Quantifying the influence of sill intrusion on the thermal evolution of organic‐rich sedimentary rocks in nonvolcanic passive margins: an example from ODP 210‐1276, offshore Newfoundland,Canada

Intrusive magmatism is an integral and understudied component in both volcanic and nonvolcanic passive margins. Here, we investigate the thermal effects of widespread (ca. 20 000 km²) intrusive magmatism on the thermal evolution of organic‐rich sedimentary rocks on the nonvolcanic Newfoundland passive margin. ODP 210‐1276 (45.41°N, 44.79°W) intersects two sills: an older, upper sill and a younger, lower sill that are believed to correspond to the high amplitude ‘U‐reflector’ observed across the Newfoundland Basin. A compilation of previous work collectively provides; (1) emplacement depth constraints, (2) vitrinite reflectance data and (3) ⁴⁰Ar/³⁹Ar dates. Collectively, these data sets provide a unique opportunity to model the conductive cooling of the sills and how they affect thermal maturity of the sedimentary sequence. A finite differences method was used to model the cooling of the sills, with the model outputs then being entered into the EASY%R_o vitrinite reflectance model. The modelled maturation profile for ODP 210‐1276 shows a significant but localized effect on sediment maturity as a result of the intrusions. Our results suggest that even on nonvolcanic margins, intrusive magmatism can significantly influence the thermal evolution in the vicinity of igneous intrusions. In addition, the presence of widespread sills on nonvolcanic passive margins such as offshore Newfoundland may be indicative of regional‐scale thermal perturbations that should be considered in source rock maturation studies.     

Thermal modelling of magmatic intrusions in the Gjallar Ridge, Norwegian Sea: implications for vitrinite reflectance and hydrocarbon maturation

Extensive magmatic activity took place in the Vøring Basin, offshore Norway, related to the Early Cenozoic rifting. The break‐up of the North‐Atlantic at the Palaeocene–Eocene transition induced strong volcanism. There are numerous magmatic sills below 3 km depth in the area. They are predominantly layer parallel and thin compared with their lateral extent. Igneous intrusions, sills and dykes affected the temperature history, and thus need to be taken into account in petroleum prospect analysis. We have calculated the temperature and maturity effects in the sedimentary layers in the Gjallar area associated with the emplacement of single sill and sill complexes. A 120‐m‐thick sill produces a theoretical vitrinite reflectance (%R₀) 0.8% higher than normal at a distance of 100 m from the sill. Vitrinite reflectance changes caused by a swarm of seven sills varying from 8 to 80 m in thickness were calculated. It is shown that the calculated thermal profile can account for the observed shift in vitrinite reflectance in the well. A two‐dimensional section crossing the Gjallar Ridge, consisting of numerous magmatic intrusions, is also modelled. The modelled geological development and temperature history over the profile show that there are significant maturation effects in the interval under investigation. Based on this work, the sill swarm observed in the area could more than double the fraction of the kerogen that has been transformed to petroleum at the (present) depth of 4 km.     

Thermal and exhumation histories of the northern subalpine chains (Bauges and Bornes—France): Evidence from forward thermal modeling coupling clay mineral diagenesis,organic maturity and carbonate clumped isotope (Δ47) data

Assessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ₄₇ clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160–180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ₄₇ thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160–180°C. High temperatures are associated with the tectonic emplacement of up to 4 km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ₄₇, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.     

Geochemical Characteristics of Oil from Oligocene Lower Ganchaigou Formation Oil Sand in Northern Qaidam Basin,China

Oil from the Oligocene oil sands of the Lower Ganchaigou Formation in the Northern Qaidam Basin and the related asphaltenes was analyzed using bulk and organic geochemical methods to assess the organic matter source input, thermal maturity, paleo-environmental conditions, kerogen type, hydrocarbon quality, and the correlation between this oil and its potential source rock in the basin. The extracted oil samples are characterized by very high contents of saturated hydrocarbons (average 62.76%), low contents of aromatic hydrocarbons (average 16.11%), and moderate amounts of nitrogen–sulfur–oxygen or resin compounds (average 21.57%), suggesting that the fluid petroleum extracted from the Oligocene oil sands is of high quality. However, a variety of biomarker parameters obtained from the hydrocarbon fractions (saturated and aromatic) indicate that the extracted oil was generated from source rocks with a wide range of thermal maturity conditions, ranging from the early to peak oil window stages, which are generally consistent with the biomarker maturity parameters, vitrinite reflectance (approximately 0.6%), and T_max values of the Middle Jurassic carbonaceous mudstones and organic-rich mudstone source rocks of the Dameigou Formation, as reported in the literature. These findings suggest that the studied oil is derived from Dameigou Formation source rocks. Furthermore, the source- and environment-related biomarker parameters of the studied oil are characterized by relatively high pristane/phytane ratios, the presence of tricyclic terpanes, low abundances of C₂₇ regular steranes, low C₂₇/C₂₉ regular sterane ratios, and very low sterane/hopane ratios. These data suggest that the oil was generated from source rocks containing plankton/land plant matter that was mainly deposited in a lacustrine environment and preserved under sub-oxic to oxic conditions, and the data also indicate a potential relationship between the studied oil and the associated potential source rocks. The distribution of pristane, phytane, tricyclic terpanes, regular steranes and hopane shows an affinity with the studied Oligocene Lower Ganchaigou Formation oil to previously published Dameigou Formation source rocks. In support of this finding, the pyrolysis–gas chromatography results of the analyzed oil asphaltene indicate that the oil was primarily derived from type II organic matter, which is also consistent with the organic matter of the Middle Jurassic source rocks. Thus, the Middle Jurassic carbonaceous mudstones and organic rock mudstones of the Dameigou Formation could be significantly contributing source rocks to the Oligocene Lower Ganchaigou Formation oil sand and other oil reservoirs in the Northern Qaidam Basin.

