TIMING CONSTRAINTS OF TECTONIC UPLIFT AND DRY GAS GENERATION: INSIGHTS FROM RE-OS PYROBITUMEN AND APATITE FISSION TRACK GEOCHRONOLOGY OF THE XUEFENG UPLIFT, SOUTH CHINA

As the key target of marine oil and gas exploration in South China, numerous hydrocarbon showings are known in the regional Paleozoic strata. However, the complex tectonic evolution, especially multiple tectonic events since the Triassic, have deeply altered and destroyed the original reservoir hydrocarbons form during the Palaeozoic. The hydrocarbons commonly exist as pyrobitumen, which are considered to have formed from bitumen contemporaneously with dry gas generation during the late stages of petroleum evolution. In order to further clarify the relationship between petroleum evolution processes and the multiple tectonic events of the South China Block, this research combines Re-Os isotope pyrobitumen and apatite fission Track geochronology. Here we focus on five different Palaeozoic palaeoreservoir at the western margin of Xuefeng uplift.

Low H/C ratio (~0.48-0.63), high Ro values (~ 1.9-2.2%) and high Tmax values (~480-520ºC) of the sampled solid hydrocarbons classify them as pyrobitumen. This is further supported by the established max reservoir burial thickness of >6000m, which reached a temperature >160 ºC in Majiang area, and with the gas composition being ~96% CH₄. This indicates that the hydrocarbons of the western margin of Xuefeng uplift have experienced a complex evolution and have been altered to pyrobitumen.

Pyrobitumen possesses ~2.5 to 15 ppb Re and 40 to 498 ppt Os. Nine of the twelve pyrobitumen samples yield a Re-Os isochron date of 78±13Ma, with an ¹⁸⁷Os/¹⁸⁸Os initial of ~1.45. The remaining three samples yield a nominally identical Re-Os date (80±13Ma), but possess a different ¹⁸⁷Os/¹⁸⁸Os initial of ~1.30. Three apatite fission track ages range from 70±6 to 123±7Ma, and all possess P(χ²) values >0.05. The mean track lengths vary from 12.7 to 13.2μm with standard deviations between 1.4 and 2.2μm. The time-temperature thermal history modelling results indicate tectonic uplift occurred between 110-80Ma. The agreement of the pyrobitumen Re-Os and apatite fission track dates, coupled with previous field evidence, fluid inclusion analysis, numerical modelling of oil composition and basin models, indicates that tectonic uplift and dry gas generation in the Northern Margin of the Xuefeng uplift occurred during the late Cretaceous to Palaeocene which coincides with the late Yanshan orogeny.