Carbonate Cements In Sandstones Research Articles

Assessment of the oxygen reactivity in a gas storage facility by multiphase reactive transport modeling of field data for air injection into a sandstone reservoir in the Paris Basin, France

The first objective of this study is to present unique field data on a three-year pilot test during which air containing 8 mol% O2(g) was injected as a cushion gas into a natural gas reservoir, a carbonate-cemented sandstone aquifer located in the Paris Basin (France). 10-year system survey showed that: the oxygen was fully depleted several months after injection completion, meanwhile CO2(g) was detected around 2–6 mol%; the pH decreased from 8 to 6, while reducing conditions shifted to mildly oxidizing ones with increasing concentration of sulfates in equilibrium with gypsum. 3 years after injection completion, the pH gradually returned to its near initial state and sulfates were reduced by 2 to 3 times. The second objective is to develop a multiphase reactive transport model based on the field data. Simulations were constructed using the HYTEC reactive transport code, progressing from 0D-batch to 2D-reservoir configurations. The model reproduced the gas-water-rock reactive sequence: 1/ full depletion of the injected O2(g) due to pyrite oxidation, 2/ leading to acidity production and dissolved sulfates, 3/ acidity buffering by calcite dissolution, 4/ followed by gypsum precipitation and CO2(g) exsolution. The model demonstrated that pyrite kinetics was the most significant factor governing not only the amount of O2(g), CO2(g) and dissolved minerals, but also the spatial extent of these chemical reactions and, hence, the gas spread inside the reservoir. The formulated advective Damköhler number for oxygen consumption indicated advection- and reaction-dominant regimes explaining the gas composition and extension. The developed field-based model could be used as a workflow for other gas storage facilities, e.g. biomethane, compressed air, and CO2. For underground biomethane storage, the O2(g) contents recommended in Europe, i.e. the EASEE-gas specification 2005-001-02, should have a low impact on gas composition and reservoir geochemistry when the reservoir contains efficient pH-buffers such as calcite.

Integrated petrographic and geochemical analysis of the Langobard age pottery of Szólád, Western Hungary

This study presents the results of the petrographic and geochemical analyses of the entire pottery assemblage discovered at the sixth-century (AD) cemetery of Szólád, Western Hungary, associated with the Langobard era in the territory of the former Roman province of Pannonia. Szólád is one of the most prominent archaeological sites of this period, where prior studies have shown that the cemetery was used for ca. one or two generations by a migrating group of diverse genetic background. The present work is the first integrated typological and archaeological science pottery analysis from the early migration period (fifth to sixth century) Hungary. We applied polarising light optical microscopy (OM), prompt gamma activation analysis (PGAA), and neutron activation analysis (NAA) on all samples and, additionally, scanning electron microscopy equipped with energy dispersive spectrometry (SEM–EDS) on one selected sample. One main fabric group with three subgroups were defined by OM, to which the majority of the samples belong. This fabric group was characterised by aplastic inclusions derived from a carbonate-cemented sandstone typical of the environs of Szólád; therefore, the vessels of this fabric group appear to have been produced locally. The remaining four samples display a variety of unique, ungrouped, fabrics (loners) indicative of different recipes and/or the presence of pottery originating from outside of the region. Our study concludes that the community associated with the cemetery favoured burying pots with the deceased that stylistically resembled archaic “Elbe Germanic” traditions, but which were in fact made locally. However, in some cases, relationships with more distant territories and cultural traditions are also represented.

Sand spikes pinpoint powerful palaeoseismicity

Sand spikes, pin-shaped, carbonate-cemented sandstone bodies of variable size widely interpreted as sedimentary concretions, have been enigmatic for nearly two centuries. We here present a high-energy mechanism for their formation. Two classic sand spike occurrences are found in the North Alpine Foreland Basin of Central Europe and at Mount Signal in southern California, USA. A distinct seismite horizon in Mid-Miocene Molasse sediments of southern Germany, genetically linked with the Ries impact event, exhibits dewatering structures and contains numerous sand spikes with tails systematically orientated away from the Ries crater. Sand spikes at Mount Signal, strikingly similar in shape to those found in Germany, have tails that point away from the nearby San Andreas Fault. Based on their structural and stratigraphic context, we interpret sand spikes as a new type of seismite and a promising tool to identify strong impact-induced or tectonic palaeo-earthquakes and their source regions in the geologic record.

