Formation Environment Of The High Quality Source Rocks In Basin OfDepression-type

Hydrocarbon-rich depression is the key to the formation of large oil and gas fields. There are already many researches about the faulted-down depression in east of China. The Triassic Yanchang Period hydrocarbon-rich depression in Ordos Basin is an important depression in west of China and meanwhile it is depression filling. However, fewer studies about it have been done. The exiting researches focus on tectonic environment, bottom topographic, geothermal field, thermal evolution, catastrophic events and so on, most of which are related about hydrocarbon accumulation and limited to respective study area. This paper selects the Triassic Yanchang Period hydrocarbon-rich depression in Ordos Basin as research object, takes geochemical Characteristics as research emphasis, recoveries the dynamic process of organic deposition, and reveals the forming rules of high quality source rocks so as to guide the petroleum exploration and perfect the theories of hydrocarbon depression.This paper summarized previous achievements about structural activity, material source, distribution of sedimentary facies, depositional sequence and volcanic hydrothermal events, analyzed inorganic and organic geochemistry of the source rocks in order to discuss the sediment environment and productivity level, and proposed freshwater stratified lake modal for the forming of high quality source rocks on this basis.According to element geochemistry results, Chang-7 excellent source rocks are rich in P and Fe, but poor in Ca, Mg and Na in major elements, which shows the prosperity of the organisms. Most of trace elements are rich in Chang-7 excellent source rocks, especially for As, U and Mo. The amount of REE is also high on the whole and its distribution patterns are characterized by rich in LREE, flat in HREE and obviously negative Eu abnormity. The ratios of V/V+Ni, Th/U, B/Ga,δU and Ceanom show that Chang-7 excellent source rocks were deposited in reductive fresh water. In addition, trace fossils and quantitative calculation show that water of the palaeo-lake was semi-deep and deep, which can form thermocline in the middle. Therefore, deep area below the thermocline in the lake was more reductive and rich in organic matter, and the condition was worse in semi-deep water.The analysis results of source rocks organic-geochemical show that Chang-7 high quality source rocks are characterized by high abundance of organic matter and are mainly type Ⅰ-Ⅱ The organic matter was mainly originated from algaes and is in the stage of low mature to mature now. Meanwhile, the excellent source rocks consist of two types, one is mudstone deposited in weak reducing environment with low TOC, low light hydrocarbon, low drimanes, high rearranged sterane, high 17a(H)-diahopanes, and the aaa20R homosteranes appear to be in "V" distribution, the other is laminated shale deposited in strong reducing environment with high TOC, high light hydrocarbon, high drimanes, low rearranged sterane, low 17a(H)-diahopanes, and the aaa20R homosteranes appear to be in "L" distribution.The Chang-7 palaeo-lake was located on the tropical zone or subtropical zone on the basis of authenticating pollen. On the condition of warm and humid climate, the algeos in the lake bloomed so that productivity was very strong and the lake was eutrophic lake to super-eutrophic lake. When there was volcanic eruption event, the volcanic ash provided a lot of nutrition for algeos so that they flourished all the time.Collisional orogeny of Yangtze plate and North China plate occurred in late Triassic, the Ordos Basin located at the back of the orogenic belt. It was deep and under compensation, planktons bloomed in the water above the thermocline, then deposited in the strong reductive bottom floor which was below the thermocline, thus organic rich laminated shales has been there. But in the semi-deep area, which was near the land, minerals from the land may destroy the organic matter, sedimental environment was not so reductive, so there was just mudstone with low TOC. When the collisional orogeny was violent, volcanic ash came into the lake water, planktons thrived in any time, they can create enough reductive environment to reserve, so there were still high quality source rocks and which were even better than organic rich laminated shales. Both the Chang-71 and Chang-72 formation were deposited in worse sedimental environment and there were lower productivity so that there were less excellent source rocks.