珠一拗陷惠州凹陷惠州26-6构造深层天然气成因及来源

Genesis and source of deep natural gas in Huizhou 26-6 structure in Huizhou Sag,ZhuⅠDepression,South China Sea

研究珠江口盆地富油凹陷惠州26-6构造深层凝析气藏天然气的类型,进一步确定天然气的来源。采用不同类型封闭体系,分别对惠州26-6构造主要烃源岩——文昌组典型半深湖相暗色烃源岩、恩平组储层原油样品开展高温热压模拟实验,建立了惠州凹陷干酪根裂解气和原油裂解气的识别标准。综合天然气组分、碳同位素、轻烃组分及凝析油地球化学特征分析,结果表明ln(C_(1)/C_(2))-ln(C_(2)/C_(3))、甲烷碳同位素-乙烷碳同位素、(δ^(13) C_(2)-δ^(13) C_(3))-δ^(13) C_(1)图版能有效区分干酪根裂解气和原油裂解气。相对原油裂解气,干酪根裂解气具有较高的ln(C_(1)/C_(2))值和较低的ln(C_(2)/C_(3))值,甲烷的碳同位素和乙烷的碳同位素较重,干酪根裂解气的(δ^(13) C_(2)-δ^(13) C_(3))值受成熟度影响更小,而原油裂解气的(δ^(13) C_(2)-δ^(13) C_(3))值随着成熟度增加而逐渐增大。惠州26-6构造深层天然气以干酪根裂解气为主,来源于藻类勃发背景下的文昌组浅湖-半深湖与半深湖-深湖相混合油型烃源岩。

Huizhou 26-6 structure in Pearl River Mouth Basin is an oil rich sag.This paper mainly studies the types of natural gas in deep condensate reservoirs in Huizhou 26-6 structure so as to determine the source of natural gas.Different types of closed systems are used to carry out high temperature and hot pressing simulation experiments on the crude oil samples from Wenchang Formation and Enping Formation main source rocks in Huizhou 26-6 structure.The experiments lead to the establishment of identification criteria of kerogen cracking gas and crude oil cracking gas in Huizhou Sag.Comprehensive analysis of natural gas composition,carbon isotope,light hydrocarbon composition and geochemical characteristics of condensate shows that ln(C_(1)/C_(2))-ln(C_(2)/C_(3)),methane carbon isotope-ethane carbon isotope,(δ^(13) C_(2)-δ^(13) C_(3))-δ^(13) C_(1) chart can effectively distinguish kerogen cracking gas from crude cracking gas.Compared with crude oil cracking gas,kerogen cracking gas has higher ln(C_(1)/C_(2))value and lower ln(C_(2)/C_(3))value,with heavier carbon isotopes of methane and ethane.The(δ^(13) C_(2)-δ^(13) C_(3))value of kerogen cracking gas is less affected by maturity,while the(δ^(13) C_(2)-δ^(13) C_(3))values of crude oil cracking gas gradually increase with the increase of maturity.The deep natural gas in Huizhou 26-6 structure is mainly kerogen cracking gas,which was likely to be originated from the mixed oil source rocks of shallow lake-semi-deep lake and semi-deep lake-deep lake facies in Wenchang Formation under the background of algae bloom.