惠州凹陷HZ25-7构造带文昌组低孔低渗砂岩储层特征和成因机理

Characteristics and origin of low porosity and low permeability sandstone reservoir of Wenchang Formation in HZ25-7 structural belt of Huizhou Depression

惠州凹陷是珠江口盆地重要的油气富集区,其中HZ25-7构造带是惠州凹陷古近系文昌组寻求油气突破的重点地区,文昌组储层特征和成因机理研究是在该区实现勘探突破的基础。基于钻井、铸体薄片、扫描电镜、储层物性及压汞等资料,对惠州凹陷HZ25-7构造带砂岩储层特征和成因机理进行了详细的研究。研究结果表明:储层岩性以岩屑砂岩和长石质岩屑砂岩为主,杂基含量高,成分成熟度和结构成熟度均较低;平均孔隙度和平均渗透率分别为12%和7.1 mD,属于低孔、低渗储层;孔隙类型以粒内溶孔和原生粒间孔为主,孔隙结构以中—小孔、细喉道为特征,孔隙连通性差。沉积条件和成岩作用是造成文昌组储层低孔、低渗的主要原因。近源的辫状河三角洲沉积背景决定了碎屑组分中软碎屑含量高,杂基充填了部分孔隙;压实作用强烈,造成大量原生孔隙减少,原始孔渗性差;可溶组分含量低,导致有机酸溶蚀对物性改善不太明显;晚期大量的黏土胶结和碳酸盐胶结,降低了储层物性,使储层变得致密。异常超压的形成有利于储层孔隙的保存。

Huizhou Depression is an important oil and gas-rich region of Pearl River Mouth Basin. HZ25-7 structural belt is an important area designed for seeking oil and gas of Wenchang Formation in Huizhou Depression. The research of reservoir characteristics and mechanism of Wenchang Formation is the foundation of oil exploration breakthrough. Based on comprehensive analysis of various data such as drilling, casting thin sections, scanning electronic microscope, physical properties analysis and mercury penetration test, this paper analyzed the reservoir characteristics and mechanism of Wenchang Formation of HZ25-7 structural belt. The results show that sandstones are mostly litharenite and feldspathic litharenite, and are generally immature both texturally and compositionally. The average porosity and permeability of the sandstones are 12% and 7.1 mD respectively, and it is emphatically defined as the low porosity and low permeability reservoir. Pore types are mainly intragranular dissolved pores and primary intergranular pores, and its micro-pore structure is typical medium-micropore and fine throat, with poor pore connectivity. Such low porosity and permeability reservoir of Wenchang Formation is controlled by depositional conditions and diagenesis. Braided fluvial deltaic depositional environment characterized by near provenance determined the large quantity of ductile grains in detrital composition and the infilling of part of primary pores by resulting matrix. Strong compaction results in primary porosity loss significantly. Due to low content in soluble grains, the improvement of physical properties mediated by organic acid is not obvious. Clay and carbonate cementations lead to continued worsening of reservoir properties and tightening, but abnormal overpressure is constructive to the preservation of reservoir porosity.