Estimation of original total organic carbon content and hydrogen index using major and trace element concentrations in the overmature Upper Ordovician–lower Silurian Wufeng–Longmaxi marine shales, southeast Sichuan Basin, south China

IF 2.7 3区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

AAPG Bulletin Pub Date : 2024-05-01 DOI:10.1306/12212322070

Xunyao Wang, Tian Dong, Sheng He, Qing He

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Abstract

Total organic carbon (TOC) content and hydrogen index (HI) are critical parameters for evaluating the hydrocarbon generation potential of source rocks and shale gas resources. However, for overmature shales, laboratory-measured residual TOC content and residual HI fail to reflect original properties. In this study, overmature Upper Ordovician to early Silurian Wufeng–Longmaxi shales from south China were selected to estimate the original TOC content and original HI based on major- and trace-element concentrations that are rarely lost during thermal alteration. Proxies were used, including biogenic silica, Cu/Al, P/Al, Mo-enrichment factor (EF), U/Th, U-EF, Al, and Ti content to document the organic matter accumulation process of the Wufeng–Longmaxi Formations. The relationships among proxies for paleoproductivity (biogenic silica), paleoredox conditions (Mo-EF), terrigenous influx (Al), and TOC content suggest that organic matter accumulation was primarily controlled by high paleoproductivity and anoxic conditions. Moreover, the ratio of biogenic silica to total silica is an effective proxy for estimating the marine organic matter fraction. The original HI values of global immature marine shales display a normal distribution; hence, the calculated marine organic matter fraction is hypothesized to conform to a normal distribution. Based on intervals (μ − σ, μ + σ) and (μ − 2σ, μ + 2σ) (μ is the deviation and σ is the variance) with the same probability, a correlation between original HI and organic matter abundance was established. The average restored original TOC and HI were 4.9 wt. % and 493 mg/g, respectively, indicating the dominance of organic matter type I–II1. The thermal maturity and hydrocarbon generation history modeling results suggest that the estimated original TOC and HI values are reasonable.

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利用主要元素和痕量元素浓度估算中国南方四川盆地东南部上奥陶统-下志留统五峰-龙马溪海相页岩的原始总有机碳含量和氢指数

总有机碳（TOC）含量和氢指数（HI）是评估源岩和页岩气资源碳氢化合物生成潜力的关键参数。然而，对于过成熟页岩，实验室测量的残余 TOC 含量和残余 HI 无法反映其原始属性。本研究选取了中国南方上奥陶统至志留纪早期的过成熟五峰-龙马溪页岩，根据热蚀变过程中很少损失的主要元素和痕量元素浓度来估算原始 TOC 含量和原始 HI。利用生物硅、Cu/Al、P/Al、Mo-富集因子（EF）、U/Th、U-EF、Al和Ti含量等代用指标记录了五峰-龙马溪地层的有机质累积过程。古生产率（生物硅）、古缺氧条件（Mo-EF）、土著涌入量（Al）和总有机碳含量等代用指标之间的关系表明，有机质的积累主要受高古生产率和缺氧条件的控制。此外，生物硅石与总硅石的比率是估算海洋有机质部分的有效替代指标。全球未成熟海相页岩的原始 HI 值呈正态分布，因此推测计算出的海相有机质部分也符合正态分布。根据概率相同的区间 (μ - σ, μ + σ) 和 (μ - 2σ, μ + 2σ) (μ为偏差，σ为方差)，确定了原始 HI 与有机质丰度之间的相关性。平均恢复的原始总有机碳和 HI 分别为 4.9 重量%和 493 毫克/克，表明有机物类型 I-II1 占主导地位。热成熟度和碳氢化合物生成历史建模结果表明，估计的原始总有机碳含量和总碳氢化合物含量值是合理的。

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来源期刊

AAPG Bulletin 工程技术-地球科学综合

CiteScore

6.60

自引率

11.40%

发文量

审稿时长

4.8 months

期刊介绍： While the 21st-century AAPG Bulletin has undergone some changes since 1917, enlarging to 8 ½ x 11” size to incorporate more material and being published digitally as well as in print, it continues to adhere to the primary purpose of the organization, which is to advance the science of geology especially as it relates to petroleum, natural gas, other subsurface fluids, and mineral resources. Delivered digitally or in print monthly to each AAPG Member as a part of membership dues, the AAPG Bulletin is one of the most respected, peer-reviewed technical journals in existence, with recent issues containing papers focused on such topics as the Middle East, channel detection, China, permeability, subseismic fault prediction, the U.S., and Africa.