Neal R. O'Brien

Pore types in the Barnett and Woodford gas shales: Contribution to understanding gas storage and migration pathways in fine-grained rocks

The identification of organoporosity (microscale and nanoscale pores within organic matter in shales), its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering three-dimensional images of the pores, such as by argon-ion milling and/or field emission scanning electron microscopy, have all been well documented and discussed for unconventional gas shales. However, other types of pores exist within shales that can be important to storage and migration of gas (and oil), and other technologies are available for their identification and imaging. The different pore types found in the Barnett and Woodford shales are described and classified in this article. Scanning electron microscopy revealed the presence of porous floccules in both the Barnett and Woodford shales, which appear similar to laboratory-produced floccules and to those in other ancient shales. Published experimental and observational studies indicate that floccules are hydraulically equivalent to coarser grains and are transported by traction processes. Current-induced microsedimentary structures and textures within the Barnett and Woodford shales, as well as the preserved floccules, suggest such processes were active during transport and deposition. Pore spaces between the floccules are open and can provide storage space as well as permeability pathways for gas molecules through the shales. Pores are also found within organic matter that occurs both as discrete particles and as adsorbed coatings around clay grains in the shale. They are referred to here as “organopores.” Porous fecal pellets are also common in the Barnett Shale. Preserved fossil fragments such as organic-walled spores and inorganic sponge spicules have hollow central chambers, which may remain partially or completely open even after burial. Intraparticle pores occur between grains of various minerals (e.g., pyrite framboids). Microchannels within shale matrix, which may be the bounding surfaces of scours or microsedimentary structures, may also provide permeability pathways for hydrocarbon migration. Mircrofractures are also common, and their initiation might be related to mineral crystal structure. When present in sufficient quantity, these pore types offer potential gas (and oil) molecule storage spaces and permeability pathways through the shales.

Argillaceous rock atlas

Fabric analysis techniques x-radiography, petrography and scanning electron microscopy descriptions miscellaneous features in argillaceous rocks case studies of specific distinctive features case study of fabric analysis in evaluating sedimentary processes and environments formation of shale by compaction of flocculated clay-A model fabrics of some hydrocarbon source rocks and oil shales fabric of geopressured shale composition of argillaceous rocks.

The Effects of Bioturbation on the Fabric of Shale

ABSTRACT X-radiography and scanning electron microscope analyses of shales from diverse sedimentary environments reveal wide variations in macro-and microfabrics attributed to the degree of bioturbation. These fabrics provide clues about the physical condition of the sediment substrate at the time of clay deposition and the relationship between bioturbation activity and the sedimentary environment. Organic-rich shales from anoxic environments are unbioturbated and characterized by fine lamination and a microfabric of parallel clay flakes. Two fabric types were found in shales formed under oxic conditions--highly bioturbated and indistinctly bedded. Fabric in shales subjected to considerable bioturbation is recognized by a homogeneous, gray tone in an X-radiograph and by a random microfabric in a scanning electron micrograph. This fabric is also very similar to mudstones formed from lithified flocculated clay; however, it does not possess the stepped domains or stepped cardhouse fabric of the latter. Indistinctly bedded shale possesses remnant original lamination and preferred particle orientation indicative of less intense bioturbation. The fabric types are useful in determining whether the sediment substrate was a soupground, softground, or firmground, as well as indicating the amount and intensity of biogenic activity during sediment deposition.

Significance of lamination in Toarcian (Lower Jurassic) shales from Yorkshire, Great Britain

Abstract Lamination in shales is an important feature useful in providing clues to ancient sedimentary environments and processes. Variations in types of lamination in Toarcian (Lower Jurassic) shales of Yorkshire, Great Britain are related to changes in sedimentary processes. X-radiography, petrography, and scanning electron microscopy techniques reveal three lamination types in the dark grey (Grey Shales) and black (Jet Rock and Bituminous Shales) Jurassic shales. Fine lamination is produced in deeper, more offshore anaerobic conditions in which suspension settling dominates. Thick lamination indicates the influence of bottom flowing currents in shallow water. Wavy lamination is produced by benthic microbial mats. The Toarcian transgressive shale sequence shows a progressive change from thick to wavy to fine lamination relating to progressive changes in the sedimentary processes operating in the environment. The examples of various unique macro- and microfabrics associated with lamination type offer a useful frame of reference for interpretation of conditions of shale formation.

