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Abstract
Chapter from: M
63: Unconformities and Porosity in Carbonate Strata
Edited By
D.A. Budd, A.H. Saller, P.M. HarrisAuthor
David K. Beach 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 1
*
Controls and
Effects of Subaerial Exposure
on Cementation and Development
of
Secondary Porosity in
the Subsurface
of Great Bahama Bank David K. Beach
Marathon Petroleum Ireland,
Ltd.
Mahon Industrial Estate
Blackrock, Cork, Ireland
*
ABSTRACT
Cementation trends and porosity profiles
across multiple subaerial unconformities demonstrate how induration created
during initial subaerial exposure played an important role in controlling
fluid flow in shallow subsurface Pliocene-Pleistocene carbonate rocks on
Great Bahama Bank (GBB). This control over fluid flow helped govern loci
of dissolution and cementation during shallow burial of these metastable
carbonates. Its role varied between meteoric vadose and phreatic, and mixing-zone
diagenetic environments. Early induration also resulted in preferential
preservation of subaerial unconformities in the subsurface.
This study of cementation and porosity
trends also revealed gradual changes in diagenetic maturity of the rocks
and progressive evolution of the pore systems with increasing depth of
burial. Subsurface cementation and secondary porosity development occurred
primarily during emergence and subaerial exposure of the bank top. Three
diagenetic stages were recognized, and were related to changing diagenetic
environments regulated by changing Pliocene-Pleistocene sea level and slow
bank subsidence. Stage I, dominated by vadose diagenesis, commenced with
initial subaerial exposure
of metastable sediments, and ended with
development of an induratedsurface breached locally by vertical solution
pipes. In Stage II, with shallow burial (surface to variably 12 to 20 m)
and under ephemeral freshwater phreatic conditions, metastable carbonate
sediments completed alteration to low-Mg calcite, and porosity inverted
from primary interparticle and intraparticle to moldic. Relatively uniform
cementation by equant calcite also occurred. In Stage III (depths to 150
to 200 m), subjection of deeper subsurface rocks to prolonged episodes
of corrosive bank-wide freshwater phreatic and mixing-zone conditions during
bank emergence resulted in extensive dissolution. Because GBB is comprised
of carbonate rock and lacks siliciclastic aquitards, freshwater lenses
and underlying mixing zones fluctuated
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