     

Conventional and Unconventional Hydrocarbon Resource Potential Evaluation of Source Rocks and Reservoirs: A Case Study of the Upper Xiaganchaigou Formation,Western Qaidam Basin,Northwest China

The Paleogene upper Xiaganchaigou Formation (E₃²) is the most important source rock and reservoir in the Qaidam Basin. However, there are few studies on the processes of hydrocarbon accumulation in this formation; therefore, its hydrocarbon resource potential has not been estimated reasonably. This paper evaluates the hydrocarbon generation properties in light of an improved hydrocarbon generation and expulsion potential model. According to the geochemical characteristics of source rocks and the petrological features of reservoirs, the potentials of different resource types, including conventional oil, tight oil and shale oil, are quantified by combining the buoyancy-driven hydrocarbon accumulation depth (BHAD) and the lower limit for movable resource abundance. The results show that the source rocks are characterized by a large thickness (more than 1000 m), moderate organic matter content, high marginal maturity and a high conversion rate (50% hydrocarbons have been discharged before R_o?=?1%), which provide sufficient oil sources for reservoir formation. Moreover, the reservoirs in the Qaidam Basin consist mainly of low-porosity and low-permeability tight carbonates (porosity of 4.7% and permeability less than 1 mD). The maximum hydrocarbon generation, expulsion, retention and movable retention intensities at present are 350?×?10⁴ t/km², 250?×?10⁴ t/km², 130?×?10⁴ t/km² and 125?×?10⁴ t/km², respectively. The thresholds of hydrocarbon generation, expulsion and BHAD were 0.46% R_o, 0.67% R_o and 0.7% R_o, respectively. Moreover, the dynamic evolution process of hydrocarbon accumulation was divided into three evolution stages, namely, (a) initial hydrocarbon accumulation, (b) conventional hydrocarbon reservoir and shale oil accumulation and (c) unconventional tight oil accumulation. The conventional oil, tight oil and movable shale oil resource potentials were 10.44?×?10⁸ t, 51.9?×?10⁸ t and 390?×?10⁸ t, respectively. This study demonstrates the good resource prospects of E₃² in the Qaidam Basin. A comprehensive workflow for unconventional petroleum resource potential evaluation is provided, and it has certain reference significance for other petroliferous basins, especially those in the early unconventional hydrocarbon exploration stage.

     

Evaluation of Upper Triassic T3x5 Source Rocks (Western Sichuan Depression, Sichuan Basin) and their Hydrocarbon Generation and Expulsion Characteristics: Implication for Tight-Sand Gas and Shale Gas Accumulation Potential Assessment

An unconventional, continuous petroleum system consists of an accumulation of hydrocarbons that is found in low-matrix-permeability rocks and contain large amounts of hydrocarbons. Tight-sand gas in the Jurassic and shale gas within the fifth member of Xujiahe Formation (T₃x⁵) are currently regarded as the most prolific emerging unconventional gas plays in China. The conventional and systematical evaluation of T₃x⁵ source rocks was carried out for the first time in the western Sichuan basin (WSD). Hydrocarbon generation and expulsion characteristics (including intensity, efficiency, and amount) of T₃x⁵ source rocks were investigated. Results show that T₃x⁵ source rocks are thick (generally >200 m), have high total organic content (TOC, ranging from 2.5 to 4.5 wt%), and dominated by III-type kerogen. These favorable characteristics result in a great hydrocarbon generating potential under the high thermal evolution history (R _o > 1.2%) of the area. An improved hydrocarbon generation potential methodology was applied to well data from the area to unravel the hydrocarbon generation and expulsion characteristics of T₃x⁵ source rocks in the WSD. Results indicate that the source rocks reached hydrocarbon expulsion threshold at 1.06% R _o and the comprehensive hydrocarbon expulsion efficiency was about 60%. The amount of generation and expulsion from T₃x⁵ source rocks was 3.14 × 10¹⁰ and 1.86 × 10¹⁰ t, respectively, with a residual amount of 1.28 × 10¹⁰ t within the source rocks. Continuous-type tight-sand gas was predicted to develop in the Jurassic in the Chengdu Sag of the WSD because of the good source-reservoir configuration (i.e., the hydrocarbon generation and expulsion center was located in Chengdu Sag), the Jurassic sandstone reservoirs were tight, and the gas expelled from the T₃x⁵ source rocks migrated for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from T₃x⁵ source rocks is up to 9.3 × 10⁸ t. The T₃x⁵ gas shale has good accumulation potential compared with several active US shale-gas plays. Volumetrically, the geological resource of shale gas is up to 1.05 × 10¹⁰ t. Small differences between the amounts calculated by volumetric method compared with that by hydrocarbon generation potential methodology may be due to other gas accumulations present within interbedded sands associated with the gas shales.     

