Illite Crystallinity Research Articles

Chlorite, white mica and clay minerals as proximity indicators to ore in the shallow porphyry environment of Quebrada de la Mina deposit, Argentina

The mineralogy and chemical composition of mica-like phyllosilicates (>2 µm) and clay fraction (<2 µm) of the shallow hydrothermal alteration zones of the Quebrada de la Mina Cu-Au deposit (San Juan Province, Argentina) were constrained by petrography, XRD, electronic microprobe and IR. The results allow us to identify: (1) relict patches of quartz + K-feldspar ± rutile (potassic) alteration, (2) a peripheral clinochlore-rich propylitic halo, and (3) a central white mica (>2 µm) and illite halo associated with phyllic and tourmaline-rich phyllic assemblages.The subhorizontal, shallow level Au-rich (>0.5 ppm Au) zone at the Quebrada de la Mina deposit coincides with the phyllic halo. The distribution of Au, Ag, Zn, Pb and As forms partially overlapping haloes toward shallower levels. Weak Cu (Mo) anomalies were found in potassic patches below the Au-rich zone. High Mn and Fe contents in clinochlore from the peripheral propylitic halo suggest a distance of ∼1 km to a porphyry Cu-(Au) mineralized core at deeper levels.White micas (>2 µm) in Quebrada de la Mina show a range of compositions between muscovite and illite. In the propylitic halo and in the patches of potassic alteration, white micas (>2 µm) show the highest values of Fe + Mg + Mn (0.40–0.55 apfu) and the lowest of Al (2.40–2.55 apfu). In the phyllic alteration zone of Quebrada de la Mina, white micas (>2 µm) with Al contents between 2.45 and 2.70 apfu and Fe + Mg + Mn between 0.35 and 0.15 apfu match the areas with high gold content (>0.5 ppm Au). The lowest values of Fe + Mg + Mn (0.11–0.05 apfu) and the highest Al contents (2.80–2.85 apfu) occur in white micas from the tourmaline-rich phyllic zone.Values of the Kübler index of illite of 0.19–0.1 2θ in the clay fraction of deep samples are indicative of higher temperature and/or higher fluid/rock ratios during illite formation, compared to values of the Kübler index > 0.2 2θ recognized in shallow samples. Thus, both the compositional variations in chlorite and white micas and the illite crystallinity could be used as a guide for exploration of copper-gold mineralization in shallow levels of porphyry systems.

Petrography, provenance, diaganesis and depositional environment of Murree Formation in Jhelum Valley, Sub Himalayas, Azad Jammu and Kashmir, Pakistan

The clastic sediments of the Murree Formation of Miocene age are exposed in Jhelum valley areas of Azad Jammu and Kashmir Pakistan. Field observations revealed the cyclic deposition in the Murree Formation. The sandstone, siltstone, and shale constitute a single cycle within the formation. This single unit is divided into five different lithofacies which constitute the Bouma sequence in the Murree Formation. The Murree Formation shows faulted contacts with Panjal Formation and Nagri Formation in the study area. The modal mineralogy data obtained from the petrography of sandstone indicates that sandstone is litharenite and lithic greywacke. The mineralogical and textural data suggests that sandstone is compositionally mature and poorly to moderately sorted. The dominantly angular to sub angular quartz grains show nearness of the source area. Fractured and sutured quartz grain reveals tectonodiagentic changes that occurred in Murree Formation. The sandstone experienced diagenetic changes. The pressure solution and cementation reduced the primary porosity of sandstone. However, alteration of feldspar and fractures in grains have produced secondary porosity. The X-ray diffraction (XRD) of the shale samples indicates that shale of the Murree Formation is argillaceous and dominated by illite clay mineral. The illite crystallinity values indicate very low grade metamorphism of Murree Formation in core of Hazara Kashmir Syntaxis. The petrographic data suggests that the provenance of sandstone is recycled orogen. Quartz is of igneous and metamorphic origin. Feldspar (albite and microcline) composition suggests its derivation from acidic igneous rocks. The rock fragments of volcanics, slate, phyllite, and schist suggest igneous and metamorphic provenance. The petrographic data suggests that at the time of deposition of Murree Formation, igneous and low grade metamorphic rocks were exposed. However, presence of some clasts of carbonates indicates that sedimentary rocks were also exposed in the source region. The quartz content and clay minerals in the shale revealed that source region was igneous and metamorphic rocks. Cyclic deposition, lithofacies, and various sedimentary structures like cross bedding, ripple marks, and calcite concretions suggest that deposition of Murree Formation occurred in fluviatile environment by meandering river system having decreasing turbidity current.

