Element Geochemistry Evaluation Of Paleogene Source Rocks In Enping Depression

Pearl River Mouth Basin is one of the most important offshore exploration fields with notable achievements and constant newly liberated exploration fields and areas, and Zhu I Depression is a main source of the oil production in this Basin.Thus, it’s worth to enforce the further research of Zhu I Depression, especially its low exploration areas. As the present key exploration field in Zhu I Depression, Enping Depression has not get enough research on source rock, which is the basis of hydrocarbon accumulation. Lacking of source rock geochemical data, the source rock characteristics showed by the organic geochemical data are not clearly, and the source rock potential requires further identification, etc. So this paper is focused on the present research on source rock in Enping Depression and put forward the idea of evaluating it through the element geochemical method.The research is focused on Wenchang Formation and Enping Formation in Enping Depression. Depending on the sample test analysis on the drilling core and cutting sample of source rock, and being guided by sedimentary geochemistry and element geochemistry, the paper thoroughly summarizes the law of major element, trace element and the inorganic carbon and oxygen isotope content in the Paleogene strata of research area, and the sedimentary environment features and its significance, Then, use the rich hydrocarbon sag, Panyu4Sag, as the analogy object, set up the analogy standards of its rich hydrocarbon elements and sedimentary environment. The paper analyzes the potential of the source rock in Enping Depression by the standards, and some major conclusions and recognitions are shown as the following:1. The major element, trace element content are different, and the characteristics differed significantly between the Wenchang Formation and Enping Formation of Paleogene strata in Enping Depression by analysis of them. The major elements includeing Fe, K, Ca, Mg, Ti, P and Mn and the trace elements includeing Li, Be, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, Tl, Pb, etc, this represents different sedimentary environments and sediment sources. Meanwhile, almost all the the carbon isotopes value in Wenchang Formation in the research area are over-5%o while which in Enping Formation are less than-5%o. This indicates that the paleoproductivity of Wenchang Formation is higher than that of Enping Formation. The oxygen isotopes characteristic of the two formations shows little obvious difference.2. Based on the relationship of the inorganic carbon and oxygen isotopes, the paper gets that the lakes of Enping Depression in late Wenchang period showed a feature of half closed-half open while in Enping period was completely open. From the values and ratios of some major elements and trace elements, the paper gets that the paleobathymctry of Enping Formation in Wenchang period was deeper than which in Enping period. And, the sedimentary environment in both Wenchang period and Enping period was reducing environment, while most time of the Wenchang period was a strong reducing sedimentary environment. Meanwhile, the paleosalinity in Wenchang period was higher than which in Enping period. The paleoclimate in Wenchang period was warm and humid while which in Enping period was baking-hot, and the overall temperatures in Enping period was higher than that in Wenchang period. Based on the parameters of the rare earth elements La/Yb, ω (LRFE)/ω (HREE), REE, the paper reaches conclusions that the sediment sources of the lower Wenchang Formation of Panyu4Sag was mainly from Panyu low uplift and base, which was a kind of local sediment source. The features of the rare earth elements of Enping Formation in Enping Depression were different from that of the lower Wenchang Formation of Panyu4Sag. The sediment source of Enping Formation in Enping Depression was mainly from the South China fold belt in the Northern fault zone of Pearl River Mouth Basin, and it was a kind of regional sediment source. The sediment source of the upper Wenchang Formation of Hnping Depression was the transition of the sediment sources of the lower Wenchang Formation and Enping Formation, including both the sediment source supply of local Panyu low uplift and the South China fold belt.3. The palaeoenvironment analogy analysis suggests that the palaeoenvironment of Enping Depression in Wenchang period and Panyu4Sag in Wenchang period was similar, while the palaeoenvironment of Enping Depression in Enping period and Panyu4Sag in Wenchang period was quite different. The analogy analysis of the nutrition water system indicates that the water cutrophication of Enping Depression in Enping period was close to that of Panyu4Sag in Wenchang period, while it was better than Enping Depression in Enping period. At the same time, the water eutrophication in different sedimentary locations of Enping Depression in Enping period showed slight difference:the water eutrophication in the sag center was better than the sag edge, while that in the sag edge was better than in the sag periphery. Through the analogy analysis of the paleoproductivity and the organic burial efficiency of Enping Depression and Panyu4Sag, the paper reaches the conclusions as follow:the paleoproductivity and organic burial efficiency of Wenchang Formation in Enping Depression was the same with that of Wenchang Formation in Panyu4Sag, while the paleoproductivity and organic burial efficiency of Enping Formation in Enping Depression was lower than that of Wenchang Formation in Panyu4Sag and Wenchang Formation in Enping Depression. For the paleoproductivity and organic burial efficiency, only some parts of the stratas of Enping Formation in Enping Depression are relatively higher than that of Wenchang Formation in Panyu4Sag and Wenchang Formation in Enping Depression, or close to it.The analogy result comprehensively suggests that Wenchang Formation in Enping Depression had a comparable hydrocarbon generation potential to Wenchang Formation in Panyu4Sag; the overall hydrocarbon generation potential of Enping Formation in Enping Depression was lower than that of Wenchang Formation in Panyu4Sag. Meanwhile, the hydrocarbon generation potential of the sag center of Enping Depression was higher than that of the sag edge, and the hydrocarbon generation potential of the sag edge was higher than that of the sag periphery. Generally speaking, Enping Depression was a rich hydrocarbon generative depression.4. Evaluating the source rocks through element geochemical method provides new ideas for evaluating the nonmarine source rocks. The element geochemical method can be used to rebuild the palaeoenvironment in which the source rocks developed and to evaluate the hydrocarbon generation potential of the source rocks. Evaluating the source rocks through element geochemical method and the conventional organic geochemistry method can get results of consistency while there are differences. Therefore, the element geochemical method can be both supplement and reference to the conventional organic geochemistry method.