Artificial Maturation of Organic Matter from Recent Venezuelan Sediments

Laboratory simulation of the catagenesis of organic matter in sedimentary rocks has been used to provide an understanding of the processes involved in petroleum generation. Several of these studies have focused on the thermal evolution of organic matter (OM) present in Recent sediments. This study examines the geochemical characteristics and experimental thermal evolution of primary organic matter from two organic facies that are thought to be major contributors to Venezuelan hydrocarbon source rocks. A third facies, generally considered unimportant for petroleum formation, is used to contrast the experimental results. Hydrous pyrolysis maturation experiments were performed for three intermediate temperatures. The products of the final 330°C stage are shown in this paper because they best illustrate the changes in the OM during catagenesis. Results from the hydrous pyrolysis experiments show that at 280°C and higher all three samples yield liquid hydrocarbons similar in composition to natural crudes and the transformed organic matter is similar to kerogen that occurs in natural source rocks. Chromatograms from the saturated fraction of extracts at 330°C are similar to natural crudes with respect to n-alkane distribution and abundance of beta; and beta;alpha; hopanes. The only difference seems to be the relative abundance of 22R over 22S isomers, which indicates immature oil. This is in contrast to indications from the R _o and T _max parameters measured on the accompanying kerogen.     

Deep-Lacustrine Shale Heterogeneity and Its Impact on Hydrocarbon Generation,Expulsion, and Retention: A Case Study from the Upper Triassic Yanchang Formation,Ordos Basin,China

The Chang-7 shale of the Upper Triassic Yanchang Formation was deposited in a deep-lacustrine environment in the southwest part of the Ordos Basin. It is characterized by a strong lithological heterogeneity, consisting primarily of pure shale and sandy laminated shale. This study explored the impact of sandy laminae in the thick pure shale on hydrocarbon generation, retention, and expulsion, which were rarely considered in previous studies. Based on core observation, thin section, and geochemical analysis, the hydrocarbon generation, retention, and expulsion characteristics were obtained for both pure shale and sandy laminated shale. In general, the Chang-7 shale stays at low mature to mature thermal evolution stage and has good hydrocarbon generation potential. It contains mainly Type II kerogen with an average total organic carbon (TOC) of 2.9% and average (S₁?+?S₂) of 8.2 mg/g. Compared with sandy laminated shale, pure shale contains more retained liquid hydrocarbon and has a higher amount of asphaltene and nitrogen–sulfur–oxygen (NSO) polarized components, indicating a relatively weak hydrocarbon expulsion process. The middle part of a thick pure shale retains more liquid hydrocarbon and has higher percentages of asphaltene and NSO polarized components than that of the top and basal part of the shale where sandy laminae occur. The difference in hydrocarbon retention capacity is interpreted to have been primarily caused by the comparatively higher reservoir quality of the sandy laminated shale, having higher amount of brittle minerals and larger pores than the pure shale. Polymer dissolution and nanopore adsorption are also key factors in hydrocarbon retention and component partition. Based on this study, we suggest that sandy laminated shale, which receives most of the hydrocarbon from adjacent pure shale, should be the current favorable shale oil exploration targets. Even though pure shale contains high hydrocarbon potential, its development is still pending improved technologies, which could solve the challenges caused by complicated geological conditions.