Precursor and ambient rock paleothermometry to assess the thermicity of burial dolomitization in the southern Cantabrian Zone (northern Spain)

Paleozoic rocks in the Cantabrian Zone, and the Variscan foreland fold-and-thrust belt on the Iberian Peninsula have been affected by a sequence of diagenetic to epizonal thermal events. Late- to Post-Variscan hot fluid circulation caused a large-scale burial dolomitization and ore mineralization, mostly in Cambrian and a Lower to Middle Carboniferous carbonate sucessions. The goal of this study is to analyze and compare the temperatures experienced by the carbonate precursor rocks, as well as the under- and over-lying siliciclastic ambient rocks to gain a better understanding of the thermicity of dolomitization. These temperatures are evaluated based on published paleothermal datasets combined with new data obtained from Rock–Eval pyrolysis and vitrinite reflectance analysis of Carboniferous rocks rich in organic matter. The overall results indicate that reworking of detrital sediments in synorogenic ambient siliciclastics results in an anomalously high thermal maturity recorded by bulk rock techniques such as illite crystallinity and Rock–Eval pyrolysis. In situ VR-derived paleotemperatures recorded by ambient siliciclastic rocks appear to be higher compared to CAI-derived temperatures for carbonate precursor rocks. This variation in thermal maturity is likely related to the analytical techniques used to obtain CAI and VR data, and the empirical equations applied to calculate corresponding paleotemperatures. Conodont fragments were not as sensitive compared to vitrinite, and the color alteration process could have suffered from hydrothermal alteration. A secondary cause might be a different response to mechanical deformation between siliciclastic and carbonate units during the Variscan and post-Variscan geodynamic evolution of the study area. Rigid precursor carbonate units experienced fluid circulation mainly along distinct and spaced fracture zones, creating fracture-related dolomite geobodies and ore mineralization. Soft ambient siliciclastic rocks experienced more diffuse fluid circulation and heat dissipation. The different paleothermometry datasets compiled for the study area indicate that the fluids circulating during Late- to Post-Variscan times, with associated fracture-related dolomitization and ore mineralization in carbonate precursors, are hydrothermal. The highest paleotemperatures were recorded in ambient and precursor rocks in the highly tectonized northern part of the study area, where several thrusts and faults allowed intense fluid circulation. Positive temperature anomalies within the precursor carbonates correlate well with the occurrence of dolomite geobodies and ore mineral deposits. Such anomalies could thus be used as an exploration tool for hydrothermal dolomite bodies in analog sub-surface settings.

Application of CASI/SASI and fieldspec4 hyperspectral data in exploration of the Baiyanghe uranium deposit, Hebukesaier, Xinjiang, NW China

