渤海海域蓬莱19-3油田原油生物降解气地球化学特征与成因

Geochemical characteristics and genesis of biodegradation gas from crude oil in PL 19-3 oilfield in the Bohai Sea

渤海海域已有天然气地球化学特征显示稠油降解气普遍与稠油油藏相伴生,其探明储量约占天然气总探明储量的12%。渤海湾盆地复杂的构造演化使生物气藏背景多样化,也使渤海海域生物气成因类型不清,其地球化学特征和成藏条件不甚明确。与非稠油降解气相比,稠油降解气中甲烷含量高、干燥系数大,以干气为主,并含有一定量的二氧化碳和氮气,甲烷碳同位素值偏轻,δ^13C1介于-35‰^-55‰,丙烷碳同位素值偏重,具有同位素倒转现象,二氧化碳碳同位素值重,δ^13Cco2介于-10‰~20‰。原油降解气的生成是喜氧微生物和厌氧微生物共同作用的结果,有效的石油生物降解作用通常发生在温度低于80℃的储层中,因此地层温度控制了微生物降解原油生成原油降解气的发生范围,此外,含油气系统内的地层水也是重要的地质影响因素,中—低矿化度的NaHCO3型或CaCl2型地层水有利于微生物的繁殖进而对原油降解生成甲烷。生物降解气是微生物代谢活动的产物,所以降解气的规模是由油藏规模决定的。在渤海海域,储层温度与地层水类型适宜、油藏规模巨大的蓬莱19-3油田最具备大规模生成稠油降解气的条件。渤海目前已发现原油地质储量约36.4×10^8t,其中埋藏深度在2 000 m以内的原油地质储量约为27.7×10^8t,根据原油的生物降解模拟实验,1 m^3原油每天可产生0.14~0.62 m^3甲烷气体,每年可产生52.01~255.5 m^3甲烷气体,若深度2 000 m为原油降解界限,据此估算渤海海域每年最多可产原油降解气0.66×10^12m^3,具有非常大的勘探潜力。

The geochemical characteristics of the existing natural gas in the Bohai Sea indicate that the biodegradation gas of heavy oil is generally associated with heavy oil reservoirs, and its proved reserves account for about 12% of the total proven natural gas reserves. The complex tectonic evolution of Bohai Bay Basin led to the diversified background of biogas reservoirs, and the uncertainties in the genetic types of biogas in Bohai Sea and the geochemical characteristics and reservoir forming conditions. Compared with the biodegradation gas from other types of oil, the biodegradation gas of heavy oil has high methane content and large dry coefficient;it is dominated by dry gas and contains a certain amount of carbon dioxide and nitrogen. The methane carbon isotope is light, with δ^13C1 between-35‰ and-55‰. The propane carbon isotope is heavy, with the phenomenon of isotope inversal. The carbon dioxide isotope is heavy, with δ^13Cco2 between-10‰ and 20‰. The degradation gas of crude oil is generated under the combined effect of aerobic and anaerobic microorganisms. The effective biodegradation of petroleum usually occurs in reservoirs with a temperature below 80℃. Therefore, the occurrence area of degradation gas generated by microorganism from crude oil is controlled by formation temperature. In addition, the formation water in the petroleum system is also an important geological influencing factor. NaHCO3-or CaCl2-type formation water with medium and low salinity is conducive to the reproduction of microorganisms and degradation of crude oil to generate methane. Biodegradation gas is the product of microbial metabolism, so the scale of degradation gas is determined by the size of reservoir. In the Bohai Sea, PL 19-3 oilfield, with optimal reservoir temperature and formation water type and huge reservoir scale, is the most suitable for large-scale generation of biodegradation gas of heavy oil. The proved geological reserves of crude oil in the Bohai Sea are about 36.4 ×10^8t, of which the geological reserves of crude oil with a burial depth of less than 2 000 m are approximately 27.7×10^8t. According to the simulation experiment of crude oil biodegradation, 0.14 to 0.62 m^3 of methane gas can be produced by 1 m^3 of crude oil per day. Annually, 52.01-255.5 m^3 of methane gas can be generated. If 2 000 m is the critical limit for crude oil degradation, it is estimated that the maximum annual amount of degradation gas from crude oil in the Bohai Sea can reach 0.66 × 10^12m^3, showing huge exploration potential.