Abstract
Organic matter heterogeneity exerts an important impact on the generation, evaluation, and exploitation of shale oil resources. In the past, only a limited number of analytical samples that represented the contribution or influence of high or low organic matter abundance intervals were used to represent an entire set of thick source rocks, and based on this limitation, occasionally researchers have reached incorrect conclusions. Here, the heterogeneity of organic matter and its significance to shale oil in the Triassic Zhangjiatan shale of the Ordos Basin were discussed based on sedimentary and geochemical analyses. Our results indicate that (1) the Chang 73 shale within the study area is characterized by high abundance, good quality, strong heterogeneity, and segmented enrichment in the vertical profile. The primary organic matter type was type II1, and this was followed by types I, II2, and III that exhibited three changing trends in the vertical profile. Organic matter maturity is primarily at the immature stage. (2) Additionally, we observed that the organic matter heterogeneity of Chang 73 shale was related to changes in lithology, sedimentary environment, paleoproductivity, terrigenous influence, and event action (such as volcanism and hydrothermal processes). The abundance of organic matter is the result of the coupling control of these processes. In the process of organic matter deposition, if volcano or hydrothermal activities occurred, the organic matter was abnormally enriched, and the total organic carbon content exceeded 20% and could even be as high as 40%. (3) Finally, we observed at least nine enrichment layers that were controlled by the coupling of different geological factors, and there were four potential target resource layers corresponding to the hydrothermal sedimentary area.
Highlights
Shale oil is abundant throughout the world, and it is considered to be a strategic alternative resource to solve the energy crisis in China (Guo et al, 2020; Zhao et al, 2020; Zou, 2017)
The total organic carbon (TOC) and total sulfur (TS) contents were obtained by carbon–sulfur detection, which were measured by induction furnace combustion (LECO) with a CS344 analyzer after the samples were treated with 10% hydrochloric acid to remove carbonates, and the analysis precisions are ±0.5%
Combined with the results of previous studies, our results indicate that it is likely that the cyclic variation of lithology and organic matter content is controlled by the paleoclimate
Summary
Shale oil is abundant throughout the world, and it is considered to be a strategic alternative resource to solve the energy crisis in China (Guo et al, 2020; Zhao et al, 2020; Zou, 2017). Based on core observations and previous studies, it was determined that Chang 73 shale ( known as Zhangjiatan shale) in the Ordos Basin possesses strong heterogeneity in both the vertical and horizontal directions (Li et al, 2016; Zhao et al, 2019; Zhu et al, 2003), and this makes it more difficult to evaluate, explore, and develop. In this study, based on centimeter-scale sampling, geochemical analysis technologies incorporating the use of carbon–sulfur detection, Rock-Eval analysis, stable carbon isotope, X-ray, and elemental analyses were used to determine the organic matter heterogeneity, genesis, and shale oil significance of the Chang 73 shale of the Hejiafang section in the Tongchuan area of the Ordos Basin (Figure 1). The results of this study will provide scientific guidance for the exploration of Chang 73 shale oil and possess certain reference significance for the exploration and development of continental shale oil in China
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