ABSTRACTHyperspectral technology can quickly identify hydrothermal altered minerals and can locate the possible hydrothermal/mineralization centres through the variations of the spectral characteristic parameters of some specific altered minerals. In this study, airborne sensors Compact Airborne Spectrographic Imager (CASI)/ Shortwave infrared Airborne Spectrographic Imager (SASI) and visible-shortwave infrared spectrometer FieldSpec4 hyperspectral data were used to study the alteration characteristics to locate the possible hydrothermal/mineralization centres for the Baiyanghe uranium deposit, Xinjiang, NW China. The results showed that the hydrothermal altered minerals mainly included white mica (it is a general term for aqueous silicoaluminate mineral group, and mainly includes muscovite and illite in the text) (Al-rich, Al-median, and Al-poor white mica), montmorillonite, kaolinite, chlorite, carbonate, and ferric iron (Fe2O3, and the ferric iron alteration usually presents red, mainly hematite in the text); and that the alteration had obvious zoning characteristics. The variations in the absorption-peak wavelength of white mica at 2200 nm, SWIR-IC (short-wavelength infrared illite crystallinity, a dimensionless quantity, and it is defined as the absorption-peak depth of illite at 2200 nm divided by the absorption-peak depth of illite at 1900 nm on a hull quotient spectrum) of white mica, and acid–base properties revealed that the possible hydrothermal/mineralization centres were located in the northern contact zone between the Yangzhuang rock body and the northern volcanic rocks of the Devonian and in the acidic alteration zone to north of the deposit about 3.0–4.0 km, where there appeared a certain number of uranium mineralization occurrences. These results pointed to the existence of multiple hydrothermal activities and could provide references for periphery uranium exploration and geological origin of this uranium area.

Mid-Miocene climatic optimum: Clay mineral evidence from the red clay succession, Longzhong Basin, Northern China

The Mid-Miocene Climatic Optimum (MMCO) is one of Earth's most recent prolonged warming events, recorded in both the deep ocean and mid- to high-latitude continents. Thick Cenozoic deposits in the Longzhong Basin, Northwest China, provide great potential for characterizing the MMCO, but have not been well documented. In this study, we carry out clay mineralogical and geochemical analyses of the Miocene red clay succession from the eastern Longzhong Basin, employing X-ray diffraction and geochemistry, and discuss the climatic characteristics of the MMCO. The results reveal that the MMCO occurred at 16–14Ma and is associated with relatively high kaolinite and smectite contents, high illite crystallinity values and kaolinite/(illite+chlorite) ratio, and lower chlorite and illite contents. These clay mineral assemblages confirm a warm and moist climate during the MMCO. Furthermore, the SiO2/Al2O3 ratio declined with increasing Al2O3, indicative of greater precipitation and intensified weathering during this period. When compared with local and global records, we find that our clay mineral records reflect regional climatic change superimposed upon global climatic trends during the MMCO. It is suggested that a high concentration of CO2, and associated global warming, may have been responsible for the MMCO, rather than the uplift of the Tibetan Plateau.

Deflating the shale gas potential of South Africa’s Main Karoo basin

&#x0D; &#x0D; &#x0D; The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects.&#x0D; &#x0D; &#x0D; &#x0D; &#x0D; Significance: &#x0D; &#x0D; &#x0D; &#x0D; This is the first report of direct measurements of the actual gas contents of southern Karoo basin shales.&#x0D; The findings reveal carbon content of shales to be dominated by overmature organic matter.&#x0D; The results demonstrate a much reduced potential shale gas resource presented by the Whitehill Formation.&#x0D; &#x0D; &#x0D; &#x0D;

Hyperspectral Alteration Information from Drill Cores and Deep Uranium Exploration in the Baiyanghe Uranium Deposit in the Xuemisitan Area, Xinjiang, China

The Baiyanghe uranium deposit is a currently important medium-sized deposit in the Xuemisitan area, Xinjiang. The hydrothermal alteration in this deposit is closely related to the uranium mineralization of the deposit. In this study, hyperspectral data are collected from drill cores in the Baiyanghe uranium deposit using a FieldSpec4 visible-shortwave infrared spectrometer to study the hydrothermal alteration. The results reveal that the altered mineral assemblages have obvious zonation characteristics: (1) the upper section comprises long-wavelength illite and minor hematite and montmorillonite; (2) the middle section contains three types of illite (long-, medium- and short-wavelength illite) and hematite; and (3) the lower section includes short-wavelength illite, chlorite and carbonate. Additionally, the variety in the characteristic absorption-peak wavelength of illite at 2200 nm gradually shifts to shorter wavelength and ranges between 2195 nm and 2220 nm with increasing depth, while the SWIR-IC (short-wavelength infrared illite crystallinity, a dimensionless quantity) of the drill holes gradually increases from 0.2 to 2.1. These patterns reflect the hydrothermal fluid activity in the deposit, which features relatively high-temperature, high-pressure hydrothermal fluid in the deeper section and low-temperature, low-pressure hydrothermal fluid in the shallower section. Additionally, the uranium mineralization is located near the fracture zone, which represents the center of hydrothermal fluid activity or mineralization. This area has abundant alteration minerals, and the minerals illite (short- and medium-wavelength), hematite and fluorite can be used as uranium-prospecting indicators for uranium exploration in the deeper sections of the Baiyanghe uranium deposit.

