川西中二叠统栖霞组白云岩特征与成因

Characteristics and genesis of the Middle Permian Qixia Formation dolostone in western Sichuan Basin

韩月卿 张军涛 何治亮 金振奎 韩文彪 高平 郝运轻 孙炜 武重阳

Yueqing HAN Juntao ZHANG Zhiliang HE Zhenkui JIN Wenbiao HAN Ping GAO Yunqing HAO Wei SUN Chongyang WU

近期在川西地区中二叠统栖霞组斩获的多个白云岩油气发现彰显了白云岩储层的油气潜力，但该组白云岩储层非均质性极强，白云岩化机制不明确，优质白云岩储层分布预测困难。研究综合分析了该组不同类型白云岩的岩石学和地球化学特征，结合沉积相背景和峨眉山大火成岩省热事件，系统总结了不同类型白云岩化作用的期次和成因。结果显示：川西地区栖霞组白云岩多为斑状或层状分布，白云石晶体以细晶为主，中-粗晶、泥-粉晶次之，伴少量鞍状白云石胶结物。泥-粉晶白云岩主要发育在栖霞组底部，白云石晶体多为半自形-他形晶，常与草莓状黄铁矿伴生，部分泥晶白云石呈球粒状、哑铃状和花椰菜状；泥-粉晶白云岩的δC（VPDB）为负值，δO（VPDB）处于正常海水数值范围内，这些特征表明泥-粉晶白云岩可能为微生物介导作用的产物。细晶白云岩在栖一段和栖二段都有发育，其白云石晶体主要为自形-半自形晶，多具有雾心亮边结构，在阴极发光下晶体内部呈棕色-暗红色，向外过渡为亮红色后再变暗；细晶白云岩具有明显的残余粒屑结构，表明前驱岩岩性为颗粒灰岩；细晶白云岩的δO（VPDB），δC（VPDB）和Sr/Sr值均指示白云岩化流体为海水，可能为浅埋藏期中等盐度回流渗透白云岩化作用而成。中-粗晶白云岩多发育在栖二段，其白云石晶体主要为非平直晶面他形晶，阴极发光下多表现为内部暗、外部亮的特征；中-粗晶白云岩的δO（VPDB）均低于正常海水数值范围，说明受到了后期流体的改造；部分中-粗晶白云岩的Sr/Sr值高于同期海水数值，呈斑状或斑马状，其溶蚀孔洞发育鞍形白云石，这些特征说明受到了来自或穿碎屑岩层热流体的直接影响；Sr/Sr值仍处于海水数值范围内的中-粗晶白云岩，未见鞍形白云石，可能指示细晶白云岩的重结晶或热调整。总体而言，川西地区栖霞组白云岩先后经历了同生期微生物介导白云岩化作用、浅埋藏期回流渗透白云岩化作用和晚期热流体调整改造作用，其中浅埋藏期回流渗透白云岩化作用是该区白云岩的主要成因。

Recent oil and gas discoveries in the Middle Permian Qixia Formation dolostone in western Sichuan Basin reveal the exploration potential of dolostone reservoirs. However， the mapping of paying dolostone reservoirs has been difficult due to their high heterogeneity and puzzling dolomitization mechanisms. In this study， the lithological and geochemical characteristics of different types of dolostone in the Qixia Formation of western Sichuan Basin are comprehensively analyzed and combined with sedimentary facies and thermal events of the Emeishan Large Igneous Province （ELIP） to summarize the diagenetic stages and genetic mechanisms of the dolostones. The results show that the Qixia Formation in western Sichuan Basin hosts mainly porphyritic or stratified dolostone that are mostly fine crystalline， sometimes medium to coarse crystalline， or micrtic to very fine crystalline， occasionally with some saddle shaped dolomite cement. The micritic to very fine crystalline dolostones， largely developed at the bottom of the Qixia Formation， consist of mainly subhedral-anhedral crystalline often accompanied by framboidal pyrite. Some micritic dolostones consist of sphaerolitic， dumbbell- or cauliflower-shaped crystalline， and have negative δC（VPDB） values and δO（VPDB） values in the range of that of normal seawater， all indicating possible products of microbial-mediated dolomitization process. Fine crystalline dolostones are developed both in the first and second members of the Qixia Formation， where they consist of euhedral to subhedral crystalline with mist-centered bright-edge structure and show change of color from brown-dark red inside to bright red outside and then dark again under cathode luminescence. Their obvious residual grain structure suggests a grainstone origin and their δO（VPDB）， δC（VPDB） as well as Sr/Sr values all indicate seawater as the dolomitization fluid possibly formed by reflux seepage dolomitization with medium salinity during shallow burial period. The medium to coarse crystalline dolostones are generally developed in the second member of the Qixia Formation. They contain mainly anhedral nonplanar crystalline which are dark from inside and bright to the outside under cathodoluminescence. Their δO（VPDB） values are lower than that of the normal sea water， indicating a modification by later fluids. Some medium to coarse crystalline dolostones with Sr/Sr values higher than that of seawater of the same period consist of porphyritic or zebra-like crystalline and often have saddle dolomites in vugs. These characteristics indicate a direct influence of thermal fluids that originated from or passed through certain clastic rocks. Those with Sr/Sr values still within the seawater range but with no saddle-shaped dolomites being observed may be the products of recrystallization or thermal adjustment of fine crystalline dolostones. In general， the dolostones of the Qixia Formation in western Sichuan Basin are mostly likely the results of a successive syngenetic microbial-mediated dolomitization， shallow burial reflux seepage dolomitization， and late thermal fluid modification， among which shallow burial reflux seepage dolomitization could be the main genetic mechanism.

10.11743/ogg20230106