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

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.

Insights into the Effect of Micropore Structure and Mineralogy on Adsorbed Tight Oil in the Huaqing Area, Ordos Basin, China

“…In this experiment, the occurrence morphology of tight oil in fresh samples was analyzed by the FTS and ESEM [6,24,[31][32][33][34][35]. According to the difference in position and shapes of adsorbed tight oil, the occurrence morphology of adsorbed tight oil in the study area was divided into five types, including the emulsion form, cluster form, throat form, thin-film form, and isolation form (Table 2 and Figure 4).…”

Control of differential diagenesis of tight sandstone reservoirs on the gas–water distribution: A case study on the Upper Paleozoic He 8 Member in the northern Tianhuan depression of the Ordos Basin

“…The two types of chlorite included chlorite films on the surface of particles and schistose crystals distributed within the intergranular pores. The chlorite films prevented the growth of authigenic quartz, thereby maintaining the early reservoir porosity (Bloch et al, 2002;Fu et al, 2020;Ma et al, 2018;Zhou et al, 2020). Dissolution pores dispersed after the discharge of silicon-alumina fluid formed by the dissolution of phase I acid fluid.…”

“…The kaolinite intercrystalline pores developed after feldspar kaolinization (Figure 9(f)). The intergrain tea leaf-like chlorites in intergranular pores and other clay minerals blocked the pores, resulting in poor pore-throat connectivity (Figure 9(g) to (j)) (Zhou et al, 2020). Due to the preservation of the original pores by chlorite film, organic acid fluids entered the reservoir and caused dissolution.…”

Effect of Authigenic Chlorite on the Pore Structure of Tight Clastic Reservoir in Songliao Basin

“…At present, the research conducted on the influence of authigenic chlorite on reservoirs is mainly focused on conventional reservoirs, and researchers have different views on the influences of authigenic chlorite on reservoirs. Most researchers believe that chlorite coatings and a certain thickness of pore-lining chlorite play a constructive role in the preservation and transformation of primary pores [4,5], and chlorite can increase a reservoir's resistance to compaction [6][7][8][9][10][11]. This is beneficial to the inhibition of quartz overgrowths [6][7][8]12] and carbonate cementation [9,10] and can protect the pores of the reservoir.…”