The East China Sea shelf basin (ECSSB) consists of three sub-basins namely, the Jeju, the Socotra and the Domi sub-basins. These three sub-basins separated by faulted-basement blocks. I have identified and mapped four regional unconformities, including the top of acoustic basement, in the northern...
The East China Sea shelf basin (ECSSB) consists of three sub-basins namely, the Jeju, the Socotra and the Domi sub-basins. These three sub-basins separated by faulted-basement blocks. I have identified and mapped four regional unconformities, including the top of acoustic basement, in the northern East China Sea shelf basin from multi-channel seismic data. These regional surfaces correspond to the Early Cretaceous (MB1), Late Eocene ? Early Oligocene (MB2), Early Miocene (MB3), and Late Miocene (MB4). The seismic units bounded by these surfaces are megasequences, representing deposition during tectonically-controlled phases of basin development. The acoustic basement is characterized by grabens and half grabens formed during the main rifting stage. The depths of the acoustic basement range from less than 1,000 m in the northwestern part of the Domi Basin to more than 4,500 m in the Socotra Basin and 5,500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1,500 m in the northwest where the acoustic basement is shallow and reaches about more than 5,500 m in the south. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area.
Reconstruction of depth-converted seismic reflection profiles from these subbasins reveals that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the Eurasian plate. The major extension took place during the early phase of basin formation. The estimated rates of extension during the initial rifting are 16 %, 6.5 %, and 3.5 % in the Domi, Jeju, and Socotra basins, respectively. The initial rifting was terminated by the regional uplift in the Late Eocene-Early Oligocene. Rifting and extension resumed in the Early Oligocene; the estimated rates of extension in the Domi, Jeju, and Socotra basins are 0.8 %, 0.8 %, and 3.5 %, respectively. The greater extension in the Socotra Basin during the Early Oligocene and Early Miocene is correlated with the increased sediment accumulation in the basin. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 meters of sediments and the basement rock were removed by erosion during and after the second phase of uplift. The maximum shortening (0.1%) associated with this uplift in the Socotra Basin occurred in the NE-SW. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Restoration suggest that the inversion magnitude increased southward in the basin with the amounts of erosion ranging from 425 m to over 900 m. The maximum contraction (0.4 %) occurred in the NW-SE in the Jeju Basin.
I identified various igneous features such as sills, volcanic edifices and stocks in the northern East China Sea shelf basin from 2-D multi-channel seismic reflection data. The stocks are tall, vertical intrusives, characterized by a seismically dead zone with upturned host rocks and uplifted overburden. The sills are imaged as high-amplitude continuous reflections that exhibit mostly concordant relationships with the host rock. The volcanic edifices, observed on the top of the shallow, eroded basement, consist of mounds and peaks, probably representing volcanoes and/or their remnants (or necks).
Shallow gas in the northern East China Sea shelf basin is associated with gas seepages, seismic chimneys, shallow enhanced reflections, and pockmarks. The gas seepages are most dominant, observed at more than 120 locations. They are characterized by small hyperbolic reflections on the seafloor, occur in groups mostly in the eastern part of the area, forming seepage fields. Seismic chimneys or columnar zones of amplitude blanking are seen dominantly in the eastern part of the study area. They are neither associated with chimneys nor faults. Shallow enhanced reflections are only seen in the eastern part and are characterized by relatively high-amplitude reflections immediately below seafloor. Pockmarks are observed only in one location in the eastern part of the study area, reaching a diameter of about 500 m.
Burial history, thermal maturity, and timing of petroleum generation were modeled at four 1-D locations in the northern East China Sea shelf basin. The 1-D models comprise two actual wells (JDZ-VII-1 and JDZ-VII-2) and two dummy wells (Dummy Well 1 and Dummy Well 2), chosen in the deepest parts of the Jeju and Domi Basins. An overall ranking of models in order of decreasing thermal maturity is JDZ-VII-1, JDZ-VII-2, Dummy well 1, and Dummy well 2 respectively.
The results indicate that burial history, thermal maturity, and timing of hydrocarbon generation significantly vary between the modeled locations and laterally throughout the studied basins. This information is of great importance for assessing the petroleum potential of these basins. Results for the synrift source rocks indicate that maximum depth of burial and associated temperatures range from 3,750 m and 120 ?C in the shallowest parts of the basin to 5,300 m and 200 ?C in the deep basin centers. The timing of onset of hydrocarbon change from 40 Ma to 28 Ma in the deep basin centers. The results of the models indicate that peak oil generation from the synrift fluvial source rocks in the deepest part of the Jeju Basin occurred from Late Eocene through Middle Eocene. Peak generation of gas from synrift source rocks occurred only at JDZ-VII-1 and JDZ-VII-2 in the Early Oligocene and in the Middle Miocene, respectively. Source rocks of the fluvial shales were buried to the oil window only at JDZ-VII-I in the Late Miocene (~11 Ma).
The main phase of oil generation in the synrift lacustrine and fluvial shales (Paleocene-Eocene in age) beneath the southern part of the study area occurred during the middle Miocene - early Oligocene period, predating the regional uplift. This significantly reduces the chances of large oil accumulations in the southern part of the basin.
