Maximum Burial Research Articles

Redbed diagenesis links district-scale mineralising fluids to Cu source rock in the Polaris Zn-Pb(-Cu) district, Arctic Canada

Red sandstone of the Proterozoic Aston Formation (Nunavut, Canada) is exposed in the same geographic area as the Storm Cu showing in the Paleozoic-carbonate-hosted Polaris Zn-Pb district (Arctic Canada). The ∼800-m-thick Aston Formation experienced successive burial and exhumation episodes, with maximum burial (>3 km) at the time of the Devonian Ellesmerian orogeny. A petrographic and in situ SIMS oxygen isotope study identified redbed diagenetic events, associated fluid types and temperatures, and their relative timing. The first hematite coat (Hem1) developed in the Proterozoic. Intrusion of Proterozoic dykes and sills heated local formation water to high temperatures (130–223 °C), resulting in precipitation of patchy Qz1 cement in a shallow-burial environment. The Fe and Si were probably derived through alteration of non-quartz silicate detritus. In the Paleozoic, a bleaching (reducing) event removed most Hem 1; the reduced fluid may have been related to regional Zn mineralisation throughout the Polaris district during the mid-Paleozoic Ellesmerian orogeny. Ensuing pressure-solution during maximum burial was probably also Ellesmerian. A second hematisation episode (Hem 2) encloses the pressure-solved quartz-grain contacts. Coeval with Hem 2 was pervasive Qz2 cementation from a low- to mid-latitude, meteoric-derived, oxidised hydrothermal (140–180 °C) fluid that supplied externally derived Si and Fe, probably in the Early Carboniferous. The nature and timing of the fluid events indicated by the Aston Formation’s diagenetic history closely match the fluid history recorded in both Cu and Zn mineralisation in the Polaris district. This relationship strongly supports the concept that Cu in sedimentary-rock-hosted ore deposits can be sourced from redbeds, even those whose depositional age and environment is much older than, and geodynamically unrelated to, the fluid event(s) that caused diagenetic reddening and Cu transfer from source rock to mineralising site.

A tale of two vein types: Bed-parallel fibrous versus blocky filled fractures in organic-rich shales and their overpressured hydrocarbon fluids

Bed-parallel fractures are a common feature of organic-rich shales, and are generally interpreted as records of fluid overpressures due to hydrocarbon generation. However, the textural features of such cemented fractures differ widely, and the relationships between these textures and the vein-forming (fracturing and filling) processes—and, critically, their respective roles in accumulation and migration of hydrocarbons—remain uncertain. Here, we compare the physical conditions and opening mode of bed-parallel cemented fractures that show either fibrous texture indicative of antitaxial growth that kept pace with fracture dilation (commonly referred to as “beef”), or blocky texture indicative of unimpeded syntaxial crystallization into an open, fluid-filled space. Specifically, we analyzed formation conditions of bed-parallel fibrous filled fractures in the Shahejie (Eocene) shales, Bohai Bay Basin, east China, and the bed-parallel blocky filled fractures in the Wufeng-Longmaxi (Ordovician-Silurian) shales, Sichuan Basin, southwest China, to evaluate root causes for the different textures of these cements and discuss their roles in self-sourced hydrocarbon accumulation. We report that fluid inclusions in each filling type show distinct and systematic differences in the magnitude of fluid overpressure during different hydrocarbon generation stages of catagenesis. The formation of bed-parallel fibrous filled fractures, including the initial hydraulic fracturing and subsequent crystallization, occurred during the main stage of liquid hydrocarbon generation (i.e., the oil window) when lithostatic fluid pressures were achieved only sporadically and modest overpressures less than lithostatic usually prevailed. The initial bed-parallel fracturing of blocky fillings, in contrast, occurred during the main gas generation stage (i.e., the gas window), and subsequent blocky crystals precipitated after this stage during uplift after the maximum burial. In this case, super-lithostatic overpressures of the CH4-saturated fluids persisted essentially continuously and actively expanded the fractures at rates that were not met by the crystallizing cements.Such a model for blocky crystallization in an open horizontal space supported by super-lithostatic fluid pressures in petroliferous systems in sedimentary basins, is analogous to some systems in metamorphic rocks including even orogenic gold deposits, and some magmatic settings, especially related to S-type granites where they form greisens and tin-tungsten deposits by blocky filling of wolframite and quartz, and I-type granitoids where coarse muscovite veins occur for example in the deep roots of porphyry copper systems, as well as numerous pegmatites. Some elements that each of these settings share with the shale-hosted, bed-parallel blocky filled fractures, include that they involve in situ fluid production (by metamorphic devolatilization or magmatic degassing) and fluid escape is stifled by low background permeability such that fluid overpressures can build up, at least periodically, to match or exceed the lithostatic load. As a result of these divergent bed-parallel fracture filling processes, the fibrous textures record conditions of modest permeability that favored accumulation of nascent liquid oil, whereas the blocky fracture cements represent open fractures that contributed to permeability and expulsion of gas. We conclude that the fibrous filled fractures record an opening mode that was beneficial for shale oil enrichment, whereas dilation of blocky filled fractures provides important conduits for hydrocarbon expulsion and release of shale gas.

