Figure
1.
Formation catalog of Pearl River Mouth Basin.
| Citation: | Ning MA, Dujie HOU, Hesheng SHI. Novel tetracyclic terpanes in crude oils and source rock extracts in pearl river mouth basin and their geological significance. Journal of Earth Science, 2014, 25(4): 713-718. doi: 10.1007/s12583-014-0468-9
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Tetracyclic terpanes have a wide distribution in most oils and source rock extracts (Peters et al., 2005). The geochemical significance of tetracyclic terpanes has been reported at home and abroad many times. Several studies indicated that high abundant C24 tetracyclic terpane might result from terrigenous organic matter (Zhang and Huang, 2005; Philp and Gilbert, 1982). The abundance of C24 tetracyclic terpane is much higher than the tricyclic terpanes in oils with coal and coaly, terri-genous organic matter input (Boreham et al., 2003; Philp and Gilbert, 1986). Some tetracyclic terpanes were thought to result from the ring opening of hopanoids (Peters et al., 2005; Tritz et al., 1999; Trendel et al., 1982; Aquino Neto et al., 1981). Grice et al. (2001) found that the stable carbon isotopic composition of C24 tetracyclic terpane had a constant δ value of around -25.5㏑. It suggested that C24 tetracyclic terpane was derived from hopanoids or terrigenous organic matter with a fairly con-stant δ13C composition. But there were also some studies showed that relatively high concentrations of C24 tetracyclic terpane in petroleum was the indication of carbonate or evapo-rate depositional environment (Clark and Philp, 1989; Connan et al., 1986). For example, the Ordovician-hosted FI oils were found having abundant C24 tetracyclic terpane, but lack of higher-plant input. Thus Ordovician-hosted FI oils were be-lieved to derive from the carbonate or evaporitic source rocks (Gong et al., 2007). High concentrations of C24-C30 tetracyclic terpanes in immature sediment of the Jinxian sag, Bohai Bay Basin were thought to derive from the diagenetic products of microbial sources in a carbonate environment (Lu et al., 2009).
The content of tetracyclic terpane is affected by both or-ganic source and maturity (Farrimond et al., 1999). C24 tetra-cyclic terpane/C30 αβ hopane ratio is often used to predict source rocks characteristics or calculate maturity (Peters et al., 2005; Farrimond et al., 1999). C24 tetracyclic terpane/C30 αβ hopane ratio shows a marked increase late in the oil window, because tetracyclic terpanes are thermally more stable than hopanes (Farrimond et al., 1999).
Early in 1992, Woolhouse et al. identified new series of tetracyclic terpane compounds in the tertiary oil called C24-des-A-oleanane, C24-des-A-lupane and C24-des-A-ursanne, respectively (Woodhouse et al., 1992). Then, des-A-oleanene was also detected in Holocene sedimentary sequence of Lake Sarbsko by Bechtel et al. (2007). Samuel et al. (2010) indenti-fied C24-des-A-oleanane and a new C27 tetracyclic terpane in Tertiary deltaic oils. But C24-des-A-oleanane and new C27 tet-racyclic terpane have not been reported in domestic oils and source rock extracts until now. When studies on the geochemi-cal characteristics of source rocks and crude oils in Zhu 1 de-pression in the Pearl River Mouth Basin are carried out, these new tetracyclic terpane compounds are detected in crude oil and source rock extracts. This article reports the identification of these new tetracyclic terpanes and their biogenic signifi-cance.
Pearl River Mouth Basin locates in northern South China Sea and the southern margin of the South China continent. It is a Meso-Cenozoic oil-bearing basin forming in Caledonian, Hercynian and Yanshan folded basement. Zhu 1 depression located in northern of Pearl River Mouth Basin, including En-ping, Xijiang and Huizhou depression. Cenozoic strata in Pearl River Mouth Basin are divided into eight groups consisting Paleocene Shenhu Formation, Eocene Wenchang Formation, Eocene-Oligocene Enping Formation, Oligocene Zhuhai For-mation, Miocene Zhujiang, Hanjiang and Yuehai formations, Pliocene Wanshan Formation (Fig. 1). Wenchang Formation is lacustrine facies with blown mudstones. Enping Formation is fluvial and swamp facies with alternating layers of mudstones, sandstones and coal seams. Wenchang Formation middle-deep lacustrine mudstones and Enping Formation coal measures mudstones are the main source rocks in this area. Zhuhai For-mation is transitional facies deposit, Zhujiang, Hanjiang, Yue-hai and Wanshan groups are marine facies.
