东营凹陷页岩油储层中方解石的成因及证据

Origin and evidence of calcite in shale oil reservoir of Dongying Sag

为明确东营凹陷页岩中方解石矿物成因和储层成岩特征,利用薄片、X衍射、碳氧同位素等分析技术,首次发现了牛页1井页岩中方解石脉具有示顶底构造和机械双晶特征,是地质构造运动产生地应力作用于页岩方解石脉上留痕的直接岩石学证据,为页岩地层地应力的研究提供了载体矿物。通过对研究区页岩中泥晶方解石和亮晶方解石的碳氧同位素对比分析发现:页岩中泥晶方解石的形成主要隶属于与甲烷细菌活动生成生物气有关的碳酸盐,亮晶方解石脉极可能为泥晶方解石溶蚀后再沉淀重结晶的产物。生烃增压等作用形成微裂缝是方解石脉形成的先决条件,有机酸对泥晶方解石的溶蚀和再沉淀是必要条件。根据发现的4个岩石学特征和碳氧同位素证据,阐述了从生烃增压、微裂缝形成、有机质热演化排出有机酸、泥晶方解石被溶蚀、方解石重结晶,直至方解石脉形成的整个同生演化过程,并探讨了页岩与相邻砂岩的碳库来源和砂泥岩协同成岩演化过程。

In order to clarify the origin of calcite minerals and the diagenetic characteristics of reservoirs in Dongying Sag,the calcite veins in Well Niuye1 are analyzed with thin section,X-ray diffraction and carbon-oxygen isotope.It is discov ered that they can indicate top-bottom structure and have mechanical twin crystals characteristics,which provides the evi dence that they are products of the in-situ stress generated by geological tectonic movement acting on the shale,and they provide carrier minerals for the study of in-situ stress in shale formation.Through comparative analysis of carbon and oxy gen isotopes of micrite calcite and sparry calcite in shale of the study area,it is found that micrite calcite in shale mainly be longs to carbonate related to biogas generated by methane bacteria.The sparry calcite veins are likely to be the product of precipitation and recrystallization of the mudstone calcite after dissolution.The micro-fractures caused by hydrocarbon gen eration and pressurization are the prerequisite for the formation of the calcite veins,and the dissolution and reprecipitation of argillaceous calcite by organic acid is a necessary condition.Based on the four petrological characteristics and the carbon-oxygen isotope,the syngenetic evolution of the hydrocarbon-generating pressurization,micro-fracture formation,or ganic acid discharged from thermal evolution of organic matter,dissolution of micrite calcite,recrystallization of calcite,and formation of calcite veins are described.The source of carbon pool of shale and adjacent sandstone and the process of synergistic diagenesis of sandstone and mudstone are discussed as well.