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Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer... The present study investigates the microfacies and the effects of diagenetic processes on the reservoir quality of limestones of the Fort Member, Jaisalmer Formation, Western Rajasthan. The study interpolates an integrated approach including field studies, petrography, staining, and X‐ray diffraction techniques. The Fort Member is composed of medium‐ to thick‐bedded, nodular highly fossiliferous limestones, which are fine‐ to medium‐grained, poorly‐ to very well‐sorted, and comprise angular‐ to well‐rounded grains of low to high sphericity. These limestones contain abundant skeletal (bivalves, brachiopods, echinoderms, foraminifera, algae, gastropods, sponge, coral, and calcispheres) and non‐skeletal grains (intraclasts, ooids, and peloids). On the basis of detailed petrographic investigations, eight microfacies have been recognized including: bioclastic wackestone, whole‐fossil wackestone, bioclastic lithoclastic packstone, aggregate grainstone, peloidal grainstone, ooidal grainstone, bioclastic grainstone, and coated bioclastic grainstone. Based on the microfacies analysis, the limestones of the Fort Member are inferred to have been deposited in shoal, lagoon, and open marine environments. The diagenetic overprints of the Fort Member limestones is characterized by the several diagenetic features such as micritization, cementation, dolomitization, compaction, dissolution, silicification, neomorphism (aragonite to calcite transformation and development of microspar), and secondary porosity. Such main diagenetic features affecting the carbonates are developed in marine phreatic, meteoric phreatic, mixed meteoric phreatic and burial diagenetic environments. Micritization of allochems, dolomitization of lime muds, and cementation by isopachous calcite, granular calcite cement, neomorphism, and recrystallization of bioclasts are developed in meteoric‐phreatic diagenesis. The meteoric‐vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Dolomitization, blocky calcite cement, and silicification processes were developed during the burial diagenesis. These diagenetic processes have direct effects on the modification of both intergranular and intragranular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization, and compaction were responsible for the reduction of the pore spaces, whereas dolomitization, dissolution, formation of moldic pores, and fracturing increased the pore volumes. Therefore, the enhanced pore volumes improved the carbonate reservoir characteristics, suggesting the limestones of the Fort Member to be a better carbonate reservoir for hydrocarbon accumulation. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geological Journal Wiley

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

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References (117)

Publisher
Wiley
Copyright
© 2021 John Wiley & Sons, Ltd.
ISSN
0072-1050
eISSN
1099-1034
DOI
10.1002/gj.3945
Publisher site
See Article on Publisher Site

Abstract

The present study investigates the microfacies and the effects of diagenetic processes on the reservoir quality of limestones of the Fort Member, Jaisalmer Formation, Western Rajasthan. The study interpolates an integrated approach including field studies, petrography, staining, and X‐ray diffraction techniques. The Fort Member is composed of medium‐ to thick‐bedded, nodular highly fossiliferous limestones, which are fine‐ to medium‐grained, poorly‐ to very well‐sorted, and comprise angular‐ to well‐rounded grains of low to high sphericity. These limestones contain abundant skeletal (bivalves, brachiopods, echinoderms, foraminifera, algae, gastropods, sponge, coral, and calcispheres) and non‐skeletal grains (intraclasts, ooids, and peloids). On the basis of detailed petrographic investigations, eight microfacies have been recognized including: bioclastic wackestone, whole‐fossil wackestone, bioclastic lithoclastic packstone, aggregate grainstone, peloidal grainstone, ooidal grainstone, bioclastic grainstone, and coated bioclastic grainstone. Based on the microfacies analysis, the limestones of the Fort Member are inferred to have been deposited in shoal, lagoon, and open marine environments. The diagenetic overprints of the Fort Member limestones is characterized by the several diagenetic features such as micritization, cementation, dolomitization, compaction, dissolution, silicification, neomorphism (aragonite to calcite transformation and development of microspar), and secondary porosity. Such main diagenetic features affecting the carbonates are developed in marine phreatic, meteoric phreatic, mixed meteoric phreatic and burial diagenetic environments. Micritization of allochems, dolomitization of lime muds, and cementation by isopachous calcite, granular calcite cement, neomorphism, and recrystallization of bioclasts are developed in meteoric‐phreatic diagenesis. The meteoric‐vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Dolomitization, blocky calcite cement, and silicification processes were developed during the burial diagenesis. These diagenetic processes have direct effects on the modification of both intergranular and intragranular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization, and compaction were responsible for the reduction of the pore spaces, whereas dolomitization, dissolution, formation of moldic pores, and fracturing increased the pore volumes. Therefore, the enhanced pore volumes improved the carbonate reservoir characteristics, suggesting the limestones of the Fort Member to be a better carbonate reservoir for hydrocarbon accumulation.

Journal

Geological JournalWiley

Published: Jan 1, 2021

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