The Discovery Of Cretaceous Marine Oil Shale In The Qiangtang Basin And Its Petroleum Geological Significance

The Mesozoic Qiangtang Basin is regarded to be of the largest oil and gas resource potential and the most hopeful exploration area on the Qinghai-Tibet Plateau. Recent years, the largest marine oil shale zone in China was found in the Shengli River, Changliang Mountain, Changshe Mountain and Tuonamu regions in the Qiangtang Basin, which was called the North Qiangtang Basin Oil Shale Zone. In addition, overlying the oil shale, where generally deposited low porosity and low permeability gypsum. This discovery, on one hand, found plenty of oil shale resources, and on the other hand, confirmed that there were good regional seals in the Qiangtang Basin and solved the problem of bad regional sealing preservation condition.In this paper, we focus on the following several research works, including1) the chronostratigraphy, stratigraphic distribution, the distribution of sedimentary facies, depositional system and the lithofacies palaeogeography characteristics during the Cretaceous marine oil shale series deposition;2) the palaeoclimate and palaeoenvironment during the Cretaceous marine oil shale series deposition;3) the mineralogy and organic geochemistry of the marine oil shale;4) the control factors and the sedimentation model of the marine oil shale series.The results are listed as follows:(1) The Cretaceous oil shale series were mainly deposited in the Upper part of the Suowa Formation (Fm). Through regional stratigraphic classification and correlation, it reveals that the Upper part of the Suowa Fm, Bailongbinghe Fm and Xueshan Fm were the contemporaneous and heteropic deposits. The Bailongbinghe Fm was rich in ammonites and distributed in the northeastern part. The Xueshan Fm mainly outcropped in the edge of this basin. Additionally, the early Cretaceous sporopollen and rich marine dinoflagellates were first found in the Anduo region of South Qiangtang Basin, which would be of important geological significance for the study of sedimentary tectonic evolution of the Qiangtang Basin.(2) During the Cretaceous oil shale series deposition, there were mainly three depositional systems, including:fluvial-delta depositional system, gulf depositional system and neritic platform depositional system, and they could be further subdivided into delta facies, fluvial facies; tidal flat facies, lagoon facies and evaporate facies; shelf facies and carbonate platform facies, respectively.(3) According to the quantitative analysis of the sporopollen fossils of the Upper part of the Suowa Fm in the Shengli River, Bandao Lake, Nadi Kangri, Longwei Lake and Tuonamu sections, and of the Xueshan Fm in the Wenquan section, it shows that the climate during the marine oil shale deposition period was warm and humid, while the climate during the gypsum and marl deposition overlying the oil shale was hot and arid. Furthermore, this vertical changed climate was universal in the whole Qiangtang Basin.(4) We selected the Changliang Mountain section and Shengli River oil shale section as study objects for trace and rare elements analysis. The result shows that during the marine oil shale deposition, the palaeoenvironment was anoxic to dysoxic reductive environment. However, the palaeoenvironment was mainly oxic during the non-oil shale deposition.(5) According to the analysis of the common organic geochemical index (TOC, Pr/Ph and C35hopane index), it revealed that the TOC content of the marine oil shale was high, which generally reflected the anoxic depositional environment. The Pr/Ph ratio of the oil shale was almost less than1.0, showing phytane advantages, and revealed a reductive and anoxic depositional environment. While during the non-oil shale deposition, the sedimentary environment changed to be oxic. In the Pr/Ph-Pr/nC17-Ph/nC18triangular diagram, most of the oil shale samples were dropped into the salt lake area, illustrating high salinity and reductive sedimentary environment, which was consistent with the C35hopane index determination result.(6) The TOC content of the Shengli River oil shale ranges from4.31%to21.37%, with an average of8.40%; the TOC content of the Tuonamu oil shale is also very high, and the maximum is25.68%, with an average of9.32%; the TOC content of the Changliang Mountain oil shale varies from2.96%to23.47%, with an average of9.56%; and the TOC content of the Changshe Mountain is between4.53%and9.49%, with an average of7.74%. The Changshe Mountain oil shale is mainly of type Ⅲ and a few type Ⅱ2and Ⅲ kerogen; the Shengli River oil shale is mainly type Ⅲ and type Ⅱ2kerogen, while the Tuonamu oil shale is mainly type Ⅲ and type Ⅰ kerogen. (7) The control factors of the development of the Cretaceous marine oil shale were analysed, including:the redox state, palaeo-productivity, paleoclimate, the mixed deposits with clay mineral, differential palaeotopography and the sea level change. There is a significant positive relationship between TOC and palaeoenvironment element index Mo (r=0.975) and V (r=0.917), while TOC shows a weak and negative relationship between the palaeo-productivity index P/Al (r=0.481) and Ba/A1(r=-0.739). Therefore, this paper proposes that the formation of the Cretaceous marine shale is mainly of preservation mode. The water environment during the organic matter deposit played a leading role in the formation of high TOC content in the oil shale. The gypsum overlying the oil shale was controlled by evaporated lagoon facies in the lateral and controlled by the palaeoclimate condition in the vertical.