Microstructural Analysis of the Transformation of Organic Macerals During Artificial Thermal Maturation
A dynamic heating experiment using a field-emission environmental scanning electron microscope (FE-ESEM) equipped with a heating stage was designed to observe microstructural changes in a variety of organic macerals identified by optical organic petrology. The same region of interest was compared before and after heating by employing correlative optical and electron microscopy. An Ar-ion milled surface was prepared from a thermally immature (0.50 %Ro vitrinite reflectance), organic-rich (5-7 wt% TOC) outcrop sample of the Boquillas (Eagle Ford) Formation. A variety of organic macerals were identified by standard optical organic petrography including: 1) structured particles of vitrinite, inertinite, liptinite, and organo-mineral aggregates, 2) diffuse amorphous organic matter, and 3) solid bitumen. The sample was heated in the FE-ESEM to 500oC at a rate of 1oC/min with a constant 2.0 Torr (266 Pa) vapor pressure. Static and video secondary electron images were captured using a heat compatible gaseous detector at 30 kV accelerating voltage. High resolution backscattered FE-SEM mosaics were also prepared before and after heating. The results of the experiment revealed that the alginite macerals were the most altered by heating with elongate voids created presumably due to the transformation of kerogen to petroleum. Volumetric changes were also observed in solid bitumen and amorphous organic matter. As anticipated, no detectable changes were observed in the inertinite and gas-prone vitrinite macerals. The voids created within the alginite macerals are atypical of the commonly observed organic pores in natural thermally mature subsurface samples. The slot-like voids associated with the alginite macerals are thought to be unlikely preserved at reservoir conditions due to closure at overburden pressure. The processes governing the development of organic matter porosity remains poorly understood because the type of organic matter pores typically observed in organic-rich mudstone reservoirs have yet to be duplicated in the laboratory by the various artificial thermal maturation experiments published to date.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018