Micromorphological, Stable Isotope, NMR, Geomicrobial and Crystallographic Analysis of Quaternary Calcrete Formation, Puerto Rico

Abstract

Pedogenic calcretes are widely used as evidence for subaerial exposure and are commonly used as sequence boundaries. Due to their association with exposure in dry regions, calcretes are commonly used as indicators for arid and semiarid paleoclimates. Yet in the literature calcretes have been reported from areas with as much as 990 mm/year, and more recently we have documented calcretes in tropical settings with precipitation over 1600 mm/year. To improve our interpretation of environment of deposition (EOD) using calcretes, this study aims to produce quantitative diagnostic criteria to identify semiarid versus humid calcretes in the rock record. Such EOD criteria is useful for carbonate reservoir characterization since calcretes have the potential to affect subsurface fluid flow. Fieldwork results reveal that semiarid calcretes are laterally extensive, continuous, and impermeable and thus, if preserved, they have the potential to compartmentalize (locally) reservoirs. In contrast, humid calcretes are patchy, discontinuous, and semipermeable which may not affect reservoir quality. Identifying exposure surfaces in these humid areas may be difficult given the paucity of the calcretes' extent. Microscopy results show that semiarid profiles are extensively recrystallized with micrite, microspar, and pseudospar leading to decreased primary porosity. Humid profiles contain well-preserved original fabrics and uncemented voids leading to preserved primary porosity. Laboratory results from NMR illustrate that semiarid calcretes have lower porosity relative to the host sediment, which is inferred to result from the extensive cementation and recrystallization, indicating high amounts of diagenesis, hence, tight carbonates (local traps?). Humid calcretes have higher porosity relative to the host sediment, reflecting the incomplete cementation and infilling of voids, indicating minimal diagenesis, thus porous carbonate deposits. Integrated calcrete field observations, micromorphology, and geophysics improve EOD interpretation of secondary carbonates in marine and terrigenous settings as well as provide useful information about carbonate reservoir characteristics.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017