Origin, Description and Characterization of Upper Cretaceous Eagle Ford Pore Networks, Karnes County, Texas

Abstract

Recent studies have demonstrated that loss of primary pores and development of secondary pores in mudrocks are primarily controlled by burial diagenesis of the sediments and thermal maturation of organic matter. Lack of quantitative data on micro- to nanometer rock properties, however, limits the ability to understand and predict petrophysical properties and fluid flow in these fine-grained rocks. To upscale these rock properties requires detailed quantification of the pore types and distribution at multiple scales. Upper Cretaceous Eagle Ford mudstone samples were collected from continuous cores taken from two adjacent (∼6 km) oil-producing wells in Karnes County, Texas to investigate small-scale variations in mineralogy, diagenesis, and pore types. Backscattered and secondary electron images were collected at 5,000X, 15,000X, and 120,000X (instrument magnification) using a field-emission scanning electron microscope (FE-SEM) to capture a broad range of visible pore sizes and pore distribution. Consistent point-count methods were used to systematically quantify pore types. Pore-tracing methods were used to validate point-count methods as well as to provide size and shape information of the organic-matter (OM) pores and mineral pores. Eagle Ford facies in the studied include: (1) thin ash beds, (2) globigerinid-bearing, laminated, argillaceous wackestone-packstone (marl) with varying organic matter content, and (3) lime packstone with varying calcite, quartz, and clay mineral content. Samples from the two cores show similar thermal maturity histories. Pores include both secondary OM pores in migrated solid bitumen and primary interparticle pores between coccolith elements (with residual OM). The Eagle Ford mineral pore network consists of mineral pores originally saturated with formation water and partly cemented mineral pores containing migrated bitumen with OM pores. SEM-image-based point-count porosity data show that Eagle Ford pore network in both wells is dominated by primary mineral pores, but that secondary OM pores in migrated bitumen are also important. This study concludes that the reservoir quality of Eagle Ford mudstones varies significantly at similar levels of thermal maturation. Pore morphology, TOC, and mineralogy all impact total porosity. A positive correlation was found between the amount of OM porosity and TOC, and between mineral porosity and volume percentage of quartz and feldspar.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016