The genesis of chlorite in tight sandstone reservoirs and its influence on reservoir properties: A case study of the lower fourth member of Shahejie Formation in Bonan Sag

Purpose: The Bonan Sag is located in the Bohai Bay Basin in eastern China, where the chlorite content is high in the lower part of the fourth member of the Shahejie Formation. In this paper, the genesis of chlorite in the study area was analyzed by means of casting thin section observation, scanning electron microscope, EPMA and LA-ICP-MS, and its influence on reservoir was discussed.

Experimental samples and analysis methods: In this paper, several samples of Well H1 and Well H2 in the lower part of the fourth member of Shahejie Formation in Bonan sag are selected. Firstly, the morphology and size of chlorite in the sample were observed and described in detail by casting thin sections and scanning electron microscopy. Then, the thin sections were polished, and the different forms of chlorite were subjected to electron probe and laser ablation with the assistance of scanning electron microscopy. The elemental composition of chlorite was measured and analyzed.

Result: Well H1 is mostly coated chlorite, attached to the edge of the particles, inhibiting the secondary enlargement and cementation of quartz and other particles, which is conducive to pore development. The H2 well is mostly rose-like chlorite, which is mainly filled in the middle of the pores, plugging the pores and reducing the porosity and permeability of the reservoir.

The Fe/(Fe + Mg) values of chlorite are generally 0.3-0.7, but they can be clearly divided into two categories: one category has an average value of 0.37, which is significantly smaller than the standard chlorite, and most of them are coated chlorite (relatively rich in Si); the average value of the other type is 0.66, which is significantly larger than that of the standard chlorite, often showing a rose-like (relatively rich in Fe).

Conclusion: The content of quartz and feldspar in well H1 is high, which is coated chlorite rich in Si. It is formed by the dissolution of feldspar and quartz during diagenesis, which has a positive effect on reservoir physical properties. The content of magmatic rock debris and metamorphic rock debris in well H2 is higher, which is Fe-rich in rose chlorite. It is formed by the alteration of magmatic rock and metamorphic rock debris during diagenesis, which has a negative effect on reservoir physical properties.

Keywords: Bonan Sag; Tight sandstone; Reservoir; Chlorite genesis;