Analysis of hydrocarbon under-filled and water-filled Miocene deepwater reservoirs, eastern Mexico offshore

Abstract

Hydrocarbon exploration in the deepwater portion of the southwestern Gulf of Mexico commenced a decade ago, but discoveries have not been able to meet the optimistic expectations based on historical success in the northern Gulf of Mexico. Despite the existence of organic-rich source rocks and high-quality Miocene reservoir rocks, there is a concern that the lack of significant hydrocarbon accumulations in some prominent traps in the northern Catemaco fold belt (Veracruz Trough) resulted from an ineffective sealing and/or trapping mechanism.By integrating drill core, 3D seismic, and well data provided by the Mexican National Hydrocarbon Commission (CNH), this paper attempts to evaluate the cause of hydrocarbon under-filled or water-filled Miocene traps within the Veracruz Trough, using the prominent structural closures drilled by Pemex-operated Kunah-1 and Yoka-1 deepwater wells as case studies. Three hypotheses are considered and analyzed for these two large traps: top seal leakage, existence of unmapped shallow spill points, and late trap formation relative to timing of hydrocarbon generation and migration.Seal rock leakage analyses, which involves downhole pressure data analysis and mercury injection capillary pressure laboratory measurements on cuttings, suggest that many of the evaluated intra reservoir and top seal rocks could have held significantly taller hydrocarbon columns than encountered by the wells. In addition, 3D seismic structural interpretation around the Yoka trap indicates that all mapped spill points are much deeper than the observed fluid contacts. However, the trap formation timing assessment shows that by the time the traps began to develop, the main oil generation phases had largely concluded, and gas generation was at a very late stage for the most prominent source rocks. Consequently, a more robust explanation for the Kunah and Yoka under-filled and water-filled structures is due to a timing mismatch, with late trap formation relative to an earlier hydrocarbon charge.