Abstract

We document massive recrystallization of low-Mg calcite lime mudstone source rocks at moderately high temperatures in the Smackover Formation of Mississippi. This process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons, secondary porosity generation in reservoirs, and global environmental change. This interpretation is based on contrasting the following petrographic and bulk-rock geochemical characteristics of organic-poor vs. organic-rich lime mudstones:

1. Total organic carbon (TOC): 0.01 vs. 0.5%

2. δ13C: +4.4 vs. +5.2‰ Peedee belemnite (PDB)

3. δ18O: +0.8 vs. -3.3‰ PDB

4. Sr content: 900 vs. 420 ppm

5. Small calcite crystals (12 μm) that have smooth crystal surface and sharp edges vs. large calcite crystals (20 μm) that have pitted surface and irregular edges

Evaluation of petrographic observations and geochemical data, as well as the absence of lime mud-producing algae, suggest that Smackover lime mud precipitated inorganically as low-Mg calcite from a near-normal salinity Late Jurassic seawater and was deposited in deep waters (>100 m) unaffected by early meteoric diagenesis. High δ18O and Sr compositions (+0.8‰ PDB and 900 ppm, respectively) of organic-poor lime mudstone layers are nearly identical with the composition of modern low-Mg calcite, suggesting preservation of original mineralogical and geochemical signatures. Low δ18O and Sr values (-3.3‰ PDB and 420 ppm, respectively) of organic-rich lime mudstones are best explained by recrystallization at moderately high temperatures (50-75 °C), driven by organic acids produced during kerogen maturation.

Observations presented in this article provide insight into a mechanism by which the maturation of organic matter can create primary migration pathways along which hydrocarbons are expelled from low-porosity and low-permeability source beds. Our observations also imply that in carbonate systems, organic acids generated during kerogen maturation are neutralized within the source rock. Therefore, generation of secondary porosity in carbonate reservoirs likely requires a source of acid unrelated to kerogen maturation. By analogy, carbonates and feldspars may be sufficiently abundant in siliciclastic source rocks to neutralize all organic acids. Acids not consumed within the source rocks react with framework grains, generating secondary porosity along migration paths or in sandstone reservoirs. Our observations also suggest that carbonate rocks and fossils in organic-rich strata are prone to diagenetic alteration by organic acids and, consequently, should be discounted as paleoclimatic indicators.

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