We present a revised Cenozoic chronostratigraphy of the Yatsuo Area, Toyama Prefecture, based on U-Pb and fission-track (FT) dating of zircon grains from tuff beds, mineralogical analysis of tuff beds, and diatom biostratigraphy. The results reveal that syn-rift volcanism (represented by the Iwaine Formation) began at ~. Ma and that the Ikahama unconformity between the syn-rift Higashibessho Formation and the post-rift Tenguyama Formation represents a relatively short-duration (~. m.y.) event ~ Ma. The Otogawa Formation unconformably overlies the Tenguyama Formation and is divided into middle Miocene (Serravallian) and late Miocene, with a possible unconformity in between. The-Ma Mita Formation unconformably overlies the Otogawa Formation. These results require previous local correlations of a key tuff bed (MT) in the Mita Formation to be reassessed. Facies and sequence stratigraphic analysis of the lowermost Nirehara Formation confirm that this formation comprises multiple depositional cycles generated by water-level fluctuations in a coastal or lacustrine fan delta. The results also suggest that the Nirehara Formation, which is unconformably overlain by the syn-rift Yatsuo Group, was formed in an early rift basin. Detrital zircon U-Pb and FT dating of sandstone within the Nirehara Formation suggests that the sandstone may be derived from the Nohi Rhyolite. Regional correlations of onshore and offshore Cenozoic chronostratigraphy in the Hokuriku district show multistage rifting during the Oligocene-middle Miocene opening of the Sea of Japan. Regional correlations also suggest that the post-rifting compressive regime was associated with fluctuations in the strength and direction of compressive stress fields.

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To generate refined depth structure for a very gentle Cretaceous reservoir on an oil field offshore Abu-Dhabi, we conducted channel velocity modelling and prestack depth migration (PSDM). The target reservoir structure on a vintage prestack time migration (PSTM) contains pull-up and push-down artefacts, induced by a couple of overburden channel velocity fills. These artefacts hindered correct representation of gentle depth structure at the target. Detailed interpretation on the PSTM volume delineates overburden channels in three upper Cretaceous formations. One of them, Fiqa channel, generates small pull-up and the others, Halul and Lafan channels generate push-down below each formation, caused by higher and lower velocity channel fills, respectively. These artefacts overlapped at some locations. Analysis of synthetic PSTM stack from forward modelling reminded us that a low-velocity channel produces obvious push-down below the channel. The shape of push-down become smaller but wider with depth and offset. The resulted push-down in full-offset stack also changed with depth. A "true" channel velocity and implementation of PSDM eliminates push-down at all depths and offsets. Bearing in mind the phenomenon confirmed on the forward modelling, we first produced channel velocity model for the target field data. With the assumption that the depth structure at a layer just below channels is flat, time thickness of channel and the amount of push-down at the layer below the channel could determine the channel velocities. Then, we estimated three channel velocities by channel time thicknesses and the amount of pull-up/push-down at the top Mid-Cretaceous shale, Nahr Umr formation that located just below these channels. On the overlapped area, a linear inversion provided each channel velocity. An application of PSDM with the channel velocity model successfully mitigated the artefact at target depth structure. Introduction We sought refined reservoir structure through channel velocity modeling and PSDM at the target oil field, located about 60km off the coast of Abu Dhabi. The lower Cretaceous reservoir (Figure 1) structure consists of a couple of very gentle domal structure-highs, whose average dips are 1-degree and heights are less than 100ft. Since subtle difference of the reservoir depth made significant change in oil saturation and production rate, identification of such structure highs is critical for seeking enhanced productivity and oil recovery. Although vintage PSTM volume produced in 2004 showed good quality to provide more accurate reservoir structure and led to success on some wells, the time structure could contain as much as 10ms (60ft with 12,000ft/s) artefacts at the reservoir, caused by overburden channel velocity fills (Figure 2). These artefacts hindered correct representation of reservoir structure and gave rise to depth uncertainty. We realized that generation of precise reservoir structure necessitates discrimination between artefacts and true structural events. There are some publications that introduce solutions of these problems. Armstrong et. al., (2000) introduced a methodology of compensating artifacts using the amount of the pull-up and push-down changed with depth on post-migrated time volume. Fujimoto et. al., (2007) removed both pull-up/push-down and PSTM velocity artifacts through high-resolution tomography and PSDM.

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