Study On Densification History Of The Fourth Member Of Xujiahe Formation In Middle Member Of The West Sichuan Depression

There is a point of contention that the tight sandstone gas reservoirs of the T3x4formation in middle member of the west Sichuan depression is conventional tightsandstone gas-reservoirs or unconventional deep basin gas. The types of gas reservoirdirectly determine the natural gas exploration strategies and resources proved degreein study area. The key to be sure the tight sandstone gas reservoirs is unconventionaldeep basin gas or not is the time of densification. So, in this paper, the densificationhistory research of the fourth member of Xujiahe formation in middle member hasbeen choosen. The tight process of study area was analyzed, using conventional thinsection, casting thin section, CLM, SEM, laser carbon oxygen isotope analysis,homogenization temperatures and the method of numerical simulation, mainlyobtained the following conclusions:(1)The mainly types of reservoir rock in T3x4formation are lithic sandstone,quartz litharenite and feldspathic litharenite, the most important type of rocks is lithicsandstone. The enriched lithic is the most obvious characteristic of the structure ofsandstone framework grain, The partial plastic sedimentary lithic have a large share inthe sandstone structrue, second metamorphic rocks and rare igneous rocks.(2)Based on the plastic characteristic of sandstone framework grain in T3x4,therocks are classified quartz sandstone, calcarenaceous sandstone and enriched plasticgrains sandstone. The numerical simulation of primary porosity evolution has beentaken. The quartz sandstone and sandstone have the strong rigidity, so they numericalsimulation as followed rigidity sandstone. The simulation result of rigid sandstoneporosity evolution shows that about47.2%of the initial porosity was lost forcompaction.the simulation of enriched plastic grains sandstone as the medium plasticsandstone mode, the result shows that the loss accounting for about70%.Obviously,compaction is the most important factor for the porosity loss and densification for enriched plastic grains sandstone.(3) The mainly authigenic minerals are carbonate cement and siliceous cement.According to laser carbon oxygen isotope analysis and homogenization temperatures,authigenic mineral deposits history has been researched. The main deposition periodof carbonate cement is upper Shaximiao and the secondary deposition period is laststage of Penglaizhen formation in enriched plastic grains sandstone. The depositionperiod of carbonate cement in calcarenaceous sandstone is more earlier, it is the laststage of T3x5formation. The deposition period of siliceous cement in enrichedplastic grains sandstone has begun in the last phase of xujiahe formation, this processcontinues until the last stage Penglaizhen formation, and The main deposition periodsiliceous cement is form upper Shaximiao to suining formation.(4) The secondary porosity is the most important porosity types in T3x4formationand take advantage of control function. It formed earlier in calcarenaceous sandstone,the dissolution may had happed during T3x5formation. The secondary porosityformed before upper Shaximiao formation in enriched plastic grains sandstone, thedissolution continues until the deposition period of Suning formation.(5) The time of overpressure form is late, the function for preservation ofprimary pores is quite limited, and the affect on T3x4also is little. Combined with Theprimary porosity evolution, authigenic mineral deposits history and the developmentof secondary pore, the analysis of densification history research of T3x4showsthat,the enriched plastic grains sandstone densification time is earliest, and the time isthe middle Penglaizhen formation,the reservior has the highlyest porosity now. Thedensification time of rigid sand is later than enriched plastic grains sandstone,the timeis the last stage of Penglaizhen formation. The densification time of calcarenaceoussandstone is the latest, and the time is the earlier stage of early Crataceous, thereservior has the lowest porosity now. The main reasons of densification in T3x4iscompaction, and authigenic mineral deposition also has an important effect.(6) the relationship between the time of reservoir densification and hydrocarbongeneration shows that the tight sandstone gas reservoirs is main “formed firstly”unconventional deep basin gas.The densification time of calcarenaceous sandstone isrelatively later,so some conventional tight sandstone gas-reservoirs may existed insuch reservoir.