The hydrocarbon source potential of Permo-Triassic rocks of the Indochina Block adjacent to the Paleo-Tethys suture in Thailand

The main Thailand petroleum systems are found in the syn-rift and post rift sections of Cenozoic rift basins. The Khorat Plateau area is a separate gas province with Permian limestone reservoirs possibly sourced from Triassic lacustrine, and Permo-Carboniferous marine shales. However, these source rocks and those of equivalent age west of the Khorat Plateau are not well described in the literature. This study investigated the source potential of outcropping Permo-Triassic shales from the western margin of the Khorat Plateau and the Khao Khwang Fold and Thrust Belt (KKFTB). The key formations sampled are: Permian: Alum Shale, Nong Pong, Sap Bon, Khao Khad, and Nam Duk formations; Triassic: Pang Asok and Hua Hin Lat Formation. Most potential Permo-Triassic source rocks sampled exhibited little potential due to the dominance of Type IV, non fluorescent amorphous, vitrinite and intertinite mascerals and low <1 % TOC content. Probably the Type IV kerogens are not original, but the product of subsurface alteration of other kerogen types (most likely Type III). The complex burial histories of the formations indicates such alteration could have occurred post-maturation. Probably maximum burial in the KKFTB was attained during the Triassic Indosinian Orogeny when upper diagenetic to anchizone conditions were attained. Significant burial also occurred when the Jurassic-Cretaceous Khorat Group (>2 km thick) was deposited. This later burial could be responsible for later maturation in areas north of the KKFTB. Simply in terms of TOC content >1, the Hua Hin Lat and Khao Khad formations have the most promising source potential.

Whitehorse Trough records Late Triassic–Cretaceous accretionary orogenesis in the Northern Canadian Cordillera via detrital mineral thermochronometry

The Whitehorse Trough formed during early Mesozoic accretion of the Intermontane terranes to northwestern North America. Here we investigate its thermal history using detrital mineral thermochronology, including 171 single-crystal (U–Th)/He zircon (ZHe) ages from 35 samples, 158 single-crystal (U–Th)/He apatite (AHe) ages from 33 samples, and apatite fission track (AFT) ages from 12 samples. ZHe single crystal ages range from 222 to 42 Ma and define Triassic–Early Jurassic, Late Jurassic, and Cretaceous–Paleogene age groups. AFT central ages range from 95 to 30 Ma with a dominant age peak at ∼50 Ma, and AHe single crystal ages range from 228 to 13 Ma with a dominant age peak between 50 and 40 Ma. Forward and inverse models of thermochronological data are compatible with two regional burial/heating stages that variably reset He in zircon. Maximum temperatures of the Whitehorse Trough strata locally exceeded 150 °C during Early Jurassic burial and shortening into a fold and thrust belt. Following Middle to Late Jurassic exhumation-related cooling and development of a prominent unconformity, Whitehorse Trough strata were buried again. Temperatures locally exceeded 150 °C during the Cretaceous, suggesting maximum burial of between ∼4 and 7.5 km. Heating and cooling rates during the Early–Middle Jurassic were ∼10 °C/myr, coinciding with deposition, fold and thrust belt development, and regional crustal thickening during the final stages of Intermontane terrane accretion. Maximum heating rates during the Cretaceous were ∼4–7 °C/myr and likely correspond to regional crustal thickening of the northern Cordillera hinterland and establishment of an outboard, Andean-type continental arc system.

The “Golden Zone” temperature distribution of oil and gas occurrence examined using a global empirical database

We present here a straightforward empirical correlation between petroleum occurrence in sedimentary basins and reservoir temperature that can be integrated with other methods of analysis to reduce exploration risk. Global amounts of recoverable oil, gas, and gas-condensate in 1175 well characterized reservoirs from 954 of the world's largest fields, constituting 54% of the world's documented recoverable conventional oil and 50% of gas, are examined in terms of present-day reservoir temperature. Most volumes (74%) occur within the range of 60–120 ± 2 °C (140–248 ±4 °F), corresponding with the isotherm-bounded depth interval in petroliferous basins previously defined as the “Golden Zone” for exploration. Only 6% of the global total occurs at higher temperatures and 20% at lower temperatures. The decrease <58 °C is gradual in most regions, whereas the drop-off of discovered volumes >122 °C is precipitous. The low-temperature limit is suggested to reflect both poor sealing capacity of diagenetically immature shales and biodegragation, but many low-temperature accumulations survive because of uplift from previous maximum burial. The high-temperature limit is interpreted as resulting mainly from diagenetically induced overpressure that causes seal failure and remigration. Similarity between the distributions of both oil and gas with temperature indicates that the controlling processes are unrelated to petroleum generation. The distributions are also similar for sandstone and carbonate reservoirs and for reservoirs with shale, evaporite, and carbonate seals, indicating little lithologic influence. Although the ultimate controls can be debated and are certainly in need of continuing study, the observed temperature dependence of petroleum volumes is entirely empirical and can be used as an independent exploration risk factor. Nevertheless, the proportions of total estimated ultimate recovery above and below the Golden Zone temperature limits vary significantly between major petroleum-producing regions, so local calibration is essential.

