Geochemically Distinct Oil Families in the Gudong Oilfield, Zhanhua Depression, Bohai Bay Basin, China

Abstract

Twenty crude oil samples were obtained from the Gudong Oilfield and their organic geochemical characteristics were analyzed. The oil samples were classified into three families by hierarchical cluster analysis and principal component analysis based on 13 source-related and depositional environment-related biomarker parameters. Oils in family I have low ratios of C 19 /C 23 tricyclic terpanes and C 24 tetracyclic terpane/C 26 tricyclic terpanes, and relatively high ratios of steranes/hopanes and C 30 4-methylsteranes/ααα20R C 29 sterane, thus indicating that microalgae were the dominant organic matter input for the source rocks of family I. The gammacerane/C 30 hopane ratios are higher than that of family II and family III, whereas the C 35 /C 34 homohopane ratios are lower, thus indicating a suboxic, brackish water environment for the source rocks. The inferred source rock is the first member of the Shahejie Formation in the Huanghekou Sag. Family II is characterized by high ratios of C 19 /C 23 tricyclic terpanes and C 24 tetracyclic terpane/C 26 tricyclic terpanes but relatively low ratios of steranes/hopanes and C 27 /C 29 αααR steranes. These findings suggest that the original organic matter of the source rocks had a greater contribution from terrigenous higher plants than from microalgal. The relatively low ratios of gammacerane/C 30 hopane and C 35 /C 34 homohopane suggest that the source rocks were deposited in an oxic environment with a low salinity, thus corresponding to the Dongying Formation in the Huanghekou Sag. Family III oils have high C 27 /C 29 ααααR steranes ratios and low C 30 4-methylsteranes/ααα20R C 29 ratios, which indicate the contribution of microalgae (especially zooplankton algae) to the source rocks. The relatively high abundance of C 35 homohopane and low gammacerane/C 30 hopane ratios suggest a weakly reducing condition with low salinity, which is in accordance with the third member of the Shahejie Formation in the Gunan Sag. The C 31 S/(S + R) homohopane ratios imply that oil samples in this study are in the mature stage, although the ratios of C 29 20S/(20S + 20R) and C 29 ββ/(αα + ββ) steranes suggest that the maturity of family II is higher than that of family I and family III.

Chemometric Classification and Geochemistry of Crude Oils in the Eastern Fukang Sag, Junggar Basin, NW China

“…Hence, it is generally used as an indicator of terrigenous higher plants, along with C 24 TeT [38,39]. The oil samples in Family B showed notably higher ratios of C 19 TT/C 23 TT (0.46~1.03) and C 24 TeT/C 23 TT (0.46~1.00) than those of Family A and Family C. Generally, C 27 and C 28 regular steranes are derived from lower aquatic organisms, while C 29 is associated with higher plants [38,40], but it should be noticed that there are many algae (e.g., brown algae, some species of green algae) that contain significant quantities of C 29 sterols [12,41]. Hao et al (2011) proposed that in the Junggar Basin, high C 28 /C 29 sterane ratios indicate algal organic matter with little or no contribution from higher-plant organic matter.…”

“…For oil-oil and oil-source correlation based on chemometric methods, due to introducing the idea of dimensionality reduction in mathematics, the influence of multiple variables can be comprehensively considered simultaneously in the familiar two-dimensional or threedimensional charts [3,4]. Thereinto, hierarchical cluster analysis (HCA) and principal component analysis (PCA) are two common and well-established chemometric methods, and they are widely applied in geochemical studies [5][6][7][8][9][10][11][12]. Principal component analysis is a multivariate statistical method by dimensionality reduction to transfer multiple possibly relevant variables into a smaller number of uncorrelated variables (principal components), so that these principal components can reflect the most information of the original data.…”

Chemometric Classification of Oil Families in the Laizhouwan Depression, Bohai Bay Basin, Eastern China

“…Petroleum system determination, migration route recognition, and satellite oilfield reconnaissance, and reservoir continuity assessment are part of extensive applications of geochemical correlation studies . Oil genetic classification largely relies on biomarker ratios in the chemometric methods such as PCA and HCA. , Other genetically related parameters such as bulk composition of oil, carbon isotopic compositions of saturated and aromatic hydrocarbons, , and Fourier transform infrared spectroscopy of asphaltene subfractions can be also integrated to the chemometric approach. The chemometric method can be used to extract major information from a large number of geochemical parameters, and plenty of information can be obtained from oil family identification for better understanding their sources, evolutions, and secondary alterations.…”