Diagenetic history of the Lower Jurassic middle member of the Marrat Formation, Magwa Field part of the Greater Burgan Field, Kuwait

The Upper Devonian Rimbey-Meadowbrook reef trend is an extensive subsurface carbonate aquifer system in the southern part of the Western Canada Sedimentary Basin and is compared with Swan Hills, Leduc and Wabamun carbonates of west-central Alberta. These Devonian levels are part of a structural homocline that crops out near its northeastern end and stretches for several hundred kilometers southwestward into the deepest part of the basin. Most Devonian carbonates are pervasively dolomitized into grey matrix dolomite, which commonly comprises about 85-90 vol % of the rocks. Despite its large regional extent and subsurface depths (0 to 6500 m), the Rimbey-Meadowbrook reef trend displays little if any systematic variations in petrography or geochemistry. Stratigraphic, petrographic and geochemical evidence suggest that the grey matrix dolomite formed at depths of about 500-1500 m from marine pore waters with relatively minor modification by water-rock interaction. Volumetrically minor amounts of calcite and dolomite cements, replacive and cement anhydrite, and elemental sulfur and hydrocarbons fill some of the remaining porosity. The volumetrically minor intermediate burial (>600-1500 m) dolomite cements and deep burial saddle dolomite and calcite cements occur as coarse-crystalline spars in secondary porosity. Intermediate burial dolomite cements display a slight decrease down the reef trend in delta 18 O values (-4 to -7 per mil PDB) with increasing depth that correspond to slightly elevated 87 Sr/ 86 Sr ratios. Fluid inclusions of these dolomite cements are highly saline and have relatively uniform homogenization temperatures (122 to 131 degrees C), suggesting that they formed from relatively hot fluids before significant basin tilting. Deep burial (>2000-3000 m) saddle dolomites are rare in the Rimbey-Meadowbrook reef trend and the available fluid inclusion homogenization temperatures increase with burial, suggesting that they precipitated under normal geothermal gradients after significant tectonic tilting. The calcites fall into two groups, a relatively early and a relatively late group. The relatively early calcites follow a 30 degrees C/km geothermal gradient when corrected for pressure, whereas the later calcites that formed as a by-product of thermochemical sulfate reduction follow a 20 to 25 degrees C/km gradient and may reflect cooler post-Laramide conditions. The deep basin of west-central Alberta includes the Obed, Kaybob South, Fox Creek, Pine Creek and Simonette fields from the Swan Hills, Leduc and Wabamun stratigraphic levels. Replacement matrix dolomites in all three stratigraphic levels are similar in terms of textures, trace elements and C, O and Sr isotopes, and to those of the Rimbey-Meadowbrook reef trend. In addition, fault-controlled, late-stage burial dolomitization is evident in the Swan Hills, Kaybob South and Simonette buildups, and in the Wabamun Pine Creek Field. Homogenization temperatures from fluid inclusions in saddle dolomites range from about 127 to 167 degrees C and late calcites have similar T h ranges from 124 to 164 degrees C. Temperatures for anhydrite are cooler, ranging from 104 to 116 degrees C, and suggest secondary alteration of inclusions at present reservoir temperatures and/or that some anhydrite was remobilized at somewhat cooler conditions after maximum burial. Downdip to the southwest in the Obed buildup, deep burial calcite cements in fractures and vugs have highly radiogenic 87 Sr/ 86 Sr ratios (0.7252) and highly depleted delta 13 C values (minimum -27.1 per mil PDB), indicating incorporation of oxidized carbon from Thermochemical Sulphate Reduction (TSR). In other buildups, delta 13 C values of late calcite cements are not as depleted in their carbon isotopic values (0.9 to -10 per mil PDB). Radiogenic Sr occurs updip to the northeast in saddle dolomite and calcite cements (Swan Hills, Kaybob South 0.7221, 0.7310; Simonette 0.7370, 0.7369; Leduc Pine Creek 0.7140, 0.7195; Wabamun Pine Creek 0.7150, 0.7188). The radiogenic 87 Sr/ 86 Sr signal appears to be derived from fluids that had interacted with the crystalline basement and/or overlying Proterozoic/Lower Cambrian clastics. It is possible that at least some radiogenic Sr was injected into the Devonian section by tectonic loading during the Laramide Orogeny. The Rimbey-Meadowbrook reef trend and the Devonian carbonates in the west-central Alberta deep basin are similar in terms of overall diagenetic phases and their characteristics. The late-stage cements exhibit somewhat higher temperatures in the deep basin of west-central Alberta. The regions differ mainly in radiogenic Sr with all west-central Alberta late-stage dolomite and calcite cements being more radiogenic, especially along established or inferred faults as in Kaybob South, Swan Hills Simonette and Wabamun Pine Creek fields. The west-central Alberta calcites also contain lighter delta 13 C (-3.5 to -10.0 per mil PDB) and even lower (0 to -27 per mil) in some high sour gas fields like Obed.

