Differences of tuffaceous components dissolution and their impact on physical properties in sandstone reservoirs: A case study on Paleogene Wenchang Formation in Huizhou-Lufeng area, Zhu I Depression, Pearl River Mouth Basin, China

The element geochemical characteristics and diagenetic alteration products of tuffaceous components in sandstone reservoirs of Paleogene Wenchang Formation in typical subsags of the Huizhou-Lufeng area of the Zhu I Depression,Pearl River Mouth Basin,were identified through microscopic analysis and quantitative analysis of main and trace elements.The impacts of dissolution of different tuffaceous components on physical properties of reservoirs were discussed through quantitative characterization of reservoir physical properties.The results show that there are mainly four types of tuffaceous components in the study area,which are acidic,intermediate,basic and alkaline tuffaceous components.The acidic tuffaceous components evolved in a process of strong alteration and weak dissolution of alteration products,with a large amount of kaolinite precipitated during alteration to disenable the improvement of porosity and permeability.The intermediate and alkaline tuffaceous components evolved in a process of strong dissolution of tuffaceous components and strong alteration of residual tuffaceous components;the dissolution of tuffaceous components created intergranular pores,but the alteration products such as autogenic quartz,apatite and illite deteriorated the pore structure;ultimately,the dissolution of tuffaceous components resulted in the increase of porosity but no increase of permeability of the reservoir.The basic tuffaceous components dominantly evolved in a process of dissolution of tuffaceous components to strong dissolution of alteration products;both tuffaceous components between particles and laumontite generated from alteration can be strongly dissolved to create pores;thus,the dissolution of tuffaceous components can significantly increase the physical properties of the reservoir.

Study on fault-controlled hydrocarbon migration and accumulation process and models in Zhu I Depression

Through the analysis of the faults and their internal structure in Zhu I Depression,it is found that the internal structure of the late fault is obviously segmented vertically.It develops unitary structure(simple fault plane)in shallow layers,binary structure(induced fracture zone in hanging wall and sliding fracture zone in footwall)in middle,layers and ternary structure(induced fracture zone in hanging wall and sliding fracture zone in middle,and induced fracture zone in footwall)in deep layers.Because the induced fracture zone is a high porosity and permeability zone,and the sliding fracture zone is a low porosity and ultra-low permeability zone,the late fault in middle layers has the character of"transporting while sealing".The late fault can transport hydrocarbon by its induced fracture zone in the side of the hanging wall and seal hydrocarbon by its sliding fracture zone in the side of the footwall.In deep layers,the late fault has the character of"dual-transportation",induced fracture zones in both sides of hanging wall and footwall can transport hydrocarbon.The early fault that only developed in the deep layers is presumed to be unitary structure,which plays a completely sealing role in the process of hydrocarbon migration and accumulation due to inactivity during the hydrocarbon filling period.Controlled by hydrocarbon source,early/late faults,sand bodies and traps,two reservoir-forming models of"inverted L"and"stereo-spiral"can be proposed in middle layers,while two reservoir-forming models of"cross fault"and"lateral fault sealing"are developed in the deep layers of Zhu I Depression.