Preservation Of The Shale Gas In Lower Cambrian Niutitang Formation,Southeast Chongqing

From large number of experimental observations and literature review,it is confirmed that the lower Cambrian Niutitang formation is very poor in gas-content,especially in comparison to Wufeng-Longmaxi formation in this area and lower Cambrian strata in northeast Chongqing.This prompted us to conduct further research on the preservation conditions of shale gas in Niutitang Formation.Through analysis composition and isotopic characteristics of a lot desorbed shale gas samples,we see that many samples are nitrogen-riched.and the nitrogen isotope values from-5‰ to 0‰,just similar to the atmospheric nitrogen isotope values.it seems that,in the geological history,shale gas reservoir in Niutitang formation may be effected by the meteoric water.On the basis of predecessors,Geochemical characters of 3 layers are funther determined and concluded that these layers have good hydrocarbon potential.But Ro in Niutitang formation is Relatively high when compared to other two formations.During the current studies,a dynamic depositional modal has been constructed for the three concerned shale layers and their hydrocarbon generation-expelling history has been evaluated by using basin imitation and hydrocarbon generation kinetics methods.It is concluded from the current studies that the lower Cambrian Niutitang formation stop generating hydrocarbon at late Permian epoch,and after that the research area are Subjected to intense tectonic activity include Indo movement,Yanshan movement and Himalayan movement.and caused research area strata uplift and erosion,and denudation amount achieve 3000 to 6000 meters,it is confirmed that late tectonic activity had tremendous impact on shale gas preservation.The current study also considers there is a connected system existing at the bottom of Niutitang formation,This paper also analyzed how the connected system was formed.Fluid history analysis are used to determine the processes involved in the deposition of such connected system at the base of Niutitang formation.There are two main types of fluid activitied in the area,Early fluids are considered coming from ancient seawater which storage in formations and lated fluids are considered mixed by ancient seawater and meteoric water.And lated fluids actived at time of stratum uplift(Yanshan Period and Xishan Period).From current studies,it is concluded that shale gas reservoir in Niutitang formation is largely effected by the meteoric water.A schematic diagram of fluid activity has been generated and it is determined that the previously well connected system at the bottom of the Niutitang formation is largely extend meteoric water activities area which has led to the broadly destruction of shale gas reservoir.