鄂尔多斯盆地寒武系储层沥青地质特征及来源分析

Geological characteristics and origin analysis of bitumen in Cambrian reservoirs of Ordos basin

本文通过对储层沥青的分布、含量、岩石学特征、成熟度、元素组成、生物标志物、同位素等的研究,剖析了鄂尔多斯盆地寒武系储层沥青的地质特征及来源。结果表明,寒武系储层沥青富集程度较高,多呈条带状和环状,赋存于微裂缝、粒间和溶蚀孔隙中。具有清晰的边界,高热演化程度(等效镜质组反射率值为1.35%~3.48%),高C原子组成的地质特征,为热裂解的焦沥青成因。寒武系储层沥青具有明显C_(27)规则甾烷优势、C_(23)三环萜烷及植烷优势的特征,表明其形成于还原环境,来源于低等藻类。较高三环萜烷/藿烷和较低藿烷/甾烷比值,表明其为海相成因。寒武系储层沥青中较高三芳甲藻甾烷参数(三芳甲藻甾烷/(C_(30)3-甲基-24-乙基三芳甾烷+三芳甲藻甾烷))和(C_(26)+C_(27))/C_(28)三芳甾烷比值,与东坡组烃源岩分布特征最为相似。此外,有机碳同位素和相似Rb/Sr、Zn/Co和Cu/Sr分布特征,进一步证实了盆地南缘和东缘寒武系储层沥青主要来源于东坡组烃源岩,即东坡组海相烃源岩生成的油气已经在鄂尔多斯盆地南缘和东缘运聚并成藏,寒武系原生油气藏的勘探潜力在鄂尔多斯盆地南部较大,不容忽视。

This article investigates the geological characteristics and sources of bitumen in Cambrian reservoirs of the Ordos basin.We analyzed the distribution,content,petrological characteristics,maturity,elemental composition,biomarkers,and carbon isotopes of reservoir bitumen.Our findings indicate a significant enrichment of bitumen in Cambrian reservoirs,primarily concentrated in strip-shaped and circular patterns in microcracks,intergranular and dissolution pores.Bitumen exhibits distinct boundaries,high thermal evolution evidenced by equivalent vitrinite reflectance value ranging from 1.35%to 3.48%,and a high carbon atom composition attributed to thermal cracking.Biomarker analysis reveals a predominance of C_(27) regular steranes,C_(23) tricyclic terpenes,and phytanes,indicative of a reducing depositional environment that originated from lower algae.The high ratio of tricyclic terpenoids to hopanes and the low ratio of hopanes to steranes further support a marine origin for the bitumen.Additionally,the high triaromatic dinosteroid parameter(triaromatic dinosteroids/(C_(30)3-methyl-24-ethyl triaromatic steroids+triaromatic dinosteroids))and(C_(26)+C_(27))/C_(28)triaromatic steroids ratio closely resemble the distribution characteristics of the Dongpo Formation source rocks.Moreover,organic carbon isotopes and similar trends in Rb/Sr,Zn/Co,and Cu/Sr ratios corroborate the conclusion that the bitumen in Cambrian reservoirs in the southern and eastern Ordos basin mainly originates from the Dongpo Formation.Consequently,these findings suggest that hydrocarbons generated from the marine source rocks of the Dongpo Formation have migrated and accumulated in the southern and eastern Ordos basin.Therefore,the Cambrian primary reservoirs in the southern Ordos basin hold significant exploration potential for hydrocarbon resources,warranting further investigation.