Morphometric and sediment source characterization of the Alaknanda river basin, headwaters of river Ganga, India

Erosion and the resulting sediment load is a silent natural hazard that can affect the hydraulic processes in a fluvial system. The physical erosion rate in the Alaknanda basin is five times higher than the global average, and Alaknanda River is a major supplier of sediments to the Ganga River. Anthropogenic intrusion in the form of construction of dams and reservoirs is influencing the natural landscape of the basin. Thus, it is necessary to prioritize erosion prone areas, understand the weathering intensity and identify source bed rocks contributing to the sediment load. The present study displays a combined approach of morphometry and geochemistry for erosion risk estimation. Nineteen morphometric parameters were evaluated for Alaknanda main channel, Mandakini, Pinder, Nandakini, Birahi Ganga and Dhauli Ganga sub-catchments. Suspended sediment samples collected during non-monsoon and monsoon seasons of the year 2014 were analyzed and quantified for sediment load, grain size distribution, clay mineralogy and rare earth elements composition. The results showed the dominance of structural, lithological and climatic control on the erosion processes. The eastern side of the Alaknanda basin was found to be more vulnerable to fluvial erosion. The mean grain size varied from 8.9 to 56.3 μm and 25.3 to 87.3 μm in the post-monsoon and monsoon season, respectively. The clay mineral assemblages, low values of kaolinite/illite ratio, illite chemistry index and illite crystallinity index along with inconsistent Eu and Ce anomaly indicate that physical and chemical weathering of felsic, mafic and carbonate rocks contributes to high sediment load carried by the Alaknanda River.

Structural boron as factor controlling illite crystallinity in a mud volcano environment (Northern Apennine, Italy)

Mud extrusion is a well-known phenomenon whereby fluid-rich, fine grained sediments ascend within a lithological succession due to their buoyancy. In this study of the terrestrial mud volcanism associated to an active thrusted area in the northern Apennines, we investigated the effect of boron incorporation in illite crystalline lattice. A significant variation was recorded on the investigated mud samples with respect to both the amount of fixed boron (118–529μg/g) and the full width at half-maximum height value (FWHM) of illite (0.29–0.62°Δ 2Θ). We attribute these variations to the interaction between boron-rich formation waters and the silty-clayey lithologies of the local sedimentary succession. The lack of any geographical correlation between the selected geochemical and mineralogical parameters is likely due to intensive thrust and fold deformation of the study area. Comparison of FWHM values with those from Dellisanti et al. (2008); Santerno succession, northern Apennine) lead us to suggest that application of FWHM as a depth indicator could be misleading within B-rich hydro-geological environments.

Very Low‐Grade Metamorphism of Clastic Rocks from the Meso‐Neoproterozoic and the Paleozoic along the Profile Yueyang‐Linxiang in Northeastern Hunan Province and Its Geological Implications

