| Zhuyi depression is a NE-SW trend depression located in the eastern of the northern depression belt in Pearl River Mouth Basin, and covers an area of about 4.2×104km2. The tectonic setting of Zhuyi depression changed from the andean-type continental margin to passive continental margin during late Cretaceous. Zhuyi depression experienced multistage tectonic movement since the Cenozoic, chronologically Shenhu movement, Zhuqiong movement, Nanhai movement, Baiyun movement and Dongsha movement. These tectonic movements could be divided into early Tertiary rifting stage and late Tertiary post-rifting stage by the deposition period of Zhuhai Formation. During Eocene to early Oligocene, Zhuyi depression was continental rift lake basins and two sets of source rocks developed:Eocene Wenchang Formation and Eocene-Oligocene Enping Formation. From late Oligocene, Zhuyi depression began to accept post-rifting deposition of Zhuhai, Zhujiang and Yuehai Formation. As the sandstone/shale interbeddings are common in Zhuhai Formation and Zhujiang Formation, the settings of source rock, reservoir and caprock of the two formations are good. Focusing on the source rock and oil from different regions of Zhuyi depression, the geochemical characteristics and distribution of source rocks were discussed, the factors that affect formation of source rock, such as tectonic subsidence, paleo-climate, structural subsidence and deposition rate, sedimentary organic matters sources and storage were studied and the sedimentary model of the two kinds of high-quality source rocks were proposed. Secondly, the oils in research area were classified based on the geochemical characteristics, and oil-sources correlation was done to find out the source of oils. Two distinct end-member oils were selected to represent the oils derived from the source rocks in Wenchang and Enping formation, respetively. Then artificial end-member oil mixing experiment was done and the plate that can determine mixed proportion was established. Lastly, the contribution of the two sets of source rocks to hydrocarbon accumulation in Zhuyi depression was investigated in detail. As a supplement, the relationship of single oil sample and geological reserves and the sources of oils in Enping sag were discussed separately. From the above research and discussion, the results that the thesis obtained are as follows;1. Geochemical and distribution characteristics of source rocks in Wenchang Formation and Enping FormationThe organic matter abundance of the two source rocks is different. The TOC, S1+S2 and chloroform bitumen "A" in Wenchang Formation are between 1.07～4.74%,1.18～21.42mg/g and 0.32～0.67% with means of 2.58%,12.76mg/g and 0.55%, respectively. The above parameters of source rocks in Enping Formation are between 1.78～4.53%,3.92～10.32mg/g and 0.26～0.40%, with means of 2.81%、6.45mg/g and 0.30%, respectively. The TOC of Enping Formation is higher than that of Wenchang formation, but its S1+S2 and chloroform bitumen "A" are lower than Wenchang Formation’s. According to lacustrine and coaly source rock evaluation criteria, the source rocks of Wenchang and Enping formations are both high-quality source rocks. The kerogen type of Wenchang source rock is mainly type Ⅱ1～Ⅰ, with a small amount of type Ⅱ2, sedimentary facies is middle-deep lacustrine facies, sedimentary organic facies is "A" and its Ro range from 1.3% to 1.8% indicating high maturity stage. The kerogen type of Enping source rock is mainly type Ⅱ2～Ⅲ and a few type Ⅱ1, sedimentary facies is lake-swamp facies, sedimentary organic facies is "C" and its Ro ranges from 0.7% to 1.5% indicating a maturity stage.The compositions of biomarkers in the two sets of source rocks are different. The parameter values of Pr/Ph, C19～C22/C23～C26-tricyclic terpanes, bicadinanes/C3o-hopane, relative content in biphenyl, dibenzofuran/dibenzothiophene in Wenchang Formation are between 0.91～5.48, 0.64～1.91,0.01～0.99,0.60～2.51% and 0.08～4.95 with means of 2.29,1.22,0.21,1.49% and 0.54, respectively. These parameters of Enping source rock are between 0.58～10.20,0.60～6.83, 0.15～9.75,3.24～6.23% and 1.24～15.23 with means of 4.99,2.55,0.99,4.73% and 2.70, respectively. Obviously, the parameter values of Enping source rock are higher than of Wenchang source rock. But the C30-4-methyl steranes/C29-regular