Quantitative assessment of the effects of zeolite alteration processes on deep clastic reservoirs – A case study of the Jiamuhe Formation in the Shawan Sag, Junggar Basin, China

Abstract

Zeolite minerals are widely developed in clastic rock reservoirs around the world, and their behavior during diagenetic evolution affects the performance of the reservoirs. Zeolite can fill the primary intergranular pores in a cemented manner and can also undergo alteration and postdissolution to increase pores during the diagenesis process, while quantitative research is still quite limited at present. For this purpose, this paper takes the typical zeolite-rich Jiamuhe Formation (P1j) in the deep layer of the Shawan Sag, Junggar Basin, as an example and carries out a series of sedimentological, petrological, mineralogical and geochemical analyses to quantitatively evaluate the effect of zeolite alteration on the reservoir. The results indicate that the deep P1j reservoir contains fan-delta facies sandstone and conglomerate, and its storage space is mainly zeolite intercrystalline pores and remaining intergranular pores. The zeolite mineral type is almost all laumontite, with very low analcite and heulandite contents (<1%). The volcanic glass provided by the medium–basic volcanic rock debris (andesite and corresponding tuff), with the continuous increase in temperature and the supply of alkali metal ions, is eroded into laumontite via clinoptilolite and heulandite. In the process of zeolite alteration, zeolite minerals appear as zonal characteristics in the vertical direction. The intercrystalline pores formed by the dehydration of zeolite minerals are the most important factors for the development of high-quality deep reservoirs. Based on a large number of statistical analyses of zeolite cementation and intercrystalline pores within the cementation, a unit change of zeolite can cause a unit change in the intercrystalline pores of 22.16%. Supposing that the laumontite mineral content of the reservoir is 30%, the porosity can be increased by 6.6%. This can provide theoretical guidance for the change in the porosity of zeolite alteration in zeolite-rich reservoirs under similar backgrounds and improve the academic understanding of this point, which is exceedingly crucial. For instance, the Permian sand-conglomerate reservoirs in the northwestern margin of the Junggar Basin also have similar zeolite alteration pathways, and the porosity can be quantitatively increased by 8.8% (the zeolite cementation is approximately 40%). Laumontite (21%) in high-quality reservoirs of the Yingcheng Formation in the Changling Sag, Songliao Basin, can increase the porosity by 4.5%.