Data for: Effects of lacustrine depositional sequences on organic matter enrichment in the Chang 7 Shale, Ordos Basin, China
Description
The paleoenvironment determines OM enrichment and preservation, and thus contributes to the different TOC contents of the three facies ( Rivera et al., 2018; Li et al., 2020; Zhang et al., 2020). Fig. 7A presents a cross-plot of the U/Th and V/Cr ratios, which are indicators of paleoredox and detrital input; these ratios suggest that the Chang 7 Shale mainly formed in a weakly reducing environment. Detrital inputs may have resulted in the temporarily turbulent environment where Facies 3 was deposited. However, Facies 1 and Facies 2 were more likely to be deposited in anoxic environments. The plot of the strontium/copper (Sr/Cu) ratio versus the magnesium/strontium (Mg/Sr) ratio (Fig. 7B) shows that overall, the Chang 7 Shale formed in a warm and humid climate, except that some of the Facies 1 and Facies 2 deposits formed in a hot and dry climate. In particular, the high Sr/Cu and Mg/Sr ratios of Facies 1 indicate high contents of carbonate minerals in a dry and hot climate with the absence of detrital inputs. Fig. 7C reveals that compared with the other facies, Facies 3 was mainly deposited in relatively shallow water. Facies 1 and Facies 2 formed in both relatively deep water and shallower water. Statistical analysis of wireline log data with calibration to a core lithology standard has been a typical method of predicting lithology (Bush et al., 1987). Using advanced methods such as spectral facies interpretation and neural network models, the sophistication of interpretation makes this approach the first choice for inter-well lithology identification and correlation (Jia et al., 2012; Torghabeh et al., 2014; Konaté Ahmed Amara et al., 2015; Singh Amrita et al., 2016; Jeong Jina et al., 2020). The most essential step of well-logging-based lithology identification is to determine the relationship between the core lithology and its sensitive logging parameters. Based on comparison of logging parameters (Table 2) such as GR, SP, AC, CNL, DEN, RT, the photoelectric absorption cross-section index, permeability, and porosity, the lithologies of Chang 7 have high GR values in common for both the sandstones and shales. However, the considerable distinctions of GR, SP, AC, CNL, and RT values between shales and sandstones make it possible to distinguish the three facies further. A logging parameter spider diagram is the lithology identification criterion based on the correspondence between the given logging data and core analysis. It may eliminate errors better and improve the accuracy of lithology identification because of the combination of multiple parameters (Zhu et al., 1992; Jia et al., 2012; He et al., 2015). The selected logging parameter spider diagram of the three facies (Fig. 8)shows that Facies 2, which is the most favorable facies for shale oil, exhibited the highest RT, GR, AC, and CNL values, and relatively low SP values. However, Facies 3 had the lowest RT, GR, AC, and CNL values, and the highest SP value.