steranes of Enping source rock is lower than of Wenchang source rock. The former ranges from 0.08 to 0.21 with mean of 0.14 and the later ranges from 0.11 to 0.26 with mean of 0.18.The biomarker composition and sedimentary facies indicate that the source rock of Wenchang Formation was deposited in reductive middle-deep lacustrine facies and its organic matter sources is mainly algae and lower aquatic organisms with some terrestrial higher plants. The source rock of Enping Formation was deposited in weak oxidizing lake-swamp facies and its organic matter source is mainly terrestrial higher plants.The thickness of Wenchang and Enping formations has the features in three aspects:Firstly, the formation thickness is controlled by sedimentary sag. The thicknesses of Wenchang Formation in LF13N sag, HZ26 sag, Xijiang main sag and EP17 sag are thicker than in other sags. The thicknesses of Enping Formation in HZ5 sag, XJ24 sag, HZ8 sag, Xijiang main sag and EP17 sag are thicker than in other sags and the lateral continuity is better than Wenchang Formation. Secondly, from Wenchang period to Enping period, subsidence and sedimentary center of Zhuyi depression had migrated from the southeast of the depression to the middle of Huizhou sag. And most of the sags formed during Wenchang period were less developed during Enping period. Thirdly, some of the sags formed in Wenchang period still developed during Enping period, such as EP17 sag and Xijiang main sag. According to the characteristics of sedimentary facies, the middle-deep lacustrine source rock of Wenchang Formation is mainly distributed in LF13N sag, HZ26 sag, Xijiang main sag and EP17 sag. The lake-swamp coal-measure source rock of the Enping formation is mainly distributed in HZ8S sag, XJ24 sag and EP17 sag.2. Development conditions, controlling factors and sedimentary model of high-quality source rocks in Wenchang Formation and Enping FormationMiddle-deep lacustrine source rocks in Wenchang FormationDeposition period of Wenchang Formation was the peak period of rifting stage, during which the NE-NEE and NWW faults were developed that led to the forming of "half graben" or "graben" structure. Wenchang Formation was developed only in the negative structure units. Sporopollen composition of mudstone indicates the paleo-climate of this period was humid and semi-humid subtropical to temperate climate. The value of tectonic subsidence rate (100-150m/Ma) and sedimentation rate (200-400m/Ma) is high. Higher tectonic subsidence rate provided enough space and depth for lacustrine basin and formed temperature stratification. Deposition rate versus TOC indicated that high deposition rate (>200m/Ma) is helpful to the formation of high-quality lacustrine source rocks.Microfossil contents show that the source of organic matters in Wenchang Formation is mainly algae and lower aquatic organisms (the proportion is about 30～50%), in which thepediastrum and pluvialis are most. C27-regluar steranes and C29-regluar steranes accounted for about 25.22～44.34% and 35.19～51.99%, respectively. C27-C29-aaa20R-regluar steranes distributed in a "V" shape. Microfossil content and steranes composition indicate that organic matter source of Wenchang Formation is mainly algae and lower aquatic organisms. The value of Pr/Ph ranges from 0.85 to 5.45, with a mean of 2.29. The relative content of fluorenes, dibenzothiophene and dibenzofuran were 22.99%,49.92% and 27.09%, respectively. The main composition is dibenzothiophene, and the content of fluorene is slightly lower than dibenzofuran. The Pr/Ph and composition of three "fluorene" show that the Wenchang source rock formed in fresh middle-deep lacustrine facies with reductive sedimentary environment, which formed good storage conditions of organic matters.Lake-swamp coal-measure source rocks in Enping FormationEnping period was in the recession of rifting, and the faults became weak. Deposition was no longer limited within the fault’s footwall, but also appeared in the higher structures. Sporopollen composition of mudstone indicates paleo-climate of this period was humid to semi-humid tropical-subtropical climate. Compared with Wenchang period, the tectonic subsidence rate (50～100 m/Ma) and sedimentation rate (50～300 m/Ma) decreased significantly. Deposition rate versus TOC indicates that