北部湾盆地涠西南凹陷深层原油和烃源岩地球化学特征及成藏模式

Geochemical characteristics and accumulation models of crude oils and source rocks in the deep strata of Weixinan Sag, Beibuwan Basin

摘要

北部湾盆地涠西南凹陷油气勘探主要集中于中、浅层,深层勘探程度低、油气发现少。根据研究区深层原油物理化学性质、族组成、饱和烃气相色谱和饱和烃色谱质谱等资料,以及烃源岩总有机碳、岩石热解、族组成、饱和烃气相色谱和饱和烃色谱质谱等资料,采用有机地球化学分析方法,研究深层原油和烃源岩地球化学特征并进行油源对比,分析深层原油成藏模式。结果表明:研究区深层原油性质较轻,运动黏度(温度为50 ℃)低,含硫质量分数很低,含蜡质量分数中等。族组成总体具有饱和烃质量分数高、饱和烃/芳烃比高,以及芳烃、非烃及沥青质质量分数较低的“两高三低”特征。深层原油划分为3种类型,Ⅰ类原油具有正构烷烃呈“双峰型”分布、三环萜烷质量分数中等、ααα(20R)C₂₇-C₂₈-C₂₉规则甾烷呈不对称“V”型分布、4-甲基C₃₀甾烷质量分数中等等特征,与流沙港组二段中上亚段泥页岩亲缘关系良好;Ⅱ类原油具有正构烷烃呈“单峰状后峰型”分布、三环萜烷质量分数较高、ααα(20R)C₂₇-C₂₈-C₂₉规则甾烷呈不对称“V”型分布、4-甲基C₃₀甾烷质量分数较高等特征,主要来自流沙港组二段下亚段油页岩;Ⅲ类原油具有正构烷烃呈“单峰状前峰型”分布、三环萜烷质量分数较高、ααα(20R)C₂₇-C₂₈-C₂₉规则甾烷呈“L”型分布、4-甲基C₃₀甾烷质量分数较低等特征,主要来自流沙港组三段上亚段泥页岩。研究区为自生自储式、旁生侧储式及自生自储和旁生侧储混合式3种类型深层原油成藏模式。该结果对北部湾盆地涠西南凹陷深层原油勘探具有指导意义。

Abstract

Oil and gas exploration in the Weixinan Sag of the Beibuwan Basin has predominantly targeted the middle and shallow layers, while deep layers remain underexplored with few oil and gas discoveries. Using analytical data on the physicochemical properties, group composition, gas chromatography (GC) of saturated hydrocarbons, and gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons, as well as total organic carbon, rock pyrolysis, group composition, gas chromatography (GC) of saturated hydrocarbons, and gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons of source rocks, this study applies organic geochemical techniques to investigate the geochemical characteristics of deep crude oil and its source correlations and its accumulation models. The results reveal that deep crude oil in the study area is relatively light, with low kinematic viscosity at 50 ℃, very low sulfur content, and moderate wax content. The group composition exhibits the "two highs and three lows" characteristics: high mass fractions of saturated hydrocarbons and high saturated hydrocarbon/aromatic hydrocarbon ratios, along with low mass fractions of aromatics, non-hydrocarbons, and asphaltenes. Deep crude oil is classified into three types. Type I crude oil shows a "bimodal" distribution of n-alkanes with significant pristane dominance, moderate tricyclic terpane content, an asymmetric "V-shaped" distribution of regular steranes (ααα(20R)C₂₇-C₂₈-C₂₉), and moderate 4-methyl C₃₀ sterane content, indicating a genetic relationship with the middle-upper sub-member mudstones of the second member of the Liushagang Formation. Type II crude oil exhibits a "single-peak with posterior dominance" n-alkane distribution, high tricyclic terpane content, an asymmetric "V-shaped" distribution, and high 4-methyl C₃₀ sterane content, primarily sourced from the lower sub-member oil shales of the second member of the Liushagang Formation. Type III crude oil is characterized by a "single-peak with anterior dominance" n-alkane distribution, high tricyclic terpane content, an "L-shaped" sterane distribution, and low 4-methyl C₃₀ sterane content, mainly derived from the upper sub-member mudstones of the third member of the Liushagang Formation. Based on these findings, three deep crude oil accumulation models are proposed for the study area: "self-sourced and self-stored", "adjacent-sourced and lateral-stored" and a mixed "self-sourced and self-stored with adjacent-sourced and lateral-stored" model. These results provide valuable guidance for deep crude oil exploration in the Weixinan Sag of the Beibuwan Basin.

关键词

深层原油 / 生物标志物 / 地球化学特征 / 油源对比 / 成藏模式 / 流沙港组 / 涠西南凹陷 / 北部湾盆地

Key words

deep crude oil / biomarker compounds / geochemical characteristics / source correlation / accumulation models / Liushagang Formation / Weixinan Sag / Beibuwan Basin

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基金资助

中国海洋石油有限公司前瞻基础研究项目(KJQZ-2024-2003)

“十四五”全国油气资源评价项目(QGYQZYPJ2022-3)