Jointly funded by the National Natural Science Foundation of China (40730844) and the Public Welfare Industry Scientific Research Special Fund Project of the Ministry of Land and Resources (200811014-02-02).
Lu Hongfeng, Liao Zhiliang, Chen Fang, Liu Jian, Chen Hong (Guangzhou Marine Geological Survey, Guangzhou 510760) About the first author: Lu Hongfeng (1976—), male, Ph.D., engaged in rock and mineral testing and geochemical research.
E-mail:luhongfeng@hydz.cnAbstract Four gas hydrate drilling cores in the Shenhu area of ??the South China Sea contain a large amount of authigenic pyrite, mainly in the form of long strips, short columns and filled foraminifera.
Pyrite mainly appears in the shallow part of sediments and in hydrate-containing layers, with the content mainly between 20% and 90%. The hydrate layer is the occurrence layer with high pyrite content.
Shallow pyrite is mainly controlled by anoxic oxidation of organic carbon and methane, while pyrite formation in deep hydrate layers is mainly affected by high methane flux.
Keywords: South China Sea gas hydrate authigenic pyrite. In 2007, the Guangzhou Marine Geological Survey carried out the first natural gas hydrate drilling in my country's waters in the Shenhu area of ??the South China Sea. The maximum depth of the core was obtained at 260m. Natural gas was discovered in two of the drill holes, SH2B and SH7B. Hydrate sample.
This paper mainly studies the authigenic pyrite in the SH1B, SH2B, SH5C and SH7B cores of natural gas hydrate drilling, and analyzes the relationship between authigenic pyrite and methane-rich environment.
1. Geological background and sampling location The South China Sea is one of the largest marginal seas in the Western Pacific, located at the intersection of the Eurasian Plate, the Pacific Plate and the Indian Ocean Plate.
Restricted by the mutual motion of the three major plates, the South China Sea has unique edge structural characteristics [1].
In the east, the South China Sea plate subducts eastward along the Manila Trench, forming an imbricated overthrust nappe accretionary wedge. A series of expansion rifting, shearing, and subsidence occur in the north and west, forming large and medium-sized sedimentary basins rich in organic matter.
Set provides the best venue.
The Shenhu Sea Area is located near the Pearl River Mouth Basin, Shenhu Uplift and Jianfeng North Basin on the northern continental slope of the South China Sea.
The seafloor topography changes relatively gently, the water depth changes range between 300 and 3500m, and the water depth line is roughly parallel to the coastline.
The terrain slopes from northwest to southeast, with an average slope drop of 13.6‰ and an average slope angle of 7°40′.
In the northwest of the sea area and near the continental shelf turning zone and the upper continental slope, the seafloor topography and slope change greatly. To the southeast, the water depth increases slowly and the topography changes more gently.
The water depth in the Shenhu hydrate drilling area is between 1000 and 1500m (Figure 1). The SH1B station has a water depth of 1264m and a hole depth of 261m; the SH2B station has a water depth of 1230m and a hole depth of 238m; the SH5C station has a water depth of 1264m and a hole depth of 175m; the SH7B station
The water depth is 1108m and the hole depth is 194m.
Among them, natural gas hydrates were found in layers around 200m and 160m at SH2B and SH7B stations respectively.
Fig.1 Location of gas-hydrate drilling, South China Sea2 Sample processing and analysis methods This paper uses the method of sediment clastic mineral identification to analyze the pyrite content of each core.
After returning from the voyage survey, the samples were stored in a freezer below 4°C to avoid the formation of late-stage pyrite from the degradation of organic matter in the core at higher temperatures.
Samples were taken from the top of the core at intervals of 20 cm or 25 cm. The sediment core was divided into many equal parts, each with a dry weight of 7.00 g, and then each sample was placed on a sieve with a particle size of 0.063 mm.
Use distilled water to clean the clay and other components, and the remaining debris (minerals and organisms) are used to identify the pyrite content.
Pyrite identification uses a LEICA MZ8 solid microscope, which has a maximum magnification of 120 times.
The analysis process mainly includes: placing each sample in the field of view of the solid microscope at a suitable magnification, carefully picking out the individual pyrites in the detrital minerals, then weighing them under an analytical balance, and finally depositing them.
The results of the pyrite content in the material are expressed as the weight percentage of the analyzed debris, which can clearly show the content changes of pyrite in the debris part after pretreatment.
The organic carbon content was analyzed using the potassium dichromate oxidation-reduction capacity method.
The principle of this method: Add a certain amount of standard potassium dichromate to concentrated sulfuric acid medium to oxidize the organic carbon in the sample into carbon dioxide under heating conditions.