第四系泥岩型生物气储层特征及动态成藏过程

Reservoir Characteristics and Dynamic Accumulation Process of the Quaternary Mudstone Biogas

第四系泥岩型生物气作为非常规油气领域一种新型的资源类型,具有巨大的资源潜力和勘探开发价值。以柴达木盆地三湖地区第四系泥岩型生物气为例,通过岩心观察、光学显微镜、扫描电镜、高压压汞、族组分分析等实验手段,分析了第四系泥岩型生物气储层特征及动态成藏过程。结果表明,第四系泥岩型生物气储层具有砂泥薄互层频繁交互、纵横向非均质性极强的特点。成岩作用处于早期阶段,R_(0)小于0.3%,岩石固结程度低,含贝类壳体没有石化,孔隙度普遍在20%以上,仍以微纳米孔隙为主,含极少量毫米孔隙。泥岩储层TOC极低,平均0.2%~0.4%,干酪根以Ⅲ型为主,主要成分是粗纤维,其次是半纤维素、有机氮,是第四系泥岩生气的主要母质来源。第四系泥岩型生物气为甲烷为主的干气,平均含量98.85%,地层水水型以CaCl_(2)型为主,酸碱度中等偏弱酸性。泥岩突破压力是甲烷滞留成藏的主要动力,低渗、富水和黏土的特征决定了泥岩具备自封闭能力。极低的气候温度、极高的水体盐度、充足的气源条件、有效的自封闭性是泥岩型生物气成藏的关键要素,以此建立了凹陷区自封闭富集带、斜坡区水封富集带、构造高点泥岩气-砂岩气叠合富集带等三种泥岩型生物气成藏模式。

Quaternary mudstone biogas is a new type of resource in unconventional oil and gas field, and has great exploration and development potential. Taking the Quaternary mudstone biogas at Sanhu(Qaidam Basin) as an example, the reservoir characteristics of the Quaternary mudstone biogas and its dynamic accumulation process are analyzed through core observation, optical microscopy, scanning electron microscopy(SEM), high-pressure mercury injection and component analysis. The results show that the reservoir is characterized by frequent sand-mud thin interbeds and strong vertical and horizontal heterogeneity. In the early-stage diagenesis, R_(0)is below 0.3%. Rock consolidation degree is low, shells are not fossilized, and porosity is generally above 20% with many micro-/nano-pores. The TOC of mudstone reservoir is very low(average 0.2%-0.4%). The kerogen type is mainly Ⅲ, and the main components include crude fiber, hemicellulose, and organic nitrogen, representing the main source of the Quaternary mudstone gas generation. The Quaternary mudstone biogas is methane-dominated(average 98.85%) dry gas. The formation water type is mainly moderately-acidic CaCl_(2). The mudstone breakthrough pressure is the main drive for methane retention and accumulation. The characteristics of low permeability, water-rich, and clay determine that the mudstone has self-sealing ability. The cold climate, high salinity, sufficient gas source, and effective self-sealing are the key factors for mudstone biogas accumulation, and three mudstone biogas accumulation modes are established:(1) self-sealing enrichment zone in sag area,(2) water-sealing enrichment zone in slope area, and(3) overlapping enrichment zone of mudstone gas and sandstone gas in structural highs.