Depositional Setting And Enrichment Mechanism Of Organic Matter Of Niutitang Formation And Longmaxi Formation In The North Of Guizhou Province

In the exploration of marine shale gas in South China, two preferred horizons were found: Niutitang Formation of Lower Cambrian and Longmaxi Formation of Lower Silurian. In practical research, we found it ubiquitous that the content of total organic carbon(TOC) in Niutitang Formation was higher than that in Longmaxi Formation. To study the difference of the richness in organic matter(OM), we chose samples of Well Renye-1 and Well Xiye-1 from the north of Guizhou Province. The mineralogy and trace element geochemistry of the two black shales were studied in order to understand the depositional environment including paleoclimate conditions, redox conditions, the influence of materials came from deep sea and ascending current.Evidence from paleomagnetism confirmed that both of these two black shales were sedimented under subtropical- tropical climate environment. Sr/Cu of samples from Niutitang Formation was 0.465 ~ 8.715, its average was 2.69. Sr/Cu of Longmaxi Formation was 1.860~16.136, with its average be 5.205. The paleoclimate of early Cambrian was more humid, which was good for the richness of OM.We found framboidal pyrite in the two series, which means both these two series were formed in reducing condition. V/Cr of samples from Niutitang Formation was 0.922~10.650, its average was 3.363. V/Cr of Longmaxi Formation was 0.904~8.767, its average was 2.464. The reducing condition of early Cambrian was stronger than that of early Silurian, so the OM was preserved in a better environment. Besides this, it was inferred other factors may lead to its richness in OM.The richness of some trace elements and Platinum Group Elements has proved that the two series were both interfused with material came from deep sea. Co/Zn of samples from Niutitang Formation was 0.007~0.347, its average was 0.098. Co/Zn of samples from Longmaxi Formation was 0.015~0.315, its average was 0.153. The Niutitang Formation was influenced stronger compared with the Longmaxi Formation. The material can promote the development of OM, which accounts for the larger TOC in Niutitang Formation.Substance in deep sea can be deliverd to the continental shelf with the help of the ascending current. So geochemistry datas of samples from continental shelf may reflect that it came from deep sea. The current was confirmed when we analysed the datas of REE of the samples from Well Renye-1, so it was richer in OM than samples from Longmaxi Formation.