Petrography and Geochemistry of Dolomites of Samanasuk Formation, Dara Adam Khel Section, Kohat Ranges, Pakistan

Replacement dolomite occurs in Jurassic Samanasuk Formation in Dara Adam khel area of Kohat ranges, North-Western Himalayas, Pakistan. This study, for the first time, document the process of dolomitization and evolution of strata bound dolomitic bodies. Field investigation, petrography and geochemistry helped in unraveling the formation of several dolomitic bodies. Petrographically dolomites comprises of: (1) medium grain crystalline planer subhedral dolomite (Dol-I); (2) fine grained crystalline anhedral non-planer dolomite rhombs (Dol-II); (3) medium to coarse grained crystalline subhedral-anhedral non-planer dolomite (Dol-III) and coarse to very coarse grained crystalline saddle dolomite cements (SD). The saddle dolomites (SD) postdate the replacement dolomites and precede telogenetic calcite (TC) cements. Stable O and C isotope analysis shows that these dolomites have δ18Ovpdb ranging from -4.09% to -10.4 whereas the δ13Cvpdb ranges from +0.8 to +2.51. Major and trace elements data show that Sr concentrations of 145.5 to 173 ppm; Fe contents of 2198 to 8215 ppm; and Mn contents of 93.5 to 411 ppm. Petrographically replacive dolomites, saddle dolomite, and δ18Ovpdb values depicts neomorphism of replacement dolomites that were formed earlier were exposed to late dolomitizing fluids. As a result of basin uplift during the Himalayan orogeny in Eocene time, dolomitization event was stopped through occurrence of meteoric water. The Main Boundary Thrust (MBT) and its splays were most likely essential conduits that channelized dolomitizing fluids from siliciclastic rocks that were buried deeply into the Jurassic carbonates rocks, leading to more extreme dolomitization.

Shortwave infrared hyperspectral imaging as a novel method to elucidate multi-phase dolomitization, recrystallization, and cementation in carbonate sedimentary rocks

Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitation, dissolution, or recrystallisation (diagenesis). Unravelling the sequence of these events is time-consuming, expensive, and relies on destructive analytical techniques, yet such characterization is essential to understand their post-depositional history for mineral and energy exploitation and carbon storage. Conversely, hyperspectral imaging offers a rapid, non-destructive method to determine mineralogy, while also providing compositional and textural information. It is commonly employed to differentiate lithology, but it has never been used to discern complex diagenetic phases in a largely monomineralic succession. Using spatial-spectral endmember extraction, we explore the efficacy and limitations of hyperspectral imaging to elucidate multi-phase dolomitization and cementation in the Cathedral Formation (Western Canadian Sedimentary Basin). Spectral endmembers include limestone, two replacement dolomite phases, and three saddle dolomite phases. Endmember distributions were mapped using Spectral Angle Mapper, then sampled and analyzed to investigate the controls on their spectral signatures. The absorption-band position of each phase reveals changes in %Ca (molar Ca/(Ca + Mg)) and trace element substitution, whereas the spectral contrast correlates with texture. The ensuing mineral distribution maps provide meter-scale spatial information on the diagenetic history of the succession that can be used independently and to design a rigorous sampling protocol.

What makes seep carbonates ignore self-sealing and grow vertically: the role of burrowing decapod crustaceans

The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15 m thick, 8 m wide column of limestone encased in deep-water marl in the middle Callovian interval of the Terres Noires Formation in the SE France Basin. The limestone body, also called “pseudobioherm”, records intense bioturbation, with predominant traces of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation was organized in four tiers. The uppermost tier, tier 1, corresponds to shallow homogenization of rather soft sediment. Tier 2 corresponds to pervasive burrows dominated by large Thalassinoides that were later passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range from −5 ‰ to −10 ‰. Tier 3 is characterized by small Thalassinoides that have walls locally bored by Trypanites; the latter represent tier 4. The diagenetic cements filling the tier-3 Thalassinoides are arranged in two phases. The first cement generation constitutes a continuous rim that coats the burrow wall and has consistent δ13C values of approximately −8 ‰ to −12 ‰, indicative of bicarbonate originating from the anaerobic oxidation of methane. In contrast, the second cement generation is dominated by saddle dolomite precipitated at temperatures >80 ∘C, at a time when the pseudobioherm was deeply buried. The fact that the tubes remained open until deep burial means that vertical fluid communication was possible over the whole vertical extent of the pseudobioherm up to the seafloor during its active development. Therefore, vertical growth was fostered by this open burrow network, providing a high density of localized conduits through the zone of carbonate precipitation, in particular across the sulfate–methane transition zone. Burrows prevented self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key aspect is the geometric complexity of the burrows with numerous subhorizontal segments that could trap sediment shed from above and, hence, prevent their passive fill.