Foraminiferal stratigraphy and palaeoenvironment of a storm-influenced marine shelf: Upper Aptian – lower Albian, Svalbard, Arctic Norway

This study presents the first record of foraminifera from the Lower Cretaceous Carolinefjellet Formation on the Svalbard archipelago, Arctic Norway. The Svalbard foraminiferal assemblages are dominated by agglutinated forms with a subordinate calcareous component. Several stratigraphically important species are recognised, and four assemblage zones are established: 1) Serovaina loetterlei Zone of middle Aptian or lower, 2) Ammobaculites wenonahae Zone of the middle Aptian, 3) Gaudryina tailleuri Zone of the upper Aptian, 4) Verneuilinoides borealis Zone of the lower Albian. The assemblages exhibit characteristic Boreal features, previously reported from the North Slope of Alaska, Arctic Canada, Western Siberia, and older Mesozoic strata in Svalbard. The study therefore allows for Circum-Arctic correlations and the extensions of stratigraphic ranges of several Boreal species.The investigated succession exhibits a large-scale fining-upwards trend. The lower part (the Dalkjegla Member) is characterised by abundant hummocky cross-stratified and wave-ripple cross-laminated sandstone beds, indicating deposition on a storm-dominated shelf. The upper part (the Innkjegla Member) is dominated by mudstones containing strata-bound siderite concretions and carbonate-cemented sandstone beds, suggesting deposition under more quiescent, middle to outer shelf conditions. An upwards increase in trace fossil diversity and degree of bioturbation, in concert with expanding foraminiferal diversity and morphogroup variability, reflects a long-term deepening of the shelf with fluctuating oxygen conditions. In combination with XRF data, diversity statistics, and morphogroup application, the present study thus sheds a new light on higher frequency palaeoenvironmental changes in upper part of the Carolinefjellet Formation.

Fault-controlled carbonate cementation: A case study from Eocene turbidite-delta sandstones (Dongying Depression) and implication for hydrocarbon migration

The subsurface distribution of carbonate cements in sandstones is cornerstone in understanding the fluid regime and origin of cements to shed the light on hydrocarbon migration. Petrography, cuttings, cores, well logs, and seismic data were integrated to better understand the distribution of carbonate cements in the Eocene turbidite-delta sandstones of the Dongying Depression. Petrographic examinations (including cathodoluminescence, CL) reveal that the carbonate cements are mainly calcite with minor ankerite that precipitated in deep burial settings. The significant abundance of carbonate cements near fault zones implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions. The extent of carbonate-cemented zones may reach hundreds of meters away from the faults, while scatter carbonate cements are much more extensive. The relationship of pervasive and scattered calcites with oil-bearing sandstones suggests that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid were displaced while the scatter calcite cements came after. The occurrence of abundant carbonate-cemented zones in the delta front oil-free sandstone intervals suggests possible hydrocarbon migration to the overlying sandstone units.

Sources of Ca2+ in the major carbonate cements in Eocene sandstones and conglomerates: Evidence from Sr isotopes, Sr/Ca ratios, and rare-earth elements

Different sources of calcium have important effects on the formation and distribution of carbonate cements. On the basis of identifying all possible sources of calcium, comprehensive quantitative analyses of Sr, C, and O isotopic values and trace elements (including rare-earth elements) were conducted to determine the Ca2+ sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs. Ferroan calcite and ankerite were the major types of carbonate cements. Ca2+ in ferroan carbonate cements in the sandstones near the mudstone-sandstone (M-S) interfaces was derived primarily from diagenetic fluids mixed with Ca2+ originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones. However, Ca2+ in the ferroan carbonate cements in the sandstones away from the M-S interfaces originated from the transformation of smectite to illite, the dissolution of carbonate minerals, and original sedimentary water in source rocks. The gradual change of Ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo-fluid in the same period. During the transformation of smectite to illite, concentration gradients were created from mudstones to sandstones. Ca2+, Mg2+, Fe2+, and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones. Pore water with Ca2+, Mg2+, Fe2+, CO32−, and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water. Due to oil emplacement and material supplied by the adjacent mudstones, the content of ferroan carbonate cements in the sandstones was lower, and the reservoir porosity was higher at a greater distance from the M-S interfaces. However, in thick-bedded sandstones, due to lack of material supply from the adjacent mudstones, Ca2+, Mg2+, Fe2+, and CO32− in the ferroan carbonate cements were mainly derived from source rocks. As a result, no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction.

A probability model to detect carbonate cementation in sandstones and other enigmatic wireline facies

Zones of carbonate cementation identified within lower Paaratte Formation sandstones of the eastern Otway Basin, Victoria, southeastern Australia, can be quantitatively detected by a wireline log-based probability model. Though trained on these zones, the same model appears to accurately predict carbonate cementation within Late Cretaceous-to-Eocene reservoir sandstones of the Latrobe Group supersequence of the Gippsland Basin, 100s of km to the east. Predicted carbonate cementation matches published evidence (a regional petrographic study) and provides a plausible interpretation for corresponding sections of Formation Micro-Imager data acquired more recently. These Latrobe Group sandstones are thought to have once been pervasively cemented prior to development of the secondary porosity responsible for providing reservoirs to the main petroleum system of the basin. However, model predictions imply that discrete, heavily cemented zones remain, which must have acted, and must still act, as local obstructions to reservoir fluid migration. These zones would also likely react with carbonic acid formed at plume fronts generated by dedicated CO2 storage operations of the future. The cemented zones that have been predicted are sparse, spatially sporadic and cannot be resolved by 3D seismic reflection survey data at present-day reservoir depths. These predictions therefore emphasise the difficulty in mapping the distribution of carbonate cemented zones. However, they provide calibration data for future mapping systems.Two different tests of the probability log model were undertaken. One benchmarks its predictions against those of a neural network trained using the original model development dataset from the Otway Basin. Predictive performance of the neural network model is significantly worse than that of the probability log model when making predictions using wireline data acquired at a well in the Gippsland Basin. The second test demonstrates that the general probability log modelling approach is amenable to classifying other cryptic ‘electro-facies’, for example those representing enigmatic or ambiguous rock types not easily interpreted from common wireline log data using standard theory. This test presents a case study by which the probability log modelling approach predicts the presence of both basic and evolved, felsic igneous sill intrusions within the stratigraphic succession of the Faroe-Shetland Basin of NW Europe.

Petrofacies prediction and 3-D geological model in tight gas sandstone reservoirs by integration of well logs and geostatistical modeling

Abstract These effective reservoir rocks that can have sufficient porosity and permeability and hydrocarbon accumulations determine economic reserves and high yield of tight oil and gas. Understanding the origin and distribution of these effective reservoir rocks is important for sorting out “sweet spots” in tight reservoirs. Multiple-scale analyses from the microscopic, via drill core to single wellbore have been conducted to examine reservoir heterogeneity of the Upper Paleozoic Shanxi Formation in the southeastern Ordos Basin. Five sandstone petrofacies have been defined in terms of texture and framework composition, detrital matrix, diagenesis and pore types: quartz arenite, tuffaceous quartz arenite, ductile lithic-lean sub-litharenite, ductile lithic-rich sandstone and tightly carbonate-cemented sandstone. The various petrofacies show fundamentally different pathways of diagenetic and reservoir-quality evolutions, due to the complexity of texture and detrital composition in the Shanxi Formation, directly related to the provenance and depositional environment. Quartz arenites and ductile grain-lean sub-litharenites both form effective reservoir rocks. By translating petrofacies to well-log signatures corrected from petrographic and core analysis data, a model based on factor analysis has been built to predict petrofacies at the well scale. For investigating the distribution of each petrofacies at the field scale, sequential indicator simulation method has been used to construct a 3-D petrofacies model with facies conditioning. The isopach maps of quartz arenites and ductile lithic-lean sub-litharenites have been made from the 3-D petrofacies model to quantify their areal distributions, and to define the “sweet spots” by combining isopach maps of these effective reservoir rocks and gas production data from well tests. The concept of reservoir petrofacies is an important tool for the characterization of tight sandstone reservoirs.

Quantified Detection of Carbonate Cementation in Sandstones using Standard Wireline Log Data

Carbonate cemented zones were identified within the lower Paaratte Formation of the eastern Otway Basin, Victoria, in southeastern Australia. A wireline log model trained on these zones can predict...

