Kitty L. Milliken

Organic matter–hosted pore system, Marcellus Formation (Devonian), Pennsylvania

The Marcellus Formation of Pennsylvania represents an outstanding example of an organic matter (OM)–hosted pore system; most pores detectable by field-emission scanning electron microscopy (FE-SEM) are associated with OM instead of mineral matrix. In the two wells studied here, total organic carbon (TOC) content is a stronger control on OM-hosted porosity than is thermal maturity. The two study wells span a maturity from late wet gas (vitrinite reflectance [Ro], 1.0%) to dry gas (Ro, 2.1%). Samples with a TOC less than 5.5 wt. % display a positive correlation between TOC and porosity, but samples with a TOC greater than 5.5 wt. % display little or no increase in porosity with a further increasing TOC. In a subset of samples (14) across a range of TOC (2.3–13.6 wt. %), the pore volume detectable by FE-SEM is a small fraction of total porosity, ranging from 2 to 32% of the helium porosity. Importantly, the FE-SEM–visible porosity in OM decreases significantly with increasing TOC, diminishing from 30% of OM volume to less than 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC. The interpretation of this anticorrelation between OM content and SEM-visible pores remains uncertain. Samples with the lowest OM porosity (higher TOC) may represent gas expulsion (pore collapse) that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (low initial hydrogen index, relatively unreactive) that is less prone to development of FE-SEM–detectable pores. In this interpretation, OM type, controlled by sequence-stratigraphic position, is a factor in determining pore-size distribution.

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Microbial precipitation of dolomite in methanogenic groundwater

We report low-temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments. In a freshwater aquifer, microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls. In the laboratory, ordered dolomite formed at near-equilibrium conditions from groundwater with molar Mg:Ca ratios of <1; dolomite was absent in sterile experiments. Geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation. We hypothesize that the attached microbial consortium reacts with the basalt surface, releasing Mg and Ca into solution, which drives dolomite precipitation via nucleation on the cell wall. These findings provide insight into the long-standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions.

Grain assemblages and strong diagenetic overprinting in siliceous mudrocks, Barnett Shale (Mississippian), Fort Worth Basin, Texas

Porosity, permeability, and total organic carbon (TOC) in a heterogeneous suite of 21 high-maturity samples (vitrinite reflectance 1.52–2.15%) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of rock texture, fabric, and composition, these factors being mostly obscured by the effects of a protracted history of diagenesis. Diagenesis in these rocks includes mechanical and chemical modifications that occurred across a wide range of burial conditions. Compaction and cementation have mostly destroyed primary intergranular porosity. The porosity (average 5 vol. % by Gas Research Institute helium porosimetry) and pore size (8 nm median pore-throat diameter) are reduced to a degree such that pores are difficult to detect even by imaging Ar ion–milled surfaces with a field-emission scanning electron microscope. The existing porosity that can be imaged is mostly secondary and is localized dominantly within organic particulate debris and solid bitumen. The grain assemblage is highly modified by replacement. A weak pattern of correlation survives between bulk rock properties and the ratio of extrabasinal to intrabasinal sources of siliciclastic debris. Higher porosity, permeability, and TOC are observed in samples representing the extreme end members of mixing between extrabasinal siliciclastic sediment and intrabasinal-derived biosiliceous debris. Reservoir quality in these rocks is neither more strongly nor more simply related to variations in primary texture and composition because the interrelationships between texture and composition are complex and, importantly, the diagenetic overprint is too strong.

Diagenetic evolution of Cenozoic sandstones, Gulf of Mexico sedimentary basin

Abstract The Gulf of Mexico sedimentary basin is a natural laboratory for the study of on-going diagenetic and incipient metamorphic processes. Sediments and rocks of Eocene through Pleistocene age have been studied from the surface to depths in excess of 6 km. Sediments heated to temperatures above 100°C have been massively transformed by mechanical compaction, cementation, and extensive alteration of detrital components. Grain dissolution, albitization, and clay-mineral transformations have reduced an initially complex detrital assemblage to quartz, albite, illite and minor carbonate at temperatures above 100°C. Volumetrically significant diagenetic processes observed in the basin include cementation by quartz, carbonate and kaolinite, grain dissolution (affecting mainly potassium feldspar, heavy minerals, and plagioclase), albitization, and the transformation of smectite to illite. Excepting carbonate cementation which shows essentially no depth-related variation, these processes occur shallower in older units, most likely in response to variations in the geothermal gradient, which is higher in the older Cenozoic depocenters. The magnitudes of the principal diagenetic processes all support the view that basinal diagenesis operates as an open system on a very large scale. Strontium isotopic data for authigenic carbonates document vertical transport on the scale of kilometers. The extent to which metamorphic processes below 6 km have effected the course of diagenesis in shallower rocks is still unproven, but current data suggest that burial diagenesis must be studied in such a context.

Pore types and pore-size distributions across thermal maturity, Eagle Ford Formation, southern Texas

Pore types, pore size, and pore abundance vary systematically across thermal maturity in the Eagle Ford Formation, Maverick Basin, southern Texas. Scanning electron imaging of 20 samples from four wells is used to assess the complex response of pores to chemical and mechanical processes, entailing both destruction of primary porosity and generation of secondary pores. Primary mineral-associated pores are destroyed by compaction, cementation, and infill of secondary organic matter, whereas secondary pores are generated within organic matter (OM). Destruction of primary pores during early burial (to ∼0.5%) occurs by compaction of ductile detrital OM and clays and, to a lesser degree, as a result of cementation and infill of secondary OM. Larger pores are associated with coccolith debris. The dominant OM is spatially isolated detrital OM “stringers.” Porosity is volumetrically dominated (average 6.2%) by relatively large, mostly interparticle mineral-associated pores (median size 51.6 nm [0.000002 in.]; detection limit near 3–4 nm [0.00000012–0.00000015 in.]). At low maturity, porosity and pore size correlate directly with calcite abundance and inversely with OM volumes. At higher maturity, further destruction of primary pores occurs through cementation, secondary OM infill, and greater compaction. Mineral-associated pores are present at high-maturity ( ∼1.2%–1.3%), but are smaller (median size 30.2 nm [0.0000011 in.]) and less abundant (average of 2.5%) than at low maturity. A large portion of OM within high-maturity samples is diagenetic in origin and has pervaded into primary pore space, coating cement crystals, and filling intraparticle pores. Substantial mineral-associated porosity is locally present in samples where incursion of primary pore space by secondary OM has not occurred. Abundant secondary porosity is generated as OM matures into the wet-gas window. Porosity in most high-maturity samples is volumetrically dominated (average of 1.3%) by smaller, OM-hosted pores (median size 13.2 nm [0.00000051 in.]).

Burial diagenesis of argillaceous sediment, south Texas Gulf of Mexico sedimentary basin: A reexamination

Cuttings from a well through a thick section of Miocene–Oligocene mudrocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochemically. On the basis of whole-rock chemical analyses, the deepest samples have lost ≈18 wt% (and approximately vol%), mostly as CaCO3, mineral-bound H2O, and SiO2, but including additional Ca, as well as Sr, light rare earth elements (REE) (La, Ce, Nd, Sm), Fe, and Li. K2O and Rb have been added to the deeper rocks. The large chemical changes are accompanied mineralogically by loss of detrital calcite, kaolinite, K-feldspar, Ca-plagioclase, and muscovite, gain of chlorite and albite, and continued reaction of smectitic illite/smectite (I/S) to more illitic (and K-rich) compositions throughout the entire depth interval of the well. The large chemical changes in this thick mud-rich interval almost certainly require advection of water (free convection?) to accomplish the mass transfer. Initial variation in sediment composition is ruled out as a cause of the observed compositional changes with increasing depth because (1) a variety of “immobile” elements (Al2O3, TiO2, Zr, Hf, heavy REE [Er, Yb], Th, and Sc) remain constant relative to each other despite their uneven distribution across various particle size fractions in the sediments; (2) deep Frio shales are unlike Quaternary Gulf of Mexico sediments or average shales; and (3) unreasonable primary mineralogic compositions would be necessary to explain the chemical composition of the deep samples. These results indicate that burial diagenesis of argillaceous sediment can be a considerably more open chemical process than is conventionally assumed, that it can account for the two major chemical cements (calcite and quartz) in associated sandstones, and that it mirrors secular changes in shales throughout geologic time.