Pore-to-regional-scale Integrated Characterization Workflow for Unconventional Gas Shales

Based on recent studies of Barnett and Woodford gas shales in Texas and Oklahoma, a systematic characterization workflow has been developed that incorporates lithostratigraphy and sequence stratigraphy, geochemistry, petrophysics, geomechanics, well log, and three-dimensional (3-D) seismic analysis. The workflow encompasses a variety of analytical techniques at a variety of geologic scales. It is designed as an aid in identifying the potentially best reservoir, source, and seal facies for targeted horizontal drilling. Not all of the techniques discussed in this chapter have yet been perfected, and cautionary notes are provided where appropriate. Rock characterization includes (1) lithofacies identification from core based on fabric and mineralogic analyses (and chemical if possible); (2) scanning electron microscopy to identify nanofabric and microfabric, potential gas migration pathways, and porosity types/distribution; (3) determination of lithofacies stacking patterns; (4) geochemical analysis for source rock potential and for paleoenvironmental indicators; and (5) geomechanical properties for determining the fracture potential of lithofacies. Well-log characterization includes (1) core-to-log calibration that is particularly critical with these finely laminated rocks; (2) calibration of lithofacies and lithofacies stacking patterns to well-log motifs (referred to as gamma-ray patterns or GRPs in this chapter); (3) identification and regional to local mapping of lithofacies and GRPs from uncored vertical wells; (4) relating lithofacies to petrophysical, geochemical, and geomechanical properties and mapping these properties. Three-dimensional seismic characterization includes (1) structural and stratigraphic mapping using seismic attributes, (2) calibrating seismic characteristics to lithofacies and GRPs for seismic mapping purposes, and (3) determining and mapping petrophysical properties using seismic inversion modeling. Integrating these techniques into a 3-D geocellular model allows for documenting and understanding the fine-scale stratigraphy of shales and provides an aid to improved horizontal well placement. Although the workflow presented in this chapter was developed using only two productive gas shales, we consider it to be more generically applicable.

Outcrop-behind Outcrop (Quarry): Multiscale Characterization of the Woodford Gas Shale, Oklahoma

An outcrop-behind outcrop study was conducted in and adjacent to a 300 100 16 m (980 330 50 ft) quarry of the gas-producing Woodford Shale to structurally/stratigraphically characterize it from the pore to subregional scales using a variety of techniques. Strata around quarry walls were described and correlated to a 64 m (210 ft) long continuous core drilled 150 m (500 ft) back from the quarry wall and almost to the Woodford-Hunton unconformity. Borehole logs obtained include neutron and density porosity (NPHI and DPHI) logs, and logs from Elemental Capture Spectroscopy (ECS™), Combinable Magnetic Resonance (CMR-Plus™), Fullbore Formation MicroImager (FMI™), and sonic scanner (Modular Sonic Imaging Platform, or MSIP™)—all manufactured by Schlumberger. The strata around the quarry are horizontally bedded. Borehole logs were used to identify a basic threefold subdivision into an upper relatively porous quartzose interval; a middle, more clay-rich, and less porous interval; and a lower interval of intermediate quartz-clay content. These intervals correspond to the informally named upper, middle, and lower Woodford. Detailed core and quarry wall description revealed several types of finely laminated lithofacies, with varying amounts of total organic carbon (TOC). The FMI log revealed a much greater degree of variability in laminations than can be readily seen with the naked eye. Organic geochemistry and biomarkers are closely tied to these lithofacies and record cyclic variations in oxic-anoxic depositional environments, which correspond to relative sea level fall-rise cycles. At the scanning electron microscopy scale, microfractures and microchannels are common and provide tortuous pathways for gas (and oil) migration through the shales. Based on FMI and core analysis, fracture density is much greater in the upper quartzose lithofacies than in the lower, more clay-rich lithofacies. A laser imaging detection and ranging (LIDAR) survey around the quarry walls documented two near-vertical fracture trends in the quartzose lithofacies: one striking N85E with spacings of 1.2 m (4 ft) and the other striking N45E related to the present stress field. The FMI analysis only imaged the latter fracture set. Both log-derived and laboratory-tested geomechanical property measurements documented a significant relationship between shale fabric (laminations and preferred clay-particle orientation) and rock strength, and a secondary relationship to mineral composition. Porosity and microfractures or microchannels also appear to influence rock strength. This integrated study has provided insight into the causal relations among Woodford properties at a variety of scales. In particular, a stratigraphic (vertical) segregation of lithofacies can be related to cyclic variations in depositional environments. The resulting stratified zones exhibit variations in their hydrocarbon source and reservoir (fracturable) potential. Such information and predictive capability can be valuable for improved targeted horizontal drilling into enriched source rock and/or readily fracturable reservoir rock in the Woodford and perhaps other gas shales.

