Geochemical characteristics and origins of natural gas in the Zhu III sub-basin, Pearl River Mouth Basin, China

The Zhu III sub-basin is an important petroliferous region in the South China Sea where hydrocarbon distribution shows strong heterogeneity with oil primary produced from the Wenchang B depression, and gases from the Wenchang A depression. However, a mixture of oil and gases are produced from the surrounding Qionghai and Shenhu uplifts. The origin, generation history and migration pathway of oil have been intensively studied but no study have been done directly on the gas, which restricts future petroleum exploration. In this study, natural gas molecular compositions were measured on 46 samples, stable carbon isotopic composition were measured on 39 samples and light hydrocarbon composition were measured on 30 samples. Results indicate that natural gases in the Zhu III sub-basin are mainly composed of hydrocarbon gases with abundant CO2 and N-2. The hydrocarbon gases are all thermogenic gases which are mainly derived from the Oligocene Enping Formation (E(3)e, 35-30 Ma). The contribution of the Eocene Wenchang Formation (E(2)w, 56-35 Ma) to natural gases increases from the Wenchang A depression to the South Boundary Fault (SBF) and surrounding uplifts. This is caused by the early generated natural gases with more contribution from the E(2)w and dissolved in oils that migrated to the surrounding Qionghai and Shenhu uplifts. Natural gases and oil in the surrounding uplifts and near the SBF, therefore, avoided secondary cracking due to the high gas migration rate through the SBF and lateral sandstone carrier beds, whereas natural gases in the Wenchang A depression mainly originated from the secondary cracking of oil. The equivalent vitrinite reflectance values of source rocks when the natural gases were generated varied from 0.6% to 1.5% based on carbon isotope differences between hydrocarbons, indicating mature to high mature stage. CO2 with abundance less than 10% mainly originated from organic matter decomposition. CO2 with abundance more than 15% was sourced from the deep mantle and crust and mainly concentrated in the deep layers near the SBF which cuts into the basement. N-2 is probably the product of in-situ reservoir alteration, such as biodegradation in shallower layers. Our study on the geochemical characteristics and origins of the natural gas will enable a better understanding of petroleum system in the Zhu III sub-basin and provide theoretical basis for optimizing future petroleum exploration strategy. Detailed study on shallow layers in the Qionghai and Shenhu uplifts and deep layers near the SBF are needed because exploration risks are relatively high in these plays.