准噶尔盆地玛湖凹陷南斜坡二叠系风城组风化壳型火山岩储层特征及控制因素

风化壳型火山岩是准噶尔盆地玛湖凹陷南斜坡二叠系风城组重要的储层类型之一,但目前对其储层特征及控制因素的研究程度较低。针对这一问题,根据岩心、钻井与录井资料、常规测井、地层微电阻率成像测井(FMI)和多种分析化验数据,对储层进行了系统的研究。研究区火山岩类型主要为玄武岩和安山岩,储集空间包括风化溶蚀孔洞、原生半充填气孔、裂缝以及少量的基质溶孔、晶间溶孔、基质收缩孔等。储层非均质性强,物性差别较大,孔隙度为0.29%~16%。储层受到早期火山作用和后期风化作用的共同控制。早期火山作用决定了气孔的发育程度,气孔主要集中在各期火山岩的上部,大多被充填-半充填形成杏仁构造和半充填气孔。后期风化作用形成了火山岩风化壳,可划分为土壤带、淋滤带、崩解带和母岩带4层结构,处于淋滤带的气孔段极易形成溶蚀孔洞,其储层物性最好,崩解带主要发育裂缝型储层。凹陷中心烃源岩生成的油气沿大型走滑断裂-小型伴生断裂-火山岩风化壳的路径高效运移,在鼻凸构造圈闭部位聚集成藏,因此处于斜坡中部鼻凸构造上的淋滤带、崩解带气孔段成藏条件最为有利,是今后勘探的重点区域。

Weathered volcanic reservoir characteristics and their controlling factors on Permian Fengcheng Formation in southern Mahu Depression,Junggar Basin

Weathered volcanic crust is one of the most important reservoir types for Permian Fengcheng Formation,however research on its characteristics and controlling factors are not enough. To solve this problem,according to core,geology logging,normal log,formation microscanner image(FMI)and assay data,this paper presents a systematic study on weathered volcanic reservoir. The volcanic rock in study area mainly included basalt and andesite,of which the reservoir space is dominated by weathered solution openings,primary semi-filled bubble,fracture,and a small amount of matrix solution pores,intercrystal pore,matrix shrinking pore. Volcanic reservoir is featured with strong heterogeneity and has great difference of physical property,from 0.29% to 16%. Reservoir characteristics are controlled by both primary volcanic process and anaphase weathering process,the former determines developing degree of bubble,and the bubble was filled or semi-filled to be amygdaloidal structure and semi-filled bubble. Anaphase weathering process makes volcanic to form a four-layer structure,including soil layer,leached zone,broken-down zone and underlying bedrock. The bubble of leached zone is very easily converted into solution openings and has good physical property,while broken-down zone is mainly characterized by fractured reservoir. Oil and gas generated from central depression source migrate along largescale strike slip faults to small associated reverse fault then to weathered volcanic crust efficiently,and finally accumulate in the trap of nose structure. Therefore,leached zone with well-developed bubble of nose structure has a good advantage of hydrocarbon accumulation,which could be prospecting exploration potential area.