ABSTRACT Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections ( n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite. Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval. The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

ABSTRACT Petrographic, geochemical, and fluid inclusion analysis of dolomite and calcite cements has been conducted on Mississippian carbonates collected from the surface and subsurface of the southern midcontinent of the United States (Oklahoma, Missouri, Kansas, and Arkansas). Fracture and vug, intergrain, and intragrain porosity are filled with calcite, authigenic quartz, and dolomite cements. Primary limestone porosity is filled partially by early marine and meteoric calcite cements. Equant (blocky) calcite cements were precipitated under seawater or mixed meteoric-seawater conditions in the phreatic zone and in the deep phreatic zone under late (burial) diagenetic conditions. Fracture- and breccia-filling saddle dolomite cements that were observed are late diagenetic and are likely related to the nearby Tri-State Mississippi Valley-type (MVT) mineral district. Carbon and oxygen isotope values of dolomite cements range from δ 18 O (VPDB) = −9.5 to −2.7‰ and from δ 13 C (VPDB) = −4.0 to −0.4‰. Values for calcite cements range from δ 18 O (VPDB) = −11.6 to −1.9‰ and from δ 13 C (VPDB) = −12.2 to +4.6‰. These values are consistent with three types of diagenetic fluids: seawater, seawater modified by meteoric water, and evolved basinal water. Analysis of fluid inclusions in late calcite, dolomite, and quartz cements indicates the presence of both dilute and high salinity end-member fluids. Homogenization temperatures (T h ) of fluid inclusions range from 57°C to 175°C and salinities range from 0 to 25 equivalent weight % NaCl. Fluid inclusion T h values and salinities are consistent with a saline basinal fluid variably diluted by fluids of meteoric or mixed seawater and meteoric origin. Petroleum inclusions were observed in late diagenetic calcite and dolomite cements.The late diagenetic cements filled porosity retained after early diagenetic cementation indicating that some original porosity in the Mississippian carbonate rocks remained open during petroleum migration. Elevated fluid inclusion T h values over a broad region, not just in the Tri-State Mineral District, imply that the regional thermal maturity of rocks may be higher than believed previously. This study indicates that the Mississippian carbonate resource play on the southern midcontinent has a very complex diagenetic history, continuing long after early diagenetic cementation. Possibly the most important diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata.

Abstract Boda Claystone is a very tight clayey rock with extreme low porosity and permeability, nano-size pores and small amounts of swelling clays. Due to this character it is ideal as a potential host rock for research into the possibilities of high-level waste deposition in geological formation. Though the research started more than 30 years ago, the genesis, the geotectonic history of the Boda Claystone Formation (BCF) and the geology of surrounding areas has only been sketched out recently. On the basis of research of the past few years the process of sedimentation of different blocks was able to be reconstructed. Equipment and methodological developments were needed for the investigation of reservoir geological and hydrodynamic behaviour of this rock, which began in the early 2000s. Based on them the pore structure and reservoir could be characterized in detail. Only theoretical approaches were available for the chemical composition of free porewater. Traditional water-extracting methods were not adaptable because of excessively low porosity and nano-scale pore size distribution. Hence, new ways have to be found for getting enough water for analysis. These new results of BCF research help to prepare more sophisticated and directed experiments, in which there is a great interest internationally.

Abstract: The Vedder (Oligocene) and Kreyenhagen (Eocene) sandstones at the Greeley oil field consist of arkosic to subarkosic arenites and wackes deposited in shallow marine environments. Burial depths of the Vedder sandstones exceed 3150 m and the reservoir temperature is 124°C. The Kreyenhagen sandstones are buried to greater than 3920 m and the reservoir temperature is estimated to be c. 135°C. These sandstones are currently at or very near their deepest burial depths. The textural relationships of the diagenetic minerals suggest syndepositional formation of glauconite, phosphate and pyrite, followed by early precipitation of pore-lining clay coatings and carbonate cements along with framework-grain fracturing and possibly dissolution. With increasing burial, dissolution of the framework grains continued, accompanied by the albitization of feldspars, the formation of K-feldspar and quartz overgrowths, the precipitation of kaolinite and other clays and possibly the precipitation of late carbonate cements. Finally, hydrocarbon migration and the formation of pyrite occurred during late diagenesis. Porosity preservation and reservoir quality are primarily the result of plagioclase dissolution occurring as the strata approached their current burial depths. Mass balance calculations indicate the significant export of aluminium out of the sands. Thus secondary porosity produced by plagioclase dissolution has replaced the primary porosity destroyed by compaction, and now accounts for the majority of the porosity in these rocks.

Abstract: A comprehensive study of the Cambrian–Ordovician Arbuckle Group suggests that multiple fluid migration events have affected reservoir porosity via fractures and preferred stratigraphic horizons. Fluid inclusion homogenization temperatures from late-stage precipitates yield temperatures higher than can be explained by burial conditions or an elevated geothermal gradient. Fluid inclusion melting temperatures yield salinity values that indicate multiple fluids evolving through time. Hydrocarbon fluid inclusions in late-stage baroque dolomite suggest oil migration concurrent with hydrothermal fluid flow. Depleted δ 13 C and δ 18 O values provide evidence for a high-temperature basinal fluid source as well as for the preferential flow of hydrothermal fluids through permeable zones in the Mississippian and Arbuckle Group, where pore systems related to paleokarst are overlain by less permeable units. Radiogenic strontium isotopic data support fluid–rock interaction with siliciclastic material or basement rock at some point during the fluid migration history. Variable 87 Sr/ 86 Sr values suggest multiple sources for the fluids responsible for the cements and a transition from an advective fluid flow system to a vertical fluid flow system. The ancient aquifer system was vertically connected during migration of hydrothermal fluids, and a temperature-controlled vertical density gradient appears to have played an important role in late-stage porosity evolution, focusing the hottest fluids in the upper sections of permeable layers.

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. The largest pores occur in the infills of foraminifer tests. The framework of crushed carbonate debris in planktonic fecal pellets shows intermediate levels of porosity, and the silicate-rich matrix that encloses framework components has the smallest average porosity. The distribution of pore types is not uniform. Our hypothesis is that facies association is an important factor that determines bulk porosity and influences reservoir performance. The observed variability in the attributes of the described distal, medial, and proximal facies associations is thought to translate into significant variability of rock properties such as total organic carbon and porosity. In turn, this variability should control the quality and distribution of the intervals that are optimum sources and reservoirs of hydrocarbons in the Eagle Ford Shale. The medial facies association most likely has the best porosity development when a favorable combination of more commonly abundant calcareous fecal pellets and organic material versus clay content is present. The systematic arrangement of facies associations into parasequences provides the basis for testing and predicting the best development of optimal reservoir facies within a sequence-stratigraphic framework in the Eagle Ford Shale.