Reservoir Degradation and Compartmentalization below Subaerial Unconformities: Limestone Examples from West Texas, China, and Oman
P. D. Wagner, D. R. Tasker, G. P. Wahlman, 1995. "Reservoir Degradation and Compartmentalization below Subaerial Unconformities: Limestone Examples from West Texas, China, and Oman", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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This paper describes how meteoric cementation enhanced the hydrocarbon trapping and/or producing potential of three limestones. Petrophysical effects of meteoric diagenesis on carbonates vary between two perfect end members of pure seal formation and pure reservoir enhancement. Net porosity and permeability changes are inferred to be a simplistic function of water availability and the exposed terrane’s chemical reactivity Meteoric tight zones form under conditions of low water availability and high terrane reactivity (e.g., a semi-dry climate exposure of Mg-calcite sediment). Solution-enhanced reservoirs form under conditions of high water availability and low terrane reactivity (e.g., a rain forest exposure of stoichiometric dolomite).
Examples of meteoric tight zones are shown in cores from west Texas, offshore China, and central Oman. Petrographic and geochemical data were used to define the causes of reservoir degradation. From an exploration/ exploitation standpoint, these intervals form potential top-seals for hydrocarbon trapping and/or intraformational permeability barriers that compartmentalize hydrocarbon production. More generally, meteoric tight zones may be a critical trapping factor in many similar hydrocarbon accumulations—both producing (but not recognized as such) and prospective. A more thorough investigation through the current inventory of fields might show meteoric seal formation is as economically important in trap formation as its much better studied “karsting” counterpart. Either end member should be easily recognized by its unusual petrographic and geochemical signature and overwhelming petrophysical effect on the rock.
Methodical searches for meteoric diagenetic traps should be most productive in areas with moderate drilling density (for rock control), relatively simple facies distributions (to minimize facies prediction problems), and accentuated paleotopography (where cross-formational meteoric tight zones can form appropriate trapping geometries). Drowning successions should also be more prospective because of a greater incidence of ephemeral exposure of unstabilized sediments during brief sea level drops.
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Looking for insights and methods useful for predicting and identifying subaerial exposure surfaces and associated porosity? This memoir addresses four major topics: Detection of unconformities; Modification of porosity during exposure; Preservation of porosity during burial; and Influence of unconformities on subsequent depositional and diagenetic patterns.