Date of Award

1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

The matrix and micritic components of "chalky" limestones feature well-disseminated pores with diameters of 5-10$\mu$m. SEM studies of porous micrites reveal numerous microporosity types, including: (1) primary intercrystalline and interparticle micropores, and (2) solution-enhanced intercrystalline, moldic and vug micropores. During early diagenesis, carbonate is transfered from particles to blocky micrite and microspar cements growing in adjacent micropores. Microporosity may be retained if cementation is arrested shortly after the stabilization process is complete. Primary, intercrystalline microporosity characterizes micritic limestones in the Shuaiba Fm. (L. Cretaceous) at Sajaa field in Sharjah, U.A.E. Calcite rhombs composing matrix average 4-6 $\mu$m in diameter. Matrix microporosity appears to have been an early-developed property of micritic limestones over several hundreds of feet of section at Sajaa. Matrix oxygen-isotope compositions ($\delta\sp{18}$O: $-4.0$ to $-5.0\sp{o}\!/\!\sb{oo}$ PDB) would result from recrystallization of Aptian marine carbonate ($-2.0\sp{o}\!/\!\sb{oo}$ PDB) in marine pore fluid ($0.0\sp{o}\!/\!\sb{oo}$ SMOW) with a porosity range of 30-50% over a temperature range of about 40$\sp\circ$-50$\sp\circ$C. This corresponds to about 2000-3000 ft of burial. Chemical compaction was responsible for occlusion of matrix microporosity in zones of intense stylolitization. Porous and nonporous micritic limestones were studied in two subsurface Miocene buildups, offshore North Sumatra. At NSB-H field, lime-mud diagenesis resulted in thorough cementation. Matrix stable-isotope signatures ($\delta\sp{13}$C: $-3.0$ to $-10.0\sp{o}\!/\!\sb{oo};$ $\delta\sp{18}$O: $-7.0$ to $-9.0\sp{o}\!/\!\sb{oo}$ PDB) reflect diagenesis in an open system of low rock/water ratio, influenced by organically-charged, shallow-meteoric fluids. In comparison, meteoric alteration of micritic limestones beneath the Pleistocene/ Holocene unconformity in Florida and the Bahamas has resulted in cementation of matrix. NSB-A field features matrix with abundant primary, intercrystalline microporosity. Marine-carbon isotopic signatures are preserved in the matrix ($\delta\sp{13}$C: +1.0 to $-3.0\sp{o}\!/\!\sb{oo};$ $\delta\sp{18}$O: $-6.0$ to $-9.0\sp{o}\!/\!\sb{oo}$ PDB), indicating that diagenesis proceeded in a somewhat closed-system of high rock/water ratio, consistent with textural constraints for limited or arrested cementation in the matrix. Dissolution-enhancement of intercrystalline microporosity at both fields followed all phases of cementation and dolomitization after significant burial. Thus, chalkification by leaching is not restricted to near-surface diagenetic settings.

Pages

369

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