Characteristics And Origin Of Clay Minerals In The Sandstone Of Taiyuan Formation, Northeastern Ordos Basin

Clay minerals in Taiyuan Formation sandstones, Northeastern Ordos Basin are generally discussed in the paper, based on x-ray diffraction (XRD), Scanning electron microscopy (SEM), energy spectrum analysis, polarizing microscope and other analyses. It is indicated that framework end-member of sandstones in the study area is characterized by rich fragments and extremely deficient feldspar, which is the final material expression of diagenetic evolution of clay minerals. Illite and kaolinite are the primary clay types. Two occurrences of kaolinite are found under the polarizing microscope, that is, tight pseudomatrix presented as multiple forms and disperse platy, booklet particles in secondary porosity. There are two morphological habits of authigenic illite, including meshwork illite (intrograin clay and reticular clay) and booklet, silk illite in secondary porosity.It is indicated by 58 XRD analyses and 396 thin-section data that there are three assemblage patterns of clay minerals occurring in Taiyuan sandstones, that is, single illite assemblage, illite and kaolinite assemblages and single kaolinite assemblage. According to thin-section data which make sense much more, single illite assemblage is the leading clay patterns (accounting for 71.46%).The lateral distribution of clay minerals shows that wells located in northern of study area mostly develop illite and kaolinite assemblages, while southern wells develop single illite assemblage; however, there are still other wells inconsistent with the above trend, which suggests that besides of sedimentary facies, diagenesis also controls the lateral distribution of clays. Vertical distribution of clay minerals indicates Taiyuan sandstone had experienced diagenetic and sedimentary processes different from other Upper Paleozoic Formations.Based on former researches and SEM photos, thin-section photos and energy spectrum analysis, kaolinite in Taiyuan reservoir sandstones is considered to form during eogenesis, while few in residual intergranular pore are later. Part of kaolinite transforms to dickite in where reservoir physical property is better. However, illitization of kaolinite blocks further growth of dickite.In deeply-buried closed system, the primary mechanism of anthigenic illite formation is finished through the process of K-feldspar + kaolinite—→illite + quartz which is divided into two phases: firstly, dissolution of K-feldspar provides K+, and then the reaction between K+ and kaolinite creates illite and quartz. There are two pathes for providing K+: K-feldspar + H+—→kaolinite + K++ Si4+, and K-feldspar + H+—→illite + K++ Si4+. Thermodynamics analysis suggests that the difference ofΔG between the above two paths is neglectable, and thus both the two paths can provide illitization of kaolinite with K+. The process of K-feldspar + kaolinite—→illite + quartz forms not only single illite assemblage but kaolinite and illite assemblages.According to the primary single illite assemblage (71.46%) and petrology characteristics extremely poor in feldspar, the process of K-feldspar + kaolinite—→illite + quartz is believed as the dominant mechanism forming authigenic illite. Combined with discussion of relationships between authigenic illite and thin-section porosity, the formation of authigenic illite is thought to promote develop of secondary porosity. And dissolution of K-feldspar along with illite formation is the dominating mechanism of secondary porosity in Taiyuan reservoir sandstone.