Worldwide carbonate rocks are occurring abundantly. These carbonate rocks are a major class of sedimentary rocks group. Carbonate are sedimentary rocks formed at (or near) the Earth surface by precipitation from solution at surface temperatures. The two broad categories are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolostone, which is composed of the mineral dolomite (Ca Mg (CO3)2). Dolomite is not a simple mineral; it can have a variety of origin, can form as a primary precipitate, a diagenetic replacement, or as a hydrothermal/metamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can also form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Potential fluid sources are seawater and subsurface fluid of marine and/or meteoric origin: and addition Mg could be released from high-Mg calcite and smectite clays. The only abundant source of Mg2+ ions for early diagenetic surface and near-surface dolomitization is seawater. Dolomitization also creates new crystals, with new rhomb growth following the dissolution of less stable precursors. Dolomitization model and formation depend on the source dolomitization site and lastly, there must exist a favorable condition for a chemical reaction. One particular type of dolomite which may be a cement or a replacement is baroque dolomite, also called 'saddle' or 'white sparry' dolomite and known to mineral collectors as pearl spar. It is characterized by a warped crystal lattice.
In Pakistan, carbonate rocks of Jurassic age are present entirely throughout the Indus Basin. The Jurassic carbonate rocks present in the Attock-Cherat requires a more detailed investigation as of recent discovery of gas in Jurassic carbonate rocks in the lower Indus basin. The evaluation of these Jurassic carbonate units in the Nizampur Basin of Attock-Cherat ranges should integrate the aspects of the depositional sediments, diagenetic history, and geochemistry to understand the reservoir behavior. In this study, the Samanasuk Formation of 90 m was sampled and measured in detail, and samples were collected with ~ 0.3 m interval and some samples were collected where reckoned necessary. Petrographic and microfacies analysis were conducted on about 100 samples; moreover, XRD analysis were performed. The present study aims to determine the depositional facies, diagenetic processes, and geochemical elements of this carbonate succession of the in an effort to explore their effect on reservoir quality. The Samanasuk Formation comprises eight microfacies assigning three facies belts including peritidal, lagoon, and shoal of a carbonate ramp. The recorded diagenetic processes include dolomitization, compaction, micritization, neomorphism, dissolution, and cementation in which dolomitization played an important role in enhancing the reservoir quality. In relation of their impacts on reservoir properties, the grainstone facies associated with peritidal facies and dolomudstone facie associated with carbonate sand and shoal shows the greatest reservoir quality, whereas lagoonal facies has the lowest reservoir quality. This study represents an approach to use the depositional facies, diagenetic alterations, and geochemical framework of carbonate succession in the reservoir characterization.
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