Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Pore Structure Of Opal-CT And Quartz Phase Porcelanites, Monterey Formation, California


In this study, we document significant variation in pore size, structure, distribution, and connectivity in quartz and opal-CT phase porcelanites of different composition and fabric. These rocks provide key reservoirs in the San Joaquin basin and elsewhere in California. In spite of similar bulk physical characteristics, porcelanites demonstrate great differences in producibility. Understanding and predicting these differences is critical to improved exploitation. Previous studies have pointed out the significance of silica phase (opal-CT or quartz) to permeability of the rocks, suggesting that an order-of-magnitude lower permeability in opal-CT phase porcelanites was likely due to difference in pore-throat size as indicated by mercury injection capillary pressure measurements. We present the first public documentation of the ultramicroscopic character of pores, primarily using secondary electron scanning electron microscopy of surfaces cut with focused ion beams and backscattered electron imaging of epoxy injected rocks. We recognize and characterize significant differences in pore structure between opal-CT and quartz-phase porcelanites, but also between rocks of the same silica phase with distinctly massive or laminated sedimentary fabric. One high-silica (11% detritus, 88% silica) quartz-phase porcelanite has a heterogeneous microstructure with low porosity matrix and high porosity lenticular patches, yielding bulk porosity of 19%. Pores are largely non-connected and form ineffective porosity. In contrast, a laminated quartz porcelanite has a heterogeneous microstructure porosity with ∼ 100 μm-thick laminations that alternate between highly porous (35%) with good interconnectivity and low porosity layers (19%) with isolated pores. This porcelanite has almost identical composition as the massive sample (12% detritus, 89%) silica but has bulk porosity of 26%. These estimates of bulk porosity from microanalysis- (19-26%) are similar to those found in previous studies by standard core analysis methods. Opal-CT porcelanites reveal an entirely different porosity microstructure, we discovered that almost half of the porosity of a high-silica sample is isolated within the cores of lepisheres by mantles of impermeable, virtually pore-free opal-CT. These lepisphere cores contain as much as 41% porosity. Better connected, and larger interlepisphere porosity makes up less than half of the total pore space that would be indicated by logging techniques.