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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract


 
Chapter from: M 63:  Unconformities and Porosity in Carbonate Strata 
Edited By
D.A. Budd, A.H. Saller, and P.M. Harris

Authors:
P.D. Wagner, D.R. Tasker, and G.P. Wahlman

Carbonate Reservoirs

Published 1995 as part of Memoir 63
Copyright © 1995 The American Association of Petroleum Geologists.   All Rights Reserved.

 
Chapter 9

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Reservoir Degradation and Compartmentalization below Subaerial Unconformities: Limestone Examples from West Texas, China, and Oman
 

P. D. Wagner
Amoco Exploration and Production Technology
Tulsa, Oklahoma, U.S.A.
D. R. Tasker
G. P. Wahlman
Amoco Exploration and Production Co.
Houston, Texas, U.S.A.


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ABSTRACT

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.

 

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