Characteristics and genetic mechanism of carbonate cements in sandstones near the overpressure top surface: A case study of the Niuzhuang Depression in Bohai Bay Basin

Carbonate cements are important authigenic minerals in the sandstone reservoir near the overpressure top surface in the Niuzhuang Depression. However, their genetic mechanism has not been studied in detail. This study uses observations of casting thin sections and cathodoluminescence and micro-area electron probe compositional data to determine that the diagenetic sequence of these carbonate cements is penecontemporaneous dolomite → calcite filling primary pores → calcite/iron-bearing calcite filling secondary dissolution pores → ankerite. The genetic mechanisms of these carbonate cements are analyzed based on the fluid inclusion temperature at different phases of carbonate cementation, carbon and oxygen isotope composition, simulation of the thermal evolution of the hydrocarbon source rock, and elemental chemical composition. Results show that the carbon isotopic compositions of the carbonate cements are affected not only by organic acids released during the maturation of organic matter but also by the dissolution of lacustrine carbonate rock in the fourth member of the Shahejie Formation. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for an obvious positive drift in the δ13CPDB values of carbonate cements near the overpressure top surface. There are four phases of carbonate cementation near the top surface of the overpressured zone, with phase IV carbonate cements mainly occurring near the overpressure top surface. Phase I carbonate cements are mainly dolomite, and their formation is related to the decomposition of organic matter by methanogenic bacteria. Phase II carbonate cements are related to Ca2+ and CO32− supersaturation during pore fluid concentration. Phase III carbonate cements result from overpressured fluid that is rich in Ca2+ and Mg2+ and feldspar dissolution caused by overpressure fluid intrusion. Phase IV carbonate cements are derived from CO32− formed by organic acid pyrolysis in late diagenesis, CO32− formed by early carbonate cement dissolution, and Mg2+ and Fe2+ plasma formed by the dissolution of metamorphic rock debris into pore fluid. The formation of phase III and IV carbonate cements is most affected by overpressured hydrocarbon fluids.

Isotopically zoned carbonate cements in Early Paleozoic sandstones of the Illinois Basin: δ18O and δ13C records of burial and fluid flow

SEM/SIMS imaging and analysis of δ18O and δ13C in sandstones from a transect through the Illinois Basin (USA) show systematic μm-scale isotopic zonation of up to 10‰ in both carbonate and quartz cements of the middle-Ordovician St. Peter and Cambrian Mt. Simon formations. Quartz δ18O values are broadly consistent with the model of Hyodo et al. (2014), wherein burial and heating in the Illinois Basin is recorded in systematically zoned quartz overgrowths. Observations of zoned dolomite/ankerite cements indicate that they preserve a more extended record of temperature and fluid compositions than quartz, including early diagenesis before or during shallow burial, and late carbonates formed after quartz overgrowths. Many carbonate cements show innermost dolomite with δ18O values (21–25‰ VSMOW) that are too low to have formed by deposition at low temperatures from ancient seawater (δ18O>−3‰) and most likely reflect mixing with meteoric water. A sharp increase in Fe content is commonly observed in zoned carbonate cements to be associated with a drop in δ18O and an abrupt shift in δ13C to higher or lower values. These changes are interpreted to record the passage of hot metal-rich brines through sandstone aquifers, that was associated with Mississippi-Valley Type (MVT) Pb-Zn deposits (ca. 270Ma) of the Upper Mississippi Valley. Local variability and individual trends in δ13C are likely controlled by the sources of carbon and the degree to which carbon is sourced from adjacent carbonate units or thermal maturation of organic matter. Quartz overgrowths in sandstones provide an excellent record of conditions during burial, heating, and pressure-solution, whereas carbonate cements in sandstones preserve a more-extended record including initial pre-burial conditions and punctuated fluid flow events.

