Genetic Origin and Diagenetic Transformation of LMC Microcrystal Textures in Limestones

Abstract

Diagenetic low-magnesium calcite (LMC) microcrystals are ubiquitous, and have been shown to significantly impact reservoir quality in ancient limestones. Previous work demonstrates that LMC microcrystals can be grouped into three major textural classes, and that these classes are strongly linked to porosity and permeability in micropore-dominated carbonates. Numerous case studies have described the origin and development of microporosity in specific carbonate reservoirs. A handful of experimental studies also exist, but they have not been systematic in their approach. As such, the precise controls on LMC microcrystal size and texture are largely unknown. This study presents findings from laboratory experiments specifically designed to evaluate the origin and development of the various types of LMC microtextures observed in nature. It specifically tests the hypothesis that the first LMC microcrystals to form during diagenetic stabilization of metastable carbonates produce the granular-euhedral texture. To investigate the controls on LMC texture, the approach involved systematically changing fluid parameters such as temperature and chemical composition, as well as reactant parameters such as minerology, texture, and grain size in laboratory stabilization experiments. Powder x-ray diffraction was used to determine mineralogy and abundance of the phases present. Resultant crystalline textures were documented using scanning electron microscopy. In the first series of experiments, 100 mg of pulverized aragonite was loaded with 15 ml of distilled water into Teflon-lined reaction vessels. To evaluate the effects of reaction temperature, stabilization experiments were run at 70, 100, 150, and 200 °C. Although the rate of transformation from aragonite to calcite increased with temperature, LMC products from all four experiments exhibited the same granular-euhedral texture observed in the rock record. Average crystal size was observed to increase with decreasing reaction temperature. These findings represent the first comprehensive effort to experimentally evaluate the controls on LMC microcrystal size and morphology

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018