deposition rate (100～300m/Ma) is beneficial to the formation of coal-measure source rock. Woody pollen and ferns account for about 50～70% and 30～40% of microfossil in Enping source rock, respectively, but the contribution of freshwater algae is little. C27-regluar steranes and C29-regluar steranes account for about 12.03～35.26% and 46.23～79.52%, respectively. C27～C29-αααa20R-regluar steranes distributed in reversed "L" shape. Biphenyl series accounts for 4.73% of total aromatics, much higher than that of Wenchang formation (1.49%). Microfossil, steranes and the composition of biphenyl series indicate that the organic matter source of Enping formation is mainly terrigenous higher plants. The value of Pr/Ph ranges from 1.05 to 10.15, with a mean of 4.99. Relative content of fluorene, dibenzothiophene and dibenzofuran is 11.54%, 23.89% and 64.57%, respectively, suggesting the source rock in Enping Formation formed in lake-swamp facies sedimentary environment, and the abundant organic matter supply made up for the deficiency of the preservation condition.According to the tectonic background, paleo-climate conditions, tectonic subsidence and deposition rate, the source of organic matters, and preservation conditions, the formation model of high-quality source rocks in Wenchang and Enping formations can be summarized as "owe compensation mode", which is high sedimentation rate in the humid climate of freshwater deep lake facies, and "balance compensation mode", which is moderate sedimentation rate humid climate limnetic facies".3、Geochemical characteristics and classification of oils, and the genetic relationship between oils and source rocks.The crude oils of Zhuyi depression are mainly come from Huizhou sag and its south edge, Panyu 4 sag, Huilu low uplift and Enping sag. Density, viscosity, carbon isotopic composition and group composition of oils from different regions are similar. Thus the oils in Zhuyi depression can be studied as a whole.The crude oils in research area can be divided into three classes based on the composition of biomarkers. The first class is mainly distributed in the Huilu low uplift, Panyu 4 sag and Liuhua uplift, including oil HZ33-1-1, oil LF13-1-1 and oil PY10-2-1. The second class is distributed in the north of Huizhou sag, represented by the oil HZ9-2-1. The third class is mainly distributed in the Huizhou sag and its south edge and the Enping sag, including oil HZ26-1-1, oil XJ23-1-4 and oil EP24-2-1. Values of C19～C22/C23～C26-tricyclic terpanes, Ci9/C23-tricyclic terpane, bicadinanes/Cio-hopane, tricyclic terpanes/pentacyclic terpanes, tricyclic terpanes/Cjo-hopane and pristane/phytane in the oils of first class are 0.54、0.39、0.14、0.17、0.45 and 2.12, respectively, which is lower than the oils of second class with values of 10.79、21.76% 0.71、0.30、 1.18 and 4.83. But the C3o-4-methyl steranes/C29-regular steranes of the second class is 0.08, which is much lower than the 1.02 of the first class. The values of the above 7 parameters in the third class are between those of the first and second class. In the physical properties of the oils, the density and viscosity of the first class are 0.86g/cnr1 and 18.85 mPa s, higher than that of the second class (0.77g/cm3 and 2.17mPa·s). The whole oil carbon isotopic composition of the first class is range from -26.6%o to -25.5%o, with a mean of -26.03%o. The second class is -28.4%o, lower than the first class. The third class range from -28.4%o to -25.0%o, with a mean of -26.49%o, higher than the second class but lower than the first class.At the composition of n-alkanes, the first class is bimodal pattern with the front peak of C15 or C17 and back peak of C27, the second class is unimodal pattern with carbon peak of C14, the third class is unbalanced bimodal pattern with strong front peak of C15 or C17 and weak back peak of C27. The values of light n-alkanes/heavy n-alkanes are different in the three classes. Values of ∑nC21-/∑C22+and (nC21+nC22)/(nC28+nC29) in the first class is 0.94 and 0.98, lower than the 2.35 and 1.64 in the second class. The third class is about 1.56 and 1.39, between the first and the second class. The absolute biomarker concentration of oil varies obviously. The first class has higher biomarkers concentration than the second class. The absolute concentration of stanes and