Abstract

Low-maturity Boquillas (Eagle Ford equivalent) outcrop samples (carbonate-rich facies) were heated anhydrously under confining pressure to study the evolution of organic-matter (OM)-hosted pores and mineral pores.

Oil and gas yields, Rock-Eval and Leco TOC analyses were used to characterize kerogen type, organic matter conversion, and thermal maturation. Calculated vitrinite reflectance (from Tmax) indicated that the outcrop Boquillas samples have reached the thermal maturity level of about 0.65 to 0.7%Ro, implying that they have reached the stage of early-hydrocarbon generation. Samples were also prepared using Ar-ion polishing to look at pore evolution under field-emission scanning electron microscopy (FE-SEM). Based on SEM petrography, we observed several diagenetic features that formed prior to hydrocarbon migration. These features include carbonate cement, quartz cement, kaolinite cement, chlorite cement and dolomite replacement.

Two major pore types were observed in outcrop samples: OM-hosted pores and mineral pores. We interpreted the occurrence of OM-hosted pores to be the result of hydrocarbon phase transformation. With increasing laboratory heating temperature, the character of OM changed through different phase transformation stages. OM connectivity also changed and new types of OM-pores were formed. Several different OM-hosted pore types coexisted at liquid generation stage, and pores were in association with solid kerogen, liquid oil, solid bitumen, solid pyrobitumen, and products from secondary cracking. The abundance and morphology of OM-hosted pores changed from no pores to large micrometer-sized OM-hosted pores, and then finally to small equant nm-sized OM-hosted pores. The small equant pores are similar to those observed in naturally matured samples and are most probably associated with methane generation (dry gas stage).

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