Research On The Characteristics Of The Ordovician Paleo-weathering Crust In The Central Area Of The Jingbian

In this paper,the paleo-weathering crust of the Lower Paleozoic Ordovician Majiagou Formation in central Jingbian County of Yanchang Gas Field is taken as the research object.Combining with the exploration practice,the characteristics of the Ordovician paleo-weathering crust in this area are analyzed by fully utilizing the data of Drilling,Well Logging and Geology,and the regularity between the distribution of gas reservoirs and the characteristics of the paleo-weathering crust is summarized.The study area is located in the middle of Yishan Slope of Ordos Basin.Paleo-weathering crust mainly develops in the upper part of Majiagou Formation.During the Ordovician sedimentary period,the area is located on the eastern slope of gentle karst structure,constituting shallow-water platform deposits of epicontinental sea and developing carbonate epicontinental platform facies.The lithology of weathered crust reservoir is mainly mudstone-fine powder dolomite.The pore type includes primary pore and secondary pore,but secondary pore is the main pore,first dissolved pore,secondly filled residual space of intergranular dissolved pore and paste mould pore.The primary pore disappeared basically.Effective fractures are mainly dissolution fracture and structural fracture,forming weathered crust reservoir with fracture-dissolution hole type as the main type.The study area is located in the karst slope zone.Two main trenches with nearly East-West distribution are developed side by side,and three types of terrain units are developed,namely,residual hill,slope and trench.Palaeogeomorphology controls the development of vertical structure of weathering crust.In this paper,the vertical structure of Ordovician ancient weathering crust in the study area is divided into soil layer,strong weathering zone,weak weathering zone and parent rock.Strong weathering zone is an advantageous position for the formation of palaeo-weathering crust reservoirs in the study area,mainly in Mawu 1+2 stratum.The favorable areas of gas reservoirs are analyzed:Ningtiaoliang Township-Ximawan-Yangmijian Township and Xiaohe Township-Qingyangcha Township.The area is favorable for gas reservoir development.