Origin and characterization of Eagle Ford pore networks in the south Texas Upper Cretaceous shelf

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ABSTRACT Recent studies have shown that the loss of primary pores and the development of secondary pores in mudrocks are primarily controlled by burial diagenesis of the mineral matrix and thermal maturation of organic matter (OM). However, the lack of quantitative data on nanometer- to micrometer-scale rock properties has limited the ability to define and predict petrophysical properties and fluid flow in these fine-grained rocks. To upscale these rock properties, quantitative data are needed at multiple scales. Representative Eagle Ford Group samples were collected from continuous cores taken from two adjacent oil-producing wells in Karnes County, Texas, to investigate small-scale variations in mineralogy, diagenesis, and pore type. Point-count and pore-tracing methods were used to systematically quantify pore types and determine the size and shape of the identified pores. The two cores from the Eagle Ford are dominated by modified mineral pores, although secondary OM pores in migrated petroleum (bitumen) are also important. The mineral-pore network includes (1) primary mineral pores originally saturated with formation water and (2) modified mineral pores containing migrated petroleum (bitumen and/or residual oil). The OM-pore network includes (1) primary OM pores and (2) secondary OM pores including relatively large, less abundant OM bubble pores and relatively small, more abundant OM spongy pores. The abundance of OM spongy pores correlates positively with total-organic-carbon (TOC) content, and that of mineral pores weakly correlates with the volume of quartz plus feldspar. Studied samples have similar thermal maturities, although samples from one deeper core are slightly more mature than the other. Except for thermal maturation, the strong, micrometer-scale heterogeneity of rock components and properties (texture, fabric, mineralogy, and TOC) impacts the abundance, distribution, and type of pores. This micrometer-scale heterogeneity in porosity and pore networks would, in turn, significantly impact matrix permeability.