Stratigraphic relations, detailed petrography, and fine-scale geochemical and isotopic analysis of diagenetic phases formed within Kimmeridgian-age reefal carbonates of the Torrecilla en Cameros Formation of Northern Iberian Ranges in Spain indicate a complex history of alteration in marine, meteoric, burial, and uplift-related settings. The reefal succession is separated from the overlying Tithonian-Berriasian fluvial and lacustrine continental deposits by a single unconformity, which is marked by brecciation and karstification. Nevertheless, a record of three distinct stages of alteration associated with this unconformity is preserved in the succession of calcite cements present within the reefal carbonates. Recognition of temporally distinct episodes of meteoric diagenesis associated with a single unconformity is atypical in carbonate rocks, but this study illustrates how such complex systems can be deconvoluted. The first episode of subaerial exposure and meteoric alteration resulted in neomorphism of marine allochems and precipitation of a first generation of nonferroan clear calcite (NFC) cement. Following local faulting and brecciation, a second NFC precipitated throughout the reefal unit. Although similar in petrographic character, this phase of cementation is distinct in its isotopic composition, reflecting changes in the regional climate of the Iberian Peninsula during the early Tithonian. Another phase of meteoric alteration of the reefal unit is recorded by renewed corrosion, including dissolution of preexisting calcite cements and precipitation of prismatic calcite, prior to the deposition of Tithonian-Berriasian lacustrine and continental sediments. These continental units, in turn, record yet another episode of alteration by meteoric waters as NFC cements formed within intragranular and dissolution porosity in the lacustrine limestones. Notably, these cements are also distinct on the basis of their petrographic and geochemical character. Following subsidence and burial during Aptian to late Cretaceous times, migration of regionally derived fluids (perhaps in response to the onset of tectonic deformation to the north) led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence. Alteration of these burial-related diagenetic phases has subsequently taken place in response to the regional uplift during the Tertiary. This final episode of meteoric alteration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite.

Petrographic, geochemical, and fluid inclusion analysis of dolomite and calcite cements has been conducted on Mississippian carbonates collected from the surface and subsurface of the southern midcontinent of the United States (Oklahoma, Missouri, Kansas, and Arkansas). Fracture and vug, intergrain, and intragrain porosity are filled with calcite, authigenic quartz, and dolomite cements. Primary limestone porosity is filled partially by early marine and meteoric calcite cements. Equant (blocky) calcite cements were precipitated under seawater or mixed meteoric-seawater conditions in the phreatic zone and in the deep phreatic zone under late (burial) diagenetic conditions. Fracture- and breccia-filling saddle dolomite cements that were observed are late diagenetic and are likely related to the nearby Tri-State Mississippi Valley-type (MVT) mineral district. Carbon and oxygen isotope values of dolomite cements range from δ18O(VPDB) = −9.5 to −2.7‰ and from δ13C(VPDB) = −4.0 to −0.4‰. Values for calcite cements range from δ18O(VPDB) = −11.6 to −1.9‰ and from δ13C(VPDB) = −12.2 to +4.6‰. These values are consistent with three types of diagenetic fluids: seawater, seawater modified by meteoric water, and evolved basinal water. Analysis of fluid inclusions in late calcite, dolomite, and quartz cements indicates the presence of both dilute and high salinity end-member fluids. Homogenization temperatures (Th) of fluid inclusions range from 57°C to 175°C and salinities range from 0 to 25 equivalent weight % NaCl. Fluid inclusion Th values and salinities are consistent with a saline basinal fluid variably diluted by fluids of meteoric or mixed seawater and meteoric origin. Petroleum inclusions were observed in late diagenetic calcite and dolomite cements.The late diagenetic cements filled porosity retained after early diagenetic cementation indicating that some original porosity in the Mississippian carbonate rocks remained open during petroleum migration. Elevated fluid inclusion Th values over a broad region, not just in the Tri-State Mineral District, imply that the regional thermal maturity of rocks may be higher than believed previously. This study indicates that the Mississippian carbonate resource play on the southern midcontinent has a very complex diagenetic history, continuing long after early diagenetic cementation. Possibly the most important diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata.

