Influence of Primary Rock Texture, Diagenesis, and Thermal Maturity on Eagle Ford Pore Systems

Ozkan, Aysen; Milliken, Kitty L.; Macaulay, Calum; Johnston, Matthew; Minisini, Daniel; Eldrett, James S.; Bergman, Steven; Kelley, Amy

Understanding controls on pore networks can shed light into hydrocarbon storage and production capacity of unconventional reservoirs such as Eagle Ford from the Maverick basin, South Texas. In shale-gas systems, nanometer- to micrometer-size pores, including those in natural fractures, form permeability pathways that allow flow of gas from the mudstone to induced fractures. We investigate influence of primary rock texture, diagenesis, and thermal maturity on evolution of pore networks in Eagle Ford samples from the outcrop (Del Rio area) and subsurface (Chittim and Piloncillo Ranch fields) utilizing light microscopy, FE SEM, cathodoluminescence, and XRD.

We divide the Eagle Ford into four facies on the basis of texture, diagenesis, mineralogy and organic content: bioturbated and laminated calcite-rich mudstones that dominate the upper Eagle Ford (UEF) and organic-rich laminated mudstones and calcite-replaced mudstones that dominate the lower Eagle Ford (LEF). UEF deposition was generally characterized by higher oxygenation levels and higher bioturbation. The LEF was deposited under lower oxygenation and higher productivity conditions; and as a result, characterized by higher organic matter abundance.

Eagle Ford samples contain a diversity of pore types including intergranular, intragranular, and organic matter-hosted pores with their relative abundances depending on thermal maturity and lithofacies type. Both inter- and intra-granular pores in the Eagle Ford are primary, corresponding to spaces between grains or within grains such as foraminifers and pellets. Bioturbated samples overall have more calcite cementation which reduces the intergranular pores. While intraparticle pores within foraminifera are sealed with calcite cement in the UEF, they are mostly filled with kaolinite and organic matter along with minor pyrite and calcite in the LEF. Organic matter-hosted pores are common in the organic-rich LEF.

The occurrence of organic matter-hosted pores is influenced by thermal maturity. This porosity type likely evolved with the thermal transformation of labile kerogen or early-formed bitumen, and therefore, they are better developed in higher thermal maturity samples. Our initial investigation suggests that organic matter type also influences abundance of organic-matter hosted pores. SEM observations suggest that pores in organic matter generally are an important component of the hydrocarbon storage and mobility network in mature organic-rich mudstones.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013