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During Triassic deposition in the Central North Sea, synforms developed on the surface of the extending Zechstein salt. In the ensuing continental environment, coarse-grained clastic sediments were deposited in such depressions. These were later preserved as sediment ‘pods’. A progressive increase in coarse clastic input to the Central North Sea Basin during the Triassic was concurrent with continued salt movement and pod subsidence. During Late Triassic times the best initial reservoir quality sands were developed in the axes of TR30 sediment pods (e.g. Marnock facies). Early Fe-rich chloritization of these high-quality reservoir facies, combined with overpressure development, has resulted in the subsequent, relatively local, preservation of high-quality Triassic reservoirs, often at substantial depths.Understanding the basin-scale halokinetic controls on Triassic sequence and facies development, as well as the subsequent diagenetic effects, is crucial to establishing a framework for Triassic reservoir prediction.We have combined the re-interpretation of newly acquired and reprocessed seismic data with sedimentological and diagenetic studies of available core material. This has enabled the generation of a model for evaluating the distribution of Triassic reservoir and hence exploration potential in the Central North Sea.

We present the results of an interpretation of 2D and 3D seismic from offshore Lebanon in which we identify a suite of 5 linearly distributed trails of fluid escape pipes with pockmarks at their upper terminus. These features transect the thick Messinian evaporites and root within prominent NE-SW oriented pre-salt folds. The pipe trails are oriented orthogonal to the strike of the pre-salt folds, with a synchronous initial expulsion episode in each trail dated at 1.7 Ma (±0.3 Ma), approximately coeval with the onset of salt-detached growth faulting along the basin margin. Each expulsion episode has been systematically offset to the NW away from the pre-salt fold by the flow of the salt, resulting in deformation of the fluid escape pipes in the salt. The orientation of the pipe trails thus provides a direct kinematic indicator for the flow direction of the salt layer during early stages of gravity-driven deformation of the salt and overburden, concomitant with basin margin uplift and tilting. The unidirectional NW oriented flow is recorded over a region of some 50 km width within the 2 translational domain of the salt tectonic deformation. Synchronicity in the onset of fluid expulsion from overpressured reservoirs within the pre-salt succession evidenced by these pipe trails and growth fault development at the basin margins implies that the pipe trails record the kinematics of the deforming salt layer throughout its post-Messinian phase of deformation. The deformed pipe trails demonstrate a Couette flow regime for the salt layer and document subtle changes in cumulative strain and velocity (2-4 mm/yr; ±0.3 mm/yr) over distances of a few km. It is proposed that this novel method of using fluid flow features as natural markers for the kinematics of deforming salt layers could be utilized in other parts of Eastern Mediterranean, as well as other salt basins on Earth.

There are increasing numbers of published examples from around the world in which significant 4D time shifts have been observed in the overburden above producing reservoirs.Indeed, this topic prompted the TLE special section "Rocks under strain" in December 2005. The significance of these 4D observations is that, if we wish to fully understand the 4D signature of compacting reservoirs, we can no longer think of the reservoir in isolation. The seismic response outside the reservoir changes because the nonreservoir rocks deform in response to reservoir activity. While these nonreservoir 4D seismic changes can obscure or contribute to the reservoir-level signal, making the 4D interpretation uncertain, if utilized appropriately they may also be used to provide information on the actual reservoir pressure changes. This new pressure information can thus be used to complement well measurements or other 4D seismicbased methods (such as the multi-attribute pressure and saturation inversion outlined by Floricich et al., 2006), providing valuable data for reservoir monitoring and management. In this paper, we build on the work of several authors who have presented methods to invert surface deformation measurements for reservoir volume or pressure change. We show how 4D seismic can extend this approach by focusing on the inversion of 3D strain deformation estimates for the overburden derived directly from the repeated seismic data. The method is then applied to Genesis Field in the Gulf of Mexico, in which there are series of compacting unconsolidated stacked turbidite reservoirs. Time-lapse time strain.Rock velocities are sensitive to changes in stress and strain so that if a volume of rock strains, the change in traveltime through it will be made up of a contribution due to the change in distance traveled by the seismic wave and a contribution due to the change in velocity. A perturbation formula relating changes in vertical traveltime t, velocity v, and vertical layer thickness z, assuming small changes in thickness and velocity, is given by Landrø and Stammeijer (2004) (1)Hatchell and Bourne (2006) went on to make the assumption that changes in thickness and velocity can be linearly related by a constant of proportionality, R, which relates the fractional change in velocity and vertical strain so that ∆v/v = -Rε zz , resulting in the following relationship (2) where we have replaced ∆z/z by ε zz , signifying the vertical component of the strain tensor. The left side of Equation 2 is the derivative of the time-shift field, which we term time strain. A method to obtain time-lapse time strains from 4D seismic is described in the companion paper in this special section ("4D time strain and the seismic signature of geomechanical compaction at Genesis"). If we have knowledge of the magnitude of R, we can obtain estimates of vertical strain directly from 4D seismic observations. With estimates of vertical strain for the overburden, a linearized inversion can be employed to obtain reservoir pressure change. Segall (1992) shows ...