The carbonate unit in the Lower-Middle Ordovician Yingshan Formation in the Tazhong region of the Tarim Basin in western China is partially to completely dolomitised. Three types of dolomite were identified in this unit: microcrystalline (<20 μm) to finely crystalline (20–50 μm) subhedral dolomite rhombs (Rd1), finely crystalline (50–250 μm) anhedral to subhedral mosaic dolomites (Rd2), and medium to finely crystalline (200–500 μm) euhedral to subhedral sucrosic dolomites (Rd3). Three types of calcite cement were also identified: coarsely crystalline mosaic calcite cement in mouldic pores (Cc1), coarsely crystalline intergrown calcite cement (Cc2), and coarsely crystalline sparry and veined calcite cement in the fractures (based on hand sample observation) (Cc3). The δ13C values and 87Sr/86Sr ratios of the Rd1 dolomite in the Yingshan Formation are consistent with those of Lower to Middle Ordovician marine limestone, suggesting that the isotopic ratios were inherited from the precursor limestone. Stratigraphic, petrographic and geochemical data constrain the formation of the Rd1 dolomite to a shallowly buried environment involving seawater with elevated salinity. The Rd2 dolomite yields lower δ18O values (−7.5 to −4.7 ‰) and contains less Sr and Na and more Fe and Mn than the Rd1 dolomite (−5.3 to −3.2 ‰). Therefore, the dolomitising fluids of the Rd2 dolomite were likely derived from the infiltration and diffusion of residual evaporitic water resulting from intense compaction at elevated temperatures. The Rd3 dolomite generally features lower δ18O values (−9.0 to −7.0 ‰) than the Rd2 dolomite, but the δ18O values of portions of the Rd2 and Rd3 dolomites overlap. Thus, the Rd3 dolomite may represent recrystallisation of the Rd2 dolomite. The Rd3 dolomite contains significantly lower Sr and Na contents and significantly higher Fe and Mn contents than the Rd1 and Rd2 dolomites, suggesting that the dolomitic fluids precipitated under reducing conditions during burial. The late-stage, medium-coarse to very coarse calcite postdates all the dolomites and is present as cement in fractures and pores. The δ18O values of the Rd3 dolomite and Cc2 calcite cements (−12.6 to −7.9 ‰) are similar, suggesting that the calcite cement in the pores may represent calcite supersaturation associated with burial dolomitisation. The Cc3 calcite cements feature slightly higher δ13C values (+0.2 to +0.4 ‰) and higher radioactive 87Sr/86Sr ratios (0.709280 to 0.709483) than those of the limestone (0.707955 to 0.708231) and Rd3 dolomite (0.707723 to 0.708345) and, in places, lower δ18O values (−11.6 to −6.3 ‰) than the latter. Consequently, the Cc3 calcite cement either formed from the upward migration of basinal fluids in a burial environment or was impacted by hydrothermal activity.

