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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received December 17, 1997;
revised manuscript received February 10, 1999; final acceptance February
28, 1999.
2Unocal, 14141 Southwest Freeway,
Sugar Land, Texas 77478; e-mail: [email protected]
3Department of Earth Science, Cambridge
University, Cambridge CB2 3EQ, United Kingdom.
4Japan National Oil Company, Technical
Research Center, Chiba, Japan.
ABSTRACT
Diagenesis and porosity development have distinct
patterns related to duration of subaerial exposure. Four stages of porosity
development are identified. (1) Very brief or no subaerial exposure (estimated
at less than 5000 yr) caused little or no diagenetic change. (2) Brief
to moderate subaerial exposure (estimated at 5000-50,000 yr) resulted in
most primary pores being filled with calcite cement, and dissolution creating
fine matrix pores (molds and intercrystalline pores). (3) Moderately long
subaerial exposure (estimated at 50,000-130,000 yr) resulted in cements
filling primary pores and some fine secondary pores, and dissolution creating
small conduit pores (vugs, fractures, fissures). (4) Prolonged subaerial
exposure (estimated at greater than 130,000 yr) resulted in most primary
and secondary matrix pores being filled with calcite cement, but dissolution
enlarged conduit pores (vugs, fractures, fissures).
Present subsurface porosity in this field preferentially
occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones
in cycles subjected to brief subaerial exposure (stages 2 and 3). Matrix
porosity (molds, intercrystalline pores) is dominant because most conduit
pores formed during prolonged subaerial exposure were filled with either
shale during subsequent transgressions or burial cements derived from pressure
solution associated with the shales. The distribution of porosity in the
Southwest Andrews area indicates that duration of subaerial exposure and
supply of clastics are major factors determining ultimate porosity in limestones
subjected to subaerial exposure and karstification.
Middle Pennsylvanian-Lower Permian limestones
in the subsurface of west Texas were studied to determine how subaerial
exposure and freshwater diagenesis (karstification) affected porosity distribution
in meter-scale cycles. Approximately 87 depositional cycles are present
in the gross reservoir interval (depths of 2600-3000 m), and each cycle
is interpreted to represent a glacio-eustatic sea level fluctuation. Using
recent radiometric age dating, average cycle duration is estimated at 160,000
yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross
reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals
in the upper part of cycles that were subaerially exposed; however, many
cycles that were subaerially exposed now lack porosity.
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