Eleven source rock samples taken from different depths of Wenchang and Enping formations in Zhu 1 depression were used in this study. Thirty crude oil samples were taken from different oilfield and oil-bearing structure in Huizhou sag, Panyu 4 sag and Enping sag, respectively (Fig. 2). Crude oil samples were mainly distributed in Enping, Zhuhai, Zhujiang-shang, Zhujiangxia, Zhujiang and Hanjiang formations.
Source rock chloroform extracts and crude oils were sepa-rated into saturated hydrocarbon fractions, aromatic hydrocar-bon fractions and non-hydrocarbon fractions using the saturate silica gel/alumina column chromatography.
The saturated hydrocarbon fractions were analyzed using an Agilent 7890A GC/5975C mass spectrometer (GC-MS) equipped with a VF-5 ms capillary column (60 m×0.25 mm×0.25 μm). The gas chromatography (GC) oven tempera-ture was held at 50 ℃ for 1 min and programmed from 50 to 120 ℃ at 20 ℃/min, then from 120 to 310 ℃ at 3 ℃/min, then maintained at this temperature for 25 min. Helium was used as a carrier gas. The mass spectrometer conditions were electron ionization at 70 eV. The GC-MS system was operated in the full scan mode and selective ion monitoring mode (SIM) of m/z=217 and m/z=191.
Partial m/z 191 mass chromatogram shows a series of terpanes, including C19-C26 tricyclic terpanes and C24 tetracyclic terpane. Novel compounds X and Y appear between C24 and C25 tricyclic terpane and after C26 tricyclic terpane, respectively (Fig. 3). Mass spectral characteristics of these compounds are listed in Fig. 4. The molecular ion peaks of compounds X and Y are m/z 330 and m/z 372, corresponding to the carbon num-ber of C24 and C27. Mass spectra of compound X have a base peak at m/z 191 and characteristic fragments at m/z 177, 315, 206, etc.. Moreover mass spectra of compound Y have a base peak at m/z 191 with characteristic fragments at m/z 357, 329, 219, 205, 177, etc.. According to the peak position in m/z 191 mass chromatogram and the mass spectra of these compounds reported (Samuel et al., 2010; Woodhouse et al., 1992), com-pounds X and Y are identified as the C24-des-A-oleanane and a new C27 tetracyclic terpane mentioned.
A strong positive correlation is observed between Y/(Y+C24) and OL in analysed oils and source rocks in this study (Fig. 5). Similarly, the ratio of X/(X+C24) and OL also have a good correlation. Compounds X and Y are similar to oleanane in the structure and their relative abundance in m/z 191 partial mass chromatogram have a good correlation with oleanane. Thus here a hypothesis that compounds X and Y are derived from the des-A degradation of oleanoid precursor is proposed.
Des-A-triterpenes have been suggested to be formed through photo-chemical or microbially-mediated oxidative loss of the A-ring from 3-oxygented pentacyclic triterpenes (Trendel et al., 1982). Unsaturated compounds such as α- and β-amyrin loss the oxygenated function at C(3) position in the first step in higher plant pentacyclic triterpene degradation (Jacob et al., 2007). It has also been assumed that photochemical alteration of 3-oxygenated triterpenoids contribute to the formation of des-A triterpenoids in natural environments (Simoneit et al., 2009). The loss of ring A is initiated by photochemical proc-esses from 3-oxy-triterpenoids (e.g., α-and β-amyrone) to the formation of 3, 4-seco-derivatives (Simoneit et al., 2009). Here compounds X and Y are considered to derive from the degrada-tion of oleanoid precursor (e.g., α- and β-amyrin or α- and β-amyrone) through microbial and/or photo-chemical media-tion. The precursor of oleanoid compound loss ring A to form 3, 4-seco-olean-12-en-3-oic acid under photochemical control. Then 3, 4-seco-olean-12-en-3-oic acid is converted to com-pounds X and Y through further degradation.
There are two main source rocks of different sedimentary facies distributing in Zhu 1 depression, namely Wenchang For-mation middle-deep lacustrine source rocks and Enping Forma-tion coal measures source rocks. Wenchang Formation middle-deep lacustrine source rocks mainly derive from planktonic algae organic matter having low abundance of compounds X and Y. High abundant compounds X and Y exist in Enping Formation coal measures source rocks with terrigenous inputs in partial m/z 191 mass chromatogram (Fig. 3). The abundance of compounds X and Y is closely related to the source of or-ganic matter and it can indicate the input of terrigenous organic matter.