terpanes, sesquiteipenoids, tricyclic terpanes and C30-4-methyl steranes are 10.25, 2.85,0.69 and 0.29mg/g respectively in the first class. The second class is much lower with values of 1.08,1.62,0.18 and 0.003mg/g. The absolute concentration of bicadinanes in the second class is 0.013mg/g, and the first class is 0.031mg/g. The absolute concentration of above biomarkers in the third class is 2.98,2.68,0.29,0.14 and 0.054mg/g, higher than the second class but lower than the first class.Composition of aromatics, fluorene, dibenzofuran and dibenzothiophene has the following characteristics:The relative amount of phenanthrenc series and chrysene series in aromatics of the first class are 29.35% and 9.44%, higher than the 15.54% and 1.32% in the second class. The relative content of naphthalene, biphenyl, fluorene, dibenzofuran and dibenzothiophene series in the first class is 30.53% ,0.90% and 0.93%, lower than the 66.79%,4,73% and 2.09% in the second class. In the third class, the relative content of naphthalene, phenanthrene, biphenyl, three fluorene and chrysene series are 44.47%,27.77%,3.18%,1.52%,6.03% and 0.21% respectively, which is between the first and the second class. Through the comparison and correlation of the source rocks and oils, different figures were built, such as multi-parameter histograms that contain C19～C22/C23～C26-tricyclic terpanes and other six parameters, dimensional cross plot that contain (C19-tricyclic terpane+C2o-tricyclic terpane+C24-tetracyclic terpane)/C26-tricyclic terpane and drimane series/M/Z:123 and four triangle figures. The oil-source correlation results show that the first and second class oils are derived from source rocks of Wenchang Formation and Enping Formation, while the third class oil is mixed with the contribution of the both sets of source rocks.4、Relative contribution of Wengchang and Enping formation source rocks to hydrocarbon accumulationsThe main hydrocarbon accumulation period of Zhuyi depression is 10.5Ma up to current, which exhibits the characteristics of new geological time and short-time accumulation. Hydrocarbon from Wenchang and Enping formations charged almost simultaneously, which means the oils come from different source rocks accumulated during the same period. The physical properties, composition characteristics, biomarkers absolute concentration of oils and the formation process of typical oil trap show that the two oil samples of HZ33-1-1 (1990-2025m) and HZ9-2-1 (DST3) can be used as the end-member oils of middle-deep lacustrine source rocks of Wenchang Formation and lake-swamp coal-measure source rocks of Enping Formation, respectively.The empirical plate was drawn from the results of end-member oil mixing experiment together with seven parameters filtered by oil-source and can be used to evaluate the contribution of the two sets of source rocks. The seven parameters are concentrations of steranes, terpanes, C30-4-methyl steranes and drimane, drimane/5-methyl decahydronaphthalene, dibenzofuran/ dibenzothiophene, C3o-4-methyl steranes/C29-regular steranes and nC21-/nC22+. The impact of "volatile differences" on the plate has been corrected. The research shows that when the volatile difference is about 20%, the contribution of Enping Formation will be overestimated by 5%. According to the calculation results of correction plate, most of the oil samples sourced from the middle-deep lacustrine source rocks of Wenchang Formation only, such as the oils produced from Huilu low uplift, Panyu 4 sag and Liuhua uplift. Only part of the oil samples in the south of Huizhou sag and Enping sag has the contribution of Enping Formation. In the south of Huizhou sag, the contribution of Wenchang Formation ranges from 31% to 67% and in Enping Formation it ranges from 57% to 80%.Contributions of the source rocks to the oil sample and geological reserves are different. Reserves of every tectonic location are quite different.90% of the geological reserves of Zhuyi depression concentrate in LH11 reef beach tectonic belt and other 4 belts. According to the calculation results, the contribution of Wenchang Formation to the hydrocarbon accumulation in Zhuyi depression is about 86% with an absolute advantage, and the contribution of Enping Formation is about 14%. |
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