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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:
J.A.D. Dickson and Arthur H. Saller 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 12
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Identification
of Subaerial Exposure Surfaces and Porosity Preservation in Pennsylvanian
and Lower Permian Shelf Limestones, Eastern Central Basin Platform, TexasJ. A. D. Dickson
Cambridge University
Cambridge, U.K.
Arthur H. Saller
Unocal Energy Resources
Brea, California, U.S.A.
*
ABSTRACT
The southwest Andrews area on the eastern
side of the Central Basin platform (west Texas) contains cyclic Pennsylvanian
and Lower Permian shelfal limestones. Limestones were deposited in shallow
marine environments during numerous highstands of sea level, but most cycles
are bounded by subaerial exposure surfaces. Reservoir porosity is developed
in only 10-45% of those depositional cycles in any given well. The purposes
of this paper are to determine: (1) features useful for identifying subaerial
exposure surfaces, (2) factors that affect  stable -isotope profiles around
subaerial exposure surfaces, and (3) circumstances critical to porosity
preservation in subaerially exposed limestones.
1. Features commonly present at or below
subaerial exposure surfaces include an abrupt change in depositional lithology,
caliche crusts, micritic rhizoliths precipitated around roots, soil-related
fractures, breccias, and mottling associated with plant roots. Rhizoliths,
caliche crusts, and breccias have developed best in wackestones and packstones.
Mottling associated with plant roots is distinct in grainstones and was
caused by heterogeneous dissolution and cementation.
2. The stable isotope signature most characteristic
of subaerial exposure is abrupt decreases in d13C
of the carbonate immediately below subaerial exposure surfaces. This signature
is displayed best in cycles with: (a) wackestones/packstones at the top,
(b) moderate duration of subaerial exposure, (c) limited overprinting by
later meteoric diagenesis, (d) little erosion during the subsequent transgression,
and (e) negligible effects of late cements on the isotopic composition
of the bulk rock.
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