This study deals with the diagenetic history of the Pachmarhi Sandstones of Satpura Gondwana Basin, Central India. The sandstones are classified as quartz-arenite and sub-arkose type. Monocrystalline quartz grains dominate the detrital mineralogy followed by polycrystalline quartz grains, feldspars (more alkali-feldspar than plagioclase), rock fragments, detrital mica and heavy minerals. The diagenetic signatures observed in the Pachmarhi Sandstones include mechanical compaction, cementation (Fe-oxide, quartz overgrowths, calcite, matrix and clay minerals), replacement and dissolution of feldspar and calcite cement. The most commonly observed type of secondary porosity is dissolution of feldspars. The porosity loss is mainly due to cementation by pore occlusion and by early stage of mechanical compaction. In addition, several clay minerals occurred as pore-filling and pore-lining cements. The nature of various types of grain- to grain contacts suggests early cementation and consequent minor compaction. Among the various cements, calcite is the earliest followed by iron oxide while silica cementation occurred probably at a late stage. The carbonate cement formed during burial by dissolution and re-precipitation represents redistributed calcite which was buried with the sandstone. The iron cement was perhaps derived from weathering and leaching of ferromagnesian minerals of overlying Deccan traps. Silica cement was derived from the corrosion of quartz and feldspar grains. Types of grain contacts, minus-cement porosity and porosity reduction indicate a shallow depth of burial conditions for these sandstones.
The Neoproterozoic Bhander Group, the youngest and most widely distributed group of Upper Vindhyans, consists of about 1000m thick succession of sandstone, shale, and limestone. Petrographic investigations reveal that the Bhander Sandstones are mineralogically mature and classi ed as quartzarenite and sublitharenite type which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Average framework composition of the Lower Bhander Sandstone is Qt 98.68 F 0.10 L 1.22 and Upper Bhander Sandstone is Qt 95.92 F 0.12 L 3.96 .Quartz is a more abundant mineral, as shown by X-ray Diffraction Analysis. Petrofacies reveal that the Bhander Sandstones were derived mainly from the source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. A scarcity of feldspar and rock fragments suggests intense chemical weathering in a warm and humid paleoclimate. Mechanical compaction, cements, authigenic clays, and dissolution and modi cation of unstable clastic grains are the major diagenetic components identi ed based on the framework grain-cement relationships. Kaolinite and silica (quartz) overgrowth are found as pore-lling and lining cements. Compaction was more effective than cementation in affecting primary porosity. Cementation decreased porosity and permeability drastically. Kaolinite lls pore spaces, reducing the sandstone's porosity and permeability. Secondary porosity developed as a result of partial to complete feldspar dissolution. The diagenetic signatures observed in the Bhander Sandstones are suggestive of deep burial. The reservoir quality of the studied sandstones is degraded by authigenic clay minerals and cementations, but enhanced by alteration and dissolution of unstable grains.
The Neoproterozoic Bhander Group, the youngest and most widely distributed group of Upper Vindhyans, consists of about 1000m thick succession of sandstone, shale, and limestone. Petrographic investigations reveal that the Bhander Sandstones are mineralogically mature and classified as quartzarenite and sublitharenite type which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Average framework composition of the Lower Bhander Sandstone is Qt98.68F0.10L1.22 and Upper Bhander Sandstone is Qt95.92F0.12L3.96. Quartz is a more abundant mineral, as shown by X-ray Diffraction Analysis. Petrofacies reveal that the Bhander Sandstones were derived mainly from the source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. A scarcity of feldspar and rock fragments suggests intense chemical weathering in a warm and humid paleoclimate. Mechanical compaction, cements, authigenic clays, and dissolution and modification of unstable clastic grains are the major diagenetic components identified based on the framework grain-cement relationships. Kaolinite and silica (quartz) overgrowth are found as pore-filling and lining cements. Compaction was more effective than cementation in affecting primary porosity. Cementation decreased porosity and permeability drastically. Kaolinite fills pore spaces, reducing the sandstone's porosity and permeability. Secondary porosity developed as a result of partial to complete feldspar dissolution. The diagenetic signatures observed in the Bhander Sandstones are suggestive of deep burial. The reservoir quality of the studied sandstones is degraded by authigenic clay minerals and cementations, but enhanced by alteration and dissolution of unstable grains.
Proterozoic Jiran Sandstone rests unconformably on Binota Shale and Khori-Malan Conglomerate. The Jiran Sandstone is comprised mainly of fine to medium-grained, varicolored, thickly bedded sandstones, showing diverse primary sedimentary structures such as ripple marks, planar, and trough cross-bedding. Petrographically, Jiran Sandstone is of mainly quartzarenite which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Quartz is more abundant mineral shown by X-ray Diffraction Analysis. The provenance, tectonic setting, and paleoclimatic condition of the Jiran sandstone were evaluated using integrated petrographic studies. Analysis pursuant, monocrystalline and polycrystalline quartz grains and heavy minerals are driven primarily from metamorphic and plutonic Precambrian basement source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. Intensive chemical weathering in warm and humid paleoclimate is indicated by lack of feldspar and rock fragments.
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