AbstractThis study uses illite crystallinity, chlorite crystallinity, illite polytypes, the b0 cell‐dimension of K‐white mica, clay mineral assemblages and mineral geothermo‐geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang in northeastern Hunan Province, China. Illite crystallinity Kübler index (KI) of the &lt;2µm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Árkai index (AI) ranges from 0.244 to 1.500°Δ2θ. This indicates that the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang were overprinted with diagenesis and anchi‐ to epimetamorphism. Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360°C. The b0 cell dimension values of illites (K‐mica) range from 0.9002 to 0.9054 nm and, on average, at 0.9030 nm for the Meso‐Neoproterozoic. Based on cumulative frequency curves of illite (K‐mica) b0 cell dimension, the peak metamorphic pressure of the Meso‐Neoproterozoic along the profile Yueyang‐Linxiang is derived of an intermediate pressure type. Most illites occur in the 2M1 polytype and some of them in a mixture of 2M1+1M types especially those in the Paleozoic. This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups. However, it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic. Crustal thickening due to “collision” between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso‐Neoproterozoic to ca. 14 km and resulted in a temperature increase in those rocks due to burial. The very low grade to low grade metamorphism overprinting the Meso‐Neoproterozoic implies that the so called “Chiangnania or Jiangnan orogen” was no relative with the “Grenvillian orogeney; instead, it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.

Asymmetry of 1 nm XRD Reflection and Measurement of Illite Crystallinity

Asymmetry of 1 nm XRD Reflection and Measurement of Illite Crystallinity

Mineralogy and geochemistry of Palaeozoic black shales from Peninsular Malaysia: Implications for their origin and maturation

Palaeozoic black shale-bearing formations comprise 25% of the sedimentary cover in Peninsular Malaysia. However, nothing has been published regarding their geology, composition and maturation. Representative samples from different ages and localities of the black shales were subjected to detailed mineralogical and geochemical investigations during this study to determine their origin. These investigations also provide an opportunity to trace the changes in depositional environments and paleoclimate during the Palaeozoic as well as the nature of unknown and hidden basement rocks in Malaysia.This paper reports nineteen Palaeozoic formations in Malaysia that contain black shales that are classified into seven categories based on their age. These are the Cambrian-Ordovician, Ordovician-Devonian, Silurian-Devonian, Devonian, Carboniferous, Carboniferous-Permian and Permian black shales. Although illite was identified in all black shales, other clay minerals including kaolinite and smectite-illite mixed layers show significant variations among the studied samples. The Al2O3/TiO2 ratios (13–33), SiO2 contents of probable magmatic source rocks (54–69wt.%) and Zr-TiO2 bivariate diagram suggest a mixture of felsic and intermediate source rocks for the studied black shales. This indicates that the hidden basement rocks in Malaysia are dominated by felsic igneous rocks. Variations in clay minerals, SiO2 and Al2O3/TiO2 ratios and upper continental crust (UCC)-normalized REE patterns suggest a local provenance for each locality rather than regional and common source rocks for all black shales. Al2O3/SiO2 ratios vary from one age to the other ranging between 0.09 and 0.37. This indicates changes in the degree of weathering of the sediment-source rocks of these shales and suggests climate changes in the Peninsular Malaysia during the Palaeozoic from relatively wet conditions during the Cambrian to relatively dry conditions during the Ordovician followed by relatively wet conditions extends from the Silurian to the Carboniferous, which followed by relatively dry conditions during the formation of Carboniferous-Permian black shales and finally to relatively wet conditions during the Permian. The V/Ni, V/(V+Ni) and V/(V+Cr) ratios as well as the positive Ce anomalies indicate that these black shales were deposited under reducing conditions. The high V/Ni ratios (>3) also suggest a purely marine source input for the organic matter. Devonian black shales show relatively high concentrations of redox-sensitive elements, which may suggest the possible existence of Devonian anoxia as well as the Devonian-Carboniferous (D-C) boundary in Malaysia. Based on the abundance of illite (>85%) and illite crystallinity (IC) values (27–46°Δ2θ), the majority of the Palaeozoic black shales in Peninsular Malaysia are probably anchimetamorphic and over-matured.

