Formation mechanisms and sequence response of authigenic grain-coating chlorite: evidence from the Upper Triassic Xujiahe Formation in the southern Sichuan Basin, China

Abstract

Authigenic grain-coating chlorite is widely distributed in the clastic rocks of many sedimentary basins around the world. These iron minerals were mainly derived from flocculent precipitates formed when rivers flow into the ocean, especially in deltaic environments with high hydrodynamic conditions. At the same time, sandstone sequences with grain-coating chlorites also tend to have relatively high glauconite and pyrite content. EPMA composition analysis shows that glauconites with ''high Al and low Fe'' content indicate slightly to semi-saline marine environments with weak alkaline and weakly reducing conditions. By analyzing the chlorite-containing sandstone bodies of the southern Sichuan Xujiahe Formation, this study found that chlorite was mainly distributed in sedimentary microfacies, including underwater distributary channels, distributary channels, shallow lake sandstone dams, and mouth bars. Chlorite had a tendency to form in the upper parts of sandstone bodies with signs of increased base level, representing the influence of marine (lacustrine) transgression. This is believed to be influenced by megamonsoons in the Middle and Upper Yangtze Region during the Late Triassic Epoch. During periods of abundant precipitation, river discharges increased and more Fe particulates flowed into the ocean (lake). In the meantime, increases or decreases in lake level were only affected by precipitation for short periods of time. The sedimentary environment shifted from weakly oxidizing to weak alkaline, weakly reducing conditions as sea level increased, and Fe-rich minerals as authigenic chlorite and glauconite began to form and deposit.

Biomarker distributions and depositional environments of continental source rocks in Sichuan Basin, SW China

“…Although extensive study has been conducted on the biomarkers of the continental source rocks in the Sichuan Basin (Lin, 2014; Shi et al., 2012; Yu et al., 2016), there are still some problems. Firstly, most studies are about the source rocks of the Late Triassic Formation, and only a few studies have been conducted on the Lower Jurassic source rocks.…”

Diagenesis and reservoir characteristics analysis of the Late Triassic Xujiahe Formation tight gas sandstone in the northern Sichuan Basin, China

“…Therefore, the grain framework did not experience effective compaction, which can be inferred from the high cementation content (Figure 9(c)). The timing of authigenic grain-coating chlorite in sandstone is believed to be related to multiple sources (Bahlis and Luiz, 2013; Billault et al., 2003; Grigsby, 2001; Sun et al., 2014; Yu et al., 2016). Thin section and SEM analysis suggest grain-coating chlorite formed before feldspar dissolution and grains came into contact, because chlorite occurs between grain contacts (Figure 9(h)).…”

“…Small amounts of grain-coating chlorite have been found in the Xujiahe sandstone, showing a positive relationship with porosity and a slight relationship with permeability (Figure 14(e) and (f)). Previous studies have focused on the relationship of authigenic grain-coating chlorite with reservoir porosity preservation (Bloch et al., 2002; Ehrenberg, 1993; Huang et al., 2004; Xiang et al., 2016; Yu et al., 2016). They reported that grain-coating chlorite is beneficial to reservoir porosity preservation by enhancing resistance of compaction and inhibit growth of quartz cement, especially in deeply buried sandstone reservoirs (Bloch et al., 2002; Ehrenberg, 1993; Huang et al., 2004).…”

Sequence architecture and sedimentary characteristics of a Middle Jurassic incised valley, western Sichuan depression, China

“…2; Wang and Meng 2008;Zheng et al 2008;Zou et al 2013;Yu et al 2016;Ye et al 2017). During the depositional period of the Shaximiao Formation, the LMTB was relatively static, whereas the Micang-Daba Mountains Tectonic Zone (MDMTZ; Fig.…”