海相页岩有机质炭化的热成熟度下限及勘探风险

Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk

以四川盆地及周缘志留系龙马溪组钻井资料为基础,利用激光拉曼、电阻率测井和物性分析等页岩源储表征技术,开展了高过成熟海相页岩有机质炭化的R_o值下限和基本特征研究。研究认为:(1)海相页岩Ⅰ—Ⅱ_1型有机质炭化的R_o值下限为3.5%。在R_o值小于3.4%阶段,一般不会出现有机质炭化现象;在R_o值为3.4%~3.5%阶段,可能出现有机质弱炭化与未炭化两种情况并存;在R_o值大于3.5%阶段,出现有机质炭化的可能性很大。(2)进入炭化阶段的富有机质页岩具有电阻率测井曲线普遍呈"细脖子型"(低—超低电阻率响应)、激光拉曼谱出现石墨峰、物性差(基质孔隙度仅为正常水平一半以下)等3个基本特征。(3)有机质炭化阶段导致页岩源储品质的损害巨大,主要表现为页岩的生烃能力衰竭、有机质孔隙和黏土矿物晶间孔的大量减少甚至消失、对天然气的吸附能力降低等。由此认为,Ⅰ—Ⅱ_1型固体有机质炭化的R_o值下限应成为古老海相地层页岩气勘探不可逾越的红线;在选区评价工作中,需加强低—超低电阻率富有机质页岩的有效性评价,排除由有机质炭化造成的高风险区;在勘探开发过程中,对钻遇的低—超低电阻率目的层应高度重视有机质炭化评价,及时调整部署方案。

Based on the drilling data of the Silurian Longmaxi Formation in the Sichuan Basin and periphery,SW China,the Ro lower limits and essential features of the carbonization of organic matter in over-high maturity marine shale were examined using laser Raman,electrical and physical property characterization techniques.Three preliminary conclusions are drawn：（1） The lower limit of Ro for the carbonization of Type Ⅰ—Ⅱ1 organic matter in marine shale is 3.5%; when the Ro is less than 3.4%,carbonization of organic matter won＇t happen in general; when the Ro ranges from 3.4% to 3.5%,non-carbonization and weak carbonization of organic matter may coexist; when the Ro is higher than 3.5%,the carbonization of organic matter is highly likely to take place.（2） Organic-rich shale entering carbonization phase have three basic characteristics：log resistivity curve showing a general ＂slender neck＂ with low-ultralow resistance response,Raman spectra showing a higher graphite peak,and poor physical property（with matrix porosity of only less than 1/2 of the normal level）.（3） The quality damage of shale reservoir caused by the carbonization of organic matter is almost fatal,which primarily manifests in depletion of hydrocarbon generation capacity,reduction or disappearance of organic pores and intercrystalline pores of clay minerals,and drop of adsorption capacity to natural gas.Therefore,the lower limit of Ro for the carbonization of Type Ⅰ—Ⅱ1 organic matter should be regarded as the theoretically impassable red line of shale gas exploration in the ancient marine shale formations.The organic-rich shale with low-ultralow resistance should be evaluated effectively in area selection to exclude the high risk areas caused by the carbonization of organic matter.The target organic-rich shale layers with low-ultralow resistance drilled during exploration and development should be evaluated on carbonization level of organic matter,and the deployment plan should be adjusted according to the evaluation results in time.