Abstract: Alteration of Fault Development on the Accumulated Oil Geochemical Features

Fu Jiamo, Pei Cunmin, Sheng Guoying, Chen Sizhong

The geochemical characteristics of crude oils and reservoir bed extracts in a well drilled in Pearl River Mouth Basin, South China Sea, were investigated in detail. The trap in which a well was drilled was cut by a sub-east-west direction normal fault. According to the seismic section study, the fault has been activated three times, which altered the oil geochemical feature dramatically.

Geochemical (GC) analytical results revealed that both the oils and the extracts were richer in saturates and poorer in polar fractions, and high saturate to aromatic ratios indicated their higher rank of maturation level. Consequently, GC analysis of the saturated fractions indicated that two major types of oil occurred in the trap. One was minor biodegraded oil, which was encountered in the upper section (2005 m and above), and the other was normal, distributed in the lower section (below 2005 m). However, because of the injection of the lower oil with much higher maturity and lower molecular weight, the topmost two oils ranked the highest maturity of the biodegraded oil, and thus their GC patterns of n-alkanes appeared saw-toothed in shape. Furthermore, GC-MS analysis of the satura es confirmed that the oils have experienced higher geotemperatures due to the discovery of diadrimanes and 8,14-secohopanes. The presence of oleanane suggested terrestrial origin of the oils, while dominant C30 4-methyl sterane in steroid kingdom revealed enriched algal input in the source rocks.

Faulting history survey revealed that the essential cause of the varying geochemical features of the biodegraded oil resulted from reaccumulation of the oil in the trap. Because of faulting, the underlying oil escaped from the initial reservoirs, migrated upward along the fault, and then favorably entered the shallowest reservoirs due to differential accumulation, resulting in the ever-biodegraded oils with high maturity and enriched lower molecular weight hydrocarbons.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994