the efficiency and accuracy of the seismic analysis by providing the ability to interactively interpret the data and measure seismic horizon Attributes
Our aim to improve the reliability of the shallow gas prediction can only be achieved by choosing an integrated approach which uses modem seismic processing and interpretation techniques, which is embedded in a seismic and geologic frame provided by exploration data. This will ultimately contribute to minimising the risks associated with shallow gas in offshore drilling.
The risk sand consequences of shallow gas blowouts that have occurred in the past during offshore drilling have increased the industry's awareness of the associated hazards, and have resulted in considerable efforts to implement and improve shallow gas operating procedures. Accidents which have occurred more recently have further underlined the necessity and importance of carefully considering methods of high resolution seismic acquisition, processing and interpretation. In order to come to a comprehensive and eventually reliable assessment of the shallow gas drilling hazards.
The outcome of the high resolution seismic interpretation relies heavily on the data quality which is, apart from the acquisition condition, governed by a careful selection of the acquisition parameters and processing steps. Operational and economic constraints frequently dictate a simplistic approach which may considerably influence seismic data quality With respect to safety in offshore drilling, this is certainly not an ideal situation. Our aim is to make the shallow gas prediction more reliable, and one of the advanced tools to persue this goal are interactive computer systems, which enable he analysis of reflection attributes.
The level of seismic interpretation varies considerably according to the methods and procedures applied The shallow gas data usually cover a small area (e g 2km x 2km), and he record length is mostly confined to less than 15 seconds two-way-traveltime. This generally allows only for a limited insight into the geological domains where processes of potentially hazardous gas generation, migration and accumulation are taking place. The shallow gas interpretation can be considerably substantiated by taking account of available exploration seismic and well data which provide additional information regarding he probability of encountering hazardous shallow gas.
