Origin and Distribution of CO2 in the Deepwater Area of the Pearl River Mouth Basin, South China Sea

Abstract

Abstract

During petroleum exploration in the deepwater area of the Pearl River Mouth Basin, South China Sea, a huge amount of CO2 gas reservoirs have been met. For the purposes of avoiding risk of high content of CO2, all the analytical data of CO2 concentrations and stable carbon isotopic ratios sampled by DST and MDT and materials of igneous distribution interpreted by residual magnetic anomaly and major deep fracture maps were collected; and noble gas and isotopic ratio measurements were also carried out.

Plot of CO2 contents vs. δ13 shows three original groups of CO2 in the deepwater area: 1) organic source; 2) inorganic source; and 3) mixing of organic and inorganic sources. The organic CO2 gases are usually with lighter δ13 values in the range of -28.2‰PDB to -10.1‰PDB and low CO2 contents of less than 20% in volume. The inorganic CO2 gases have δ13 values in the range of -7.89‰PDB to -1.2‰PDB and a wide range of CO2 contents from 0.56% to 99.53% in volume. The mixings of organic and inorganic CO2 gases have δ13 values in the range of -10.00‰PDB to -8.00‰PDB and the CO2 contents from 60% to 80% in volume.

Noble gases measurements indicate that R/Ra ratios range from 1.14 to 6.92, and the CO2/3He ratios from 3.36×107 to 7.1×109. Both of the plots of δ13 vs. 3He/4He and 3He/4He vs. CO2/3He ratios illustrate that the inorganic CO2 derived from the magmatic degassing. But the calculated mantle-derived CO2 contribution ratios are in the range of 14.04% to 85.76%, which implies that the mantle alkaline magmas had been contaminated with the crustal rocks during the ascending process. Parts of CO2/3He ratios are less than 109 mean the loss of the mantle-derived CO2 during the migration and accumulation in reservoirs.

Using 40Ar/36Ar ratios of noble gas to calculate the inorganic CO2 generating ages are about 38.79Ma. The homogenization temperatures of fluid inclusions projecting on burial curves gives the oldest charging age of inorganic CO2 is about 20.5Ma, which suggests that the magmatic degassed CO2 spent about 10 Ma to transport from the degassing kitchen to the Cenozoic reservoirs.

Integration of major deep fractures with igneous rocks interpreted by residual magnetic anomaly and drilling meeting, the inorganic and mixing CO2 are usually distributing on the top of igneous rock or laterally close to the major deep fractures which connect with igneous bodies. It is, therefore, operable to effectively avoid high risk of CO2 in the future exploration.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016