Morphology of Hydrocarbon Droplets During Migration: Visual Example from the Monterey Formation (Miocene), California

In this paper, we describe the various morphologies of hydrocarbon residue found in the Miocene Monterey Formation in California. Scanning electron microscope photomicrographs clearly show various shapes of hydrocarbon droplets in microchannels in the Monterey Formation and hence provide insight not only into the shape of hydrocarbon droplets during migration, but also about the migration pathways themselves. The small (5-10 µm) diameter droplets are observed to have various shapes that include spherical droplets, amalgamated droplets, sausage-shaped droplets, and elongate rod-shaped droplets. Observations suggest that hydrocarbon droplets move through the rock matrix into microchannels where they coalesce into larger droplets and then move into larger fractures. Dif erent morphologies are simply a result of amalgamation of simple droplets during migration.

Origin of Pennsylvanian Underclays in the Illinois Basin

Experimental evidence suggests the detrital origin of underclays of the Illinois Basin. Flocculated clay materials were deposited in glass tubes and the resulting clay-mineral orientation determined by X-ray techniques. Random orientation in the flocculated clay was preserved in the dried clay, which had dewatered slowly under minimal overburden pressure. Samples were compressed to study the influence of water content and pressure on the preservation of random orientation. Clay with lower water content showed less reorientation due to compression than clay with a higher content, suggesting that the amount of liquid water in proportion to adsorbed water in the system may influence clay-flake orientation. The soil concept in explaining the origin of underclay is discussed. Evidence indicates that the nonbedded character of underclays is due to the random orientation of clay flakes, which could be a primary feature resulting from the slow deposition of flocculated clay during which the clay dewaters under minimal overburden pressure. Observations by the author and others do not support the concept that most underclays in the Illinois Basin are soils subjected to subaerial weathering and reqorking by plant rootlets.

Middle and Late Pennsylvanian Flint Clays

ABSTRACT Nearly all literature relating to flint clays of Pennsylvanian age in the eastern United States concerns deposits occurring in the lower part of the Pennsylvanian System. This report describes the geologic occurrence and the physical and mineralogical features of flint clay deposits of middle and late Pennsylvanian age in the eastern United States, many of which have not previously been described as flint clay. The brecciated texture, conchoidal fracture, and variegated color--including shades of yellow, brown, gray, green, and red--of these clays make them distinctive. They are composed primarily of well crystallized kaolinite. The flint clays are often closely associated with coals, and at several localities they occur in channels or lenses where the normal continuity of the coal bed has been interrupted by erosion or nondeposition. The flint clay is believed to have formed by alteration of ordinary fine-grained Pennsylvanian sediments in acid swamps in a manner similar to that recently proposed by Patterson and Hosterman (1960) to explain the formation of early Pennsylvanian flint clay in the Olive Hill district, Kentucky.

A fabric classification of argillaceous rocks, sediments, soils

Abstract A microfabric classification of argillaceous rocks and nonlithified clay sediments and soils is proposed based upon fabric determined from scanning electron micrographs and by using a signal intensity gradient technique. This technique generates a visual image of the actual particles plus a computer-generated rose diagram illustrating their orientation. The latter may be quantified and expressed as a fabric index I f number. Analysis of numerous argillaceous rocks, sediments, soils from various sedimentary environments reveals the following fabric classification: preferred, intermediate and random. The classification quantifies clay fabrics and provides a frame of reference for other clay scientists to use in describing microfabrics.