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Abstract

Subsurface Upper Jurassic Smackover limestones of the Gulf Coast of the U.S.A. are an ideal laboratory for the study of the burial diagenesis of platform carbonates because of the enormous quantity of material available occasioned by extensive hydrocarbon production, the wide range of burial depths over which they occur, and the relative simplicity of their depositional setting. The model presented is based on several long term studies of these limestones. Pathways for burial diagenetic fluids were established early on during the sedimentological evolution of the platform and were driven by relative sea level fluctuations. Regionally extensive high porosity trends, that subsequently acted as conduits for diagenetic fluid flow, were generally restricted to siliciclastic lowstand fans and wedges, and highstand blanket carbonate sands. Transgressive systems tracts are generally muddy and act as aquicludes, or in some cases, hydrocarbon source rocks. Relative sea level lowstands can dramatically influence later diagenetic fluid flow by modifying original porosity distribution and permeability characteristics through dissolution and cementation. Carbonate mineralogies are generally stabilized during this early, surface-related diagenetic phase.

Burial diagenesis in platform limestones proceeds in two main phases: prehydrocarbon migration diagenesis and post-hydrocarbon migration diagenesis. Pre-hydrocarbon migration diagenesis is driven by chemical compaction in an initially open system, dominated by marine connate and mixed marine connate and meteoric water. This hydrologic system became compartmentalized during hydrocarbon trap formation. New pore fluids, derived from underlying siliciclas- tics below and from the dewatering of adjacent basinal deposits with faults as conduits, moved into porous platform carbonates and mixed with evolved connate waters. Calcite cements were predominantly sourced from pressure solution of host limestones. However, their composition also reflects the importation of Fe, Mn, radiogenic Sr, Pb, Ba, etc., from subjacent lowstand siliciclastic sequences and adjacent basinal deposits. Hydrocarbon precursors, such as H2S and organic acids were also introduced, leading to sulfide formation and minor secondary porosity generation. Migration and trapping of hydrocarbons arrested further diagenesis within the carbonate reservoir sequence.

Post-hydrocarbon migration diagenesis is driven by thermal destruction of reservoir hydrocarbons under the influence of high temperatures. The resulting high H2S environment led to metal fixation by sulfide precipitation, thermochemical sulfate reduction, late calcite cementation, and sulfur-driven methane destruction. Progressive loss of porosity through compaction as well as the emplacement of the by-products of late diagenesis, such as calcite cements and pyrobitumen, resulted in progressive closure of the diagenetic system. Trace element and isotopic compositions of late, post-hydrocarbon calcite cements reflect these processes.

This model should be applicable to hydrocarbon and sulfate-bearing carbonate shelf-ramp depositional sequences featuring high energy, highstand systems tracts, developed in response to 3rd order relative sea level fluctuations.

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