Geochemical characteristics and possible sources of crude oils in the Baiyun deep-water area of the Pearl River Mouth Basin, South China Sea

Two classification methods, the distribution of biomarkers and the alternating least squares (ALS) chemometric analysis were used to investigate the genetic characteristics of oils collected from the Baiyun deep-water area of the Pearl River Mouth Basin in the South China Sea. Results of the two methods were generally consistent that most of the oils were originated from the shallow lacustrine source rocks, but oils from the West Sub-Sag (WSS) and the South Sub-Sag (SSS) have a distinct geochemical characteristic with low abundances of bicadinanes and oleanane, and relatively lighter delta C-13 value (<-28.0 parts per thousand). The ALS chemometric analysis can quantitatively determine the sources of the oils. It shows that mixing has commonly occurred between the oils, and three end-member oils were identified (i.e., EM1, EM2, and EM3), which were derived from the semideep-deep Wenchang Formation source rocks that are characterized by a predominance of C-27 steranes and low concentrations of bicadinanes and oleanane, the shallow lacustrine Enping Formation source rocks in the Main Sub-Sag (MSS) that are rich in bicadinanes but depleted in oleanane, and the shallow lacustrine Wenchang and/or Enping formations source rocks in the East Sub-Sag (ESS) that have a relatively high abundance of bicadinanes, a moderate content of oleanane, and a predominance of C-29 steranes, respectively. The oils from the Panyu Low Uplift (PLU) are dominated by 83% of EM2 oil. Oils from the SSS and WSS are dominated by EM1 oil (68.0%-93.1%). Oils from the ESS and eastern part of the MSS (E-MSS) are mixed oils consisting mainly of EM2 and EM3 oil, with an increasing proportion of EM3 moving from the E-MSS to central ESS. The maturity of the oils was assessed based on diamondoid indices, and oils from the PLU have the highest maturities (1.43%-1.59% Rc), whereas oils from the SSS have the lowest maturities (1.23%-1.25% Rc), corresponding to the different maturities of their source rocks. The semideep-deep lacustrine oils discovered in the WSS and SSS highlight the hydrocarbon generation potential of these types of source rocks in the Wenchang Formation, where further oil exploration should be focused

Discussion on integrated geological and geophysical identification method for spatial distribution of favorable source rocks in depression with lack of drilling data: a case study of Enping 17 Sag, Zhu Ⅰ Depression, Pearl River Mouth Basin

Due to the lack of drilling data in the Zhu 1 Depression of the Pear River Mouth Basin, the Enping 17 Sag is taken as an example in this paper to solve the problem of comprehensive identification of underground distribution of high-quality source rocks. Based on the analysis of the geological background, sedimentary characteristics and petrophysical characteristics of source rocks, seismic inversion is guided by the constraints of the petrophysical characteristics of the source rocks in different sedimentary environments. According to the statistics of the relationship between the lithofacies of drilled source rocks and the inversion resistance, a semi-quantitative prediction of the spatial distribution of favorable source rocks is made by geological modeling, and the rationality of prediction method and results is analyzed in combination with the recognition of the geochemical origin type and source of crude oil, to discuss the geological and geophysical identification method of the underground distribution of high-quality source rocks with lack of drilling data. The more reductive the depositional environment of source rocks in the Paleogene Wenchang Formation, the lower the impedance value of high-quality source rocks, and the greater the impedance difference between poor source rocks or non-source rocks. The third, fourth and fifth members of Wenchang Formation are the main source rocks of the Enping 17 Sag, which can be supported by the study of crude oil biomarkers. In the small sags of the Pearl River Mouth Basin with basically clear petroleum geological background conditions, establishing the quantitative and semi-quantitative relationship between the lithofacies of source rocks and the inversion impedance value is helpful to quickly clarify the development interval and scale of main source rocks. This method has a good prospect of popularization and application

Geochemical characteristics of light hydrocarbons in condensate oil from the southwestern Huizhou of the Pearl River Mouth Basin

The southwestern Huizhou is an important crude oil enrichment area in the Zhu Ⅰ Depression of the Pearl River Mouth Basin. However, condensate oil and gas have been discovered in the upper memberof Neogene Zhujiang Formation, Paleogene Enping and Wenchang formations, and basement buried hill reservoirs in recent years. In this study, the origin of condensate oil was revealed based on the integration of light hydrocarbons and biomarkers characteristics, which were obtained from GC of whole-oil and GC-MS of saturated fractions.The results, including light hydrocarbon composition, C7 is o-alkanes, naphthene, n-alkanes and other biomarkers indicate that the condensate oil in the H6-1 structure came from the semi-deep to deep lacustrine source rock in Wenchang Formation, which was generated by sapropelic kerogen with relatively high maturity (1.2%-1.3% Ro). In contrast, the condensate oil in the H1-1 structure originated from the shallow lake to peat facies source rock in Enping Formation, the oil was rich in alkanes with low-carbon number which was generated by humic kerogen with relatively low maturity (0.9%-1.1% Ro).The results suggest that the formation of condensate oil in the southwestern Huizhou is both controlled by organic matter type and thermal evolution stage of source rock. This study provides a geochemical basis and support for the evaluation and analysis of condensate oil and gas exploration potential in hydrocarbon-rich depressions