Description: We have performed 2D and 3D gas hydrate (GH) petroleum systems modeling for the Pleistocene turbiditic sedimentary sequences distributed in the Daini-Atsumi area in the eastern Nankai Trough to understand the accumulation mechanisms and their spatial distribution related to geologic and geochemical processes. High-resolution seismic facies analysis and interpretations were used to define facies distributions in the models. We have created a new biogenic methane generation model based on the biomarker analysis using core samples and incorporated it into our model. Our 2D models were built and simulated to confirm the parameters to be used for 3D modeling. Global sea level changes and paleogeometry estimated from 3D structural restoration results were taken into account to determine the paleowater depth of the deposited sedimentary sequences. Pressure and temperature distributions were modeled because they are the basic factors that control the GH stability zone. Our 2D modeling results suggested that the setting of biogenic methane generation depth is one of the most important controlling factors for GH accumulation in the Nankai Trough, which may be related to the timing of methane upward migration (expulsion) and methane solution process in pore water. Our 3D modeling results suggested that the distribution of sandy sediments and the formation dip direction are important controlling factors in the accumulation of GHs. We also found that the simulated amount of GH accumulation from the petroleum systems modeling compares well with independent estimations using 3D seismic and well data. This suggests that the model constructed in this study is valid for this GH system evaluation and that this type of evaluation can be useful as a supplemental approach to resource assessment.

Description: The first offshore gas hydrate production test was conducted within the gas-hydrate-concentrated zone (reservoir) of the eastern Nankai Trough, which is considered to be a stratigraphic accumulation. However, the accumulation mechanism for this concentrated zone was not yet well understood. We used core and geophysical log data sets to determine the subsurface geologic architecture and stratigraphic evolution most likely responsible for the stratigraphic accumulation of gas hydrate in the eastern Nankai Trough. Seven depositional sequences were identified based on grain size, bed thickness, sedimentary structure, and stacking patterns. The sequence boundaries were also identified by terminations of seismic reflection. These sequences were attributed to a fourth to fifth-order eustatic sea-level changes because the stacking pattern cycle was in phase with global oxygen isotope curves; the cycle was also identified in the onshore formation during the same period. The reservoir was interpreted as falling-stage systems tract (FSST) and lowstand systems tract (LST). FSST and LST consisted mostly of trough-fill channel deposits. The deposits were represented by alternations of very fine- to fine-grained sand and silt. The reservoir is located in association with the structural wing of the Daini-Atsumi Knoll. The uplift of the knoll was strongly controlled by tectonic events associated with subduction of the pacific plate during Pleistocene time. The muddy deposits above the reservoir were interpreted as condensed section. We identified channel facies pinched out against structural highs, and together, these result in stratigraphic traps. Consequentially, the gas hydrate trapping system was constrained by sedimentary facies, systems tracts, and geographic and tectonic setting. Concepts and data generated in this study can be used for gas hydrate petroleum system analysis such as basin simulation.