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Abstract
Chapter from: M
63: Unconformities and Porosity in Carbonate Strata
Edited By
D.A. Budd, A.H. Saller, and P.M. HarrisAuthors:
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. |
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Chapter 9
*
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.
*
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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