川北元坝地区长兴组白云石化作用机制及其对储层形成的影响

通过岩心观察和薄片鉴定,在岩石学和矿物学特征分析基础上,结合白云石有序度测定,碳、氧同位素、锶同位素和稀土元素组成及配分模式分析,详细研究了川北元坝地区长兴组白云石化作用的特征、机制及模式,结果表明长兴组发育微晶白云石(岩)、粉-中晶他形白云石、粉-中晶自形白云石和异形白云石等四种类型,它们的有序度由低变高;白云石的碳、氧同位素、锶同位素和稀土元素组成及配分模式特征表明,长兴组微晶白云石(岩)、粉-中晶他形白云石、粉-中晶自形白云石主要形成于浓缩海水环境、正常海水环境、或者与正常海水相似的地层水环境中,并遭受过热液地质作用的改造,从微晶白云石(岩)→粉-中晶他形白云石→粉-中晶自形白云石是一个沉积埋藏过程中多阶段白云石化作用的产物,异形白云石则由热液作用形成;根据长兴组白云石的矿物学和地球化学特征及白云石化作用与层序和沉积相之间的关系,分别可以用准同生期蒸发泵白云石化作用模式、准同生期渗透回流白云石化作用模式、成岩早期浅埋藏状态下地层水白云石化作用模式和成岩晚期热液白云石化作用模式来解释微晶白云石(岩),粉-中晶他形白云石、粉-中晶自形白云石和异形白云石的形成。白云石化作用是有利于长兴组储层形成的建设性成岩作用。

Dolomitization mechanisms and influence on reservoir development in the Upper Permian Changxing Formation in Yuanba area, northern Sichuan Basin

The dolomitization characteristics and mechanisms of the Upper Permian Changxing Formation in the northern Sichuan Basin were studied in detail, and dolomitization models were established based on the analyses of rock structure, mineralogy of dolomite, combined with the analysis of carbon, oxygen and strontium isotopic geochemistry, REE composition and patterns. According to rock structure and mineralogy, four types of dolomites were recognized: (a) micritic dolomite; (b) fine-medium sized anhedral dolomite; (c) fine-medium sized euhedral dolomite, and (d) coarse saddle dolomite. Carbon, oxygen, and strontium isotopic geochemistry as well as REE composition and patterns indicate that the first three types of dolomites of the Changxing Formation were formed in normal or concentrated marine or similar formation water environments and later modified by hydrothermal processes The dolomites are products of multistage dolomitization processes. Based on the mineralogical and geochemical characteristics of the dolomites and their close distributional relationship with sequence stratigraphy and sedimentary facies, three dolomitization models has been established for dolomitization interpretation: (a) penecontemporaneous evaporative pumping dolomitization model for micritic dolomite formation; (b) penecontemporaneous seepage reflux dolomitization model for fine-medium sized anhedral dolomite formation; and (c) shallow-burial formation water dolomitization during early diagenetic stage for fine-medium euhedral dolomite. Coarse saddle dolomite was generated by hydrothermal dolomitization during the late diagenetic stage. Dolomitization is one of the constructive diagenetic processes for reservoir development in the Changxing Fromation.