Fluid Histories of Middle Ordovician fault–fracture hydrothermal dolomite oil fields in the southern Michigan Basin, U.S.A.

ABSTRACT Dolomitized fault–fracture structures in the Trenton and Black River formations (TBR) are the type example for “hydrothermal” petroleum reservoirs world-wide. However, fluid histories of these structures are only partially understood. Trenton and Black River reservoirs in the southern Michigan Basin are composed of fault-associated, vertical dolomite bodies that are highly fractured and brecciated. Open spaces are partially to completely filled by saddle dolomite and less frequently by calcite cement. Cathodoluminescence microstratigraphies of void-filling carbonate cements are not correlatable between oil fields. Fluid inclusion homogenization temperatures (Th) measured in carbonate cements indicate two fluid endmembers: a warm fluid (∼ 80° to 180° C) and a hot fluid (180° to ∼ 260° C). Increasing Th proximal to the underlying Proterozoic Mid-Michigan Rift (MMR) suggest that the hot fluids emanated from the rift area. Included fluids are saline (16.1–49.4 wt. % NaCl equivalent), and salinity likely is sourced from overlying Silurian Salina Group evaporites. First melting temperatures (Tfm), interpreted as eutectic temperatures (Te), of fluids range from –112° C to –50° C, indicating a complex Na–Ca–KCl brine; the expected composition of dissolved Salina salts. Lower Te proximal to the MMR suggest the rift as a source of additional complexing ions. C and O isotope values for carbonate cements are depleted with respect to δ18O (–6.59 to –12.46‰ VPDB) relative to Ordovician seawaters, and somewhat depleted with respect to δ13C (–1.22 to +1.18‰ VPDB). Equilibrium calculations from δ18O and Th values indicate that cement precipitating waters were highly evolved (+1.3 to +14.4‰ δ18O‰ VSMOW) compared to Ordovician and Silurian seawaters (–5.5‰ δ18O‰ VSMOW). Strontium isotope values indicate two fluid sources: Proterozoic basement and Late Silurian evaporites. Values of 87Sr/86Sr for cements in the Freedom, Napoleon, Reading, and Scipio fields (0.7086–0.7088) are influenced by warm water sourced from Silurian strata, and values for cements in the Albion, Branch County, and Northville fields (0.7091–0.7110) record continental basement signatures. Cement precipitating fluids in TBR oil fields likely have similar sources and timing. However, water–rock interactions along fault pathways modified source waters, giving each oil field a unique petrographic and geochemical signature. Fluid movement in TBR oil fields likely were initiated by reactivation of basement faulting during Silurian–Devonian tectonism.

Stylolites and stylolite networks as primary controls on the geometry and distribution of carbonate diagenetic alterations

There is an ongoing debate on whether stylolites act as barriers or conduits for fluids, or even play no role in terms of fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study (Maestrat Basin, E Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during the geological evolution of a basin. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and calcitization processes. Anastomosing stylolites, which pre-date burial dolomitization, likely acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front weaves up and down following consecutive anastomosing stylolites, which are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Contrarily, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers again when the burial and stress conditions changed. Stylolites within dolostones close to faults are found corroded and filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent calcitization reactions and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.

Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer

This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite-cemented breccias at much shallower depths than previously realized.

Indication of hydrocarbon migration in the Western Mecsek Mountains evidenced by fluid inclusion chemostratigraphy

Primary and secondary hydrocarbon-bearing fluid inclusion (HCFI) assemblages occur in the Middle Triassic Lapis Limestone in the Szuadó Valley of the Western Mecsek Mts. The primary HCFIs were trapped in saddle dolomite crystals, and the secondary HCFIs were enclosed in calcite neospar and fracture-filling calcite. Solid bitumen is also present along fractures. The volatile compounds liberated from fluid inclusions are characterized by non-hydrocarbon and hydrocarbon species. The fluorescent properties of HCFIs, the occurrence of the solid bitumen, as well as the composition of inclusion oils indicate the migration of light oils through the Lapis Limestone. Petrographic observations suggest a prolonged oil charge event, which resulted in HCFIs beeing trapped under evolving diagenetic conditions.