As one of the predominant diagenetic products in clastic rocks, calcite cements are typical fingerprints of cement‐forming fluids and are key controls on reservoir quality. The Puig‐reig anticline, in the south‐eastern Pyrenees (Spain), exposes excellent outcrops of conglomerates, sandstones and claystones, which were deposited from a proximal to medial fluvial system and underwent folding, fracturing and cementation. This anticline constitutes an appropriate case study to investigate the origin and distribution of calcite cements during folding evolution and how they affect reservoir quality. Based on structural, petrographic and geochemical analyses (carbon, oxygen, strontium and clumped isotopes and elemental composition), five generations of calcite cements (‘Cc0’ to ‘Cc4’) have been identified, filling intergranular porosity of host rocks, faults and four fracture sets (F1 to F4). Calcite cement Cc0 precipitated in intergranular porosity from meteoric fluids in the phreatic zone during the early diagenetic stage. During the most intense phase of thrusting and folding, Cc1 precipitated in intergranular porosity, faults and F1 to F4 fracture sets from hydrothermal fluids that migrated from deeper areas of the Pyrenean chain. During the late stage of fold growth, Cc2 precipitated in faults and their associated fractures in the anticline crest from hydrothermal fluids but at shallower burial depths than that of Cc1. Calcite cement Cc3 mainly precipitated in fractures with the same strike as F1 and F4 fracture sets in the north‐western part of the anticline, from formation fluids that probably migrated through the frontal thrust of the south‐eastern Pyrenees. During the continuous fold denudation, Cc4 precipitated from meteoric fluids in F1 to F4 fracture sets across the anticline. Results indicate that at foreland basin margins, external fluids coeval with compressional deformation and/or alteration of detrital carbonates contribute to intensive calcite cementation. This can result in an overall occlusion of porosity and significantly damaged reservoir quality.

Skeletal aragonite dissolution from hypersaline seawater: a hypothesis

Carbonate-cemented stylolites and fractures in the Upper Jurassic limestones of the Eastern Iberian Range, Spain: A record of palaeofluids composition and thermal history

Origin of hydrothermal dolomitization in the Huize Zn–Pb ore district, SW China: Insights from in situ U–Pb dating, fluid inclusion, and C–O–Sr–Mg isotope analyses

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Nature and distribution of diagenetic phases and petrophysical properties of carbonates: The Mississippian Madison Formation (Bighorn Basin, Wyoming, USA)

Methane‐derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane‐enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south‐eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X‐ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ13CVPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan‐shaped aragonite (δ13CVPDB of −52‰ to −30‰) and high‐magnesium calcite cements, with the latter often growing from low‐magnesium calcite spherules. Botryoidal low‐magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low‐magnesium calcite breccias, high‐magnesium calcite nodules (δ13CVPDB of −35‰ to −20‰) and cements, and partially replaced fan‐shaped aragonite cements. Carbonate pavements consist of low‐magnesium calcite microsparite, micritic dolomite and high‐magnesium calcite. Fan‐shaped aragonite is locally present as pore‐lining cement. Iron oxides are often seen coating the low‐magnesium calcite, high‐magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high‐magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.

Deeply buried Precambrian carbonate reservoirs host economic petroleum reserves. The upper Ediacaran Dengying Formation (maximum burial depth >8 km) is predominantly composed of dolostone with porosity ranging from 2 to 4%, and minor limestone with a porosity of <2%. Detailed petrological observations and high-resolution geochemical analyses show the evolution from depositional fluids to late diagenetic fluids. Dolostone is subdivided into dolo-mudstone, dolo-laminite, stromatolitic dolostone, thrombolitic dolostone and oncoidal dolostone. These dolostones were precipitated from the Ediacaran seawater and yield rare earth elements and yttrium (REY) patterns similar to those of modern seawater. In the early diagenetic regimes, fibrous dolomite cement, characterised by superchondritic Y/Ho ratios, was first precipitated from marine porewaters, and then fine-crystalline dolomite cement, characterised by low Y/Ho ratios, was formed during meteoric diagenesis. Bladed dolomite cement was subsequently precipitated from (modified) seawater-dominated porewaters. In the intermediate burial regimes, medium- to coarse-crystalline dolomite cement was formed during reduction of Mn oxides. In the deep-burial regimes, saddle dolomite was precipitated from hydrothermal fluids and yields REY patterns characterised by MREE enrichment. Its superchondritic Y/Ho ratios were likely caused by the fractionation between Y and Ho during the co-precipitation of fluorite. Calcite cement has similar parent fluids to those of saddle dolomite. These dolomite phases have similar δ13C values, while their δ18O values gradually decrease during the paragenetic sequence. Multiphase dolomitisation led to the increase in Ca/Mg ratios of formation water and reservoir performance. This study is significant for those concerned with the deeply buried dolostone reservoirs. KEY POINTS Microbialites and five types of dolomite cements have different REY patterns. Co-precipitation of fluorite led to the superchondritic Y/Ho ratios of dolomite. Dolomite and calcite grains show micro-scale geochemical variations. This dolostone reservoir underwent multiphase fluids, including meteoric water, seawater and hydrothermal fluid.