The effects of uplift and erosion on the petroleum systems in the southwestern Barents Sea: Insights from seismic data and 2D petroleum systems modelling

In the past three decades, the hydrocarbon prospectivity has been enigmatic in the southwestern Barents Sea as gas discoveries have dominated over oil. However, more recent discoveries indicate oil potential, especially in the Polheim Platform and Hoop Complex areas. Cenozoic uplift and erosion episodes are critical because they control the duration and depth of maximum burial of source rocks and the reduction of porosity of reservoir rocks due to diagenetic processes at larger depth and increased temperatures. Modelling of interpreted horizons from the southwestern Barents Sea provide a best-fit realization of the basin-scale sedimentary filling from the post-rifting in the Jurassic until the Last Glacial Maximum. 2D basin modelling was performed across one regional seismic section crossing the southwestern Barents Sea. For calibration of the 2D model, 1D modelling was performed for a selected number of wells in the region by integrating different estimates of net apparent erosion (velocity inversion of wells, interpreted profiles and structure maps as well as from temperature and vitrinite reflectance data) in order to assess the burial, thermal and maturity history of the source rocks at various well locations as well as along the 2D profile. Simulation results show that the Upper Jurassic source rock is immature to marginal mature in the central and eastern parts of the Norwegian Barents Sea. The western part is characterised by high chargeability of traps associated with rich source kitchens and porous reservoir rocks. Although not proven by commercial discoveries, there are evidences that the Permian Ørret and the Lower-Middle Triassic basal Klappmyss and basal Kobbe shales may be important gas-prone source rocks in the eastern Norwegian Barents Sea. Consequently, the exploration must rely on the Triassic and/or Palaeozoic source rocks in the eastern area.

DEVELOPMENT OF FIBROUS CALCITE VEINS RELATED TO HYDROCARBON GENERATION AND OVERPRESSURING IN ORGANIC‐RICH SHALE SOURCE ROCKS: THE VACA MUERTA FORMATION, NEUQUÉN BASIN, ARGENTINA

Fibrous calcite bed‐parallel veins (BPVs) are a typical feature of the Upper Jurassic – Lower Cretaceous Vaca Muerta Formation in the subsurface of the Neuquén Basin (Argentina). The formation is considered to be the main source rock in the basin as well as an important unconventional play. This study examines the growth of BPVs through an analysis of core from three wells located along a transect extending for some 150 km from the NE Platform near the basin margin in the east to the Agrio fold‐and‐thrust belt at the Andean deformation front in the west. The main objective is to integrate fluid inclusion data with the palaeothermal and palaeopresure evolution obtained from a regional‐scale 2D basin and petroleum systems model to examine the timing of fracture development and its relationship with hydrocarbon generation in the Vaca Muerta Formation through time.The apertures of BPVs were measured in more than 360 m of core from three wells (wells A, D and E). This data was combined with optical petrography to investigate the number of calcite cementation events, and the temperature of cement precipitation based on fluid inclusion data. The organic geochemical and mineralogical characteristics of the Vaca Muerta source rock were also analysed. The integrated results were incorporated into a poro‐elastic basin model to investigate the impact of horizontal shortening due to Andean compression on pore pressure development and fracturing in the Vaca Muerta Formation. This framework allowed the timing of BPV formation to be determined together with possible mechanisms governing overpressure conditions through time.Near the Andean deformation front in the west of the modelled section where the Vaca Muerta Formation is in the wet gas window (well D) and dry gas window (well A), BPVs are characterized by two or more generations of calcite fibres indicating multiple growth phases. Calcite which precipitated during cementation event 1 (E1) in the internal zones of BPVs consists of crystals oriented perpendicular to fracture walls, indicating perpendicular vein opening. Calcite precipitated during cementation event 2 (E2) in the outer zones of BPVs includes curved and oblique crystals. During this phase, shear occurred between the opening vein walls as a result of horizontal shortening. Cementation event 3 (E3) is characterized by an equant mosaic of calcite crystals which preserve intracrystalline porosity. E1cements formed between 110 and 90 Ma with trapping temperatures of ∼112 °C (upper Vaca Muerta, well A) and ∼125 °C (lower Vaca Muerta, well D). Fracturing resulted from disequilibrium compaction and from volumetric expansion due to primary cracking of kerogen within the oil window. E2 cements record a trapping temperature of ∼159 °C and formed between 70 and 55 Ma (lower Vaca Muerta, well D) during maximum burial of the Vaca Muerta Formation, synchronous with the secondary cracking of retained liquid hydrocarbons and the beginning of Andean compression. E3 cements (upper Vaca Muerta, well A) have a trapping temperature of ∼162 °C, and formed between 65 Ma and 53 Ma synchronous with the generation of thermogenic gas.By contrast, in the east of the modelled section in the less deformed foreland area of the Neuquén Basin where the Vaca Muerta Formation is in the early oil window (well E), BPVs are composed of a single generation of calcite fibres (E1)with a trapping temperature of ∼118 °C. The E1 cement is characterized by calcite crystals which are oriented perpendicular to fracture walls with no evidence of shearing. According to model simulations, cementation here occurred between 64 Ma and 53 Ma during maximum burial and was related to overpressures which resulted from both disequilibrium compaction and primary transformation of kerogen into oil.The data presented suggests that in some intervals of the Vaca Muerta Formation, a slight increase in TOC content is accompanied by an increase in vein thickness, with the highest number of cementation events occurring towards the Andean deformation front in the west of the study area compared to the foreland in the east.

