The Formation Mechanism Of Authigenic Quartz In Lacustrine Shale And Influence To Reservoir Property

As the typical lacustrine shale,shale in Yanchang formation of Ordos basin and Lucaogou formation of Santanghu basin provides us an requirement to research the formation mechanism of authigenic quartz in lacustrine shale.Shale in Lucaogou formation contains a large content of tuff,which is different from shale in Yanchang formation.Hence,the research about these two sets of lacustrine shale will help us to understand the diversity of authigenic quartz formation mechanism better.According to the research of organic geochemistry and reservoir petrology about these two sets of lacustrine shale,this study classifies lacustrine shale into two types:shale affected by volcanism and shale unaffected by volcanism.On the basis of classification and research of reservoir property,adsorbability and rock mechanical property about lacustrine shale,this research figures out the occurrence of silica in both the two types of shale,discloses the mechanism and controls of authigenic quartz formation,indicates the influence of authigenic quartz on reservoir property,and illustrates the significance of authigenic quartz to shale oil accumulation.Silica in both two types of shale displays in three occurrences,which is amorphous silica,detrital quartz and authigenic quartz.shale unaffected by volcanism contains large content of amorphous silica.Authigenic quartz in shale unaffected by volcanism is mainly observed in silty mud stone inter-beds,and lives together with authigenic clay minerals.shale affected by volcanism contains large content of authigenic quartz,which is usually developed in tuffaceous shale.These authigenic quartz exhibits good crystalline morphology,usually being short prismatic and allotriomorphic granular rather than typical hexagonal column.The diameters of these authigenic quartz particles vary greatly,ranging from several to several tens microns.The source of silica in shale unaffected by volcanism is different from that in shale affected by volcanism.Silica in shale unaffected by volcanism is formed during clay minerals transformation and dissolution of detrital quartz,while silica in shale affected by volcanism is mainly formed during tuff devitrification.Lacking enough space is the key factor that inhibits the formation of authigenic quartz from amorphous silica.Organic acid emerged during thermal evolution of organic matter.The organic acid provides an acid environment for authigenic quartz formation,and accelerates the dissolution of tuff to develop more silica.Organic pores formed during thermal evolution provide space for authigenic quartz growth.Hence,space and organic matter is the main controls to the formation of authigenic quartz.Favorable reservoirs hardly develop in shale unaffected by volcanism because of low content authigenic quartz and large content of clay minerals.However,shale affected by volcanism contains large content of authigenic quartz.The formation of authigenic quartz improves reservoir property and increases space for petroleum accumulation.The formation of authigenic quartz also declines the compressive strength and result in easy formation of fracture in strata.Hence,the formation of authigenic quartz is useful to reservoir fracturing and improvement.