柴达木盆地东坪地区原油裂解气的发现及成藏模式

Discovery and accumulation model of oil cracking gas reservoirs in Dongping area,Qaidam Basin

柴达木盆地东坪地区发现了中国陆上地质储量最大的基岩气田——东坪气田,其天然气来源于侏罗系高成熟—过成熟阶段的裂解气,但深层裂解气藏的成因较为复杂,特别是在东坪气田以西的坪西和尖顶山构造带,由于不发育侏罗系,新发现的基岩气藏来源不明,影响了深层天然气的勘探认识。利用天然气组分和同位素分析数据,结合东坪地区地质特征对深层裂解气开展分析,建立了天然气的成藏模式。研究表明,东坪地区深层基岩气藏具有原油裂解气,基岩储层中发育沥青包裹体,表明该地区发育古油藏裂解气。柴达木盆地北缘的侏罗系烃源岩以湖相泥岩为主,有机质丰度高、类型好,经历了长期的深埋过程,具备形成原油裂解气的物质基础和温度条件。东坪地区深层基岩气藏的成藏具有早期充油、后期高温裂解、晚期调整的特征。东坪地区原油裂解气的发现拓展了柴达木盆地北缘天然气的勘探领域,对深化柴达木盆地深层天然气勘探具有重要指导意义。

The Dongping bedrock gas field,with the largest onshore geological reserves in China,was discovered in Dongping area of Qaidam Basin.Natural gas in the Dongping gas field is sourced from Jurassic cracking gas in the high or over mature stage.However,the genesis of deep cracking gas reservoirs is complicated,especially in Pingxi and Jiandingshan tectonic belts west of the Dongping gas field,where the Jurassic is not developed and the source of newly discovered bedrock gas reservoirs is unknown,thus affecting the exploration of deep gas.Based on the natural gas compositions,isotope data and geological characteristics,the deep cracking gas is analyzed and the accumulation model of natural gas is established in Dongping area.Studies show that the deep bedrock gas reservoirs in Dongping area have the characteristics of crude oil cracking gas,and the development of bitumen inclusions in the bedrock reservoir indicates that cracking gas from ancient oil reservoir is developed in this area.The Jurassic source rocks in the northern margin of Qaidam Basin are mainly lacustrine mudstones with good organic matter and high abundance.The source rocks have undergone a long-term deep burial process,so that they have the material bases and temperature conditions for the formation of crude oil cracking gas.The formation of deep bedrock gas reservoirs in Dongping area is characterized by oil filling in the early period,cracking at high temperature and adjustment in the later period.The discovery of oil cracking gas in Dongping area expands the exploration field of natural gas in the northern margin of Qaidam Basin,and has important guiding significance for deepening the exploration of deep natural gas in Qaidam Basin.