Hydrocarbon charge and petroleum system evolution of the Montney Formation: A multidisciplinary case study of the Blueberry sub-play in Northeast British Columbia, Canada

Abstract The origin, timing, and evolution of hydrocarbons in the early Triassic Montney Formation siltstone has long been debated and is poorly understood. A proper understanding of the timing and processes involved in hydrocarbon emplacement forms an important part of in-situ fluid characterization. Presented here is a case study from northeastern British Columbia. This study integrates basin modeling, carbon isotope geochemistry, fluid inclusion microthermometry, organic and inorganic petrography, sedimentology, and stratigraphy to reconstruct the burial history and relative timing of geological events and mechanisms involved in developing the present-day Montney hydrocarbon system in the Blueberry sub-play. Petrography from whole core indicates that the organic material present is largely pyrobitumen. The lack of primary kerogen suggests that the original oil charge must have migrated into place prior to increasing thermal maturity and conversion to pyrobitumen at maximum burial. In addition, patterns in stable carbon isotope curves from extracted core fluids show a stratified oil column, indicating a lack of vertical migration that would have homogenized the hydrocarbons. Thin section petrography and facies analysis show a laminated fabric with strong permeability anisotropy suggesting preferential migration along bed-parallel permeability pathways, which would have prevented significant vertical mixing. Any interpretations about the origins of the Montney Formation hydrocarbons in this study area must satisfy these observations. Given the data available, the most likely interpretation is a bed-parallel eastern migration from a downdip source. Collective understanding of these datasets helps illustrate the processes involved in the evolution of the hydrocarbon system present in the Montney Formation. Enhanced understanding of the Montney hydrocarbon migration history and reservoir evolution is important for understanding the lateral and vertical variations in the producing gas-condensate-ratios across the Montney formation fairway.

Late diagenetic evolution of Ordovician limestones in the Baltoscandian basin revealed through trace-element mapping and in situ U–Pb dating of calcite

Carbonate rocks are important paleo-environmental records, with bulk chemical and isotopic compositions commonly used to deduce temperatures, redox conditions, etc. Diagenetic (re)crystallization of clasts and cements is common, may perturb the chemical and isotopic systems typically interpreted to reflect conditions at the timing of deposition, may occur essentially synchronous with or long after deposition, and is difficult to date. Advances in in situ carbonate trace-element mapping and U–Pb dating by LA-ICP-MS provide opportunities to determine the absolute timing of diagenesis which can then be related to the age of deposition of carbonate rocks. We applied these techniques to two samples of well-preserved Ordovician bioclastic limestones from the Viki drill core, western Estonia (Baltoscandian basin) with precisely constrained depositional ages (ca. 459 and 444 Ma) and comprehensively detailed post-depositional histories which indicate that they were never deeply buried nor significantly heated following deposition. Previously published K–Ar data for K-bentonites from the Viki core and elsewhere in the Baltoscandian basin do, however, indicate diagenesis long before the timing of maximum burial. Optical petrography and backscatter-electron (“BSE”) imaging reveal low-porosity “BSE-bright” calcite spar cement in pore spaces between “BSE-dark” micro-porous carbonate bioclasts. Trace-element mapping of several areas (several mm2 each) in each thin section by LA-quadrupole-ICP-MS reveals variably elevated Mn/Sr, U concentration, and 238U/206Pb in the calcite spar cement. These maps were subsequently used to guide the placement of laser spots for U–Pb dating by LA-multicollector-ICP-MS. Primary bioclastic carbonate in both samples has low U/Pbc (with 238U/206Pb < ~6.5) and, thus, yields imprecise Concordia-intercept dates, which nonetheless overlap with the depositional ages. Calcite spar cement has higher U/Pbc (with 238U/206Pb up to ~15.7) and yields intercept dates of ca. 414–412 Ma in each sample. Individual common-Pb-corrected dates—particularly those for domains with the highest U/Pbc—are as young as ca. 376 Ma. The timing of diagenetic calcite (re)crystallization and cementation identified here is tens of Myr later than deposition and overlaps with the previously published K–Ar dates, and also with the timing of continent collision during the Caledonian orogeny in Scandinavia. The calcite U–Pb and K-bentonite K–Ar chronometers yield consistent records of what may have been tectonic-driven diagenesis far inboard (>600 km) from the orogenic front. Furthermore, this work demonstrates that apparently pristine carbonates may have experienced recrystallization (or at least chemical-isotopic perturbation) in open systems long after deposition and should be carefully scrutinized before using bulk-sample paleoenvironmental proxies.

Using Clumped Isotopes to Reconstruct the Maximum Burial Temperature: A Case Study in the Sichuan Basin

For strata that have experienced continual burial in the early stage and uplift in the late stage, the present-day temperature is lower than the maximum burial temperature (MBT), which is a key parameter for studying the hydrocarbon generation history of source rocks in petroliferous basins. In this paper, a new method for reconstructing the MBT is proposed based on the solid-state reordering model of carbonate clumped isotopes (Δ47). The MBT reconstructed using the Δ47 was compared with the MBT constrained using the traditional Easy%Ro model. The clumped isotope temperature (TΔ47) of the Permian micritic limestone from the Xibeixiang outcrop (about 62°C) is much higher than its initial formation temperature (20–25°C), suggesting that the limestone experienced partial solid-state reordering during the late burial process. The MBT of the calcite obtained from the solid-state reordering model is 139–147°C, which is quite similar to the MBT determined using the Easy%Ro model (139.5–147.5°C). TΔ47 of the Permian and Triassic limestone and calcite cements in the Puguang gas field are 150–180°C, while TΔ47 of the micritic dolostone is about 70°C, suggesting that the Δ47 of the limestone and calcite cements experienced complete solid-state reordering and the dolostone only experienced partial solid-state reordering. The MBT of the dolomite determined using the solid-state reordering model is 200–220°C, which is also similar to the MBT determined using the Easy%Ro model (202–227°C). Therefore, the case studies from the Sichuan Basin suggest that Δ47 can be used to reconstruct the MBT of ancient carbonate strata lacking vitrinite and detrital zircon data. However, different types of carbonate samples should be used to reconstruct the MBT for strata that have experienced different temperature histories. Micritic limestone and very finely crystallized dolostone can be used to reconstruct the MBT of strata that have experienced MBTs of &amp;lt;150–200°C and &amp;gt;200–250°C, respectively.

Long‐distance fluid migration defines the diagenetic history of unique Ediacaran sediments in the East European Craton

AbstractThe diagenetic history of the Ediacaran sedimentary rocks in the East European Craton (EEC) over the area extending from Arkhangelsk (Russia) in the north to Podolia (Ukraine) in the south was revealed by means of the XRD characterization and K–Ar dating of clay fractions, mudstone porosity measurements and organic geochemistry investigations. Mudstone porosity measurements produced direct evidence of shallow maximum burial of the Ediacaran sediments on the craton (Russia, Lithuania, Belarus, Volyn), not exceeding 1.5 km, and much deeper burial at the cratonic margin, in Podolia and Poland. In general, illitization of smectite and biomarker indices indicates more advanced diagenesis at the cratonic margin.K–Ar dating of authigenic illite–smectite and aluminoceladonite revealed the Palaeozoic age of mineral diagenesis (ca. 450–300 Ma) both on the craton and its margin, with older ages generally observed in the north. When the maximum palaeotemperatures were evaluated from illite–smectite and biomarkers, based on the calibrations from the conventional burial diagenetic sections, a major mismatch was detected for the cratonic area: 100°C–130°C from illite––smectite and tens of oC lower from the lipid biomarkers. This diagenetic pattern was interpreted as the result of short‐lasting (in ky scale) pulses of potassium‐bearing hot fluids migrating from the Caledonian and Variscan orogens deep in the craton interior, effectively promoting illitization in porous rocks without altering the organic matter. Analogous short pulses of fluids were responsible for numerous diagenetic phenomena, including Mississippi Valley‐Type ore deposits, in the American Midwest, in front of the Appalachians. K–Ar dating indicates that the entire Proterozoic sedimentary cover of the Great Unconformity on the EEC remained untouched by measureable post‐sedimentary changes until the early Palaeozoic, thus for over 1000 My, which is an unprecedented finding.

