Offshore high-density 3D seismic acquisition and processing technologies and its applications in lithologic oil and gas reservoir exploration in the Pearl River Mouth Basin (East)

A new 3D seismic reflection data volume acquired in 2012 has allowed for the detailed mapping and characterization of gas hydrate distribution in the Pearl River Mouth Basin in the South China Sea. Previous studies of core and logging data showed that gas hydrate occurrence at high concentrations is controlled by the presence of relatively coarse-grained sediment and the upward migration of thermogenic gas from the deeper sediment section into the overlying gas hydrate stability zone (BGHSZ); however, the spatial distribution of the gas hydrate remains poorly defined. We used a constrained sparse spike inversion technique to generate acoustic-impedance images of the hydrate-bearing sedimentary section from the newly acquired 3D seismic data volume. High-amplitude reflections just above the bottom-simulating reflectors (BSRs) were interpreted to be associated with the accumulation of gas hydrate with elevated saturations. Enhanced seismic reflections below the BSRs were interpreted to indicate the presence of free gas. The base of the BGHSZ was established using the occurrence of BSRs. In areas absent of well-developed BSRs, the BGHSZ was calculated from a model using the inverted P-wave velocity and subsurface temperature data. Seismic attributes were also extracted along the BGHSZ that indicate variations reservoir properties and inferred hydrocarbon accumulations at each site. Gas hydrate saturations estimated from the inversion of acoustic impedance of conventional 3D seismic data, along with well-log-derived rock-physics models were also used to estimate gas hydrate saturations. Our analysis determined that the gas hydrate petroleum system varies significantly across the Pearl River Mouth Basin and that variability in sedimentary properties as a product of depositional processes and the upward migration of gas from deeper thermogenic sources control the distribution of gas hydrates in this basin.

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The role of fluid migration in the occurrence of shallow gas and gas hydrates in the south of the Pearl River Mouth Basin, South China Sea

Interpretation ◽

10.1190/int-2016-0197.1 ◽

2017 ◽

Vol 5 (3) ◽

pp. SM1-SM11 ◽

Cited By ~ 4

Author(s):

Jinxiu Yang ◽

Xiujuan Wang ◽

Jiapeng Jin ◽

Yuanping Li ◽

Jie Li ◽

...

Keyword(s):

Gas Hydrates ◽

River Mouth ◽

Pearl River ◽

Fluid Migration ◽

Pearl River Mouth Basin ◽

Gas Reservoir ◽

Shallow Gas ◽

The Pearl River ◽

Gas Chimneys

High-amplitude anomalies (HAAs), bottom-simulating reflectors (BSRs), vertical acoustic blanking features, polygonal faults, and mass transport complexes are observed in high-resolution 3D seismic data from the south of the Pearl River Mouth Basin, South China Sea. We have focused on the role of fluid migration in the occurrence of shallow gas and gas hydrates. HAAs are interpreted to be shallow gas accumulations or gas hydrates, depending on their polarity and the presence of BSRs. Acoustic blanking features are probably caused by the presence of gas chimneys and overlying free gas due to features including chaotic and weak seismic reflections, pull-downs, and long distances of vertical extension. These gas chimneys root into the underlying late Oligocene gas reservoir. There is a close spatial relationship among the gas reservoir, gas chimneys, shallow gas accumulations, and BSRs, suggesting a genetic link. We have developed a schematic model to explain the processes of fluid migration, which supplied gas to the shallow gas accumulations and the gas hydrate system. When the gas reservoir was critically pressured, the focused fluid migration pathways may be formed or be reactivated, and gas was transported vertically along these conduits to the shallow strata. Then, there were two destinies for the migrated gas at shallow depths. One is that gas was trapped below low-permeability seals, such as mudstone or gas hydrates, forming shallow gas accumulations or free gas zones. The other one is that gas entered the gas hydrate stability zone and formed gas hydrates. In addition, this research indicates that the shallow hydrocarbon resource in this area may not be very prolific probably due to limited fluid migration.

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