Research In Characteristic Of Deep Lacustrine Sediment And Tuff Deposits Of Yanchang Formation, Upper Triassic In Ordos Basin

Ordos Basin is the second largest petroliferous basin in China, the oil-bearing area, oil reserves and production of which are increasing year by year. Large oil and gas fields have been found almost distribute in the deep lacustrine and its marginal area.According to differences in topography of basin basement, palaeogeomorphology and features of sedimentary systems in different periods, the paper expounds detailed the features of deep lacustrine sediments and their distribution ranges. Plenty hydrocarbon source rocks are generated in deep lacustrine sediments. Through a large number of documents and materials, the paper compares the geochemical indicators of organic matter of source rocks in Yanchang Formation, and analyzes the differences among main source rocks. As the result, the best source rocks are developed in Chang 7 oil layer group.By means of the analysis of outcrops, cores and logging data, the paper believes that there are 3 kinds of deep lacustrine gravity currents in Yanchang Formation in Ordos Basin, turbidity current, debris flow and Liquefaction flow, of which the turbiditie is the dominating reservoir in deep lacustrine sediments. The paper analyzes the petrological characteristics, sedimentary structures and types of turbiditie, and divides them into turbiditie with feeder channel and turbiditie without feeder channel. The turbiditie with feeder channel is the primary type in the basin, which can be divided into inshore turbidite fan and distal turbidite fan.According to petrological characteristics of tuff interbed, outcrops, cores and logging data, the paper discusses the properties and distributions of tuff, and believes that, in Chang 7 period, the distribution range of tuff is similar to that of oil shale. Through the identification of the sedimentary facies in joint profiles of tuff in the basin, the paper considers that tuff can develop not only in low-energy deep lacustrine environment, but also in high-energy delta and shallow lacustrine environment.Through the research on geochemistry of tuff interbed (major element, trace element and lanthanon), the paper thinks that it is sedimentary tuff, which was carried from crater by long way. Trace elements, such as Rb, Sr, Hf and La, are relatively enriched, but Y, Eu and Lu are relatively depleted. The REE pattern presents as a type of LREE enrichment and HREE loss. Negative anomaly of Eu appears in all the tuff samples, which could be caused by volcanic activity.