--> Abstract: Icehouse Carbonate-Platform Meteoric Bulk-Porosity Evolution: Insight from Forward-Modeling with Carb3d+, by Richard J. Paterson, Fiona F. Whitaker, Peter L. Smart, and Gareth D. Jones; #90078 (2008)

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Icehouse Carbonate-Platform Meteoric Bulk-Porosity Evolution: Insight from Forward-Modeling with Carb3d+

Richard J. Paterson1, Fiona F. Whitaker2, Peter L. Smart3, and Gareth D. Jones4
1ExxonMobil International Limited, Leatherhead, United Kingdom
2Department of Earth Science, University of Bristol, Bristol, United Kingdom
3School of Geographical Sciences, University of Bristol, Bristol, United Kingdom
4Chevron Energy Technology Company, San Ramon, CA

Icehouse carbonate diagenesis is complex. Prolonged subaerial exposure can impart a strong early meteoric-diagenetic signature through a carbonate platform, a consequence of high-frequency high-magnitude sea-level cycles. We have used CARB3D+ to forward-model the evolution of porosity in a generic platform using rates of diagenesis derived from hydrochemical studies of the modern Bahamas (high-stand island) and Guam (uplifted analogue for lowstand island).

There is an apparent contradiction between the significant net dissolution evident from calcium concentrations in modern carbonate groundwaters under all climates (at rates of up to several %/ky according to hydrochemical studies), and the prodigious amount of apparently meteoric cementation in the rock record (with reduction of depositional grainstone porosities of > 45 % to limestone porosities of < 35 % before burial diagenesis). Using modern rate data for subsurface diagenetic processes, a range of porosities can be simulated depending upon assumptions made regarding both hydrological routing of waters through the vadose zone and the character of freshwater-lens diagenesis. However, using most realistic scenarios, it is difficult to simulate pre-compaction porosity values of less than 60 %. Only by specifying an external input of calcium carbonate at least equal to the amount discharged from the meteoric system, can geologically reasonable porosities be modeled. The most plausible input into the open-system is calcium carbonate derived from land-surface dissolution, and we explore implications of reprecipitation within the vadose zone and freshwater lens. Because the source of this surface-derived carbonate is missing from the rock record, only by forward modeling and examining the rock record for evidence of missing section can we explore this process and its importance for subsurface porosity evolution.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas