Robert G. Loucks

Bio: Robert G. Loucks is an academic researcher from University of Texas at Austin. The author has contributed to research in topics: Sedimentary depositional environment & Facies. The author has an hindex of 37, co-authored 143 publications receiving 7904 citations. Previous affiliations of Robert G. Loucks include ARCO.

Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale

Abstract: Research on mudrock attributes has increased dramatically since shale-gas systems have become commercial hydrocarbon production targets. One of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks. Our work on siliceous mudstones from the Mississippian Barnett Shale of the Fort Worth Basin, Texas, shows that the pores in these rocks are dominantly nanometer in scale (nanopores). We used scanning electron microscopy to characterize Barnett pores from a number of cores and have imaged pores as small as 5 nm. Key to our success in imaging these nanopores is the use of Ar-ion-beam milling; this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high-magnification imaging. Nanopores are observed in three main modes of occurrence. Most pores are found in grains of organic matter as intraparticle pores; many of these grains contain hundreds of pores. Intraparticle organic nanopores most commonly have irregular, bubblelike, elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm. Internal porosities of up to 20.2% have been measured for whole grains of organic matter based on point-count data from scanning electron microscopy analysis. These nanopores in the organic matter are the predominant pore type in the Barnett mudstones and they are related to thermal maturation. Nanopores are also found in bedding-parallel, wispy, organic-rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter, but this mode is not common. Although less abundant, nanopores are also locally present in fine-grained matrix areas unassociated with organic matter and as nano- to microintercrystalline pores in pyrite framboids. Intraparticle organic nanopores and pyrite-framboid intercrystalline pores contribute to gas storage in Barnett mudstones. We postulate that permeability pathways within the Barnett mudstones are along bedding-parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores.

Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores

Abstract: Matrix-related pore networks in mudrocks are composed of nanometer- to micrometer-size pores. In shale-gas systems, these pores, along with natural fractures, form the flow-path (permeability) network that allows flow of gas from the mudrock to induced fractures during production. A pore classification consisting of three major matrix-related pore types is presented that can be used to quantify matrix-related pores and relate them to pore networks. Two pore types are associated with the mineral matrix; the third pore type is associated with organic matter (OM). Fracture pores are not controlled by individual matrix particles and are not part of this classification. Pores associated with mineral particles can be subdivided into interparticle (interP) pores that are found between particles and crystals and intraparticle (intraP) pores that are located within particles. Organic-matter pores are intraP pores located within OM. Interparticle mineral pores have a higher probability of being part of an effective pore network than intraP mineral pores because they are more likely to be interconnected. Although they are intraP, OM pores are also likely to be part of an interconnected network because of the interconnectivity of OM particles. In unlithifed near-surface muds, pores consist of interP and intraP pores, and as the muds are buried, they compact and lithify. During the compaction process, a large number of interP and intraP pores are destroyed, especially in ductile grain-rich muds. Compaction can decrease the pore volume up to 88% by several kilometers of burial. At the onset of hydrocarbon thermal maturation, OM pores are created in kerogen. At depth, dissolution of chemically unstable particles can create additional moldic intraP pores.

Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas

Abstract: The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.

Paleocave carbonate reservoirs; origins, burial-depth modifications, spatial complexity, and reservoir implications

Abstract: Paleocave systems form an important class of carbonate reservoirs that are products of near-surface karst processes and later burial compaction and diagenesis. Features and origins of fractures, breccias, and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems. Information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale, pore networks, and spatial complexities. Spatial complexities in paleocave reservoirs result from near-surface and burial processes. Near-surface processes include dissolutional excavation, clastic sedimentation, chemical precipitation, and localized fracturing, brecciation, and collapse of cave walls and ceilings. Burial processes begin as cave systems subside into the subsurface. Remaining cave passages commonly collapse and early-formed breccia clasts are rebrecciated. Differential compaction of strata around and over collapsed passages produces fractures, crackle breccias, and mosaic breccias. Near-surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity. Hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed-paleocave systems. The coalescing of passages in a cave system into larger, connected porosity zones results from a combination of multiple, cave-forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured. This combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across, commonly forming large exploration targets. Final size, pore-network types, and spatial complexities of coalesced collapsed-paleocave systems are products of their evolution from near-surface development through burial into the deeper subsurface. The coalesced collapsed-paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved.

Carbonate Depositional Sequences and Systems Tracts—Responses of Carbonate Platforms to Relative Sea-Level Changes

Abstract: Standard carbonate facies models are widely used to interpret paleoenvironments, but they do not address how carbonate platforms are affected by relative changes in sea level. An understanding of how the subtidal carbonate "factory" responds to relative sea-level changes and the role played by other environmental factors towards influencing the formation of carbonate platforms allows one to differentiate platform types and it helps establish a basis for constructing depositional sequence and systems tract models. The combination of in-situ production of carbonate sediment, which is also subject to transport, and local variations in depositional processes result in the formation of a wide variety of stratal patterns, some of which are unique to carbonate systems. Fundamental carbonate-depositional principles and geologic-based observations were used to construct depositional sequence and systems tract models for a variety of rimmed shelves and ramps. The models show how, for example, depositional sequences made up of (1) carbonate, (2) carbonate-siliciclastic, or (3) carbonate-evaporite-siliciclastic facies are produced by depositional systems responding to lowstand, transgressive, and highstand conditions. Lowstand: Carbonate sediment production is reduced on rimmed shelves because a relatively small area of shallow seafloor is in contact with the carbonate "factory." Reduced sedimentation and subaerial exposure foster the retreat of shelf edges and slopes by erosion and slope failure during lowstands. As a result, thick debris-flow eposits may form. Karst development is important in humid climates and can affect large areas of a subaerially exposed platform. If siliciclastic sediments are available, they are delivered to the shelf edge and slope by fluvial-deltaic systems or, in arid climates, by wadis and advancing ergs. Under arid conditions, lowstand evaporites may fill an isolated or completely silled basin. Transgression: Carbonate sedimentation initiates in restricted environments and later as more open conditions develop, open marine facies, including patch reefs, may locally develop atop flooded platforms and ramps. Retrogradational End_Page 3-------------------------- parasequences comprising shallow-water carbonates form and subsequently drown, and shelf edges tend to aggrade, backstep, and drown if the rate of sea-level rise is high. Highstand: Seaward-prograding carbonate or siliciclastic coastal sediments and landward-prograding carbonate rimmed shelf edges may partially fill inner to outer shelf seas. Under arid conditions, evaporites and red beds commonly fill wide and shallow salinas. These strata onlap subaerially exposed rimmed shelf edges and prograding grainstone islands in ramps. Shelf edges and shorelines tend to prograde under the influence of high rates of carbonate sedimentation across the shelf and shelf edge. Slope and basinal environments receive excess shelf- and shelf-edge-derived sediment. Factors listed above must be integrated with established facies models in order to arrive at comprehensive sequence and systems tract models. As should be the case with all models, however, they are not meant to serve as rigid templates within which all carbonate sequences must fit. Modification may be needed to accommodate each case. Once they are deemed applicable to a specific case, they function as working hypotheses to help geologists visualize how and why carbonate strata were laid down and fit together as they do. As a general predictor of facies, carbonate depositional sequence and systems tracts models may be used in conjunction with seismic records to identify depositional systems and to locate reservoir-, seal-, and source-prone facies.

Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment

Abstract: Shale-gas resource plays can be distinguished by gas type and system characteristics. The Newark East gas field, located in the Fort Worth Basin, Texas, is defined by thermogenic gas production from low-porosity and low-permeability Barnett Shale. The Barnett Shale gas system, a self-contained source-reservoir system, has generated large amounts of gas in the key productive areas because of various characteristics and processes, including (1) excellent original organic richness and generation potential; (2) primary and secondary cracking of kerogen and retained oil, respectively; (3) retention of oil for cracking to gas by adsorption; (4) porosity resulting from organic matter decomposition; and (5) brittle mineralogical composition. The calculated total gas in place (GIP) based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf/section (5.78 × 10 9 m 3 /1.73 × 10 4 m 3 ). We estimate that the Barnett Shale has a total generation potential of about 609 bbl of oil equivalent/ac-ft or the equivalent of 3657 mcf/ac-ft (84.0 m 3 /m 3 ). Assuming a thickness of 350 ft (107 m) and only sufficient hydrogen for partial cracking of retained oil to gas, a total generation potential of 820 bcf/section is estimated. Of this potential, approximately 60% was expelled, and the balance was retained for secondary cracking of oil to gas, if sufficient thermal maturity was reached. Gas storage capacity of the Barnett Shale at typical reservoir pressure, volume, and temperature conditions and 6% porosity shows a maximum storage capacity of 540 mcf/ac-ft or 159 scf/ton.

Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale

Abstract: Research on mudrock attributes has increased dramatically since shale-gas systems have become commercial hydrocarbon production targets. One of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks. Our work on siliceous mudstones from the Mississippian Barnett Shale of the Fort Worth Basin, Texas, shows that the pores in these rocks are dominantly nanometer in scale (nanopores). We used scanning electron microscopy to characterize Barnett pores from a number of cores and have imaged pores as small as 5 nm. Key to our success in imaging these nanopores is the use of Ar-ion-beam milling; this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high-magnification imaging. Nanopores are observed in three main modes of occurrence. Most pores are found in grains of organic matter as intraparticle pores; many of these grains contain hundreds of pores. Intraparticle organic nanopores most commonly have irregular, bubblelike, elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm. Internal porosities of up to 20.2% have been measured for whole grains of organic matter based on point-count data from scanning electron microscopy analysis. These nanopores in the organic matter are the predominant pore type in the Barnett mudstones and they are related to thermal maturation. Nanopores are also found in bedding-parallel, wispy, organic-rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter, but this mode is not common. Although less abundant, nanopores are also locally present in fine-grained matrix areas unassociated with organic matter and as nano- to microintercrystalline pores in pyrite framboids. Intraparticle organic nanopores and pyrite-framboid intercrystalline pores contribute to gas storage in Barnett mudstones. We postulate that permeability pathways within the Barnett mudstones are along bedding-parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores.

Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores

Abstract: Matrix-related pore networks in mudrocks are composed of nanometer- to micrometer-size pores. In shale-gas systems, these pores, along with natural fractures, form the flow-path (permeability) network that allows flow of gas from the mudrock to induced fractures during production. A pore classification consisting of three major matrix-related pore types is presented that can be used to quantify matrix-related pores and relate them to pore networks. Two pore types are associated with the mineral matrix; the third pore type is associated with organic matter (OM). Fracture pores are not controlled by individual matrix particles and are not part of this classification. Pores associated with mineral particles can be subdivided into interparticle (interP) pores that are found between particles and crystals and intraparticle (intraP) pores that are located within particles. Organic-matter pores are intraP pores located within OM. Interparticle mineral pores have a higher probability of being part of an effective pore network than intraP mineral pores because they are more likely to be interconnected. Although they are intraP, OM pores are also likely to be part of an interconnected network because of the interconnectivity of OM particles. In unlithifed near-surface muds, pores consist of interP and intraP pores, and as the muds are buried, they compact and lithify. During the compaction process, a large number of interP and intraP pores are destroyed, especially in ductile grain-rich muds. Compaction can decrease the pore volume up to 88% by several kilometers of burial. At the onset of hydrocarbon thermal maturation, OM pores are created in kerogen. At depth, dissolution of chemically unstable particles can create additional moldic intraP pores.

The Rock Physics Handbook

Abstract: Responding to the latest developments in rock physics research, this popular reference book has been thoroughly updated while retaining its comprehensive coverage of the fundamental theory, concepts, and laboratory results. It brings together the vast literature from the field to address the relationships between geophysical observations and the underlying physical properties of Earth materials - including water, hydrocarbons, gases, minerals, rocks, ice, magma and methane hydrates. This third edition includes expanded coverage of topics such as effective medium models, viscoelasticity, attenuation, anisotropy, electrical-elastic cross relations, and highlights applications in unconventional reservoirs. Appendices have been enhanced with new materials and properties, while worked examples (supplemented by online datasets and MATLAB® codes) enable readers to implement the workflows and models in practice. This significantly revised edition will continue to be the go-to reference for students and researchers interested in rock physics, near-surface geophysics, seismology, and professionals in the oil and gas industries.