     

Application of multi‐kinetic apatite fission track and (U‐Th)/He thermochronology to source rock thermal history: a case study from the Mackenzie Plain,NWT, Canada

Shale of the Upper Cretaceous Slater River Formation extends across the Mackenzie Plain of the Canadian Northwest Territories and has potential as a regional source rock because of the high organic content and presence of both oil‐ and gas‐prone kerogen. An understanding of the thermal history experienced by the shale is required to predict any potential petroleum systems. Our study integrates multi‐kinetic apatite fission track (AFT) and apatite (U‐Th)/He (AHe) thermochronometers from a basal bentonite unit to understand the timing and magnitude of Late Cretaceous burial experienced by the Slater River Formation along the Imperial River. We use LA‐ICP‐MS and EPMA methods to assess the chemistry of apatite, and use these values to derive the AFT kinetic parameter r_mr0. Our AFT dates and track lengths, respectively, range from 201.5 ± 36.9 Ma to 47.1 ± 12.3 Ma, and 16.8 to 10.2 μm, and single crystal AHe dates are between 57.9 ± 3.5 and 42.0 ± 2.5 Ma with effective uranium concentrations from 17 ppm to 36 ppm. The fission track data show no relationship with the kinetic parameter D_par and fail the χ²‐test indicating that the data do not comprise a single statistically significant population. However, when plotted against their r_mr0 value, the data are separated into two statistically significant kinetic populations with distinct track length distributions. Inverse thermal history modelling of both the multi‐kinetic AFT and AHe datasets, reveal that the Slater River Formation reached maximum burial temperatures of ~65–90 °C between the Turonian and Paleocene, indicating that the source rock matured to the early stages of hydrocarbon generation, at best. Ultimately, our data highlight the importance of kinetic parameter choice for AFT and AHe thermochronology, as slight variations in apatite chemistry may have significant implications on fission track and radiation damage annealing in apatite with protracted thermal histories through the uppermost crust.     

Distinction Between Oil Expulsion History and Gas Expulsion History

The process of organic matter transformation into oil and gas is also a balance process of hydrocarbon transformation. This article probes to distinguish the oil expulsion history from gas expulsion history based on the hydrocarbon generation, hydrocarbon residual, and hydrocarbon expulsion processes of the source rocks. In this method, the first step is to study the hydrocarbon expulsion rate by means of hydrocarbon generation potential method; the second step is to study the oil generation rate by means of the heating–pressuring experiment method; the third step is to study the oil residual rate by means of the mathematical method. The difference between the values of oil generation rate and oil residual rate is defined as the oil expulsion rate, while that between the values of hydrocarbon expulsion rate and the gas expulsion rate is defined as the gas expulsion rate. Then, combined with the geological parameters of source rocks, the oil and gas expulsion history can be obtained. This study on Es₁ Source rocks, Nanpu Sag, Bohai Bay Basin, China shows that the primary expulsion period of Es₁ source rocks is Guantao–Minghuazhen period.     

One-dimensional models of groundwater flow, sediment thermal history and petroleum generation within continental rift basins

We investigate the effects of convective heat transfer on the thermal history of sediments and petroleum formation within continental rift basins using one-dimensional mathematical modelling. The transport equations used in this study to describe vertical groundwater flow and conductive/convective heat transfer are solved by the finite element method. Sediment thermal history is quantitatively represented using first-order rate kinetic expressions for kerogen degradation and an empirical fanning Arrhenius model for apatite fission track annealing. Petroleum generation is also represented in the model by a suite of first-order rate kinetic expressions. The analysis provides insights into how pore fluid circulation patterns are preserved in the rock record as anomalies in palaeogeothermometric data within continental rifts. Parameters varied in the numerical experiments include the ratio of conductive to convective heat transfer (thermal Peclet number; Pe) and the composition of the disseminated organic matter in the sediment (type II and III kerogen). Quantitative results indicate that vertical groundwater flow rates on the order of a mm/yr cause a change in computed vitrinite reflectance of the rocks and a shift in the depth to oil generation by as much as 3000 m. Differences in thermal gradients between recharge and discharge areas (Pe= 0.6) also change the width of the zone of oil generation by a factor of two. Even more dramatic, however, are the large changes in predicted apatite fission track length distributions and model ages between recharge and discharge areas. For example, a sediment package buried to a depth of 2400 m over 200 Myr within the groundwater recharge column had a fission track length distribution with a computed mean and standard deviation of 12.83 μm and 0.77 μm, respectively. The fission track model age for this sediment package was 209 Ma. The same sediment package in the discharge area has a distribution with a mean track length of 5.68 μm, a standard deviation of 3.37 μm, and a fission track model age of 2.6 Ma. Transient groundwater flow simulations, in which fluid circulation ceases after a period of time within the rift basin, are also presented to illustrate how disturbances in palaeogeothermometric parameters are preserved on geological time-scales. Vitrinite reflectance profiles require about 10 Myr to return to conductive conditions within groundwater recharge areas while the convective disturbances are preserved indefinitely along the discharge column, as long as further subsidence does not occur. Ancient groundwater flow systems are preserved as anomalies in computed apatite fission track model ages and distributions much longer after groundwater flow stops, relative to organic-based geothermometers. Significant differences exist in model ages between recharge (145 Ma) and discharge (90 Ma) areas 200 Myr after flow has ceased. However, calculated fission track histogram distributions are virtually identical in recharge and discharge areas after about 50 Myr. Our study suggests that ancient groundwater flow systems can be detected by comparing thermochronometric data between suspected recharge and discharge areas within continental rifts. Vitrinite reflectance profiles, observed offsets in the depth to the onset of petroleum generation, and apatite fission track annealing studies are all well suited for detecting groundwater flow systems which have been relatively long lived (10⁷ years). Apatite fission track age data are probably best suited for identifying ancient groundwater flow systems within rifts long (>200 Myr) after flow ceases.     

