Geological Characteristics And Significance Of Ancient Weathering Crust In North Qiangtang Depression

The Qiangtang basin is the largest continental marine hydrocarbon-bearing sedimentary basin in China.The Mesozoic strata are mainly exposed,located in the eastern part of the Tethys tectonic domain of the global oil and gas resources.The basin is rich in hydrocarbon source rocks from Paleozoic to Mesozoic,although the area has a very rich source rock,but the reservoir is relatively dense and lack of the formation of high quality reservoirs in the study area,and Xiaochaka Formation and that between the group the discovery of the ancient weathering cranberries undoubtedly provides an opportunity to solve this problem.In recent years,China has found a large number of oil and gas reservoirs associated with the ancient weathering shell,such as the Sichuan Basin,the high stone 1 well,the double exploration 2 wells,or the Ordos Basin Majiagou Formation,the Songliao Basin Yingcheng Formation,etc.,shows that the palaeo-weathering crust in the oil and gas accumulation of the important significance,However,for the carbonate weathering crust,its heterogeneity is very strong,the distribution is rather complex,which undoubtedly increased the palaeo-weathering crust oil and gas accumulation research difficulty.Based on the tectonic evolution of the North Qiangtang depression,this study is based on field geological survey,combined with geology,stratigraphy,petrography,mineralogy,and reservoir geology of the North Qiangtang depression palaeo-weathering crust for a multidisciplinary comprehensive study,the results show thatthe positive weathering of the study area is mainly affected by regional zonal development(Mainly in the area of Xiao Cha card formation)in the study area,the karst collapse breccia is an important marker for the identification of the shell;The palaeo-weathering effect in the study area shows the characteristics of the north and west strong south and east weak,The geochemical characteristics showed a slight negative anomaly of Ce,Eu was obvious negative anomaly,Indicating that the shell in the formation process by the oxidation of the environment at that time,element differentiation.In addition,the ancient weathering crust as a potential reservoir is mainly developed with rich dissolution holes,among them,the large dissolution cave is a potential high quality reservoir space in the shell which has not yet been exposed,A large number of gap development provides a good migration channel for the seepage fluid of the palaeo-weathering crust,which ensures the development of the dissolution pore system,there is no better way to measure the age of weathering crust,the upper limit age is determined only by the age of upper and lower strata,as for its lower limit age has yet to be further studied.