澳大利亚苏拉特盆地Walloon煤组成藏条件及富集模式

中侏罗统Walloon亚群煤组是澳大利亚苏拉特(Surat)盆地煤层气藏的主要储层,为一套典型的低煤阶高产煤层。为了明晰该煤组的煤层气成藏条件及富集模式,基于录井、实验和测试资料,研究了该煤组的展布特征及烃源岩条件、储集条件与保存条件,结合对该煤组的天然气成因及含气性特征分析结果,对比分析了该盆地东北部3个煤层气开发区在气藏富集模式上的差异,并归纳了Walloon煤组成藏及富集的有利条件。结论认为:(1)构造运动、地下水条件以及显微组分为Walloon煤组的生气创造了有利条件,该煤组的顶、底板及夹层岩性致密,封闭条件较好;(2)Walloon煤层气藏是一个形成在单斜构造上由水动力和岩性共同封闭的以次生生物成因气为主的混合成因气藏,煤层气的富集受局部构造、煤层生气能力和地下水动力3种因素的控制,并可分为背斜、向斜和斜坡3种模式,其中背斜模式、向斜模式的煤层气富集程度相对更高;(3)与其他煤层气藏相比,Walloon煤层气藏的成藏和富集同时具有双重碳源、高效生气、适中水动力和双重封闭4个有利条件。

Coalbed methane accumulation conditions and enrichment models of Walloon coal measure in the Surat Basin,Australia

The coal measure of Middle Jurassic Walloon Sub-Group is the main reservoir of coalbed methane (CBM) reservoirs in the Surat Basin, Australia, and it is a set of typical low- rank, high-yield coal beds. To figure out the CBM accumulation conditions and enrichment models of this coal measure, its distribution characteristics, source rock characteristics, reservoir characteristics and preservation conditions were investigated based on mud logging, experiments and testing data. Then, based on the analysis results of gas origin and gas bearing property of this coal measure, three CBM development blocks in northeastern Surat Basin were analyzed and compared in terms of its gas enrichment model and special conditions for CBM accumulation and enrichment. It is indicated that tectonic movements, groundwater conditions and macerals provide favorable conditions for Walloon coal measure to generate gas. The roof, floor and interbed of this coal measure are lithologically tight, presenting a good sealing capacity. Besides, the Walloon CBM reservoir is a gas reservoir of hybrid origin with the secondary biogenic methane as the dominant component in a monocline structure and it is sealed jointly by hydrodynamic and lithology. CBM enrichment is dominated by three factors, i.e., regional structure, methane generating capacity of coal bed and underground hydrodynamic, and it can be divided into models, i.e., anticline model, syncline model and slope model, among which the CBM enrichment degree of the former two models is higher. Finally, different from other CBM reservoirs, Walloon CBM reservoir satisfies four special conditions in CBM accumulation and enrichment, including dual-carbon source, high-efficiency methane generation, moderate hydrodynamic and double sealing, so multiple methane generating systems are formed in extra thin coal beds.