Studies of relative amplitude processed sections over a Central North Sea Diaper structure have identified shallow gas reservoirs within Quaternary and Tertiary sediments in the interval up to 600 metres below seabed. They are believed to have formed within silt and sand layers in an otherwise clay sequence
Two pathways for the migration of thermogenic gas into the shallow section are identified, namely Tertiary fault planes and vertical columns of gasified sediment, or (seismic) gas chimneys.
Migration of gas into individual reservoirs has taken place either vertically to a seal or partial seal, or horizontally along silt and sand layers, away from the vertical migration paths. This has resulted in differing seismic responses from the upper and lower limits of the reservoirs Estimates of the partial pressure exerted by the gas column at the structural high point, and the volume of gas in place have been made for selected individual reservoirs.
High resolution seismic data was acquired by Britsurvey over a diapir structure in the Central North Sea, with the prime objective of identifying any potential shallow gas reservoirs and assessing them as drilling hazards. Details of the equipment spread are shown in Figure 1 together with the acquisition parameters Line spacing was 100 metres increasing to 50 metres in some places, and all lines were migrated during processing (Fig. 1 is available in full paper) The data, which was interpreted in conjunction with exploration seismic lines and five well logs from within the area of coverage, shows a late Cenozoic sequence of clays and claystones with subordinate sit and sand bodies in the form of lenses and layers, typical of the Central Graben of the North Sea. In this case, however, movement of the underlying diapir had caused doming and halokinetic fracturing of the formation Figure 2 is a section which shows the main structural features identified (Fig. 2 and 3 are available in full paper) The study identifies shallow gas reservoirs in several contrasting geological settings, and highlights variations in their seismic attributes in relation to the migration paths involved in their formation. Estimates of the pressure and volume of gas within individual reservoirs have been made for comparison purposes.
In this case particular Importance was given to depth conversion Velocity calibrations from two of the wells were compared with an average velocity function derived from the stacking velocities used in processing (Figure 3) A velocity profile was subsequently derived based mainly on the well velocity calibration, although since 375 metres below seabed IS the upper limit of this calibration, stacking velocities were used above this depth
Data interpretation has concentrated on the upper 800 milliseconds TWTT of data (corresponding to just over 600 metres below seabed), where there are a large number of amplitude anomalies, many of which are interpreted as shallow gas reservoirs (Figure 4)