ABNORMAL ORGANIC-MATTER MATURATION IN THE YINGGEHAI BASIN, SOUTH CHINA SEA - IMPLICATIONS FOR HYDROCARBON EXPULSION AND FLUID MIGRATION FROM OVERPRESSURED SYSTEMS

Three superimposed pressure systems are present in the Yinggehai Basin , South China Sea. A number of commercial, thermogenic gas accumulatio ns have been found in an area in which shale diapirs occur. Because th e reservoir intervals are shallow and very young, they must have fille d with gas rapidly. The thick (up to 17 km) Tertiary and Quaternary se dimentary succession is dominated by shales, and is not disrupted by m ajor faulting in the study area, a factor which seems to have had an i mportant effect on both hydrocarbon generation and fluid migration. Or ganic-matter maturation in the deepest, most overpressured compartment has been significantly retarded as a result of the combined effects o f excess pressure, the presence of large volumes of water, and the ret ention of generated hydrocarbons. This retardation is indicated by bot h kerogen-related parameters (vitrinite reflectance and Rock-Eval T-ma x); and also by parameters based on the analysis of soluble organic ma tter (such as the C15+ hydrocarbon content, and the concentration of i soprenoid hydrocarbons relative to adjacent normal alkanes). In contra st to this, organic-matter maturation in shallow, normally-pressured s trata in the diapiric area has been enhanced by hydrothermal fluid flo w, which is clearly not topography-driven in origin. As a result the h ydrocarbon generation ''window'' in the basin is considerably wider th an could be expected from traditional geochemical modelling. These two unusual and contrasting anomalies in organic-matter maturation, toget her with other lines of evidence, suggest that there was a closed flui d system in the overpressured compartment until shale diapirs develope d. The diapirs developed as a result of the intense overpressuring, an d their growth was triggered by regional extensional stresses. They se wed as conduits through which fluids (both water and hydrocarbons) ret ained in the closed system could rapidly migrate. Fluid migration led to the modification of the thermal regime and the enhancement of organ ic maturation, as well as the accumulation of commercial volumes of ga s in a relatively short the interval.