The silicified evaporite syndrome; two aspects of silicification history of former evaporite nodules from southern Kentucky and northern Tennessee

ABSTRACT Silicified evaporite nodules from Mississippian rocks of south-central Kentucky and adjoining Tennessee typify in many respects silicified evaporite nodules which have been reported in rocks of widely differing ages and diagenetic histories. The silicified evaporite syndrome is an assemblage of characteristics shared by evaporite-replacement quartz nodules and includes both mineralogical and textural aspects. The sequence of textures in most nodules begins with interlocking quartzine spherules near the outer nodule edge and ends with a mosaic of equant megaquartz in the nodule center. Proportions of the different textures vary considerably between nodules. Anhydrite inclusions are abundant in the centers of megaquartz crystals. Megaquartz in the nodules is characterized by strongly un ulose, radial extinction. Euhedral terminations often have a cubic appearance. Many nodules contain crusts of zebraic chalcedony on the euhedral megaquartz bordering internal cavities. Oxygen isotope analyses are useful in understanding the silicification history. Microcrystalline quartz and fibrous quartz types have O18 values which are heavier than those of megaquartz. The microcrystalline quartz nodules, associated with the silicified evaporites, formed earliest in fluids related to sea water. Length-slow fibrous quartz types formed later in water of intermediate composition. Megaquartz developed at somewhat higher temperatures and in fluids derived from meteoric water. Zebraic chalcedony in the nodules is also of meteoric origin. All silicification of these samples has probably occurred at temperatures of less than 40° C.

Petrography and Composition of Authigenic Feldspars, Oligocene Frio Formation, South Texas

ABSTRACT Detrital feldspar assemblages (1 to 4 km burial depth) in Oligocene sandstones from South Texas are extensively altered by dissolution. In some samples, cementation in secondary pores, principally by albite but also by authigenic K-feldspar and other minerals, occurs subsequent to the dissolution and can lead to wholesale replacement textures. Solid-state diffusion of cations has not been important in the replacement process. Information from conventional light microscopy, SEM examination, and back-scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time, even on the scale of a thin section. Dissolution of detrital feldspars and precipitation of relatively pure albite and K-feldspar take place ver a large depth interval and thus can be markedly separated in time. Vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain. More rapid dissolution results in formation of larger-scale intragranular secondary pores, commonly filled by subsequent precipitation of clear albite within the former grain volume. Electron microprobe analyses show that the composition of vacuolized albite and authigenic K-feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars. Regardless of their cause, these subtle variations in composition have important ramifications for basin-scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar.

Porosity and permeability variations in fractured and liesegang-banded Breathitt sandstones (Middle Pennsylvanian), eastern Kentucky: diagenetic controls and implications for modeling dual-porosity systems

Abstract Middle Pennsylvanian fluvial sandstones in eastern Kentucky (Breathitt Group) manifest visible evidence of alteration related to fluid flow localized through near-vertical joints. Fracture-related alterations involve both physical and chemical modifications that together create dramatic permeabilisty variations at the outcrop scale. On the fracture surface, infiltered detritus combined with mineral and organic coatings have reduced pore sizes and, hence, permeabilities (0.03–0.44 md) by an order of magnitude over values characteristic of the adjacent sandstone (0.32–1.53 md). Prominent zones of orange-brown discoloration contain evidence of oxidation reactions and form an envelope of variable thickness around the fractures. Authigenic iron oxides are not uniformly distributed within these zones, but rather are concentrated as local bands of pervasive mineralization commonly known as liesegang bands. Petrographic evidence suggests that most of the iron that now resides in oxidized authigenic phases was derived from solutes mobilized through dissolution of older iron-bearing authigenic minerals. Permeability and pore sizes within the oxidation zone are bimodal and vary from high values similar to those in adjacent unoxidized sandstone to low values, associated with the zones of pervasive mineralization, that approach those values observed for the fracture skin. The large magnitude of permeability variation around fracture systems in these sandstones documents the presence of a dual porosity system and suggests that fluid and contaminant transport cannot be realistically modeled using average rock properties.