Diagenesis and Fluid Flow Variability of Structural Heterogeneity Units in Tight Sandstone Carrier Beds of Dibei, Eastern Kuqa Depression

Tight sand gas plays an important role in the supply of natural gas production. It has significance for predicting sweet spots to recognize the characteristics and forming of heterogeneity in tight sandstone carrier beds. Heterogeneity responsible for spatial structure, such as the combination and distribution of relatively homogeneous rock layers, is basically established by deposition and eodiagenesis that collectively affect the mesogenesis. We have investigated the structural heterogeneity units by petrofacies in tight sandstone carrier beds of Dibei, eastern Kuqa Depression, according to core, logging, and micropetrology. There are four types of main petrofacies, that is, tight compacted, tight carbonate-cemented, gas-bearing, and water-bearing sandstones. The brine-rock-hydrocarbon diagenesis changes of different heterogeneity structural units have been determined according to the pore bitumen, hydrocarbon inclusions, and quantitative grain fluorescence. Ductile grains or eogenetic calcite cements destroy the reservoir quality of tight compacted or tight carbonate-cemented sandstones. Rigid grains can resist mechanical compaction and oil emplacement before gas charging can inhibit diagenesis to preserve reservoir property of other sandstones. We propose that there is an inheritance relationship between the late gas and early oil migration pathways, which implies that the sweet spots develop in the reservoirs that experienced early oil emplacement.

Diagenesis and reservoir quality evolution of the Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, East China

The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites, texturally and compositionally immature. These sandstones have a wide range of porosity (0.4–37%) and permeability (0.004–6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level. The reduction in reservoir quality is a product of several digenetic processes; these include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite and ankerite in mesodiagenetic stage.Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones. Carbonate cementation, together with compaction reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones with distance to sandstone/mudstone interface less than one meter always have lower porosity than central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities (<5%), indicating that thin sandstone beds (<2 m) were totally destroyed by cementation and compaction, and only thick sandstone beds (>2 m) can be potential effective reservoirs.Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is significantly degraded.

Reservoir quality and petrophysical properties of Cambrian sandstones and their changes during the experimental modelling of CO2storage in the Baltic Basin

The objectives of this study were (1) to review current recommendations on storage reservoirs and classify their quality using experimental data of sandstones of the Deimena Formation of Cambrian Series 3, (2) to determine how the possible CO2 geological storage (CGS) in the Deimena Formation sandstones affects their properties and reservoir quality and (3) to apply the proposed classification to the storage reservoirs and their changes during CGS in the Baltic Basin. The new classification of the reservoir quality of rocks for CGS in terms of gas permeability and porosity was proposed for the sandstones of the Deimena Formation covered by Lower Ordovician clayey and carbonate cap rocks in the Baltic sedimentary basin. Based on permeability the sandstones were divided into four groups showing their practical usability for CGS (‘very appropriate’, ‘appropriate’, ‘cautionary’ and ‘not appropriate’). According to porosity, eight reservoir quality classes were distinguished within these groups. The petrophysical, geochemical and mineralogical parameters of the sandstones from the onshore South Kandava and offshore E6 structures in Latvia and the E7 structure in Lithuania were studied before and after the CO2 injection-like alteration experiment. The greatest changes in the composition and properties were determined in the carbonate-cemented sandstones from the uppermost part of the South Kandava onshore structure. Partial dissolution of pore-filling carbonate cement (ankerite and calcite) and displacement of clay cement blocking pores caused significant increase in the effective porosity of the samples, drastic increase in their permeability and decrease in grain and bulk density, P- and S-wave velocity, and weight of the dry samples. As a result of these alterations, carbonate-cemented sandstones of initially ‘very low’ reservoir quality (class VIII), ‘not appropriate’ for CGS, acquired an ‘appropriate’ for CGS ‘moderate’ quality (class IV) or ‘very appropriate’ ‘high-2’ reservoir quality (class II). The permeability of the clay-cemented sandstones of ‘very low’ reservoir quality class VIII from the lower part of the E7 reservoir was not improved. Only minor changes during the alteration experiment in the offshore pure quartz sandstones from the E6 and E7 structures caused slight variations in their properties. The initial reservoir quality of these sandstones (‘high-1’ and ‘good’, classes I and III, respectively, in the E6 structure, and ‘cautionary-2’, class VI in the E7 structure) was mainly preserved. The reservoir sandstones of the Deimena Formation in the South Kandava structure had an average porosity of 21%, identical to the porosity of rocks in the E6 structure, but twice higher average permeability, 300 and 150 mD, respectively. The estimated good reservoir quality of these sandstones was assessed as ‘appropriate’ for CGS. The reservoir quality of the sandstones from the E7 offshore structure, estimated as ‘cautionary-2’ (average porosity 12% and permeability 40 mD), was lowest among the studied structures and was assessed as ‘cautionary’ for CGS. Petrophysical alteration of sandstones induced by laboratory-simulated CGS was studied for the first time in the Baltic Basin. The obtained results are important for understanding the physical processes that may occur during CO2 storage in the Baltic onshore and offshore structures.

