西沙中新世藻礁白云岩储层特征及成因模式

西琛1井中新世生物礁被认为是植物藻礁白云岩,但为一孔之见。基于西沙群岛两大环礁3个岛屿的3口晚中新世—中中新世钻井生物礁全取心岩心的资料,开展藻礁白云岩的储层对比及成因模式分析,证实白云岩在各自埋深、厚度等方面虽不一致但特征较为相近,均处于中新世,多属藻礁白云岩,对于理解西沙群岛新近纪生物礁的形成演化具有重要意义。其中,涉及白云岩3层,总厚724.65 m,通过常规岩石薄片、扫描电镜、X光粉晶衍射和物性测试分析发现,3口井的研究井段以藻礁白云岩为主,但深度不同;白云石矿物隐晶—粉晶—细晶状,发育粒间孔、藻架孔和生物体腔孔等原生孔隙和粒内溶孔、铸模孔、藻体溶孔及裂缝等次生孔隙;并在多个层段形成粒内溶孔—晶间孔—藻体溶孔组合;具有高孔、高渗显著特征,为有利油气储层。本段白云岩锶含量低,是新近纪全球冰川性海退事件的结果,构成混合水白云化作用机制。

RESERVOIR CHARACTERISTICS AND GENESIS OF MIOCENE ALGAL REEFAL DOLOSTONE IN THE XISHA ISLANDS

The Miocene reef deposits penetrated by the Well Xichen-1 have been treated as a kind of algal reef dolostone for long. This article is a summary of the data from three drilling holes on three islands of two atolls of middle Miocene-late Miocene. Comparative analysis suggests that the basic characteristics of the algal reef dolostone are rather similar each other even small differences occur in their buried depth and thickness. Most of the deposits, according to the data from drilling holes, are algal reefs and their derives, which are of significance to the understanding of the Neogene evolutionary history of the Xisha Islands. There are three dolomite layers, with a total thickness of 724.65 m. 381 pieces of samples have been taken for conventional laboratory study of thin sections, scanning electronic microscopy (sem), X-ray powder diffraction and physico-property test and analysis. Priority is given to the algal reef dolostone itself. They are buried in different depths and consist of dolomite mud with various primary pores, such as intergranular pore and visceral foramen, and secondary pores, such as moldic pores, intergranular emposieu and tectonic fissures. High porosity and permeability layers may be formed by the combination of primary and secondary pores to form favorable oil and gas reservoir. The dolomite is low in strontium content, of which the origin is related to the mixing of fresh water and sea water during the glacial regression.