南堡凹陷深层火山岩天然气成因与成藏模式

南堡凹陷深层火山岩油气藏具有较大勘探潜力，其天然气以烃类气体为主，甲烷含量主要在70%～90%，重烃含量在4%～27%，为湿气；非烃含量较低。天然气中甲烷碳同位素主要分布在−35‰～−43‰，乙烷碳同位素主要分布在−28‰～−24‰。这些天然气相当于有机质在镜质体反射率 R_o为 1.0%～1.5%阶段形成的天然气，为烃源岩在成熟-高成熟阶段生成的油型气，与凝析油或原油伴生，因此天然气碳同位素偏重。南堡凹陷深层火山岩天然气以凝析气为主，结合单体烃碳同位素，对天然气和伴生凝析油的成因与来源进行了分析，结果表明,南堡凹陷深层火山岩油气同源，沙河街组一段（简称沙一段）以下天然气主力源岩主要为沙河街组三段（简称沙三段）烃源岩，以凝析气为主；沙一段及以浅地层天然气主力源岩主要为沙一段烃源岩，以油为主。沙一段厚层泥岩是一套优质烃源岩，处于生烃高峰，同时形成烃浓度封闭，又是区域优质盖层。这些来自于凹陷深部烃源岩生成的高成熟油气，主要通过继承性发育的深大断裂运移到火山岩复合岩性圈闭中聚集成藏，火山岩油气成藏受火山岩单体和火山岩相双重控制。

Natural gas origin and gas accumulation model for deep volcanic rocks in Nanpu sag,Bohai Bay Basin

A breakthrough has been made recently in natural gas exploration of deep volcanic rocks in Nanpu sag of Bohai Bay Basin, East China, indicating a very high potential in natural gas exploration of deep volcanic rocks in Nanpu sag. Therefore, it is quite important to study the origin and hydrocarbon accumulation model of natural gases for the future exploration. Natural gases accumulate mainly in deep volcanic rock reservoirs within the Paleogene Shahejie Formation. They are dominated by hydrocarbon gases in chemical composition, of which the methane content varies from 70% to 90%, ethane and heavy hydrocarbons are relatively high, ranging between 4%~27%, the dry coefficient varies from 0.71 to 0.96, indicating mainly a wet gas. Carbon isotopes of natural gases are relatively heavy, with δ¹³C₁ mainly varying from−35‰ to−43‰ and δ¹³C₂ mainly from−24‰ to−28‰. According to plots of C₁/(C₂+C₃) versus δ¹³C₁ and δ¹³C₁ versus δD, the natural gas in deep volcanic rock reservoirs belongs to an oil-type condensate gas. The calculated thermal maturity of the gas ranges between 1.00%~1.50% by the δ¹³C−R_o method, it matches the thermal evolution of typeⅡ₁ source rocks in the Es₃ member. In addition, Pr/Ph ratios of associated condensates, light hydrocarbon parameters and hydrocarbon isotopes also show that the deep volcanic rock gas is an oil-type gas and it is dominated by a condensate gas. Carbon isotopes of individual hydrocarbons of associated condensates show that the gas in deep volcanic rocks is generated from source rocks of the Es₃ and Es₁ members, namely the natural gas below the Es₁ stratum is mainly derived from source rocks of the Es₃ member and composed of mainly a condensate gas, while the natural gas above the Es₁ stratum is mainly derived from source rocks of the Es₁ member, which generate chiefly oils. Thick mudstones of the Es₁ member serve as not only a high-quality cap rock but also a good source rock. The high maturity oil and gas generated from deep source rocks in the sag migrate laterally through inherited abysmal faults to and accumulate in deep volcanic rock traps, hydrocarbon accumulations are controlled both by volcanic individuals and by volcanic phases.