Ediacaran Araba Complex of Jordan

ABSTRACTThe Ediacaran Araba Complex in Jordan is defined and described for the first time in lexicon style, with an emphasis on the sedimentary, volcanic and volcaniclastic units outcropping adjacent to Wadi Araba, and from seismic and deep exploration well data. The Araba Complex ranges in age from ca. 605 to 550 Ma and comprises a major cycle of sedimentary, volcanic and volcaniclastic, and igneous rocks emplaced in an overall extensional tectonic regime that followed intrusion and amalgamation of the granitoid and metamorphic Aqaba Complex, a part of the Gondwanan Arabian-Nubian Shield (ANS; ca. 900 to 610 Ma).The Araba Complex is bounded by two major erosional unconformities, the newly defined Ediacaran Araba Unconformity (ca. 605 Ma) at its base, underlain by the Aqaba Complex, and the post-extensional, regional lower Cambrian Ram Unconformity (ca. 530 Ma) that is marked by the widespread deposition of thick alluvial and marginal-marine siliciclastics (Ram Group).Two sub-cycles can be recognised in the Araba Complex mega-cycle. The earliest (Safi Group) followed suturing and extensional rifting of the Aqaba Complex that resulted in rapid basinal subsidence and the deposition of coarse-grained, polymict conglomerates (Saramuj Formation) in predominantly proximal, but evolving to more distal, alluvial fan settings. The early extensional basin appears to have been orientated approximately north-south (depocentre to the west) and can be traced from north Sinai to Lebanon, approximately parallel to the present-day Dead Sea Transform. Rounded clasts, up to boulder-size, include a variety of local to regionally-derived basement lithologies, including granites, diorites, metamorphic rocks; doleritic and rhyolitic dyke rocks. Rapid isostatic uplift and weathering of the granitoid basement resulted in high sediment flux that kept pace with rapid basin subsidence; this, in turn, led to erosion and partial peneplanation of the hinterland ANS. Regional detrital zircon ages from the conglomerate clasts and matrix indicate age ranges from ca. 650 to 600 Ma with a minor cluster between 750 to 700 Ma, indicating mostly a local or, at least, near-field provenance. Subsequent to this early, rapid basin-fill, continued crustal extension resulted in tapping of rhyolitic and basaltic effusive volcanics and volcaniclastics (Haiyala Volcaniclastics and Museimir Effusives, ca. 598–595 Ma), including flow-banded rhyolitic lavas and air-fall tuffs, the latter deposited in a lacustrine or shallow-water environments.The second Araba sub-cycle (595–586 Ma) is characterised by renewed basinal subsidence, very low burial metamorphism to about 6 km depth, and associated stock-like intrusion of the Qunaia Monzogabbro (595 ± 2 Ma) that resulted in thermal contact metamorphism of the Saramuj conglomerate, as well as granite plutons (e.g. Feinan-Humrat intrusions) and dolerite dykes. The second cycle is characterised by renewed extension, rifting and the deposition of volcanic rocks, agglomerates (Aheimir Volcanics) and monomict conglomerates (Umm Ghaddah Formation) that were sourced, locally, from volcanic rocks on the rift margins.To the east, in the sub-surface of south-central Jordan, the early Safi sub-cycle is absent. Deep exploration wells and seismic data in the Jafr area demonstrate that the Araba Complex comprises terrestrial lavas (Ma’an Formation) with weathered soil horizons, unconformably overlying weathered Aqaba Complex granitic basement (Araba Unconformity). Seismic data in the Jafr region records the eruption of lavas in north-south trending graben and half-graben settings, and possible northwest-trending bounding faults similar to the Najd basins in Saudi Arabia. Again, in contrast to the outcrop areas to the west, the upper part of the Araba Complex, hereabouts, consists of fine-grained, in part carbonate-cemented sandstone and claystone, together with anhydrite (Jafr Formation) suggesting a shallow-marine or coastal sabkha setting, and a possible link to similar shallow-marine extensional basin-fills that developed widely within NW-trending Najd basins across the ANS in Saudi Arabia (e.g. Jibalah and Antaq basins). To date, no Ediacaran biotas have been described from the Araba Complex, but the Jafr Formation, which post-dates the appearance of soft-bodied faunas around 579 Ma, and which was probably deposited in marginal-marine environments, is a potential candidate for these enigmatic fossils.Subsequent to the final Araba extensional rifting phase, renewed regional uplift, far to the south, of the ANS hinterland during the early Cambrian, led to widespread deposition of alluvial and shallow-marine siliciclastics as a progradational ‘sand-sea’ (Ram Group) that blanketed the now peneplained Aqaba Complex in south Jordan and surrounding countries (Ram Unconformity). However, the younger Ediacaran Araba Complex outcrops adjacent to Wadi Araba remained, in places, as a relatively immature palaeotopography. It was not until early mid-Cambrian times (ca. 509 Ma), during the Burj marine transgression that this late Ediacaran palaeotopography was finally buried.The Araba Complex in Jordan with its multi-cycle development provides an insight to the regional development of Ediacaran extensional basins in the Arabian-Nubian Shield, an important phase in the evolution and transition from Neoproterozoic to Phanerozoic crustal tectonics and associated basin-fill.