Diagenesis and burial history modeling of heterogeneous marginal marine to shoreface Paleocene glauconitic sandstones, Taranaki Basin, New Zealand

ABSTRACTPaleocene marginal marine to shoreface glauconitic sandstones (F-Sands) of the Farewell Formation from the Maui Field in Taranaki Basin, New Zealand, demonstrate a diagenetic evolution driven by major shifts in acidic pore-water composition, rate of burial, and clay-mineral authigenesis. Mechanical compaction is the principal porosity-reducing mechanism during the first 2500 m of burial of the F-Sands. Continued mechanical compaction with long-grain contacts, concavo-convex contacts, and deformed liable grains are common throughout the F-Sands. Late-stage flow of dissolved CO2 in the pore fluids of the Farewell Formation is thought to have been generated from thermal decarboxylation of coaly source rocks. The circulation of these CO2-rich fluids will have dissolved into undersaturated pore fluids and partially catalyzed dissolution of feldspar and quartz, producing ions for the precipitation of kaolinite and chlorite. Timing of the diagenetic reactions, as determined using paragenetic observations, fluid-inclusion analysis, and burial history modeling, suggests that the quartz cements formed at a late stage (&amp;gt; 100°C, corresponding to 0–7 Ma) and is consistent with the migration of hydrocarbons, and associated CO2, into the F-Sand reservoir. Significant secondary porosity is generated through the dissolution of feldspar, which is preserved due to late-stage of occurrence at close to present-day maximum burial. Dissolved solutes in the F-Sands sandstones are being preferentially precipitated in interbedded and surrounding fine-grained heterolithic siltstone to very fine-grained sandstone beds, leading to enhanced heterogeneity and preservation of secondary porosity. This study provides an improved understanding for diagenetic reconstruction of marginal marine to shoreface facies.

Thermo‐tectonic history of the Northeast Sichuan Basin, Southwest China, using apatite fission‐track analysis, vitrinite reflectance, and basin modelling: Implications for hydrocarbon prospectivity

Analysis of apatite fission‐track and vitrinite reflectance combined with 1‐D basin modelling was used to reconstruct the thermo‐tectonic history of the northeast Sichuan Basin, China. The study indicates that the basin has evolved through multi‐phase tectonic regimes involving a series of subsidence and tectonic uplift phases exerting significant control on the thermal history. Heating and cooling processes were largely related to deep burial and intense exhumation, respectively. Exhumation that extended from 122 to 2.5 Ma has resulted in at least 4 km of sedimentary removal. The rate of exhumation and removal (denudation) was found to have reached 58 and 35 m/Myr, respectively. The results show that the denudation magnitude is significantly higher in the eastern part reaching 5.5 km, compared with 2.2 km in the western part of the study area. The fission‐track ages demonstrate the occurrence of three cooling phases: initial‐gradually increasing (~120–70 Ma), stagnant (70–15 Ma), and rapid (15 Ma to present). This suggests that cooling was associated with different compressive thermo‐structural events that began in the mid‐Lower Cretaceous (~122 Ma) marking the subsidence climax period. With the sedimentary section approximating 8‐km thickness by the time of maximum burial, the modelled average maximum temperature and vitrinite reflectivity coefficient reached roughly 232°C, and 3.5% for the Lower Triassic Feixianguan Formation (T1f1), thus indicating the conditions of dry gas generation. The Meso‐Cenozoic thermo‐structural evolution that postdated the onset of hydrocarbon generation has been critical for the evolution of the petroleum play, including the Feixianguan Formation in the marine carbonates of the Early Triassic and the Xujiahe Formation in the non‐marine sandstones of the Upper Triassic. This resulted in the modification of primary hydrocarbon accumulations, mostly in the lower petroleum play, and development of secondary accumulation in the newly formed structural traps in the upper petroleum play. Therefore, accurate thermo‐tectonic history reconstruction is valuable for understanding the geological history and hydrocarbon prospectivity.

The Hewett Field satellites: Big Dotty, Little Dotty, Deborah, Della, Dawn and Delilah, Blocks 48/29a, 48/30a, UK North Sea

Abstract Six satellite fields have been developed through the Hewett Field facilities: Big Dotty, Little Dotty, Deborah, Della, Dawn and Delilah. Little Dotty has produced from both the Leman Sandstone Formation (LSF) and Bunter Sandstone Formation (BSF) whilst the other satellites are exclusively LSF developments. The LSF reservoir quality exhibits a marked contrast across the Dowsing Fault Zone, which separates the inboard satellites to the SW from the outboard satellites to the NE. The inboard satellites, Big Dotty, Little Dotty and Dawn, display the best reservoir quality, reflecting their lesser depth of maximum burial. These fields share a strong aquifer, exhibited a rapid water-cut development and are now shut-in. The greater depth of maximum burial experienced by the outboard satellites, Deborah, Della and Delilah, is reflected in poorer reservoir quality along with weaker aquifers that are also more compartmentalized. These remain in production and will achieve higher recovery factors. Big Dotty was developed from a wellhead platform whereas the other fields were developed as subsea tie-backs. Collectively, these satellite fields have produced some 0.9 tcf of gas, playing an important strategic role in offsetting the production decline in the Hewett Field and extending the life of the asset.

