The Evalution Of The Advantageous Carboniferous Period Stratum For Carboniferous Period Stratum In Zhongwei And Aroud Area

Zhongwei and its adjacent exploration areas locate in the triangle connectingzone of the North Qilian orogenic belt, the Alxa plate and the North China block. Thesurvey region covers carboniferous layer with extraordinarily thick sediment. Ingeneral, it is in a period of high sea level and develops multi cycles of sea levelchanges and indicates the existence of source rocks. Based on the above information,it is necessary to carry out research on source rocks in the survey region which it alsomeaningful for guiding oil and gas exploration.The Carboniferous in the region are divided to Qianheishan Formation,Chouniugou Formation, Jingyuan Formation, Yanghugou Formation and TaiyuanFormation from the bottom to the top. The Taiyuan Formation belongs to layer-crossformation. The paper conducts researches on stratigraphic, sedimentology and sealevel changes of all carboniferous layers in the survey region on the basis of previousresearches and analyze the change rule of formation development, sedimentary facies’evolution, sea levels and source rocks characteristics. Then the paper summarize thechange rule of source rocks development and their characteristics. Based on theresearch, the paper divides development intervals of different source rocks in theregion into different levels.As to argillaceous source rocks, Early Carboniferous favorable sedimentary faceis Lagoon. The terrigenous clastic sediments is in the under-compensation state withless argillaceous sediments inputting. The argillaceous source rocks’ development haspoor material basis and low abundance of organic matter. The most TOC%distributesbelow the range of0.5%. The thickness of argillaceous source rocks, formation andresidual stratum is small. Although the organic matter belong to I type and is in overmature stage, most of these source rocks are invalid. The Early Carboniferous has poor developed argillaceous source rocks. The Late Carboniferous endogenouscarbonate deposits become thin and disappears. The terrigenous clastic sedimentsincrease and is in weak under-compensation or weak compensation, the material basisof argillaceous source rocks’ development become better. The favorable sedimentaryfaces is Lagoon, Deltaic plain and Shallow sea with widespread distribution. And theargillaceous source rocks has high abundance of organic matter with the averagevalue of TOC%exceeding1.5%. And the thickness of argillaceous source rocks,formation and residual stratum is big. The Late Carboniferous has better developedargillaceous source rocks than Early Carboniferous. From the Jingyuan Formationperiod to the Taiyuan Formation period, because of the sea level rising, the organicmatter abundance become higher. But the type of organic matter evolve fromⅠto Ⅲand the thickness of argillaceous source rocks, formation and residual stratum becomesmall make the hydrocarbon potential becoming smaller. In the Upper Carboniferousthe Jingyuan Formation’s argillaceous source rocks belonging toⅠtype with theaverage value of TOC%reaching1.86%have the best hydrocarbon generation ability.It’s the main source rocks developed layer. The Yanghugou Formation’s argillaceoussource rocks belonging toⅡtype with the average value of TOC%reaching2.14%have the second best hydrocarbon generation ability. It’s the secondary argillaceoussource rocks developed layer. The Taiyuan Formation’s argillaceous source rocksbelongs toⅡ2and Ⅲ type with the highest TOC%average value reaching2.39%.However, the thickness of argillaceous，formation and residual stratum is muchsmaller than Jingyuan and Yanghugou Formation. That limits its hydrocarbongeneration ability. It’s the three-level argillaceous source rocks developed layer.Regarding to coal source rocks, the Carboniferous interval all develope Peat flatand Marsh mud flat. The Early Carboniferous has less terrigenous clastic sediments.The material basis of coal source rocks is bad. The coal-bearing source rocks is poorlydeveloped with low organic matter abundance. The Upper Carboniferous oal-bearingsource rocks having more terrigenous clastic sediments with high organic matterabundance is better developed. But the Carboniferous sea level falls quickly, so thecoal accumulation is short. That makes the coal sources thinly and limit its hydrocarbon generation ability.Carbonate source rocks only developed in the Yanghugou period and Taiyuanperiod which have favorable environments for sediments. The abundance of organicmatter is poor in general with small thickness and limited in distribution. Carbonatesource rocks poorly developed over the whole carboniferous layers.