Various biomarker parameters of analysed oils and oil families are shown in Table 1. On the basis of sterane and triterpane biomarker compositions, the analysed oils can be grouped into three families, namely types A, B, and C.
Type-A oil mainly originated from the middle-deep lacus-trine source rock in Wenchang Formation.
Type-B mixed oil came from the middle-deep lacustrine source rocks in Wenchang Formation and Enping Formation coal measures source rocks.
Type-C oil was mainly derived from Enping Formation coal measures source rocks with terrigenous high plants inputs.
A good correlation is observed between the ratio of X/(X+C24) and Y/(Y+C24). Figure 6 reflected that X/(X+C24) and Y/(Y+C24) ratios of three types of crude oils are roughly distributing in three different ranges. X/(X+C24) ratio value of Type-A oil rang from 0.19 to 0.42, Type-B oil rang from 0.27 to 0.61, Type-C oil rang from 0.72 to 0.81. The ratio of X/(X+C24) and Y/(Y+C24) increase with the increasing inputs of terri-genous organic matter in crude oils.
Three types of crude oils derived from different sources have different abundance of compounds X and Y. The abundance of peaks X and Y of crude oils in partial m/z 191 mass chromatogram gradually increases from Type-A to Type-C (Fig. 7). Type-C oil with terrigenous high plants or-ganic matter inputs has the highest abundance of compounds X and Y in partial m/z 191 mass chromatogram.
And likewise, the ratio of Y/(Y+C24) also has a certain re-lationship with two parameters C19/C23 and T/C30H (Fig. 8). The C19 tricyclic terpane was thought to originate mainly from high plants (Peters et al., 2005). Bicadinanes (W, T) appear to indicate a resinous polymer input from high plants and they are common in Southeast Asian oils and rock extracts (Van Aarssen et al., 1990). T/C30H and C19/C23 ratio can be used as effective parameters to reflect terrigenous organic matter inputs. T/C30H, C19/C23 and Y/(Y+C24) ratios increase with increasing terri-genous organic matter inputs in crude oils. In general when two terrigenous compounds C19 tricyclic terpane and bicadinane-T are high in crude oils, there are also corresponding high abun-dant compounds X and Y.
Figure 8b shows that T/C30H ratio value vary greatly in Type-C oil while their Y/(Y+C24) ratio value is high. This is mainly because that two Type-C oil samples were from Enping Formation source rocks in different regions (Fig. 2). HZ19-2-1 well is located in the western Huizhou depression. Enping Formation source rocks and the oil derived from it in this re-gion contain high abundant bicadinane-T and C19 tricyclic ter-pane. C19/C23 ratio value of Enping Formation source rocks in the western Huizhou depression rang from 0.8 to 2.2. T/C30H ratio value rang from 1.1 to 3.5. However, HZ9-2-1 well is located in the northern Huizhou depression. Enping Formation source rocks and the oil derived from it in this region also have high abundance of C19 tricyclic terpane. But Enping Formation source rocks in this region have lower abundance of bicadinane-T than Enping Formation source rocks in western Huizhou depression. C19/C23 and T/C30H ratio value of Enping Formation source rocks in the northern Huizhou depression is 3.5 and 0.7, respectively. Different abundance of bicadinane-T of Enping Formation source rocks in different regions lead to T/C30H ratio difference in Type-C oil.
(1) Compounds X and Y are detected in source rock extracts and crude oils in Pearl River Mouth Basin. According to the peak position in m/z 191 mass chromatogram and mass spectra of these two compounds reported in the literature, compounds X and Y are identified as the C24-des-A-oleanane and a new C27 tetracyclic terpane.
(2) Here compounds X and Y are thought to be derived from the des-A degradation of oleanoid precursor.
(3) The abundance of compounds X and Y in crude oil and source rocks is closely related to the source of organic matter. High abundant compounds X and Y indicates the input of ter-rigenous organic matter.
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Ning MA, Dujie HOU, Hesheng SHI. Novel tetracyclic terpanes in crude oils and source rock extracts in pearl river mouth basin and their geological significance. Journal of Earth Science, 2014, 25(4): 713-718. doi: 10.1007/s12583-014-0468-9
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