Control Effect Of Water-Rock Chemical Reaction On Reservoir Development In Shale Reservoir Of Fengcheng Formation In Mahu

Mahu Sag is the most hydrocarbon-rich sag in Junggar Basin,and Fengcheng Formation is the main source rock.The shale reservoir of Fengcheng Formation has experienced multiple fillings of hydrothermal fluids,organic acids and other fluids,triggering a variety of water-rock chemical reactions,forming an interbedded reservoir of shale and argillaceous dolomite,and developing a large number of alkaline minerals such as reedmergnerite.Mineral change caused by water-rock chemical reaction plays an important role in pore evolution and is the key to the formation of high-quality reservoirs.However,the diagenesis of shale reservoirs is long and complex.The water-rock chemical reaction is comprehensively controlled by temperature,fluid properties,lithology and other factors.The dissolution,precipitation and transformation of minerals in reservoirs by different types of fluids are complex.Therefore,it is the key to reveal the development mechanism of high-quality shale reservoirs to clarify the controlling factors of water-rock chemical reaction in shale reservoirs,the mechanism of mineral dissolution,precipitation and transformation caused by it,and the process of porosity development.This paper selects the shale reservoir of Fengcheng Formation in Mahu Sag of Junggar Basin as the research object,and studies the chemical effect caused by hydrothermal fluid in the process of hydrothermal upwelling.Firstly,the chemical mechanism of early hydrothermal activity on reservoir development are explored,find out the control factors of water-rock chemical reaction in this process,and the mineral transformation law caused by water-rock chemical reaction under different temperature,ion concentration and fluid rate is analyzed to clarify the influence of hydrothermal activity on shale reservoir development.Secondly,for a large number of reedmergnerite precipitated after hydrothermal activity,the genesis and enrichment mechanism of reedmergnerite are revealed.Finally,in view of the Fengcheng Formation containing reedmergnerite reservoir,the law of mineral dissolution,precipitation and transformation in the reservoir is explored,the dissolution reaction of organic acids on different contents of reedmergnerite is analyzed,and the influence of acidic fluid on the development of reedmergnerite enrichment reservoir is clarified.Studies have shown that:（1）the deep hydrothermal solution of Fengcheng Formation is uprising to the bottom of the lake basin,resulting in an increase in salinity,and micrite dolomite is directly precipitated from the water body and interbedded with organic matter;at the same time,the hydrothermal metasomatic albite inside the fault forms fine crystalline dolomite,which hinders fluid migration and is not conducive to reservoir development.（2）The larger the hydrothermal flow rate is,the more right the water-rock chemical reaction equilibrium moves,and the faster the reaction rate is.The ion concentration was positively correlated with the reaction rate,but there was a maximum reaction rate.The hydrothermal flow rate and ion concentration control the reaction rate by affecting the ion concentration on the contact surface of water-rock chemical reaction.（3）Deep hydrothermal upwelling makes abundant Na⁺,B³⁺,Si O₃^2-in the water combine to form a small amount of reedmergnerite precipitation.Subsequently,organic acids were combined with hydrothermal solution to reduce the p H value of the fluid,and CO₂ was added as a mineralizer to promote the metasomatic carbonate of reedmergnerite,and finally the enrichment of reedmergnerite occurred.（4）The degree of porosity development is related to the content of reedmergnerite;the higher the content of reedmergnerite,the greater the porosity increment,the more favorable for reservoir development;the degree and range of favorable porosity development in the reedmergnerite layer with higher content are better than those in the reedmergnerite layer with lower content.