Comprehensive Evaluation Of Tight Carbonate Cap Rock In The Fuyuan-Yueman Block In Halahatang Depression Of Tabei Uplift

At present,the research of deep carbonate caprock in Halahatang depression is relatively not deep.In order to study sealing mechanism of carbonate caprocks and their control effect on oil and gas stratification enrichment,we take Fuyuan and Yueman block as an example.By taking full advantage of geological,seismic and logging data,we found that there were two sets of reservoir cap assemblages in the study area.Mudstone of Sangtamu formation and the marlite of Tumuxiuke formation of Upper Ordovician are the most important regional caprocks,and Ordovician carbonate caprock has the characteristics of large thickness and stable distribution,and there are a few faults developing all around the study area,which are little faulted by fracture.The high resistant and tight limestones developing within Yijianfang Formation and Yingshan Formation are important local cap or intercalation,and they show spatial distribution featuring in lateral unstableness and vertical superimposition,which played an important role in the stratification and enrichment of oil and gas.The microscopic sealing properties of two types of caprock were studied from microscopic pore-throat features and physical seal features.Firstly,through scanning electron microscopy and cast film sheet,we found that they were compact in structure and had a relatively low degree of porosity and fracture development.Besides,they were mostly filled with bright-calcite calcareous or argillaceous and had good sealing properties.Secondly,the displacement pressure characteristics and distribution of two caps were studied by direct displacement method and fitting formula,the Tumuxiuke formation displacement pressure value is higher,and the maximum displacement pressure is up to 12 MPa,screening the main capping sealing capacity evaluation parameters for the quantitative evaluation of the plugging capacity of Tumuxiuke formation,and the evaluation results show that the plugging capacity of the Tumuxiuke formation in the Yueman block is superior to that of Fuyuan block.Most of the region's evaluation index is over 5 and the underlying oil and gas of Yijianfang Formation are mainly distributed in the area where the comprehensive evaluation value is greater than 5.The formation of dense high resistant layer in Yijianfang Formation and Yingshan Formation is due to differences of diagenesis,and the sealing mechanism is mainly dependent on pressure difference sealing oil and gas.When the displacement pressure difference between the high resistant layer and the underlying reservoir are more than 2.5MPa,the high resistant layer can be used as a local cap or intercalation and has a certain blocking effect on the underlying oil and gas.Based on the known research achievements,analysis of the formation and evolution of caprocks and fractures and expulsion hydrocarbon history of source rocks,it is concluded that the hydrocarbon in the reservoirs of Hanilcatam area are derived from Middle-Upper Ordovician source rocks in South West Manjiaer sag.The sealing ability of carbonate cover is earlier than hydrocarbon expulsion,and the multistage strike-slip faults are the main migration channels for effective source rock communication.Finally,the comprehensive analysis results were used to predict the favorable area of hydrocarbon accumulation.