The oldest oil accumulation in China: Meso-proterozoic Xiamaling Formation bituminous sandstone reservoirs

Abstract

Based on analysis on the coexistent relationship between igneous rock intrusion and bituminous sandstone of the Xiamaling Formation in northern Hebei (Qnx), the accumulation time of the Xiamaling Formation bituminous sandstone reservoir is defined by the methods of organic petrology and isotope chronology. According to field investigation, observation of thin-section and the shallow drilling show, and based on the previous results of oil seepages and bitumen spots in the Qnx base sandstone, thirty-one new oil seepages (bitumen spots), including twenty-one visible black bitumen spots, were discovered in the Qnx. The distribution of paleo-reservoirs in the Qnx bituminous sandstone was identified, and 4 paleo-reservoirs, i.e. Shuangdong paleo-reservoir in Pingquan, Longtangou paleo-reservoir in Lingyuan, Lujiazhuang paleo-reservoir and Tashan paleo-reservoir in Kuancheng, have been named. The reserves of the paleo-reservoirs are 0.7-1.0 billion tons, which are calculated by the volume method with the solid bitumen content. Organic petrology analysis shows that two kinds of solid bitumen with diferent reflectivity characteristics were developed. Class structure of natural coke is discovered in solid bitumen, which is the first direct evidence to prove that severe thermal alteration occurs in the Qnx base sandstone bitumen. The age of Qnx gabbro-diabase baddeleyite by SHRIMP U-Pb dating is 1 327 Ma±2 Ma, which indicates that the accumulation time of the Xiamaling Formation bituminous sandstone is 1 400 Ma to 1 327 Ma. So, it is the oldest oil accumulation in China. 摘要: ：从冀北坳陷下马岭组（Qnx）沥青砂岩与火成岩侵入体的共存关系入手，利用有机岩石学、同位素年代学方法准确厘定Qnx沥青砂岩油藏的成藏时限。根据野外调查、实验室薄片镜下观察及浅钻结果，在前人已发现Qnx底砂岩油苗/沥青点的基础上，新发现Qnx油苗/沥青点31处（其中肉眼可见黑色沥青点21处），基本查明了Qnx沥青砂岩油藏平面分布规律并命名了4个古油藏：平泉双洞、凌源龙潭沟、宽城芦家庄和宽城塌山古油藏。根据沥青含量，采用体积法估算Qnx沥青砂岩古油藏的规模在7´108～10´108 t。大量样品的有机岩石学分析发现，Qnx沥青砂岩中发育两类具有不同反射率特征的固体沥青，在固体沥青中发现了类天然焦结构，首次以直接证据证实Qnx底砂岩沥青遭受过严重的热蚀变效应。Qnx辉长辉绿岩斜锆石SHRIMP U-Pb测年结果为1 327 Ma±2 Ma，限定Qnx沥青砂岩油藏的成藏时间为1 400～1 327 Ma，确定Qnx沥青砂岩油藏是迄今为止中国最古老的油藏。图7表1参30 Key words: bituminous sandstone, Mesoproterozoic-Neoproterozoic, Xiamaling Formation, igneous rock, oldest reservoir, accumulation time, Yanshan