Different types of Paleogene source rocks and characteristics of pyrolysis gas generation of crude oil in Zhuyi Depression, Pearl River Mouth Basin
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摘要:
HZ26-6凝析气田为珠江口盆地东部珠一坳陷首次获得重大发现的凝析气藏，打破了近40年来珠一坳陷围绕原油展开勘探的传统局面，揭示了该区富油洼陷巨大的天然气勘探潜力。然而，对于珠一坳陷文昌组中深湖相泥岩这一主力油源岩的生气潜力以及生气特征，以及恩平组河沼相煤系烃源岩供气能力的认识则较为模糊，缺乏系统的生气机制研究。此类问题则可直接确定珠一坳陷天然气成因归属及勘探潜力。针对该问题，采用黄金管-高压釜封闭体系热模拟实验，对珠一凹陷典型文昌组和恩平组不同类型烃源岩和原油热裂解生成天然气过程及天然气碳同位素演化特征进行系统分析，并探讨惠州凹陷HZ26-6凝析气田天然气成因及主力气源岩。研究认为，珠一坳陷恩平组湖沼相煤系泥岩最大累积烃类气体产率为230.8 mL/g.TOC，甲烷产气率在Ro为1.1%之后呈指数增长，呈现出持续生气特征，且以干酪根裂解气为主；文昌组中深湖相泥岩最大累积烃类气体产率达428.30 mL/g.TOC，在～1.6% Ro阶段后开始大量产气，且高产气率实际上主要由油的二次裂解贡献为主；原油裂解气最大产率达693.6 mL/g.TOC，远高于文昌组及恩平组泥岩最大产气率，不同类型烃源岩及原油样品生成天然气组分碳同位素演化特征差异性明显；珠一坳陷文昌组偏腐泥型泥岩在1.3%～1.6% Ro阶段所生天然气甲烷及乙烷碳同位素组成与HZ26-6凝析气藏天然气具有较好的匹配性，为HZ26-6凝析气藏天然气的主力气源岩，恩平组湖沼相煤系泥岩及原油裂解气对该气藏未见明显成藏贡献。
Abstract:HZ26-6 condensate gas field is the first major discovery of condensate gas reservoir in Zhu Ⅰ Depression in the Pearl River Mouth Basin （East）, breaking the traditional situation of exploration around crude oil in Zhuyi depression in recent 40 years, and revealing the huge natural gas exploration potential of oil-rich depression in this area. However, systematic research on the gas generation mechanism and gas generation potential of deep lacustrine source rocks in Wenchang Formation （Fm） and river biogas coal-bearing source rocks in Enping Fm are scarce. Therefore, we conducted a thermal simulation experiment of gold-tube-autoclave closed system to analyze systematically the process of natural gas generation from different types hydrocarbon sources in typical Wenchang Fm and Enping Fm in Zhu Ⅰ depression and the evolution characteristics of carbon isotopes of natural gas, to explore the genesis of natural gas and main source rocks in Huizhou 26-6 condensate field. Results shows that the maximum cumulative hydrocarbon gas yield of lacustrine coal-bearing mudstone of Enping Fm in Zhu Ⅰ depression is 230.8 mL/g.TOC, and the methane gas production rate increases exponentially after 1.1% Ro, being characteristic of continuous gas generation, mainly kerogen pyrolysis gas; The maximum cumulative hydrocarbon gas yield of deep lacustrine mudstone in Wenchang Fm is 428.30 ml/g.TOC. A large amount of gas is produced after the ～1.6% Ro stage, and the high gas yield is actually mainly contributed by the secondary cracking of oil; The maximum yield of crude oil cracked gas is 693.6 mL/g.TOC, which is much higher than that of mudstone in Wenchang Fm and Enping Fm. In addition, differences are obvious in the evolution characteristics in carbon isotopes of natural gas components generated by different types of source rocks and crude oil samples. The carbon isotopic compositions of methane and ethane of the natural gas generated by the partial sapropelic mudstone of Wenchang Fm in Zhu Ⅰ depression in the stage of 1.3%～1.6% Ro are well matched with the natural gas of Huizhou 26 condensate gas reservoir, the main gas source rock of Huizhou 26 condensate gas reservoir. The lacustrine coal-bearing mudstone and raw oil pyrolysis gas of Enping Fm have no obvious contribution to the gas reservoir formation.
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