Since the discovery of giant Aptian pre-salt reservoirs in Brazilian margin basins, the study of lacustrine carbonates has drawn great attention from the scientific community. Comparatively, minor attention was given to the characterization and genesis of the Mg-silicates (e.g., stevensite, kerolite) which are commonly associated with these carbonates. A systematic petrological study was performed in the Aptian Barra Velha Formation (BVF) within distinct structural compartments of the giant Lula Field in the Santos Basin, in order to recognize the patterns of primary formation and diagenetic alteration of these Mg-silicates. Mg-silicates occur as peloids, ooids, intraclasts, and fine-grained laminated deposits, either mixed in variable proportions with other particles, such as carbonate bioclasts and volcanic rock fragments, or constituting specific intrabasinal deposits. In the BVF interval, clay peloids and laminated deposits are associated with spherulitic and fascicular calcite aggregates, as substrate and hosts for these precipitates. Ooids are interpreted as formed at the sediment–water interface by the nucleation of concentric envelopes on the surface of particles (heterogeneous nucleation), through repeated rolling under gentle wave and current action. Laminated deposits, interpreted as precipitated directly from the water column (homogeneous nucleation) in highly supersaturated and low-hydrodynamic-energy environments, constitute extensive deposits in the BVF. Peloids were probably formed in intermediate energy conditions. Some ooidal arenites show porosity from the dehydration and contraction, and/or the dissolution of ooids. In some rocks, these pores are filled with fibrous calcite, while the remaining Mg-silicates are replaced by dolomite, calcite, or silica. A similar diagenetic pattern occurs in the laminated deposits, where magnesite and dolomite fill shrinkage pores formed along their characteristic wavy laminae. Owing to their elevated solubility, most of the Mg-silicates were dissolved, or intensely replaced by calcite, dolomite, or silica. The detailed petrologic analysis indicates that the original volumes of Mg-silicates were substantially larger, and that their deposition was widespread in the basin, including on structurally high areas. The types and intensity of diagenetic alteration of the Mg-silicate deposits are distinct for each structural compartment, being more intense towards the highs and closer to the overlying evaporites, which imposed a strong influence on reservoir quality.

中文翻译：

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来自巴西桑托斯盆地 Lula 油田的 Aptian 盐下镁硅酸盐沉积和成岩作用的古环境见解

自从在巴西边缘盆地发现巨大的阿普蒂安盐下储层以来，湖相碳酸盐岩的研究就引起了科学界的高度关注。相比之下，较少关注通常与这些碳酸盐相关的镁硅酸盐（例如，硅镁石、黑橄榄石）的表征和成因。为了识别这些镁硅酸盐的原生形成和成岩作用模式，对桑托斯盆地巨型卢拉油田的不同构造隔间内的 Aptian Barra Velha 组 (BVF) 进行了系统的岩石学研究。镁硅酸盐以球粒、鲕粒、内碎屑和细粒层状沉积物的形式出现，或者以不同比例与其他颗粒混合，例如碳酸盐生物碎屑和火山岩碎片，或构成特定的基底内沉积物。在 BVF 层段，粘土球体和层状沉积物与球晶和束状方解石聚集体有关，作为这些沉淀物的基质和宿主。鲕粒被解释为通过在温和的波浪和电流作用下的反复滚动，通过颗粒表面的同心包络成核（异质成核）在沉积物-水界面形成。层状沉积物，解释为在高度过饱和和低流体动力环境中直接从水柱中沉淀（均匀成核），构成 BVF 中的广泛沉积物。球状体可能是在中等能量条件下形成的。一些鲕粒砂岩因脱水和收缩和/或鲕粒溶解而显示出孔隙率。在一些岩石中，这些孔隙充满了纤维状方解石，而剩余的硅酸镁则被白云石、方解石或二氧化硅所取代。类似的成岩模式出现在层状沉积物中，其中菱镁矿和白云石填充了沿其特征波浪纹层形成的收缩孔隙。由于它们的溶解度提高，大多数硅酸镁被溶解，或被方解石、白云石或二氧化硅强烈取代。详细的岩石学分析表明，镁硅酸盐的原始体积要大得多，并且它们的沉积在盆地中很普遍，包括在构造高的区域。镁硅酸盐矿床的成岩蚀变类型和强度对于每个构造隔室都是不同的，向高处更强烈，更靠近上覆的蒸发岩，