Authigenic minerals and diagenetic evolution in altered volcanic materials and their impacts on hydrocarbon reservoirs: evidence from the lower Permian in the northwestern margin of Junggar Basin, China
文摘
Potentially, volcanic and volcaniclastic rocks are significant hydrocarbon reservoir rocks. In these rocks, the low-temperature alteration (<200 °C) and diagenetic transformation of volcanic materials are universal and usually improve the reservoir quality greatly. Based on observations of available cores, thin sections and scanning electron microscope (SEM) data, this paper describes the complex set of diagenetic reactions in the volcanic material-bearing hydrocarbon reservoirs and their role in creating secondary porosity, as well as providing a scientific geological foundation for quality evaluation and prediction in similar reservoir rocks. Volcanic materials in volcanic and volcaniclastic rocks of the lower Permian in the northwestern margin of the Junggar Basin underwent devitrification, chloritisation, zeolitisation, albitisation and calcification. The analysis of porosity data based on cores indicates that in an open hydrologic system, grain-coating chlorites and the dissolution of minerals can contribute to good porosity, especially in tuff-bearing clastic rocks. Appreciable primary pores are preserved by the formation of early chlorite coats and pore linings. Acidic water dissolved zeolite, albite and calcite that precipitated under alkaline conditions in sedimentary facies and eruptive facies, creating dissolution pores. The nonisopyknic transformation of albitisation created abundant secondary pores in both eruptive facies and effusive facies. Too much cement of chlorite and heulandite (>4 % vol) damaged the reservoir quality. The dissolution of laumontite was significant for deep reservoir quality. Coarse conglomerate with soluble analcime cements developed many secondary dissolution pores and were the most favourable clastic reservoir.