Application of chemometrics to quantitative source assessment of crude oils from the Zhanhua Depression, Bohai Bay Basin, northeast China

https://doi.org/10.1016/j.jseaes.2021.104875Get rights and content

Highlights

  • The geochemical characteristics of 65 crude oils from the Zhanhua Depression were investigated.

  • The relative contributions of different source rocks for oils in the Zhanhua Depression were assessed.

  • Multidimensional scaling was employed to reveal the affinities of oils in two-dimensional diagram.

  • This is the first time that circos has been applied in petroleum geochemistry.

Abstract

The chemometric methods alternating least squares and multidimensional scaling were employed in this study to quantify the relative contribution of the sources and identify the affinities of sixty–five crude oil samples collected in the Zhanhua Depression. The circos diagram was used to reflect the relative contributions of different source rocks. Based on these techniques, two end–members (EMs) were identified which represent the geochemical characteristics of two regional source–rocks (Es3L and Es4U). The EM1 composition assigned to Es3L is characterized by relatively low C35/C34 homohopane ratio and relatively low abundance of gammacerane. This EM is consistent with a source facies that was deposited in freshwater lacustrine environment that contained a non–stratified water column and suboxic to dysoxic bottom waters. Distributions of the regular steranes and tricyclic terpanes, the relatively low steranes/hopanes ratio suggest the organic matter input into this source facies were combined microalgal material and terrestrial organic matter. Oils from the Bonan Sag and Gubei Sag were derived mainly from this source facies. In contrast, EM2 has been assigned to the Es4U source rock and is characterized by relatively high abundance of gammacerane, C35/C34 homohopane ratio greater than one, suggesting deposited in a saline to hypersaline lacustrine environment that contained a stratified water column with strongly reducing bottom waters. Distributions of the regular steranes and tricyclic terpanes as well as relatively high steranes/hopanes ratio suggest the organic matter deposited in this setting was mainly derived from microalgal/bacterial sources. Oils from the Luojia Nose and Chanjiazhang High were derived mainly from this source facies. Mixed contributions of these two source facies (end–members) account for the compositions of crudes produced in the Yihazhuang Arch and Gudao High. In the case of the Yihezhuang Arch the crudes appear to have near equal contributions from the two source faces, whereas crudes from the Gudao High appear to have slightly higher contributions from the Es3L source facies than from the Es4U source facies.

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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