Geochemical Characteristics And Identification Of Mixed Crude Oil In The Pearl River Mouth Basin

The Pearl River Mouth Basin (PRMB) is one of the most important petroliferous basins in China, especially for the offshore oil. According to previous studies, three sets of source rock were found in PRMB, which is Wenchang formation of marine facies, Enping formation of marine-continental transitional facies and Zhuhai formation of marine facies. As the crude oil generated from source rock will migrate through complicated geological channel to the reservoir, thus the geochemical parameters in different crude oil samples will vary. However, several geochemical parameters were used in oil-source and oil-oil correlation commonly, which is too simple to completely represent the complex crude oil. During the process of oil exploration, the results about the oil-source and oil-oil correlation are beneficial for the evaluation of petroleum resources and determination of exploration strategies. Thus, the research on the geochemistry characteristics in crude oil and identification of mixed crude oil are of great value for the evaluation of petroleum resources and determination of exploration strategies in PRMB.In this study, the corresponding pretreatment and analysis method was used according to different types of compounds in crude oils. Based on the reliable analyzed result, the PRMB comprehensive evaluation system (CES) had been established by using chemometrics to screening significant in numerous geochemical indexes. With the developed model, the crude oils were easily discriminated into three oil classes, including Enping Formation, Wenchang Formation and mixed oils generated from Enping Formation and Wenchang Formation. Finally, de-convolution of the mixed oils had been done. Consequently, this thesis will offer a new technique for establishing CES and de-convolution of mixed crude oil in PRMB. The main contents and results were as follows:(1) According to different types of compounds used in this study, the corresponding pretreatment and analysis method was used, thus the data is reliable. Moreover, a novel and high efficient method using GP-MSE to pretreated oil samples for the determination of diamondoids was established. The method provides a faster (7 min), low-cost, solvent-saving technique (0.5mL n-hexane), more sensitive and environmentally friendly method than traditional methods. The limit of detection (LOD) for adamantane and diamantane in crude oil was 64ng/g and 88ng/g, respectively, and the relative standard deviation (RSD) was 1.57% and 2.63%, respectively. In addition, satisfactory recovery and repeatability were obtained and the RSD about several relevant geochemical parameters in crude oils were all below than 8%. The method provides some advantages over the widely used column chromatography and it can also be extended to the analysis of other components in crude oils and other complex samples.(2) The Wenchang and Enping source rock were deposited in different environments and have significant different biomarker assemblages. Oils from the lacustrine shales and mudstones of the Eocene Wenchang Formation have significant abundant with sesquiterpenes, C30-4-methylsteranes,2-methylhopane, methyltrimethyltride cylchromans(MTTCs), 9-methylphenanthrene, dibenzothiophene, benzohopanes, triaromatic steroid and methyltriaromatic steroid, and with relatively few bicadinanes, adamantanes(0.43μg/g), terphenylene, oleanane and 3,3’-dimethylbiphenyl. Oils from the shallow lacustrine-deltaic coal-bearing sequence of the Eocene-Oligocene Enping Formation have significant abundant with pristine, naphthalene, terphenyl, bicadinanes,1,2,5-trimethylnaphthalene,1,2,7-trimethylnaphthalene, C19 tricyclic terpane, oleanane and adamantanes(17.21μg/g), however, with relatively few sesquiterpenes, C30-4-methylsteranes,2-methylhopane, benzenehopane, triaromatic steroid, methyl triaromatic steroid and MTTCs.(3) The PRMB comprehensive evaluation system(CES) had been established.36 geochemical parameters were selected by Chemometric techniques. In this system saturated hydrocarbon, aromatic hydrocarbon and diamondoids were considered, which might can completely represent the complex crude oil in sedimentary environment and material sources.(4) Established the de-convolution model for the mixed oils in PRMB. Standard discriminant analysis (SDA) and principal component analysis (PCA) were used to distinguish the oil sources. With the CES and developed model, the crude oils were easily discriminated into three oil classes, including Enping Formation, Wenchang Formation and mixed oils generated from Enping formation and Wenchang formation. The overall correct classification rate is 97.8% for SDA and 100% for the PCA. Finally, the de-convolution model had been established using CES and the classification result and all mixed oils had been calculated.(5) The de-convolution result show that the contribution of the Enping Formation source rock in PRMB can not be ignored. Based on the CES and classification result, all mixed oils were deconvolved. And result shows that:nearly half of mixed oil samples got lower contribution of Enping Formation source rock(7 samples,<30%),,4 mixed oil samples have between 30-50%, and other 5 mixed oil samples have over 50%. Combined with the distribution and contribution, we infer that Qionghai uplift and Huibei half graben to Huizhong half graben may be a prospective area for the Wenchang source and Enping source, respectively. All this indicated that the contribution of the Enping Formation source in PRMB had great potential.