The East China Sea shelf basin (ECSSB) consists of three sub-basins namely, the Jeju, the Socotra and the Domi sub-basins. These three sub-basins separated by faulted-basement blocks. I have identified and mapped four regional unconformities, including the top of acoustic basement, in the northern East China Sea shelf basin from multi-channel seismic data. These regional surfaces correspond to the Early Cretaceous (MB1), Late Eocene ? Early Oligocene (MB2), Early Miocene (MB3), and Late Miocene (MB4). The seismic units bounded by these surfaces are megasequences, representing deposition during tectonically-controlled phases of basin development. The acoustic basement is characterized by grabens and half grabens formed during the main rifting stage. The depths of the acoustic basement range from less than 1,000 m in the northwestern part of the Domi Basin to more than 4,500 m in the Socotra Basin and 5,500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1,500 m in the northwest where the acoustic basement is shallow and reaches about more than 5,500 m in the south. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area.
Reconstruction of depth-converted seismic reflection profiles from these subbasins reveals that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the Eurasian plate. The major extension took place during the early phase of basin formation. The estimated rates of extension during the initial rifting are 16 %, 6.5 %, and 3.5 % in the Domi, Jeju, and Socotra basins, respectively. The initial rifting was terminated by the regional uplift in the Late Eocene-Early Oligocene. Rifting and extension resumed in the Early Oligocene; the estimated rates of extension in the Domi, Jeju, and Socotra basins are 0.8 %, 0.8 %, and 3.5 %, respectively. The greater extension in the Socotra Basin during the Early Oligocene and Early Miocene is correlated with the increased sediment accumulation in the basin. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 meters of sediments and the basement rock were removed by erosion during and after the second phase of uplift. The maximum shortening (0.1%) associated with this uplift in the Socotra Basin occurred in the NE-SW. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Restoration suggest that the inversion magnitude increased southward in the basin with the amounts of erosion ranging from 425 m to over 900 m. The maximum contraction (0.4 %) occurred in the NW-SE in the Jeju Basin.
I identified various igneous features such as sills, volcanic edifices and stocks in the northern East China Sea shelf basin from 2-D multi-channel seismic reflection data. The stocks are tall, vertical intrusives, characterized by a seismically dead zone with upturned host rocks and uplifted overburden. The sills are imaged as high-amplitude continuous reflections that exhibit mostly concordant relationships with the host rock. The volcanic edifices, observed on the top of the shallow, eroded basement, consist of mounds and peaks, probably representing volcanoes and/or their remnants (or necks).
Shallow gas in the northern East China Sea shelf basin is associated with gas seepages, seismic chimneys, shallow enhanced reflections, and pockmarks. The gas seepages are most dominant, observed at more than 120 locations. They are characterized by small hyperbolic reflections on the seafloor, occur in groups mostly in the eastern part of the area, forming seepage fields. Seismic chimneys or columnar zones of amplitude blanking are seen dominantly in the eastern part of the study area. They are neither associated with chimneys nor faults. Shallow enhanced reflections are only seen in the eastern part and are characterized by relatively high-amplitude reflections immediately below seafloor. Pockmarks are observed only in one location in the eastern part of the study area, reaching a diameter of about 500 m.
Burial history, thermal maturity, and timing of petroleum generation were modeled at four 1-D locations in the northern East China Sea shelf basin. The 1-D models comprise two actual wells (JDZ-VII-1 and JDZ-VII-2) and two dummy wells (Dummy Well 1 and Dummy Well 2), chosen in the deepest parts of the Jeju and Domi Basins. An overall ranking of models in order of decreasing thermal maturity is JDZ-VII-1, JDZ-VII-2, Dummy well 1, and Dummy well 2 respectively.
The results indicate that burial history, thermal maturity, and timing of hydrocarbon generation significantly vary between the modeled locations and laterally throughout the studied basins. This information is of great importance for assessing the petroleum potential of these basins. Results for the synrift source rocks indicate that maximum depth of burial and associated temperatures range from 3,750 m and 120 ?C in the shallowest parts of the basin to 5,300 m and 200 ?C in the deep basin centers. The timing of onset of hydrocarbon change from 40 Ma to 28 Ma in the deep basin centers. The results of the models indicate that peak oil generation from the synrift fluvial source rocks in the deepest part of the Jeju Basin occurred from Late Eocene through Middle Eocene. Peak generation of gas from synrift source rocks occurred only at JDZ-VII-1 and JDZ-VII-2 in the Early Oligocene and in the Middle Miocene, respectively. Source rocks of the fluvial shales were buried to the oil window only at JDZ-VII-I in the Late Miocene (~11 Ma).
The main phase of oil generation in the synrift lacustrine and fluvial shales (Paleocene-Eocene in age) beneath the southern part of the study area occurred during the middle Miocene - early Oligocene period, predating the regional uplift. This significantly reduces the chances of large oil accumulations in the southern part of the basin.
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