The thermal maturity of sedimentary basins as revealed by magnetic mineralogy

The thermal evolution of sedimentary basins is usually constrained by maturity data, which is interpreted from Rock-Eval pyrolysis and vitrinite reflectance analytical results on field or boreholes samples. However, some thermal evolution models may be inaccurate due to the use of elevated maturities measured in samples collected within an undetected metamorphic contact aureole surrounding a magmatic intrusion. In this context, we investigate the maturity and magnetic mineralogy of 16 claystone samples from Disko-Svartenhuk Basin, part of the SE Baffin Bay volcanic margin. Samples were collected within thermal contact metamorphic aureoles near magma intrusions, as well as equivalent reference samples not affected by intrusions. Rock-Eval pyrolysis (T_max), and vitrinite reflectance (Ro) analysis were performed to assess the thermal maturity, which lies in the oil window when 435°C ≤ T_max ≤ 470°C and 0.6%–0.7% ≤ Ro ≤ 1.3%. In addition, we performed low- (<300K) and high-temperature (>300K) investigations of isothermal remanent magnetization to assess the magnetic mineralogy of the selected samples. The maturity results (0.37% ≤ Ro ≤ 2%, 22°C ≤ T_max ≤ 604°C) show a predominance of immature to early mature Type III organic matter, but do not reliably identify the contact aureole when compared to the reference samples. The magnetic assemblage of the immature samples consists of iron sulphide (greigite), goethite and oxidized or non-stoichiometric magnetite. The magnetic assemblage of the early mature to mature samples consists of stoichiometric magnetite and fine-grained pyrrhotite (<1 μm). These results document the disappearance of the iron sulphide (greigite) and increase in content of magnetite during normal burial. On the other hand, magnetite is interpreted to be the dominant magnetic mineral inside the contact aureole surrounding dyke/sill intrusions where palaeotemperatures indicate mature to over-mature state. Interestingly, the iron sulphide (greigite) is still detected in the contact aureole where palaeotemperatures exceeded 130°C. Therefore, the magnetic mineralogy is a sensitive method that can characterize normal burial history, as well as identify hidden metamorphic contact aureoles where the iron sulphide greigite is present at temperatures beyond its stability field.     

New insights on measured and calculated vitrinite reflectance

Measurement of dispersed vitrinite reflectance in organic sediments is one of the few regional data sets used for placing bounds on the thermal history of a sedimentary basin. Reflectance data are important when access to complementary information such as high‐quality seismic data is unavailable to place bounds on subsidence history and in locations where uplift is an important part of the basin history. Attributes which make vitrinite reflectance measurements a useful data set are the relative ease of making the measurement, and the availability of archived well cores and cuttings in state, provincial, and federal facilities. In order to fully utilize vitrinite data for estimating the temperature history in a basin, physically based methods are required to calibrate an equivalent reflectance from a modelled temperature history with measured data. The most common method for calculating a numerical vitrinite reflectance from temperature history is the EASY%R_o method which we show systematically underestimates measured data. We present a new calculated reflectance model and an adjustment to EASY%R_o which makes the correlation between measured vitrinite values and calculated vitrinite values a physical relationship and more useful for constraining thermal models. We then show that calibrating the thermal history to vitrinite on a constant age date surface (e.g., top Cretaceous) instead of calibrating the thermal history in depth removes the heating rate component from the reflectance calculation and makes thermal history calibration easier to understand and more directly related to heat flow. Finally, we use bounds on the vitrinite–temperature relationships on a constant age date surface to show that significant uncertainty exists in the vitrinite data reported in most data sets.     

Stable isotope constraints on the fluid source of hydrothermal breccia pipes in the Tankwa Karoo depocentre,South Africa: Breakdown of authigenic minerals during sill intrusion

