渤海海域莱州湾凹陷沙河街组硅化碎屑岩地质特征及其储层意义

渤海海域莱州湾凹陷硅化碎屑岩以粒间石英晶体孔隙式充填、强烈石英次生加大为特征,局部地区甚至发生低变质作用,表现为变余砂状结构,这在国内外含油气盆地中是极为少见的。运用地震、测井、岩石学和单颗粒锆石U-Pb测年等技术手段,揭示了硅质碎屑岩的地质特征,首次通过锆石测年证实了莱州湾凹陷沙河街组沉积时期存在火山活动。硅质热流体活动证据主要体现在硅质脉体发育,呈多期充注特征,石英颗粒次生加大强烈,岩石甚至出现变余砂状结构;粒间粘土几乎全部热蚀变为黑云母和白云母,并可见"帚状"正在生长的云母类矿物;岩石中发育大量集块状和集簇无序状黄铁矿,不同产状的黄铁矿为多期次热液成因;热场发射扫描电镜下识别出黄铁矿-重晶石-独居石-方铅矿-金红石-硬石膏热液矿物组合。深部硅质热流体进入储层,碎屑石英颗粒产生强烈次生加大,加大石英和硅质沉淀完全充填粒间孔隙,对储层起破坏性作用,但硅化作用大大增强了地层的脆性,在后期构造运动作用下产生大量断裂和微裂缝网络。另外,热液流体溶蚀了储层中的长石类矿物,产生大量长石粒内溶蚀孔和铸模孔,在裂缝沟通下形成了孔隙-裂缝型优质储层。

Geological features and their participation in the formation of silicified clastic reservoirs in the Shahejie Formation of Laizhouwan Sag,Bohai Sea

Silicified clastic rocks in the Laizhouwan Sag, Bohai Bay, are characterized by intergranular pores among quartz crystals filled with siliceous materials and by secondary intensive quartz overgrowth.Some areas even exhibit low-level metamorphism of blastopsammitic texture, which is rarely seen in petroliferous basins.The geological features of the siliceous clastic rocks in the sag are revealed by means of seismic and log interpretations, petrological analyses and zircon U-Pb dating.It has been confirmed for the first time that volcanic activities did occur during the deposition of the Shahejie Formation in the sag.Siliceous thermal fluid activities are verified by well-developed siliceous veins of multi-stage filling and charging, intensive overgrowth of the secondary quartz, and the blastopsammitic texture of rocks.Almost all the clay minerals in intergranular pores were thermally altered to biotites and muscovites, and growing "broom-shaped" mica minerals are observed.A large number of pyrite are developed in agglomerates or random clusters.Pyrite with different occurrence is derived from multi-phase hydrothermal solution.The hydrothermal mineral assemblage of pyrite-barite-monazite-galena-rutile-anhydrite is identified under the field-emission scanning electron microscope.With siliceous hot fluids from deep entering reservoirs, the overgrown clastic quartz and siliceous precipitates worked together to fully stuff the intergranular pores, causing deterioration in physical properties of the reservoirs.However, the silicification saved the situation by greatly enhancing the reservoir brittleness, which generated numerous fracture and micro-fracture networks during later tectonic movements.In addition, the hydrothermal fluids dissolved the feldspar minerals in the reservoirs, resulting in a large amount of dissolution pores and moldic holes in feldspar grains.Thus high-quality reservoirs of pore-fissure type are formed with well-connected fracture networks.