Sylfjellet: a new outcrop of the Paleogene Van Mijenfjorden Group in Svalbard

A hitherto unrecognized Paleogene outcrop has been discovered at Sylfjellet, a mountain located at the northern side of Isfjorden, Svalbard. The strata, which cover an area of 0.8 km2, have until now been assigned to the Lower Cretaceous succession of the Adventdalen Group. In this study, the Sylfjellet site was studied in detail to provide an updated structural and sedimentological description of strata and lithostratigraphy. The age and burial history of the investigated succession were constrained by absolute (U/PB) and relative dating methods in addition to vitrinite reflectance analyses of coal seams. The results show a Paleogene age of the deposits, which is supported by the occurrence of an angiosperm pollen grain, plant macrofossils, and a tephra layer of early Selandian age (61.53 Ma). The 250 m-thick succession of Sylfjellet is assigned to the Firkanten, Basilika and Grumantbyen formations. This succession unconformably overlies the Lower Cretaceous Helvetiafjellet Formation. Sylfjellet is incorporated into the West Spitsbergen Fold-and-Thrust Belt and interpreted to be a fourth structural outlier of the Van Mijenfjorden Group. Vitrinite reflectance data indicate that at least 2000 m overburden has been eroded above the Sylfjellet coal seams, and that maximum burial of the strata predates folding and thrusting in the area.

A history of pore water oxygen isotope evolution in the Cretaceous Travis Peak Formation in East Texas

Abstract Oxygen isotope analyses of diagenetic cements can provide detailed evidence of sedimentary burial processes and conditions, as the δ18O values of precipitating minerals reflect contemporaneous local δ18Owater and temperature conditions. Uncertainties in the timing and rates of pore water δ18O evolution in sedimentary basins can complicate interpretation of these records. Fracture-bridging (0.5–1 mm) quartz cements observed in sandstones of the Cretaceous Travis Peak Formation in the East Texas basin show clear growth-zoning by cathodoluminescence and contain detailed fluid inclusion records of temperature that make them excellent candidates for interrogating prolonged histories of basin temperature and the evolution of δ18O in basin pore water. New secondary ion mass spectrometer (SIMS) δ18Oquartz isotopic data from fluid inclusion-rich quartz bridges in Travis Peak sandstones record a steady increase of pore water δ18O values from ∼5 to 7‰ (VSMOW; Vienna Standard Mean Ocean Water) as the sandstone warms from ∼130 to 150 °C. To help evaluate whether this trend could be generated solely from local water-rock interactions in response to burial compaction, a one-dimensional closed system isotopic burial model was created to simulate how δ18Owater values change in a quartz-dominated sandstone during diagenesis. Using both directly measured and inferred rates of Travis Peak compaction, the magnitude of change in δ18Owater that we calculate from quartz bridge geochemistry cannot be reasonably modeled solely by local quartz mechanical compaction, pressure solution, and cementation processes, necessitating significant fluxes of silica and high-δ18O water from outside of the sandstones prior to maximum burial. This indicates that even units which appear surrounded by significant barriers to fluid flow (i.e., mudrock-bounded channel sandstones) may have been infiltrated and diagenetically modified by large fluxes of fluid on geologic time scales.

Burial and exhumation history controls on shale compaction and thermal maturity along the Norwegian North Sea basin margin areas

Abstract The North Sea area has been subjected to significant erosion and subsequent deposition of sediments in the basin margin and deeper basin areas, respectively, during the late Neogene. A large amount of Cretaceous-early Quaternary sediments have been removed below the angular unconformity along the west and southwest coast of Norway and deposited in the huge North Sea Fan at the mouth of the Norwegian Channel. At the same time, a considerable thickness of early Quaternary-Paleocene sediments was also eroded towards the east in the central North Sea and subsequently deposited in the deeper basin areas to the west. This study seeks to estimate exhumation from compaction and thermal maturity based techniques by using sonic velocities of shales/carbonates and vitrinite reflectance data in a large number of boreholes in the central, eastern and northern North Sea. The results indicate no or minor exhumation in the Central Graben and flanking high areas, whereas more than ∼1 km sediments are eroded in the basin margin areas towards the Norwegian coast. More than ∼500 m sediments are eroded in the Egersund Basin and Stord Basin areas. A similarity of exhumation estimates from the Early Cretaceous-Early Miocene shales and Late Cretaceous-Early Paleocene carbonates indicates maximum burial sometime after the Early Miocene in most of the central and northern North Sea areas. However, the maximum burial throughout the North Sea Basin may be diachronous. Seismostratigraphic analysis indicates maximum burial sometime during the Oligocene in the Sorgenfrei-Tornquist Zone area in the eastern North Sea. Maximum burial in the Norwegian-Danish Basin varies from Miocene-Pliocene in eastern parts to early Pleistocene in western parts, whereas sediments are currently at their maximum burial in the Central Graben and southern Viking Graben areas. Restoration of surface elevations to their original position before the onset of erosion indicated large subaerially exposed areas in the Norwegian-Danish Basin and along the southwest coast of Norway. This is also supported by predominantly coastal and/or deltaic environments in the Norwegian-Danish Basin area during the late Neogene. These subaerially exposed areas may be linked to the regional tilting and erosion of the basin margin areas to the east and progressive basinward migration of deposition centres to the west since the Oligocene. The exhumation had significant effects on the petroleum system in the basin margin areas by cooling down the source rock. However, the deeper burial of sediments may also have changed the rheological properties of sediments from more ductile to brittle due to compaction and diagenetic processes which makes them more failure prone during exhumation leading to hydrocarbon leakage or seal failure in case of CO2 injection.

