Important mineralogical factors for fluid saturation, specific surface area and pore size distributions based on gas adsorption, cation exchange capacity and 2D dielectric microscopy: a case study of quartz phase porcelanites in the Miocene Monterey Formation

Abstract

The diagenetic processes and unique mineralogical composition of the Miocene Monterey formation have attracted scientists from a wide array of backgrounds endeavoring to resolve its deepest mysteries. While the formation has historically been an excellent source rock, sourcing many of the conventional reservoirs found in California, its diverse lithologies in conjunction with the presence of systematic natural fractures, also make it an effective reservoir. The complexities in formation evaluation stem from the formation's heterogeneous nature, being highly fractured, containing thin beds (multiple electrofacies), organic matter, interbedded carbonates, various clay types in a wide array of distributions, conductive minerals and possibly being fractionally-wet. It is the primary scope of this study to investigate the impact of mineralogical complexity on oil and brine saturation distribution in quartz phase porcelanite core. The impact of mineralogical complexity on preferential fluid imbibition was investigated with spontaneous imbibition experiments using brine and mineral oil, and assessed with dielectric microscopy and QEMSCAN®. A 2D high frequency dielectric scan of a porcelanite sample exposed to brine and mineral oil showed that carbonate rich layers imbibed more mineral oil than quartz rich areas. Quartz rich areas required higher porosity and/or microfractures to imbibe any mineral oil at all. It is conceivable that in the absence of connected fractures, dolomitic facies in the Monterey could have higher oil saturations than quartzitic facies. Furthermore, quartz phase porcelanites with some carbonate content can possibly be considered fractionally-wet, requiring lower expulsion pressures to imbibe the rock with hydrocarbons. A secondary scope of this study was to investigate a suitable laboratory technique to characterize specific surface areas of porcelanite containing kerogen. Ethylene glycol monomethyl ether (EGME) derived specific surface areas are an overestimate because of the reaction with the kerogen itself. Surface areas in this study were measured by nitrogen gas adsorption and compared to total specific surface areas derived from the EGME method. Cation exchange capacity (CEC) measurements were obtained for the same samples to develop a correlation between CEC and Brunauer-Emmet-Teller (BET) specific surface areas in quartz phase porcelanite. Pore size distributions were also obtained from nitrogen gas adsorption, and were used to investigate the controls of quartz and carbonate content on the pore structure of porcelanite. The main conclusions from these various techniques on Miocene Monterey formation samples are: 1. Dielectric microscopy at high frequencies shows that carbonate rich layers have a high affinity for oil, while quartz rich matrix requires higher porosity and/or microfractures for oil imbibition 2. Total specific surface area (TSSA) of a kerogen-rich porcelanite sample can be quickly estimated with the BET surface area-CEC correlation developed in this study 3. Clay content and clay type seem to increase the micro and meso pore content (IUPAC), while carbonate content seems to increase the macro pore content (IUPAC)