Clay mineralogical characteristics at the Permian–Triassic Shangsi section and their paleoenvironmental and/or paleoclimatic significance

High–resolution clay mineralogical investigations were conducted on the Permian–Triassic (P–Tr) carbonate– and siliceous rocks from the Shangsi section, South China. The clay mineralogy is dominated by illite and illite/smectite mixed–layers (I/S) throughout the sampled section with subordinate chlorite and vermiculite emerging from bed 24 (approaching the End–Permian mass extinction horizon–EPME) onwards. Positive correlations of clay fraction with mainly land–derived TiO2 and magnetic susceptibility (MS) indicate a primary continental origin of the clay minerals. Rock–Eval pyrolysis analysis, palynomorph coloration, scanning electron microscope (SEM) and Transmission Electron Microscope (TEM) observations, all suggest that the rocks experienced thermal diagenesis which mainly promotes illitization processes of smectite clay minerals while discrete clay minerals are merely affected. Despite this diagenetic effect, clay minerals still carry valuable paleoenvironmental and paleoclimatic information in the Shangsi section. The overall illitic composition of the clay minerals and nearly absence of kaolinite suggest a general semi–arid condition during the P–Tr interval. Appearance of chlorite from bed 24 onwards suggests further aridification compared to the Late Permian. Occurrence of vermiculite from beds 24–28 in association with abundant volcanogenic materials, most negative δ13Ccarb values as well as enhanced soil–derived moretane abundance, suggests catastrophic soil erosion and likely soil acidification during the P–Tr transition in the source area(s) in relation with the active volcanism in that time. Further, decrease of chemical weathering and increase of physical weathering are indicated by increased input of primary clay minerals such as chlorite, a better illite crystallinity and increase of Al2O3/TiO2 ratios. A climate cooling period corresponding to the Clarkina changxingensis–Clarkina deflecta conodont zone to the lower Clarkina zhangi conodont zone is indicated by higher abundance of illite, lower terrigenous TiO2, MS and Al2O3/TiO2 in comparison with the overlying uppermost Permian strata.

Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity, SW Sardinia, Italy

In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is related to an early Ordovician mild compressional phase, known as the “Sardic phase”. The pelitic samples from the structurally lower Cabitza Formation consist of alternating reddish phyllosilicate-rich and whitish phyllosilicate-poor, sialic layers, whose S0 bedding plane is parallel to a pre-Variscan S1P schistosity overprinted by the Variscan S1V schistosity. Pelitic samples from the Monte Argentu Formation are characterized by a Variscan S1V axial plane schistosity. Samples from the two formations consist of quartz and phyllosilicates. The latter are potassic white mica, chlorite, paragonite, locally kaolinite, and pyrophyllite. The illite crystallinity values determined for the Cabitza samples are 0.25–0.31, with an average of 0.29; meanwhile, the Monte Argentu samples produce values of 0.33–0.38, with an average of 0.35. The chlorite crystallinity and b0 of potassic white mica values show greater heterogeneity in the Cabitza than the Monte Argentu samples. The b0 values and P–T pseudosections allow us to confirm that there is no significant difference in the P–T metamorphism conditions between the Cabitza and Monte Argentu samples. The Iglesiente region, which is considered to be the rift zone behind the Middle Ordovician Sarcidano–Barbagia volcanic arc, underwent the “Sardic phase”, giving rise to E–W folds. These were first overprinted by weak E–W, and then by stronger N–S-oriented Variscan deformation events.

Magnetic evaluation of the palaeothermal variation across the Karoo Basin, South Africa

The Karoo Basin of South Africa is of economic importance for its large coal reserves but has in recent years also been in the spotlight due to the possibility of extensive shale gas reserves. Reconstruction of the thermal history of the Karoo Basin is essential for evaluating the potential hydrocarbon generation within this Late Carboniferous – Middle Jurassic sedimentary basin. Magnetic techniques provide an alternative approach in comparison to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper variations in the thermal history across the Karoo Basin as a result of heating by the Karoo LIP are evaluated using different magnetic “geothermometers”. These include palaeomagnetism (baked contact test), thermomagnetic analysis (alteration index method) and anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in identifying a variation in metamorphic effects adjacent to contact aureoles, only the alternating index (A 40 ) provides a means of estimating peak temperatures. Our results indicate a regional elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. This study shows that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining in strata between these intrusions. An increase in the paleotemperatures from 200°C in the southwest to 400°C in the northeast of the basin is observed. We hypothesize that this trend is mainly due to differences in thermal conductivity of the different sedimentary rock types across the basin as the Karoo Basin transgresses from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone towards the northeast.