The Karoo Basin covers much of South Africa and is an area of prospective shale gas exploration, with the Whitehill Formation the target shale unit. However, the sedimentary succession, including the Whitehill, has been intruded by a series of sills and dykes associated with the Karoo Large Igneous Province (~183 Ma), which are expected to have modified the thermal history of the basin dramatically. Here, we investigate a secondary effect of these intrusions: a series of hydrothermal vent complexes, or breccia pipes, focusing on using O, H, and C isotopes to constrain the origin and evolution of fluids produced during the intrusion of basaltic sills. A cluster of breccia pipes have been eroded down to the level of the Ecca Group at Luiperdskop on the western edge of the Karoo basin; a small isolated pipe of similar appearance crops out 13 km to the east. The Luiperdskop pipes are underlain by a Karoo dolerite sill that is assumed to provide the heat driving fluidization. The pipes consist of fine‐grained matrix and about 8% clasts, on average, of mostly sedimentary material; occasional large rafts of quartzite and dolerite are also present. The presence of clasts apparently from the Dwyka Group is consistent with the depth of formation of the pipes being at, or near, the base of the Karoo Supergroup, between 400 and 850 m below present surface. The presence of chlorite as the dominant hydrous mineral is consistent with an emplacement temperature between 300 and 350°C. The major and trace element, and O‐ and H‐isotope composition of the Tankwa breccias is homogenous, consistent with them being derived from the same source. The δ¹⁸O values (vsVSMOW) of the breccias are relatively uniform (7.1‰–8.7‰), and are similar to that of the country rock shale, and both are lower than expected for shale. The water content of the breccia is between 2.7 and 3.1 wt.% and the δD values range from ?109‰ to ?144‰. Calcite in vesicles has δ¹³C and δ¹⁸O (VSMOW) values of ?4.2‰ and 24.0‰, respectively. The low δD value of the breccia rocks does not appear to be due to the presence of methane in the fluid. Instead, it is proposed that low δD and δ¹⁸O values are the result of the fluid being derived from the breakdown of clay minerals that formed and were deposited at a time of cold climate at ~290 Ma.     

Spatial distribution of micrometre‐scale porosity and permeability across the damage zone of a reverse‐reactivated normal fault in a tight sandstone: Insights from the Otway Basin,SE Australia

Knowledge of the permeability structure of fault‐bearing reservoir rocks is fundamental for developing robust hydrocarbon exploration and fluid monitoring strategies. Studies often describe the permeability structure of low porosity host rocks that have experienced simple tectonic histories, while investigations of the influence of faults with multiple‐slip histories on the permeability structure of porous clastic rocks are limited. We present results from an integrated petrophysical, microstructural, and mineralogical investigation of the Eumeralla Formation (a tight volcanogenic sandstone) within the hanging wall of the Castle Cove Fault which strikes 30 km NE–SW in the Otway Basin, southeast Australia. This late Jurassic to Cenozoic‐age basin has experienced multiple phases of extension and compression. Core plugs and thin sections oriented relative to the fault plane were sampled from the hanging wall at distances of up to 225 m from the Castle Cove Fault plane. As the fault plane is approached, connected porosities increase by ca. 10% (17% at 225 m to 24% at 0.5 m) and permeabilities increase by two orders of magnitude (from 0.04 mD at 225 m to 1.26 mD at 0.5 m). Backscattered Scanning Electron Microscope analysis shows that microstructural changes due to faulting have enhanced the micrometre‐scale permeability structure of the Eumeralla Formation. These microstructural changes have been attributed to the formation of microfractures and destruction of original pore‐lining chlorite morphology as a result of fault deformation. Complex deformation, that is, formation of macrofractures, variably oriented microfractures, and a hanging wall anticline, associated with normal faulting and subsequent reverse faulting, has significantly influenced the off‐fault fluid flow properties of the protolith. However, despite enhancement of the host rock permeability structure, the Eumeralla Formation at Castle Cove is still considered a tight sandstone. Our study shows that high‐resolution integrated analyses of the host rock are critical for describing the micrometre‐scale permeability structure of reservoir rocks with high porosities, low permeabilities, and abundant clays that have experienced complex deformation.     

Palaeogene igneous rocks reveal new insights into the geodynamic evolution and petroleum potential of the Rockall Trough, NE Atlantic Margin

