SEAM Time Lapse, a collaborative technical project of SEG and the Society of Petroleum Engineers, created integrated geologic, reservoir, and geophysical models to simulate temporal changes in the geometry and physical properties of a complex reservoir with the detail and accuracy needed to explain the subtle effects seen in time-lapse surveys of real oil fields. The geologic model consisted of 2 billion grid cells, representing a region 12.5 × 12.5 km in horizontal extent and 5 km in depth and including a 420 m thick reservoir with upper and lower units separated by an impermeable shale layer and offset by faults. Deepwater clastic turbidite channels and lobes were used to create a typical shallow Gulf of Mexico reservoir that also can serve as an analogue of other turbidite fields around the world. Stratigraphic detail within the reservoir was retained in the simulation model through careful finite-element meshing. The reservoir simulation computed the fully coupled three-phase fluid flow and linear geomechanical response in a production scenario involving 11 production wells and six water-injection wells penetrating the reservoir's three compartments. Seismic surveys were simulated with isotropic elastic-wave modeling before the start of production and after 27.5 months of production at a simulated rate of about 67,500 barrels per day, with about 32,500 barrels per day of water injection. Rock properties were updated by petrophysical models calibrated to turbidite systems in the Gulf of Mexico. Analysis of the model and simulations improves understanding of the complex interaction of fluid effects, pressure changes, and rock deformation. For example, compaction in the reservoir may cancel time-lapse fluid effects, and compaction or dilation in the surrounding shales may override the observed reservoir signal in the seismic bandwidth. Strain-induced velocity changes in the shales have a much larger effect on estimated time-lapse time shifts than do strain-induced changes in path lengths. Geomechanical effects impact the interpretation of 4D seismic difference attributes and require careful consideration. The integrated, full-physics SEAM approach used for this model provides a unique data set to explore these complex production effects and the value of 4D seismic surveys. The models and data, which also include simulations of time-lapse gravity and electromagnetic surveys, are publicly available through SEAM.
Abstract We modeled the velocity structure of the Huatung Basin and Gagua Ridge using offshore wide‐angle seismic data along four ∼E‐W transects. These transects are accompanied by several multichannel seismic reflection (MCS) profiles that highlight the shallow deformation in this area east of Taiwan. Although it is agreed that the Gagua Ridge was the product of a transient compressional episode in the past, relatively few data have been collected that reveal the deeper structure resulting from this enigmatic process. The velocity models show evidence for normal, to thin, oceanic crustal thicknesses in the Huatung Basin and West Philippine Basin. Moho reflections from the associated MCS profiles confirm the thickness observed in the velocity models. The velocity models indicate significant crustal thickening associated with the Gagua Ridge, to 12–18 km along its entire length. Most importantly, the two central velocity models also show a significant asymmetry in the crustal thickening, suggesting a westward underthrusting of >20 km of WPB oceanic crust beneath that of the Huatung Basin. This geometry is extremely unexpected given interpretations that indicate the Huatung Basin could be significantly older than the West Philippine Basin (Early Cretaceous versus Eocene). Our observations, along with recent geophysical data concerning the age of the Huatung Basin, indicate that the Gagua Ridge was the result of a failed subduction event during the Miocene that may have existed simultaneously and for a short time, competed with the Manila subduction zone to the west in accommodating convergence between the Eurasia and Philippine Sea plates. In this scenario, the present‐day Gagua Ridge represents a snapshot of a failed subduction initiation preserved in the geologic record.
Abstract We use offshore multichannel seismic (MCS) reflection and wide‐angle seismic data sets to model the velocity structure of the incipient arc‐continent collision along two trench perpendicular transects in the Bashi Strait between Taiwan and Luzon. This area represents a transition from a tectonic regime dominated by subduction of oceanic crust of the South China Sea, west of the Philippines, to one dominated by subduction and eventual collision of rifted Chinese continental crust with the Luzon volcanic arc culminating in the Taiwan orogeny. The new seismic velocity models show evidence for extended to hyperextended continental crust, ~10–15 km thick, subducting along the Manila trench at 20.5°N along transect T1, as well as evidence indicating that this thinned continental crust is being structurally underplated to the accretionary prism at 21.5°N along transect T2, but not along T1 to the south. Coincident MCS reflection imaging shows highly stretched and faulted crust west of the trench along both transects and what appears to be a midcrustal detachment along transect T2, a potential zone of weakness that may be exploited by accretionary processes during subduction. An additional seismic reflection transect south of T1 shows subduction of normal ocean crust at the Manila trench.
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