Deep-burial dissolution in an Oligocene-Miocene giant carbonate reservoir (Perla Limestone), Gulf of Venezuela Basin: Implications on microporosity development

摘要

Sedimentary rocks tend to progressively lose porosity with depth due to mechanical and chemical compaction. In carbonates, this trend is hard to predict since many factors can create porosity in the burial setting. The Oligocene-Miocene Perla Limestone, a giant gas reservoir located in the Gulf of Venezuela Basin, shows a complex porosity system marked by a strong diagenetic control. Despite exhaustive depositional facies modelling carried out in the carbonate reservoir, inconsistencies remained when distributing petrophysical properties with depositional facies. These inconsistencies become more important in areas strongly affected by intense diagenetic processes. After the initial investigation, burial dissolution was identified as the main diagenetic process affecting porosity in this succession. To understand the origin of the burial dissolution process and its distribution along the reservoir, detailed petrographic, mineralogical and isotopic studies were carried out on recovered cores from different exploration wells. Results show a series of diagenetic features which were associated with the evolution of the strata including burial dissolution, dolomitization and compaction as the main processes. After testing several hypotheses, the results support an inorganic-CO2 model as responsible for the burial dissolution process in the Perla Limestone. According to this model, deep-burial dissolution created pervasive microporosity, with subordinated moldic and vuggy pores that enhanced the reservoir quality in localized areas of the carbonate succession. These features are located where the ascent of hydrothermal CO2-rich fluids funneled by discontinuity surfaces (faults, fractures, stylolites, etc.) reached the Perla Limestone.