Formation mechanism of authigenic chlorite in tight sandstone and its influence on tight oil adsorption, Triassic Ordos Basin, China

Qianshan Zhou,Xiaofeng Ma,Chao Li,Chengze Li,Guojun Chen,Chengfu Lv

doi:10.1177/0144598720947140

Abstract

In this study, the formation mechanism of authigenic chlorite in tight reservoirs and its influence on the adsorption capacity to tight oil have been analyzed. The occurrence states of chlorite and the formation mechanism have been analyzed by thin section (TS) and field emission scanning electron microscopy (FE-SEM) measurements. Due to the alteration of volcanic rock fragments, the mudstone pressurized water, and the dissolution of early chlorite, the material source has been provided for the formation of chlorite. The formation time of chlorite with different occurrence states is in the following order: grain-coating chlorite → pore-lining chlorite → pore-lining chlorite in dissolved pores → rosette chlorite. Authigenic chlorite developed in the reservoirs has influenced the change of the reservoir quality in two respects. On the one hand, authigenic chlorite can protect the residual pores, improve the anti-compaction capacity of the reservoir, and provide certain inter-crystalline space. On the other hand, it can hinder pore space and inhibit throat, resulting in a decrease in the connectivity of pores and the increase in the heterogeneity of the reservoir. Tight oil absorbed by the chlorite is mainly in the form of the thin film and aggregates. Through in situ testing of environmental scanning electron microscope (ESEM) and energy dispersive spectrum (EDS), the adsorption capacity of chlorite with different occurrence states to tight oil, being in the following order: rosette chlorite > pore-lining chlorite > pore-lining chlorite in dissolved pores > grain-coating chlorite. Furthermore, the controlling factors on reservoir quality, the content of chlorite and content of Fe and K have been investigated, and the adsorption capacity of different chlorite types has been studied, which can provide guidance for analysis of the control factors on the difference in adsorption capacity of different occurrence states of chlorite to tight oil in tight reservoirs.

Highlights

With the successful exploration and development of unconventional oil and gas, tight oil and gas have become more and more important in world energy, being the priority choice for energy replacement (Alotaibi et al, 2015; Denne et al, 2014; Jia et al, 2012; Kuhn et al, 2012; Landon et al, 2001; LeFever et al, 2013; Sonnenberg et al, 2018; Wu et al, 2016, 2019; Zou et al, 2012)
The content of volcanic rock fragments ranges from 1.3% to 21.1%, the content of sedimentary rock fragments ranges from 0.3% to 7.0%, and the content of metamorphic rock fragments ranges from 3.4% to 35.6%
The results indicate that tight oil adsorbed on the chlorite in the reservoirs, which is consistent with previous study (Qian et al, 2017)

Summary

Introduction

With the successful exploration and development of unconventional oil and gas, tight oil and gas have become more and more important in world energy, being the priority choice for energy replacement (Alotaibi et al, 2015; Denne et al, 2014; Jia et al, 2012; Kuhn et al, 2012; Landon et al, 2001; LeFever et al, 2013; Sonnenberg et al, 2018; Wu et al, 2016, 2019; Zou et al, 2012). The content and type of clay minerals have a significant influence on the reservoir’s physical properties. Chlorite is the most common clay mineral, and its content and distribution might control the reservoir’s physical properties. It is very important to systematically study the occurrence states and formation mechanism of chlorite.Chlorite is a specific type of clay mineral, with typically high content of iron and magnesium (Worden et al, 2020). Chlorite is an important clay mineral in coarse-grained clastic sediments such as sandstones, especially in the lacustrine sandstone reservoirs (Worden et al, 2020). As an important anti-compaction mineral in sandstones, chlorite has attracted widespread attention for more than 50 years (Huang et al, 2004; Tian et al, 2008; Worden et al, 2020; Yao et al, 2011; Zhou et al, 2016)

Results

Discussion

Conclusion