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Pore Morphology and Distribution in the Cretaceous Eagle Ford Shale, South Texas, USA

Robert M. Reed and Stephen C. Ruppel
Bureau of Economic Geology, The University of Texas at Austin, Box X, University Station, Austin, TX 78713

The Upper Cretaceous Eagle Ford Shale is dominantly an organic-rich, calcareous mudstone (Harbor, 2011) found in the Maverick Basin and adjoining areas of south Texas. For hydrocarbon production, the unit functions as a source, seal, and reservoir.

Samples were Ar-ion-milled for pore imaging as described in Loucks et al. (2009). Ar-ion milling is a sample-preparation technique that provides a flat surface free of artifacts related to mechanical polishing, such as plucking and gouging. Ion-milled sample surfaces were examined primarily using an FEI Nova NanoSEM 430 field-emission scanning electron microscope (SEM). Mineral identifications and phase identification maps were made using a Bruker XFlash® SDD energy dispersive spectroscopy system.

More than 50 samples were collected from core of 10 different wells located throughout the Maverick Basin and the adjoining San Marcos Arch (Fig. 1). Sample depths range from 3313 ft (1,009.8 m) to 13,825 ft (4213.9 m). Vitrinite reflectances estimated (VRoe) from Tmax range from 0.5% to 2.4%. A series of SEM images was collected from each sample to characterize the types and distribution of pores.

The Eagle Ford exhibits a range of nanometer- and micrometer-scale pores. In most samples, interparticle pores are the dominant pore type. However, in many samples intraparticle pores are also present in grains and organic matter. Calcareous foraminifera with partly open and partly cemented chambers are a distinctive feature of several facies. Multiple pore-filling cements, including kaolinite, calcite, bitumen, pyrite, chlorite, and quartz indicate a complex diagenetic history for these mudrocks.

Interparticle pores are common in Eagle Ford samples from all depths. Silty calcareous samples typically have more interparticle pores between the various grain types. Pore diameters range from nanometer-scale to several micrometers. Pore development is heterogeneous both between samples and commonly within samples.

Pores in organic matter (Loucks et al., 2010; in press) are developed in most samples, particularly higher thermal maturity samples. Low thermal maturity samples do not contain significant numbers of organic matter pores. The divide between samples with organic pores and samples without significant organic pores seems to be in the vicinity of 0.8% VRoe. However, even in high maturity samples there is typically some small amount of organic matter that does not contain pores.

 

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012