川中地区北部震旦系灯二段储层沥青特征及意义

Characteristics and significance of reservoir solid bitumen in the second member of Sinian Dengying Formation in the north of central Sichuan Basin

川中地区北部震旦系灯二段天然气勘探不断取得突破,已成为灯影组勘探的主要接替区。钻井揭示该地区灯二段储层发育大量固体沥青,沥青特征对于灯影组天然气成藏研究具有重要意义。通过采集研究区关键探井灯二段储层沥青样品,基于光学显微观察、扫描电镜、色谱—质谱和激光拉曼等分析测试手段,对川中地区北部灯二段储层沥青地球化学特征和发育机制等进行探讨。研究结果表明,灯二段沥青主要以溶蚀孔(洞)和裂缝形式赋存,其次为晶(粒)间孔,形态以点状、球状、板状、脉状等为主。岩石沥青含量高,单井平均沥青含量为2.96%~5.13%。重排甾烷和拉曼光谱表明灯二段沥青处于高成熟阶段,拉曼光谱计算沥青反射率分布范围为2.49%~4.09%,沥青主要源于原油热裂解成因。灯二段沥青生物标志化合物21α(H)-C_(29)降藿烷、C_(35)藿烷与C_(34)藿烷含量、Ts/Tm等多个指标与高石梯—磨溪地区灯四段沥青不同,推测灯二段沥青母源与灯四段存在差异,有下寒武统麦地坪组和震旦系陡山沱组等烃源岩的贡献。灯二段储层可能发育2期沥青,指示了地质历史时期中的2期原油充注过程和硫酸盐热化学还原反应(TSR)作用。

The second member of Dengying Formation in the north slope area of central Sichuan Basin has made continuous breakthroughs in natural gas exploration in recent years.In this area,a large amount of reservoir solid bitumen was developed in the second member of Sinian Dengying Formation.Characteristics of solid bitumen are of great significance for the study of natural gas accumulation in Dengying Formation.In this paper,many solid bitumen samples of the second member of Sinian Dengying Formation were sampled from primary exploration wells.Based on a series of testing including optical microscopy,scanning electron microscopy,chromatography-mass spectrometry and laser Raman analysis,the geochemical characteristics and development mechanism of reservoir solid bitumen were studied.The results show that the solid bitumen mainly occurs in the form of dissolved pores(cave)and fractures,followed by intergranular pores.The solid bitumen morphology is characterized by point,ball,plate and vein.The average solid bitumen content of single well distributes from 2.96%to 5.13%.The solid bitumen stays in the high maturity stage,and the reflectance ratio calculated by laser Raman parameter ranges from 2.49%to 4.09%.Thermal cracking of crude oil formed the solid bitumen.In terms of biomarkers,the 21α(H)-C_(29)norhopane,C_(35)hopane and C_(34)hopane,and Ts/Tm of the solid bitumen of the second member of Dengying Formation are different from that of the fourth member of Dengying Formation in Gaoshiti Moxi area.It is speculated that the solid bitumen partly sourced from the Lower Cambrian Maidiping Formation and Sinian Doushantuo source rocks.Two types of solid bitumen were developed in the second member of Dengying Formation,indicating two possible stages of oil charging process.Potentially,a certain degree of thermochemical sulfate reduction(TSR)occurred.