Key Dating Methods of Reservoir Accumulation in Marine Strata and Their Applications to Natural Gas of the Changxing Formation in Yuanba Gas Field

Wang Jie; Liu Wenhui; Tao Cheng; Tenger; Xi Binbin; Wang Ping; Yang Huamin

Abstract：Petroleum characteristics and accumulation of the marine superimposed basins show strong complexity,which makes it difficult to determine key time of reservoir accumulation.It is urgent to establish effective dating methods for hydrocarbon source age,hydrocarbon generation and expulsion time,reservoir adjustment and stabilization time.On the basis of helium age accumulation effect and reservoir preservation mechanism,the helium dating model and age estimating formulas are put for-ward to determine the time of reservoir formation and stabilization retention after large scale of petroleum charging in this pa-per.According to the relationships between the 40Ar/36Ar ratio of natural gas,potassium abundance of source rock and the ge-ological time,argon isotopic estimation model on the source rock age is fitting by many parameters.Based on the closure tem-perature on U-Th/He dating system of apatite and zircon being consistent with the temperature scope of hydrocarbon genera- tion in sedimentary basin,uplift time and reservoir transformation time induced by tectonic uplift are revealed by means of time and closure temperature difference of close system in apatite and zircon,and geothermal gradient of sedimentary basin.By means of a series of condition test,pretreatment techniques including asphaltene extraction,acid dissolving,rhenium-osmium purity,enrichment and separation are established.Re-Os isotopic dating can be used to directly date hydrocarbon source rock, solid bitumen,oil and oil sand and offer hydrocarbon generation and migration time.According to priority of the formation time or occurrence time of geological part of reservoir,the key dating technologies to determine source rock age,hydrocarbon gener-ating time,charging time,reservoir adjustment and stabilization time are established.By means of dating technologies,the key time including source rock age,hydrocarbon charging time,gas reservoir adjustment and stabilization time are determined in Yuanba gas field in Sichuan basin.It is indicated that the main gas source is composed of the Longtan Formation hydrocarbon source rocks,and two stages crude oil charging time being from 220 Ma to 175 Ma and from 168 Ma to 140 Ma,large scale crude oil began to crack into gas between 140 Ma and 118 Ma.Tectonic uplift began to occur since 97 Ma,especially since 15 Ma,gas and water interface began to adjust until gas reservoir kept stabilization between 12 Ma and 8 Ma.