Data acquired from petroleum exploration well 164/7‐1 drilled in the UK sector of the Rockall Trough have yielded fresh insights into the igneous and thermal history of this frontier region. The well targeted a large four‐way dip closed structure of presumed Mesozoic age named ‘The Dome Prospect’. The structure is now known to have a magmatic, rather than a purely structural origin, which was the preferred pre‐well interpretation. The well encountered 1.2 km of Palaeocene age basaltic lavas, overlying Late Cretaceous mudstones which were intruded by over 70 dolerite sills ranging from <1.5‐ to 152‐m thick. ⁴⁰Ar/³⁹Ar dating of the dolerite intrusions indicates an Early Palaeocene age (63–64±0.5 Ma), which are among the oldest ⁴⁰Ar/³⁹Ar dates recognised in the North Atlantic Igneous Province. Radiometric dating of the overlying basaltic lavas proved unsuccessful, because of excessive alteration. Biostratigraphic dating of underlying and overlying sedimentary strata was utilised to constrain the age of the lavas to Late Paleocene to Early Eocene age (～55 Ma). Despite being related to two distinct events separated by ～8 Ma, the intrusives and extrusives are compositionally similar. The basaltic rocks from well 164/7‐1 possess Sr–Nd isotopic, major and trace‐element geochemical compositions similar to other volcanic and intrusive rocks of the British Tertiary Igneous Province and represent partial melts of both lithospheric and asthenospheric mantle associated with the proto‐Icelandic mantle plume head. Joint consideration of thermal maturity, potential fields and 3D seismic data indicate a deeper igneous body in addition to the sills encountered in well 164/7‐1. Jack‐up and arching mechanisms associated with both scales of intrusive body are believed to have developed the dome structure. The preferred interpretation is of a mafic laccolith, 17 km in diameter, ～7 km thick, intruded at 64.5 Ma, situated ～2.5 km below the bottom of the well. 3D thermal modelling suggests that all of Tranche 52 was thermally affected by the intrusion of the magmatic body. The thermal aureole, between 27 and 51 km in diameter, is not thought to play an important role in the hydrocarbon prospectivity of the surrounding Tranches in the NE Rockall Basin. Results show that hydrocarbon exploration prospects that are circular in map view should be interpreted with caution on volcanic continental margins. In sedimentary basins, where salt domes and shale diapirs are absent and igneous rocks prevalent, periclinal structures such as ‘The Dome Prospect’ should undergo a thorough multi‐disciplinary risk assessment.     

Limnogeology in Brazil’s “forgotten wilderness”: a synthesis from the large floodplain lakes of the Pantanal

Sediment records from floodplain lakes have a large and commonly untapped potential for inferring wetland response to global change. The Brazilian Pantanal is a vast, seasonally inundated savanna floodplain system controlled by the flood pulse of the Upper Paraguay River. Little is known, however, about how floodplain lakes within the Pantanal act as sedimentary basins, or what influence hydroclimatic variables exert on limnogeological processes. This knowledge gap was addressed through an actualistic analysis of three large, shallow (<5 m) floodplain lakes in the western Pantanal: Lagoa Gaíva, Lagoa Mandioré and Baia Vermelha. The lakes are dilute (CO₃ ²⁻ > Si⁴⁺ > Ca²⁺), mildly alkaline, freshwater systems, the chemistries and morphometrics of which evolve with seasonal flooding. Lake sills are bathymetric shoals marked by siliciclastic fans and marsh vegetation. Flows at the sills likely undergo seasonal reversals with the changing stage of the Upper Paraguay River. Deposition in deeper waters, typically encountered in proximity to margin-coincident topography, is dominated by reduced silty-clays with abundant siliceous microfossils and organic matter. Stable isotopes of carbon and nitrogen, plus hydrogen index measured on bulk organic matter, suggest that contributions from algae (including cyanobacteria) and other C₃-vegetation dominate in these environments. The presence of lotic sponge spicules, together with patterns of terrigenous sand deposition and geochemical indicators of productivity, points to the importance of the flood pulse for sediment and nutrient delivery to the lakes. Flood-pulse plumes, waves and bioturbation likewise affect the continuity of sedimentation. Short-lived radioisotopes indicate rates of 0.11–0.24 cm year⁻¹ at sites of uninterrupted deposition. A conceptual facies model, developed from insights gained from modern seasonal processes, can be used to predict limnogeological change when the lakes become isolated on the floodplain or during intervals associated with a strengthened flood pulse. Amplification of the seasonal cycle over longer time scales suggests carbonate, sandy lowstand fan and terrestrial organic matter deposition during arid periods, whereas deposition of lotic sponges, mixed aquatic organic matter, and highstand deltas characterizes wet intervals. The results hold substantial value for interpreting paleolimnological records from floodplain lakes linked to large tropical rivers with annual flooding cycles.     

The Astrakhan Arch of the Pricaspian basin: Geothermal analysis and modelling

The Astrakhan Arch (ASAR) region contains one of the largest sub‐salt carbonate structures of the Pricaspian salt basin (located to the northwest of the Caspian Sea), where prospects for hydrocarbon generation and accumulation in the Devonian to Carboniferous deposits are considered to be high. We evaluate the regional vertical temperature gradient within stratigraphic units based on the analysis of 34 boreholes drilled in the region. To show that the thermal gradient is altered in the vicinity of salt diapirs, we study measured temperatures in six deep boreholes. We develop a three‐dimensional geothermal model of the ASAR region constrained by temperature measurements, seismic stratigraphic and lithological data. The temperatures of the sub‐salt sediments predicted by the geothermal model range from about 100 °C to 200 °C and are consistent with the temperatures obtained from the analysis of vitrinite reflectivity and from previous two‐dimensional geothermal models. Temperature anomalies are positive in the uppermost portions of salt diapirs as well as within the salt‐withdrawal basins at the depth of 3.5 km depth and are negative beneath the diapirs. Two areas of positive temperature anomalies in the sub‐salt sediments are likely to be associated with the deep withdrawal basins above and with the general uplift of salt/sub‐salt interface in the southern part of the study region. This implies an enhancement of thermal maturity of any organically rich source rocks within these areas. The surface heat flux in the model varies laterally from about 40 to 55 mW m^?2. These variations in the heat flux are likely to be associated with structural heterogeneities of the sedimentary rocks and with the presence of salt diapirs. The results of our modelling support the hypothesis of oil and gas condensate generation in the Upper Carboniferous to Middle Devonian sediments of the ASAR region.     

