Data for: Effects of lacustrine depositional sequences on organic matter enrichment in the Chang 7 Shale, Ordos Basin, China

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.古环境决定了有机质（organic matter, OM）的富集与保存过程，进而造成三种沉积相的总有机碳（Total Organic Carbon, TOC）含量存在差异（Rivera等，2018；Li等，2020；Zhang等，2020）。图7A为U/Th与V/Cr比值的交会图，二者均为反映古氧化还原条件与陆源输入的替代指标；该交会图结果显示，长7页岩主要形成于弱还原环境。陆源输入可能导致了沉积相3沉积时的环境处于短暂动荡状态；而沉积相1与沉积相2则更倾向于形成于缺氧环境中。锶铜比（Sr/Cu）与镁锶比（Mg/Sr）的比值交会图（图7B）显示，整体而言长7页岩形成于温暖湿润的气候背景，仅部分沉积相1与沉积相2的沉积物形成于炎热干燥的气候。其中，沉积相1较高的Sr/Cu与Mg/Sr比值，指示其形成于无陆源输入的炎热干燥环境，且碳酸盐矿物含量较高。图7C显示，相较于其余两种沉积相，沉积相3主要沉积于相对较浅的水体环境；而沉积相1与沉积相2则形成于相对深水与较浅水体两种环境。以岩心岩性为标准刻度的测井曲线数据统计分析，是岩性预测的经典方法（Bush等，1987）。借助频谱相解释、神经网络模型等先进技术，解释方法的不断成熟使得该方案成为井间岩性识别与对比的首选（Jia等，2012；Torghabeh等，2014；Konaté Ahmed Amara等，2015；Singh Amrita等，2016；Jeong Jina等，2020）。基于测井的岩性识别最核心的步骤，在于明确岩心岩性与其敏感测井参数之间的对应关系。通过对比自然伽马（Gamma Ray, GR）、自然电位（Spontaneous Potential, SP）、声波时差（AC）、补偿中子测井（CNL）、密度测井（DEN）、电阻率测井（Resistivity, RT）、光电吸收截面指数、渗透率与孔隙度等测井参数（表2）可知，长7层的砂岩与页岩均普遍具有较高的GR值。但砂岩与页岩在GR、SP、AC、CNL与RT参数上的显著差异，为进一步区分三种沉积相提供了可行条件。测井参数雷达图是基于给定测井数据与岩心分析结果对应关系的岩性识别标准。由于整合了多组测井参数，该方法可更好地消除误差，提升岩性识别精度（Zhu等，1992；Jia等，2012；He等，2015）。针对三种沉积相选取的测井参数雷达图（图8）显示：沉积相2是最有利的页岩油沉积相，其RT、GR、AC与CNL值均为最高，SP值相对较低；而沉积相3的RT、GR、AC与CNL值均为最低，SP值则最高。