An SEM Study of Porosity in the Eagle Ford Shale of Texas—Pore Types and Porosity Distribution in a Depositional and Sequence-stratigraphic Context

Abstract

Although typically considered with a focus on high-resolution petrography, shale porosity should not be thought of as a stand-alone petrographic feature. Shale and mudstone porosity is the outcome of a long succession of processes and events that span the continuum from deposition through burial, compaction, and late diagenesis. For the Eagle Ford Shale this journey began with accumulation in intra-shelf basins at relatively low latitudes on a southeast-facing margin during early parts of the late Cretaceous. To understand the factors that generated and preserved porosity in this economically important interval, a scanning electron microscope study on ion-milled drill-core samples from southern Texas was conducted to understand the development of petrographic features and porosity and place them in stratigraphic context. The studied samples show multiple pore types, including pores defined by mineral frameworks (clay and calcite), shelter pores in foraminifer tests and other hollow fossil debris, and pores in organic material (OM). In many instances, framework and shelter pores are filled with OM that has developed pores due to maturation. Large bubble pores in OM suggest that hydrocarbon liquids were left behind in or migrated into these rocks following petroleum generation and that the bubbles developed as these rocks experienced additional thermal stress. These larger OM pores indicate deeper seated interconnection on ion-milled surfaces and in three-dimensional image stacks.

Surfactant inhibition mechanisms of carbonate mineral dissolution in shale

“…They show that the sample is comprised of 67% calcite, 8.7% quartz, 6.2% Illite and Mica, 4.2% Illite/Smectite mixture, 5% K-feldspar, 4.1% plagioclase, and minor amounts of pyrite, kaolinite, chlorite, apatite, and siderite. This composition is similar to that measured by others[6][7][8], except for the lack of small amounts of gypsum and dolomite.The cation exchange capacity of the Eagle Ford shale is 89.1 meq/kg. The Ca and alkalinity (HCO3 -) released into brine by Eagle Ford shale upon exposure to 0.5 M MgCl2 after rinsing three times in brine were also measured.…”

“…In the former they are rich sources of oil and natural gas, and in the latter they serve as cap rock to prevent carbon dioxide escape. Shales are complex assemblages of fine mineral fragments and organic matter of varying reactivity [5], and in many cases are dominated by highly reactive carbonate minerals (e.g., Eagle Ford shale > 60% calcite [6][7][8], Wolfcamp and Marcellus shales >80% calcite+dolomite). Shales are exposed to injected fluids during hydraulic fracturing and geological carbon sequestration leading to dissolution and secondary mineral precipitation.…”

“…Per XRD, the only Ca containing mineral measured was calcite. However, the presence of Mg and SO42-indicates minor amounts of dolomite and gypsum are present, and they have been identified in other samples of Eagle Ford shale[6][7][8].Also per Figure2, Ca, Mg, and SO4 2concentrations in brine and IOS brine are different. For Ca, this difference is only significant at 3 hours, and then it becomes indistinguishable with respect to the measurement error.…”

Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south‐central Utah: Implications for the pursuit of climate and burial histories

“…Key products of these reactions include bicarbonate ions, reduced manganese and iron, and sulphide, all of which can be incorporated into early diagenetic minerals such as calcite, pyrite, siderite and dolomite (Berner, 1984;Raiswell, 1988;Macaulay et al, 1993;Raiswell and Fisher, 2000;Taylor et al, 2000). Over the past decade, an increasing number of case studies have suggested that significant amounts of kaolinite in some ancient mudstone successions is of early diagenetic origin (Aplin and Macquaker, 2012;Macquaker et al, 2014;Taylor and Macquaker, 2014;Schieber et al, 2016), instead of being introduced to sedimentary basins by rivers that had intensely weathered soils in their catchment areas (Thiry, 2000;Junttila et al, 2005;Wendler et al, 2016). Over the past decade, an increasing number of case studies have suggested that significant amounts of kaolinite in some ancient mudstone successions is of early diagenetic origin (Aplin and Macquaker, 2012;Macquaker et al, 2014;Taylor and Macquaker, 2014;Schieber et al, 2016), instead of being introduced to sedimentary basins by rivers that had intensely weathered soils in their catchment areas (Thiry, 2000;Junttila et al, 2005;Wendler et al, 2016).…”

“…That clay minerals can also form during this shallow burial stage has been known for some time (Odin, 1990;Mackenzie and Kump, 1995;Michalopoulos and Aller, 1995) but still is not considered of significant importance in many published studies of mudstones. Over the past decade, an increasing number of case studies have suggested that significant amounts of kaolinite in some ancient mudstone successions is of early diagenetic origin (Aplin and Macquaker, 2012;Macquaker et al, 2014;Taylor and Macquaker, 2014;Schieber et al, 2016), instead of being introduced to sedimentary basins by rivers that had intensely weathered soils in their catchment areas (Thiry, 2000;Junttila et al, 2005;Wendler et al, 2016).…”