Canning Basin - Petroleum Systems Analysis

New high resolution geochemical information was acquired for fluids from recent Canning Basin wells and interpreted in context with previous work on fluid typing and correlation by Geoscience Australia (GA). Ungani field oils are interpreted to be derived from the same/very similar clastic source rock, comprising bacterial and marine algal matter deposited under anoxic to sub-oxic conditions, and generated within the peak oil window for a high quality marine source rock. Observed low GOR’s are the consequence of both source rock type and gas removal. Liquids from the Yulleroo field are derived from a similar source to the Ungani oils, with the addition of dry gas from a higher maturity and/or more gas prone source, and generated and expelled at slightly higher maturity. The current lean gas condensate phase is the result of the addition of dry gas combined with minimal water washing. In nearly all aspects the Ungani and Yulleroo liquids resemble the L4 family previously attributed by GA to a probable Carboniferous age source. Gas-condensate from the Valhalla North-1 field was derived from a more mature source rock deposited in an oxic environment and containing more gas prone (terrigenous) organic matter. Map based modelling incorporated eleven 1D models in the Ungani-Yulleroo and Valhalla-Asgard region, and a map based burial history and maturity model. The source rock model was derived from the liquid geochemistry results rather than the poor quality source rock potential data gathered to date. Burial history modelling and maturity modelling at the top of the Lower Laurel Carbonate shows maturity for gas expulsion in the main trough and oil to light oil expulsion on the flanks of the basin. Maximum burial in the basin took place immediately prior to the Fitzroy Uplift, resulting in the main phase of oil generation and expulsion taking place around 200Ma.

Characterization of the natural fracture system of the Eagle Ford Formation (Val Verde County, Texas)

The Eagle Ford Formation has attracted considerable industry attention as a self-sourced unconventional shale reservoir. The productive interval in the Eagle Ford Formation is the transgressive systems tract, which contains parasequences whose lithologic content varies upward with increasing proportions of limestones. Optimum success in both exploration and production depends on the adequate characterization of fracture systems as a function of lithology. The outcrops present along US Highway 90 in Val Verde and Terrell Counties, Texas, provide considerable insight into the regional natural fracture system of the Eagle Ford Formation. Fracture-orientation analysis reveals two sets of conjugate hybrid shear fractures and two sets of regional fractures. Abutting relationships suggest that hybrid shear fractures formed first, followed by the thoroughgoing northeast-striking fracture set, and finally by a northwest-striking set, which tends to be confined to individual mechanical units. The orientation of these fractures suggests that they formed during post-Laramide stress relaxation and progressive exhumation. Spacing-frequency distribution analysis of the fracture population reveals a mature hypersaturated fracture system that likely formed at depth by overburden load and/or fluid pressure near maximum burial. Our results indicate that the Eagle Ford Formation displays a well-developed fracture network regionally distributed in the Val Verde Basin, and likely present in the productive Eagle Ford play. These observations provide evidence for pathways and vertical connectivity for potential fluid pathways throughout the Eagle Ford Formation.

High-temperature alteration and porosity generation in Upper Ordovician microbial reefs, Hudson Bay intracratonic Basin, Arctic Canada

Upper Ordovician biohermal mounds of the Red Head Rapids River Formation in the Hudson Bay Basin consist of microbial and sponge boundstone and cementstone. The cementstone is made up of isopachous layers and botryoids of former aragonite now calcite. Secondary dissolution porosity and small fractures are cutting through the bioherm. Secondary pore-fillings consist of drusy calcite cement and subsequent bitumen. The δ18OVPDB and δ13CVPDB values of late cements are invariably more negative than those of the marine cements. Fluid inclusion microthermometry documents the presence of two distinct populations of homogenization temperatures, one in secondary pore-filling burial calcite cement (Th average 93 ± 10 °C) and a second one in recrystallized marine calcite cements (Th average 118 ± 25 °C). The combined δ18OVPDB and Th data suggest that burial cements precipitated from a fluid having δ18OSMOW values between +0.5 and −2‰, whereas the marine cement data indicate resetting of the fluid inclusions in the presence of a high temperature, δ18OSMOW heavy brine (+2.5 to +12‰). The lower temperature fluid inclusions of the late pore-filling cement agree with recent Apatite Fission Track data suggesting early oil window temperature at maximum burial. The higher Th values recorded in the marine cement represent resetting of initial or entrapment of new fluid inclusions from fracture-controlled circulation of basement-derived fluids. The petrographic and geochemical data suggest that fracture-controlled high temperature brine circulation occurred after the inception of burial and recrystallization of original marine aragonite to calcite, which resulted in the generation of significant secondary porosity that was later filled by lower temperature burial cements and hydrocarbons.