ベトナム・クーロン堆積盆地の15-2鉱区における湖成根源岩・産出油

抄録

Japan Vietnam Petroleum Co., Ltd. (JVPC) confirmed oil accumulation in fractured granitic basement and Miocene sandstone at the Rang Dong structure in Block 15-2, offshore S. R. Vietnam. Since 1998, Rang Dong oil field is producing oil from both reservoirs.
The Cuu Long Basin has thick and wide syn-rift deposits of the Oligocene lacustrine source rock shale which is confirmed in over 30 wells drilled in Block 15-2. This shale is dark brown, very hard with oil shows, high resistivity and slow sonic velocity. The source rock potential is very high with 1-4 wt% of TOC and 200-700 mg/g of HI. The kerogen type is dominantly type I and is highly oil prone. Although the shale encountered in the Rang Dong field are immature, it is estimated to be mature to overmature at the center of the basin and near mature at the proximity of the Rang Dong field.
The characters of the oil produced from the fractured reservoir and the Miocene sandstone reservoir in the Rang Dong oil field are commonly high wax, low sulfur and 35-43 deg. API gravity. In another structure, condensate (52 deg API) was found in the weathered zone of basement. The oil maturation confirmed by biomarkers suggests that the oil produced from basement has higher maturity than Miocene oil.
From Sterane triangle plot, these produced oils are confirmed to have been generated by the same source rock which is the Oligocene lacustrine shale.
In a few wells, a fracture-filling “Tar-like Material”, a possible obstacle for production, was locally found in the fracture zone of the basement. It is largely insoluble in the organic solvent and more mature than the produced oil. Biomarkers and geochemical data suggest that it is generated by some sort of cracking byproduct.