Application of chemometrics to quantitative source assessment of crude oils from the Zhanhua Depression, Bohai Bay Basin, northeast China
Graphical abstract
Introduction
The Bohai Bay Basin is one of the most petroliferous basins in China (Hao et al., 2010). The Jiyang Superdepression in the southeast of the Bohai Bay Basin (Fig. 1a) covers an area of 25510 km2 (Wang et al., 2015) and consists of four depressions from south to north, that is the Zhanhua, Dongying, Huimin, and Chenzhen depressions, and many secondary structural units (Zhang et al., 2011). The Zhanhua Depression located in the east of the Jiyang Superdepression is a faulted lacustrine depression developed under the background of Palaeozoic structural activities (Shi et al., 2005). The hydrocarbon accumulation and reservoirs formation in the Zhanhua Depression has a close relationship with fault activity (Wang et al., 2015; Sun et al., 2015; Shi et al., 2005). The petroleum in different reservoirs show different geochemistry characteristics due to the mixture during or after hydrocarbon accumulation.
Although several studies were carried out on the Zhanhua Depression, the geochemical characteristics of the crude oils are not understood. It is known that the oil and gas in most reservoirs are derived from deep source rocks (Xie et al., 2006) and the hydrocarbon subsequently migrated up through fault–fracture mesh networks to different sags (Wang et al., 2005). But the relative contributions of different source rocks have not been identified. Chemometric methods have unique advantages with respect to the comprehensive consideration of the effects of multiple parameters, genetic classification and correlation of mix oils (Peters et al., 2007, Peters et al., 2008a). The statistical algorithms employed during chemometric analysis provide a more quantitative estimation of the contribution of source rocks for the mixed oils than that obtained through traditional qualitative geochemical methodologies. In this study, we have applied chemometric methods to organic geochemical parameters from a suite of crude oils collected in the Zhanhua Depression. Alternating least squares (ALS, Peters et al., 2008b, Zhan et al., 2016a) was employed to determine the sources of oils and multidimensional scaling (MDS, Wang et al., 2016) was used for oil–oil correlation. The visualization tool, circos, was used to reveal the relationships between endmembers and oils.
Section snippets
Geological setting
The Jiyang Superdepression is bounded by various structures (Huang and Liu, 2014): Tan–Lu Fault to the east, arcuate Chengning Uplift to the west and north, and Qi–Guang Fault to the south (Zhang et al., 2012). The geologic framework of the Jiyang Superdepression is complex due to the counter–inclined block faulting, basin–inclined fault depression, and monolithic depression that occurred from the Paleogene to Neogene (Li et al., 2004). The Zhanhua Depression lies at the northeast of the Jiyang
Samples
Sixty–five crude oil samples were collected from the Zhanhua Depression, Bohai Bay Basin, China. They were recovered from different depths and reservoirs. In total, 29, 18, 7, and 11 crude oil samples were collected from the Bonan–Gubei Sag, Yihezhuang Arch, Gudao High, and Luojia–Chenjiazhuang High, respectively. The sample locations are shown in Fig. 1b. The basic information of the oil samples, including reservoir layer, depth, and level of biodegradation, is presented in Table 1.
GC analysis of the whole oils
The whole
Stable carbon isotope composition
The stable carbon isotopes of the oils are mostly depending on the organic matter input and depositional environment of the source rocks (Sun et al., 2000). The stable carbon isotope ratio (δ13C) compositions of the whole oils range from –28.5‰ to –24.3‰, with an average of –26.3‰ (Table 1). The δ13C values of the saturates and aromatic hydrocarbons range from –29.3‰ to –25.1‰ and –28.0‰ to –23.4‰, with averages of –27.3‰ and –25.7‰, respectively. The oil samples in the study show relatively
Conclusions
The contributions of the Es4U and Es3L shale to crude oils collected from the Zhanhua Depression were determined using ALS and circos diagrams and the genetic relationships of the crude oils were distinguished using MDS.
The dominant contributor to oils from the Bonan Sag and Gubei Sag was the Es3L source rock. The major contributor to oils from the Luojia Nose and Chanjiazhang High is the Es4U source rock. The contribution of the Es4U shale is a slightly higher than that of the Es3L shale for
CRediT authorship contribution statement
Xiao–Hui Lin: Investigation, Writing - original draft. Zhao–Wen Zhan: Formal analysis, Writing - review & editing. Yan–Rong Zou: Conceptualization, Methodology, Software. Tian Liang: Investigation, Writing - review & editing. Ping’an Peng: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We appreciate Prof. Liu Keyu and the two anonymous reviewers for their detailed and constructive comments that significantly improved the quality of the manuscript. We acknowledge Dr. Yao–Ping Wang for the kind help with this study. This study was funded by the Natural Science Funding Council of China (Grant No. 41621062), SKLOG project (SKLOGA 201701 SKLOGC 201704), and GIGCAS 135 project (Grant No. Y234021001).
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