Petroleum source rock potential of the Silurian Dadaş shales in the Hazro and Korudağ regions of Southeast Anatolia, Turkey

Shales of the Silurian Dadaş Formation exposed in the southeast Anatolia were investigated by organic geochemical methods. The TOC contents range from 0.24 to 1.48 wt% for the Hazro samples and 0.19 to 3.58 wt% for the Korudağ samples. Tmax values between 438 and 440 °C in the Hazro samples indicate thermal maturity; Tmax values ranging from 456 to 541 °C in the Korudağ samples indicate late to over-maturity. Based on the calculated vitrinite reflectance and measured vitrinite equivalent reflectance values, the Korudağ samples have a maximum of 1.91%R(g-v), in the gas generation window, while a maximum value of 0.79%R(amor-v) of one sample from the Hazro section is in the oil generation window. Illite crystallinity (IC) values of all samples are consistent with maturity results.Pr/Ph ratios ranging from 1.32 to 2.28 and C29/C30 hopane ratios > 1.0 indicate an anoxic to sub-oxic marine-carbonate depositional environment.The Hazro shales do not have any shale oil or shale gas potential because of their low oil saturation index values and early to moderate thermal maturation. At first glance, the Korudağ shales can be considered a shale gas formation because of their organic richness, thickness and thermal over-maturity. However, the low silica content and brittle index values of these shales are preventing their suitability as shale gas resource systems.

Mineralogy, geochemistry, and depositional environment of the Beduh Shale(Lower Triassic), Northern Thrust Zone, Iraq

Integrated mineralogical and geochemical methods are utilized to investigate the provenance, paleoweathering, and depositional setting of shale from the Lower Triassic Beduh Formation in the Northern Thrust Zone, Iraq. The ~64-m-thick Beduh Formation consists of calcareous shale and marl intercalations with thin calcareous sandstone interbeds. X-ray diffraction analysis revealed that clay minerals comprise illite, kaolinite, and chlorite, with a minor mixed layer of illite/smectite and illite/chlorite. Calcite and quartz are the main nonclay species with subordinate amounts of feldspar and hematite. The mineralogical and geochemical parameters of the shale (e.g., high content of illite and moderate illite crystallinity index, Al2O3/TiO2, Th/Co, Cr/Th, and LREE/HREE ratios) indicate that they were derived from felsic and intermediate components. This is supported by the enrichment of LREEs, negative Eu anomaly, and depletion of HREEs. The discriminant function-based major element diagrams indicated that the origin of sediments was probably from passive (the Arabian Shield and the Rutba Uplift) and active (volcanic activity) tectonic environments. The source of sediments for the Beduh Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin. Paleoweathering indices such as the chemical index of alteration and chemical index of weathering, as well as the A-CN-K (Al2O3-CaO+Na2O-K2O) diagram of the shale of the Beduh Formation suggest that the source terrain was moderately to intensely chemically weathered. The Cu/Zn, U/Th, Ni/Co, and V/Cr ratios and negative Eu anomaly indicate the deposition of sediments under an oxygen-rich environment.