Low resistivity zones at contacts of igneous intrusions emplaced in organic-rich formations and their implications on fluid flow and petroleum systems: A case study in the northern Neuquén Basin,Argentina

Igneous sills and laccoliths emplaced in sedimentary basins may significantly impact petroleum systems, both positively and negatively. Igneous intrusions provide heat to maturate regionally immature organic-rich host rocks, act as fractured reservoirs hosting commercial accumulations of hydrocarbons, and form structures affecting fluid flow and trapping at different scales. Nevertheless, the petrophysical implications of igneous intrusions on their host rock are poorly known. In this study, we analyse 200 wells in the Río Grande Valley oil field, Neuquén basin, Argentina, where the main reservoirs are in fractured igneous sills. This dataset represents a globally unique possibility to characterize the igneous–host rock interaction using both wireline logs and core material. We identify a systematic Contact Low Resistivity Zone (CLRsZ) at both the upper and lower contacts of the sills emplaced in the organic-rich Vaca Muerta and Agrio Formations. We characterize the nature of these CLRsZ and their petrophysical properties by integrating resistivity and gamma ray well logs, petrographic analyses, petrophysical tests and geochemical analyses. The low resistivity signal of the CLRsZ is dominantly carried by massive-sulphide deposits, mainly pyrite, observed both in the host rock and the chilled margin of the sills. Well log images and porosity-permeability analysis on core plugs show that both the sills and their associated CLRsZ can act as carrier for fluid flow and reservoir for hydrocarbons storage. The thickness of the upper and lower CLRsZ correlates linearly with the thickness of the sill, and the volume of both the upper and lower CLRsZ represents ca. 40% with respect to the volume of their associated sill. The thickness of the CLRsZ represents ca. 13% of the thickness of contact aureole induced by the sills. In the CLRsZ, a great proportion of kerogen was transformed to hydrocarbon, so that CLRsZ were restricted to the innermost contact aureole of the sills. Our results show that the CLRsZ can have major implications on fluid flow and should be considered in reservoir models in volcanic basins hosting sills emplaced in organic-rich formations.     

Characterization of Late Cretaceous to Miocene source rocks in the Eastern Mediterranean Sea: An integrated numerical approach of stratigraphic forward modelling and petroleum system modelling

Stratigraphic forward modelling was used to simulate the deposition of Upper Cretaceous, Eocene and Oligo‐Miocene source rocks in the Eastern Mediterranean Sea and, thus, obtain a process‐based 3D prediction of the quantity and quality distribution of organic matter (OM) in the respective intervals. Upper Cretaceous and Eocene models support the idea of an upwelling‐related source rock formation along the Levant Margin and the Eratosthenes Seamount (ESM). Along the margin, source rock facies form a narrow band of 50 km parallel to the palaeo shelf break, with high total organic carbon (TOC) contents of about 1% to 11%, and HI values of 300–500 mg HC/g TOC. On top of the ESM, TOC contents are mainly between 0.5% and 3% and HI values between 150 and 250 mg HC/g TOC. At both locations, TOC and HI values decrease rapidly towards the deeper parts of the basin. In the Oligo‐Miocene intervals, terrestrial OM makes up the highest contribution to the TOC content, as marine organic matter (OM) is diluted by high‐sedimentation rates. In general, TOC contents are low (<1%), but are distributed relatively homogenously throughout the whole basin, creating poor quality, but very thick source rock intervals of 1–2 km of cumulative thickness. The incorporation of these source rock models into a classic petroleum system model could identify several zones of thermal maturation in the respective source rock intervals. Upper Cretaceous source rocks started petroleum generation in the late Palaeocene/early Eocene with peak generation between 20 and 15 Ma ca. 50 km offshore northern Lebanon. Southeast of the ESM, generation started in the early Eocene with peak generation between 18 and 15 Ma. Eocene source rocks started HC generation ca. 25 Ma ago between 50 and 100 km southeast of the ESM and reached the oil to wet gas window at present day. However, until today they have converted less than 20% of their initial kerogen. Although the Miocene source rocks are mostly immature, Oligocene source rocks lie within the oil window in the southern Levant Basin and reached the onset of the wet gas window in the northern Levant Basin. However, only 10%–20% of their initial kerogen have been transformed to date.