The objective of this study is to test the influence of some key input parameters in basin modelling and to evaluate the resultant effect of varying these parameters on the model. 3D basin modelling and petroleum system analysis of the northern North Sea has been carried out using the PETROMOD software. The model was calibrated using well 34/8-7 in the Visund field. Different input parameters such as heat flow, source rock properties, fault properties, paleo water depth, source rock kinetics, migration methods and different erosion scenarios have been varied and their effects on the model assessed. The effect of the various input parameters has been assessed in terms of hydrocarbon volumes in the Kvitebjørn and Visund fields, source rock maturity, transformation ratios, hydrocarbon saturations and the time hydrocarbon generation began in the Draupne and Heather Formation source rocks. Increase in heat flow increases source rock maturity, the start of hydrocarbon generation, transformation ratios and results in the generation of a lot more gas than oil. Hydrocarbon generation starts at shallower depths with higher heat flow. Increasing Total Organic Content (TOC) and Hydrogen Index (HI) generally results in increase in the volume of hydrocarbons generated. The increase in HI, however, results in the generation of a lot more oil than gas. High TOC and HI also increase the hydrocarbon saturations in the source rock. Increasing Paleo Water Depth (PWD) has a marginal effect on the model. It increases the volume of oil and decreases the volume of gas marginally. Varying the PWD has no significant effect on source rock maturity, transformation ratios and hydrocarbon saturations. Opening the fault planes resulted in an increase in the volume of hydrocarbons generated. The increase was more evident in the volume of oil than gas. This increase in volumes is a consequence of additional migration pathways created by the faults. Varying the erosion thickness of the Draupne Formation did not have any effect on the model.

3D stratigraphic, structural, thermal and migration modelling has become an essential part of petroleum systems analysis for passive margins, especially if complex 3D facies patterns and extensive volcanic activity are observed. A better understanding of such underexplored offshore areas requires a refined 3D basin modelling approach, with the implementation of realistically sized volcanic intrusions, source rocks and reservoir intervals. In this study, an integrated modelling workflow based on a Great Australian Bight case study has been applied. The 244800-km2 3D model integrates well data, marine surveys, 3D stratigraphic forward modelling and 3D basin modelling to better predict the effects of 3D facies variations and heat flow anomalies on the determination of the source rock-enriched intervals, the source rock maturity history and the hydrocarbon migration pathways. Plausible sedimentary sequences have been estimated using a stratigraphic forward model constrained by the limited available well data, seismic interpretation and published tectonic basin history. We also took into account other datasets to produce a thermal history model, such as the location of known volcanic intrusion, volcanic seamounts, bottom hole temperature and surface heat flow measurements. Such basin modelling integrates multiple datatypes acquired in the same basin and provides an ideal platform for testing hypotheses on source rock richness or kinetics, as well as on hydrocarbon migration timing and pathways evolution. The model is flexible, can be easily refined around specific zones of interest and can be updated as new datasets, such as new seismic interpretations and data from new sampling campaigns and wells, are acquired.

The diagenetic history of Triassic sandstone from the Beacon Supergroup, Victoria Land, Antarctica, can be divided into three main phases of shallow burial diagenesis, contact diagenesis (temperatures of 200–300°C), and post‐contact diagenesis, on the basis of petrographic and geochemical analyses. Shallow burial diagenesis is characterised by minor compaction, K‐feldspar alteration to illite, and quartz cementation. Contact diagenesis is related to emplacement of dolerite intrusions and basalt flows during Gondwana break‐up at 180 Ma. This high‐temperature diagenetic phase is dominated by zeolite cementation, even in sandstone poor in zeolite precursor materials. Elevated thermal conditions associated with the igneous intrusions are suggested by increased illite crystallinity, but strong evidence for contact metamorphism is missing. Post‐contact diagenesis is signified by zeolite and K‐feldspar dissolution, and local quartz, calcite cementation, and minor albite and K‐feldspar precipitation. This diagenetic phase is possibly related to renewed (hydro‐) thermal activity in Victoria Land in association with rifting of New Zealand and Australia from Antarctica at c. 96 Ma.