南羌塘坳陷古油藏带布曲组白云岩储层特征及控制因素分析

Characteristics and controlling factors of the dolostone reservoirs in the Buqu Formation,southern Qiangtang depression

羌塘盆地布曲组白云岩储层是整体致密背景下的"甜点"储层,深入研究其发育的控制因素,是进行有利区预测的基础。通过野外地质剖面观察、岩心观察、铸体薄片鉴定,结合实测分析,对白云岩储集空间进行分类,并分析其发育的控制因素。结果表明,南羌塘坳陷古油藏带布曲组白云岩储层发育在碳酸盐岩台地的台内滩中,高位体系域晚期的滩间洼地局限水体提供白云化流体。储层中发育晶内溶孔、晶间孔、晶间溶孔、溶蚀孔洞,物性较好。储层质量受沉积作用、成岩作用及构造作用共同控制。台内滩发育的颗粒灰岩能够保存更多的连通孔隙,为后期流体改造提供运移通道,滩后局限水体提供白云石化流体。浅埋藏阶段白云石化是储集空间发育的最有利因素。燕山运动Ⅱ幕的构造挤压形成热点,提供形成孔隙充填鞍形白云石的热流体,同时该流体能够溶蚀围岩。古油藏破坏过程中的酸性流体和大气淡水对储层进行溶蚀改造,对储层具有关键性改造意义的是浅埋藏阶段的白云石化和第一次生烃前的有机酸溶蚀。

The dolostone reservoirs are interpreted as the ＂sweet spot＂ reservoirs in the Buqu Formation, southern Qiangtang depression. The study of the controlling factors of the dolostone reservoirs may be helpful to the prediction of the favourable exploration areas. The results of research in field investigation and thin section examination in this study have disclosed that the dolostone reservoirs in the Buqu Formation, southern Qiangtang depression are mostly developed in the intraplatform shoals of the carbonate platforms. The restricted waters in the intershoal sags in the later stages of the highstand systems tract may provide dolomitized fluids. The dolostone reservoirs have well-developed intracrystal solution openings, intercrystal pores, intercrystal solution openings and dissolved pore spaces. The controlling factors are represented by depositional, diagenetic and tectonic processes. The interconnected pores are well preserved in the grainstones widespread in the intraplatform shoals, and thus may be served as the migration passages for the reworking of fluids. The dolomitization during the shallow burial diagenesis may be most favourable for the development of the reservoir spaces of the dolostones. The tectonic compression during the second episode of the Yanshanian movement may provide the thermal fluids for the generation of the saddle dolomite. The acidic fluids produced by the destruction of the fossil oil reservoirs and meteoric fresh water may play an important part in the dissolution and reworking of the dolostone reservoirs. The key factors include dolomitization in the shallow burial diagenesis and organic acidic dissolution before the first hydrocarbon generation.