塔河地区鹰山组灰岩白云石化成因及其对储层的影响

Genesis of dolomites in limestone of Yingshan Formation and their effects on poroperm characteristics of carbonatereservoir in Tahe area

白云石化是碳酸盐典型的成岩作用类型之一,中-下奥陶统鹰山组浅海碳酸盐岩是塔河地区重要的含油气储层,普遍发育多种类型的白云石化作用。本文针对塔河地区鹰山组灰岩中沿缝合线分布这一特定类型的白云石化进行了研究。岩石学基础上的阴极发光分析、^(86)Sr/^(87)Sr比值以及δ^(13)C-δ^(18)O的研究表明,这种类型的白云石化发育于相对还原的浅埋藏成岩环境,孔隙中的残留海水是主要的白云石化流体来源,碳酸盐矿物(主要是高镁方解石)稳定化过程中释放的Mg^(2+)可能是白云石化主要的物质来源,而埋藏过程中逐渐升高的温度一定程度上也促进了白云石化的发生。成岩流体及Mg^(2+)有限的供给量限制了白云石化作用的整体规模。显微结构与岩石物性分析表明,该类白云石化对储层物性参数的绝对贡献相对有限,但可能成为小规模油气运移以及深部热流体活动的潜在通道。

Dolomitization is one of typical diagenesis in carbonate rocks, the Middle-Lower Ordovician shallow marine carbonate rocks of Yingshan Formation in Tahe area is important oil- gas bearing reservoirs and pervasively dolomitized. Focused on the special dolomitization confied by stylolites in the limestone of Yingshan Formation, this paper attempts to reveal the diagenetic conditions and dolomitization processes of this type of dolomites. Based on detailed petrological studies, cathodoluminescence, 86Sr/87 Sr ratio,δ13C-δ18O isotope and oxygen thermometer of dolomites and matrix limestone are analyzed, and these geochemical results indicate that dolomites confined by stylolites in limestone of Yingshan Formation occurred in shallow burial diagenetic environment which was relatively reductive, and the remnant sea water in micro-pores supplied the fluids for dolomitization, and Mg2＋ released by high-Ma calcite stabilization during burial diagenesis probably supplied Mg2＋ for dolomitization. However, relatively limited diagenetic fluids and Mg2＋ supply in the relative closed diagenetic conditions limited the scale of dolomitization and the development of dolomite. Analysis of poroperm of dolomitic limestone and pure limestone show that the contribution of dolomitization on absolute poroperm is relatively limited. However, some oil filling in dolomitization parts in limestone probably indicate that this kind of dolomitization can afford potential conduits for small scale oil-gas migration and deep hydrothermal fluids flow.