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Pore Systems in a Volcanogenic-Grain-Bearing Mudrock: The Jurassic-Cretaceous Vaca Muerta Shale of Argentina

Abstract

This study examined 21 large-area, Ar-ion-milled samples from three cores in the Jurassic-Cretaceous Vaca Muerta Shale of the El Trapial area, Neuquén Basin, Argentina, with an emphasis on pore characterization. Sample depths range from 2450 to 4008 m (8038 to 13,150 ft) and thermal maturities range from peak oil to gas condensate. Lithologic heterogeneity is significant, with variation from terrigeneous-rich mudrocks (tarls), to carbonate-rich mudrocks (argillaceous carls), to bio-siliceous-rich mudrocks (argillaceous sarls). Mafic volcanogenic debris is a common component. Pores can be classified as organic-matter (OM) pores, intraparticle pores and interparticle pores. Of particular interest for the OM pores is a depth-related increase in OM porosity. For the shallowest three samples, OM pores are rare. Deeper samples show varied but higher levels of OM pore development. Overall, the positive depth dependence of OM pores is more pronounced than in sample sets from other formations we have studied. OM pores are found both in OM interpreted to be kerogen and OM interpreted to be diagenetic organic matter (solid bitumen). Pore morphology tends to vary between these two cases, with kerogen-hosted pores being more elongate and larger (up to micrometers in length) and bitumen-hosted pores being more spherical and smaller. Intraparticle pores are present in most samples. However, intraparticle pores are generally minor and may not connect well to pores in the rest of the rock. Mobile organic matter (solid bitumen) has filled many intraparticle spaces that were once present. However, some intraparticle pores remain, especially associated with pyrite (both framboids and larger masses) and phosphate grains. Some intraparticle pores appear to be related to incomplete albite replacement of detrital feldspars or feldspar-rich volcanic rock fragments. Interparticle pores very widely from sample to sample, and even for different areas within samples. The controlling factor seems to be the degree to which ductile or mobile OM has filled in the spaces between grains, especially clay mineral particles. It is not uncommon to find areas where porous OM fills only some interparticle spaces and interparticle pores are still present.