Study On The Formation And Evolution Of Gaoshiti-moxi Area And Differential Enrichment Of Oil And Gas In Dengying Formation From Central Of Sichuan Basin

In recent years,great breakthroughs have been made in oil and gas exploration of Dengying Formation of Sinian system in Gaoshiti Moxi area.However,with the deepening of exploration work,the productivity of different blocks shows obvious differences,and the mechanism of oil and gas reservoir formation differences needs to be further clarified.At present,the focus is mainly on reservoir,source rock and so on,but the research on structural formation and evolution and oil-gas fluid system is very weak,and it is rarely involved to solve the problem of oil-gas reservoir forming difference by combining the two.This paper takes this as a breakthrough point,through the co MParative analysis of the sequence of filling minerals in the pores and fissures of the platform margin zone and the platform interior zone,it shows that they have similar fluid filling sequence,and there are three generations of mineral filling commonly:Dolomite?pyrolyzed asphalt?quartz/dolomite/calcite,which represents the three phases of fluid filling:the first phase of brine fluid filling occurs in front of the oil generating stage,especially in order to form dolomite The second stage is the filling of petroleum fluid,marked by asphalt;the third stage is the salt water fluid,which mainly occurs in the period of adjustment and accumulation of ancient gas reservoir behind the gas generating stage,represented by quartz,dolomite or calcite rich in methane inclusions.It is found that Gaomo oil and gas reservoir has experienced the evolution rule from ancient oil reservoir?ancient gas reservoir?adjustment and formation of ancient gas reservoir?present gas reservoir.According to the petrographic observation of the fluid inclusions in quartz,calcite or dolomite formed during the adjustment of ancient gas reservoir,it is found that the fluid inclusions in liquid single-phase methane and gas-liquid two-phase brine coexist,in which the homogenization temperature of liquid methane inclusions is between-82.7?to-110.8?,and the density is concentrated in the range of 0.18 g/cm~3 to 0.28 g/cm~3,the homogenization temperature of its associated salt water inclusions is concentrated at 180??240?,the peak temperature is about 220?,and the salinity is basically below 15%.Based on the analysis of the burial history of regional thermal evolution,it is determined that the adjustment of paleogas reservoir occurred in the tertiary Quaternary.Through ancient pressure recovery,it is determined that the pressure of the ancient gas reservoir is 53.66MPa?132.42MPa,and the ancient pressure coefficient is concentrated in 1.3?1.9,which has the characteristics of overpressure,and the high pressure coefficient area is concentrated in the platform margin zone.According to the characteristics that the present gas reservoir is a normal pressure gas reservoir,it shows that the process from the ancient gas reservoir to the present gas reservoir is an adjustment from the super pressure reservoir to the normal pressure gas reservoir;according to the calculation results of the paleofluid potential,the migration direction of the paleofluid is inferred.The Gaoshiti Moxi structure has undergone multiple structural changes.From Sinian to Early Cambrian,under the influence of Xingkai taphrogenesis,Mianyang-Changning intracratonic sag was formed on the west side of the study area.The intracratonic sag controlled the distribution of high-quality reservoirs and source rocks in the study area.During this period,Dengying Formation experienced Tongwan uplift and denudation,resulting in the development of regional dissolution holes in Dengying Formation.Before the Permian sedimentation,Leshan-longnusi Sinian palaeouplift was basically finalized.At this time,Moxi-longnusi area is located in the axis of the Sinian palaeouplift,while Gaoshiti area is located in the slope area to the south of the ancient uplift.Before the deposition of the upper Triassic,the Moxi-longnusi area grew into a paleo oil and gas trap,and the Gaoshiti area gradually entered the range of uplift oil and gas trap.From the Late Triassic to now,under the influence of Yanshan tectonic movement and Himalayan tectonic movement,Weiyuan structure has been rapidly uplifted to the highest position of the study area.At the same time,the internal differentiation of Gaoshiti-Moxi structure has occurred,and Gaoshiti area has replaced Moxi area to become the local structural high position.According to the time point of structural evolution and the time point of hydrocarbon accumulation process,it shows that structural evolution controls all stages of hydrocarbon accumulation:(1)in the paleo reservoir period,Moxi area in the Late Triassic is locally high in structure,which is conducive to the relative accumulation of oil and gas,and the high-quality source rocks are concentrated along the Mianyang-Changning intracratonic sag,which guarantees the material basis for the dominant reservoir formation in the platform margin zone;(2)In the paleo gas reservoir stage,the overall structural feature of the study area has little change from Jurassic to Cretaceous,and the relatively stable structural environment provides the preservation conditions for the early gas reservoir formation;(3)in the adjustment stage of ancient gas reservoir,the rapid uplift of Weiyuan area since Himalayan period has become the highest level of Sinian palaeouplift structure,forcing the natural gas to gather in Weiyuan gas field,Which causes the loss of Gaoshiti Moxi Gas Reservoir.At the same time,Gaoshiti structure grows into the highest in the study area,it promotes the adjustment and accumulation of natural gas to Gaoshiti area,resulting in the difference of current production capacity in Gaoshiti-Moxi area.The differential enrichment of oil and gas reservoirs is mainly controlled by Mianyang-Changning intracratonic sag and Weiyuan anticline structure:(1)the Mianyang-Changning intracratonic sag promotes the platform edge zone to have better oil and gas geological conditions,on the one hand,the Mianyang-Changning intracratonic sag concentrates to produce high-quality source rock,and the source rock and the platform edge zone connect laterally to ensure the source conditions;on the other hand,the platform edge zone close to the Mianyang-Changning intracratonic sag is high and steep,it promotes the development of favorable lithofacies such as mound algal complex in the margin of the platform,and the transformation by karstification is more significant,which ensures the reservoir conditions;(2)In Himalayan period,The rapid uplift of Weiyuan structure caused the migration of natural gas from Gaoshiti-Moxi gas reservoir to Weiyuan gas reservoir,it also causes the loss of high pressure in the Gaoshiti-Moxi reservoir.At the same time,the internal structure of Gaoshiti Moxi area is different.At this stage,the Gaoshiti area finally evolved into the high part of the study area and became the favorable accumulation area of the present gas reservoir.