The Effect of Dolerite Intrusions on the Hydrocarbon Potential of the Lower Permian Whitehill Formation (Karoo Supergroup) in South Africa and Southern Namibia: A Preliminary Study

Recent interest in the shale gas potential in the main Karoo Basin of South Africa has focused attention mainly on the carbonaceous Whitehill Formation (Ecca Group, Karoo Supergroup). To unravel the metamorphic effect of the Lower Jurassic Karoo igneous intrusions on the Lower Permian Whitehill Formation, outcrop samples were taken at various distances (ranging from 1 to >30 m) from the intrusive contacts of dolerite sills in the western part of the main Karoo Basin (Northern Cape, South Africa) and in the Karasburg Basin (southern Namibia). Furthermore, to establish the baseline petrological and geochemical composition of the Whitehill Formation, samples have also been collected at localities unaffected by dolerites in the southern main Karoo Basin. Mineral assemblages from all sample localities plot within the chlorite zone and show very low grade metamorphism. Mineral assemblages and illite crystallinity data indicate that the Whitehill Formation has undergone (a) anchizone metamorphism (250 to 300°C) in the southern main Karoo basin as well as in areas close to the dolerite intrusions, and (b) moderate burial diagenesis (~180°C) in areas away from the immediate effect of dolerites in the Northern Cape and southern Namibia. Regional evaluation of unconventional gas potential of the Karoo basins of southern Africa ought, therefore, to consider the subsurface thickness and spacing of Karoo intrusive bodies within the target carbonaceous formations.

Thermal History Reconstruction Based on Multiple Paleo‐Thermal Records of the Yazihe Area, Western Sichuan Depression, SW China

AbstractThe thermal history of boreholes in the western Sichuan depression was reconstructed using paleo‐temperature indicators, such as vitrinite reflectance (Ro), apatite fission track (AFT) and the illite crystallinity index (IC index). The maximum paleo‐temperatures reconstructed based on different indicators were compared. The results showed that the western Sichuan depression has experienced a continuous process of denudation and cooling since the late Cretaceous (K3), resulting in a geothermal gradient reduction from ∼26 °C·km−1 to ∼22 °C·km−1 and the removal of 1.3 to 1.9 km of sediment. The surface area has undergone continuous uplift and denudation from ∼80 Ma to 40 Ma, followed by a thermal quiet period until 2.5 Ma. Since ∼2.5 Ma, the geothermal gradient has risen by ∼5 °C·km−1. In addition, there was high consistency among the paleo‐temperatures that were reconstructed based on different indicators. Relative to Ro and AFT, the illite crystallinity index is still controversial; more research is necessary before it can be used as a maturity and geothermal indicator for sedimentary rock.

Porosity and pore size distribution of deeply-buried fine-grained rocks: Influence of diagenetic and metamorphic processes on shale reservoir quality and exploration

Significant exploration risks are associated with the pursuit of deeply-buried shale gas reservoirs due to pore volume reduction and changes in pore size distribution. These changes in pore character result in decreases in gas in place and permeability. A suite of shale, low grade, pelitic metamorphic and a granite outcrop samples from various location in North America have been selected to span the later stages of diagenetic, epimetamorphic (epizone) and anchimetamorphic (anchizone) processes to evaluate the changes in the inorganic pore volumes and size distributions. Diagenetic/metamorphic ranking of samples were determined by the illite crystallinity method. Pore volumes reduce with increasing maturity/metamorphic grade. The loss of mesopore volume (2–50nm) with increasing maturity is the cause of the reduction in porosity. The reduction in mesopore volume is interpreted to be due to the authigenic recrystallization and growth of the clay minerals. As maturity/metamorphic grade increases there is a relative increase in the macropore (>50nm) and micropore (<2nm) size fractions. The increase in micropore volumes may be attributed to the development of secondary porosity within the kerogen. At higher maturity/metamorphic grade (i.e., illite crystallinity<0.2 Δ2θ) porosity values range between 0.9% and 3.6% indicating that fracture porosity is not the only mechanism of gas storage in deeply buried shale (and pelitic metamorphic rocks) reservoirs. Matrix porosities in these higher maturity/metamorphic samples are comparable to matrix porosities of the Horn River shales of British Columbia and other shale reservoirs. Similar to the Horn River and Doig–Montney shales, the reduction in mesopore volumes may reduce the matrix permeability of these rocks and fracture stimulation will be an integral component of the completions program to access hydrocarbons.