ABSTRACT: On the Current Geological Knowledge of East China Sea and Its Petroleum Potential
Edmund Z. Chang, Stephan A. Graham, Simon C. Brassell
The East China Sea covers an area of 770,000 km2 from the mainland to the Ryukyu archipelago and including the Taiwan Strait. About 60% of the East China Sea is underlain by the continental shelf, with an average water depth of 72 m and a maximum width of 550 km. The Okinawa Trough, parallel to the Ryukyu archipelago, is floored by new oceanic crust with a maximum water depth of 2717 m and a higher than average heat flow of 2.97 HFU.
There are two sedimentary basins in the East China Sea. They are the Cenozoic continental-shelf basin in the west and the Okinawa basin in the east. The basement of the former is mainly Pre-cambrian; whereas that of the latter consists of Pre-cambrian basement, Upper Paleozoic cover strata, Mesozoic volcanics, and Cenozoic active continental margin assemblages. Based on interpretation of seismic data, the Upper Cretaceous as the oldest sedimentary infilling probably exists in the southwestern part of the East China Sea, where the shallow-marine and coastal-marsh deposits of Paleocene age unconformably or disconformably overlie on the older strata. The Eocene reflects an environment similar to the Paleocene age in the southwestern part of the East China Sea, but in the northeastern par , the Eocene is predominantly lacustrine deposits with some marine intercalations. The thicknesses of Paleocene and Eocene are 3000-4000 m and 1000-3000 m respectively. Oligocene strata were deposited mainly in the eastern part of the continental shelf, with a thickness of 1000-2000 m and thinning to the west. The sedimentary facies show a north-south differentiation from marine beds to transitional facies. The Oligocene is unconformable with the Eocene but is usually gradational upward into the Miocene with only local disconformity. The Miocene consists of fluvial-alluvial, swamp to lacustrine deposits with marine intercalations. The thickness of the Miocene varies from 2600 m to near 4000 m. Above another regional unconformity, the Pliocene is widespread over the East China Sea and app ars to be alternating marine and terrestrial deposits 400-700 m thick. The marine facies of the Quaternary is widely distributed too. Its average thickness is 350-400 m on the continental shelf, but it is much thicker in the Okinawa Trough, where undifferentiated marine Pliocene and Quaternary are 2-6 km thick.
The East China Sea has undergone several phases of tectonism. Its evolutionary history can be divided into a fault depression stage from late Mesozoic to Eocene, a down-warping stage from Oligocene to Miocene, and a regional subsidence stage from Pliocene to the present time. The difference of structural styles between the eastern and western parts of the East China Sea is thought to be of first-order importance. The major structural units from west to east are the Zhe-Min Uplift, the Shelf Basin, the Shelf Marginal Uplift, and Okinawa Trough. In the Shelf Basin itself, a NNE-oriented rise divides the basin into eastern and western depressions. Each of the depressions can be subdivided into sags and highs in a N-S direction. The western depression is characterized by half-grabens fill d by Upper Cretaceous, Paleocene, and Eocene sediments. The eastern depression shows similar structural and sedimentary features as in the western belt, but the overlying Oligocene and Miocene sediments, up to 5000 m thick, are folded into anticlines that, in places, are related to reverse faults or current faults.
Petroleum exploration thus far has revealed four sets of source rocks in the East China Sea. They lie in the Paleocene, Eocene, Oligocene, and Miocene. Upper Cretaceous source rocks remain to be proved. The geothermal gradient is 3.40-3.75° C/100 m and thus the current generation threshold is 2400-3000 m deep. The most important play-types are anticlines in the east and buried hills and fault blocks in the west, but nonstructural traps are not negligible in the whole Shelf Basin.
About 15 wells have been drilled to the present. Among them only five are dry. The others have had high-flow rates during testing. Maximum rates are about 12,000 bopd (condensate) at Pinghu 4 and 25 mm cftd gas at Pinghu 2. The first gas-condensate field on Pinghu structure in the middle of the Shelf Basin is now under feasibility study and will be developed soonest. Considering the high rate of success during early exploration and the discoveries in Pinghu and in Taiwan Strait, it is quite reasonable to be optimistic about petroleum prospects in the East China Sea.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990