Selective dissolution of feldspars in the presence of carbonates: The way to generate secondary pores in buried sandstones by organic CO2

Carbonates are suggested to dissolve rapidly than feldspars by laboratory experiments. Petrography texture of selective dissolution of feldspars in the presence of carbonates, however, is widespread in buried sandstones and even shales, inspiring a revisit to the chemistry of burial secondary pores.Feldspar dissolution, precipitation of secondary minerals (quartz, clays), and carbonate cementation are common chemical reactions in the Eocene sandstones in the northern Dongying Sag. Petrography evidence demonstrates the selective dissolution of feldspars in the presence of carbonate minerals (both detrital and authigenic minerals) in these buried sandstones. The equilibrium constant of calcite leaching reactions is much smaller than that of K-feldspar leaching reactions. Numerical simulations of chemical reactions in K-feldspar-calcite–CO2–H2O systems utilizing the Geochemist's Workbench 9.0 (GWB) indicate that only a small amount of calcite was dissolved at the onset of simulation processes, while much K-feldspar was dissolved with precipitation of quartz, clays and some calcite for extended periods of time. Precipitation of secondary calcite could also promote feldspar dissolution. Simulation of reactions in a simplified sandstone system with constraints of present-day pore water and partical pressure of carbon dioxide (pCO2) in the northern Dongying Sag indicates that the pore waters are close to equilibrium with calcite. Petrography evidence and modeling results share consistence in confirming that only feldspar could be dissolved extensively, with precipitation of quartz, clays and some carbonate minerals.The organic CO2-leaching theory (Schmidt and McDonald, 1979) regarding the dissolution of mainly carbonates and, to a lesser degree, feldspars to form secondary pores has been questioned due to supposed extensive internal consumption of organic CO2 in source rocks by carbonate minerals. The CO2 in sandstones and shales and the isotopic composition of CO2 and carbonate cements in sandstones, however, suggest migration of organic CO2 from mudstones (even with large amounts of carbonate) to sandstones. Calculations in this study indicate that, with suppression of carbonate dissolution in sedimentary rocks, many secondary pores can be generated in sandstones through selective dissolution of feldspar by organic CO2 from thermal evolution of organic matter. Selective dissolution of feldspars, rather than carbonates, in sedimentary rocks is then likely a general mechanism to decipher the chemistry of secondary porosity.Other secondary porosity generation mechanisms were also reevaluated with numerical simulations. Results demonstrate that reverse weathering reactions of silicates may take place with increasing of temperatures, but cannot provide enough H+ to dissolve carbonates at elevated temperatures. Additionally, cooling of hot fluids can dissolve only insignificant amounts of carbonate unless significant amounts of water are provided.

Chemical Composition and Isotopic Characteristics of the Carbonate Cements in Sandstone Reservoir Layer of Dongying Sinking

The chemical composition of the carbonate cements in reservoir sandstones was determined by the EPMA. The generation sequence is: calcite, dolomite, ferrous calcite, ankerite, and the latest are coarse-grained and fine-grained calcites which are veined. In the MnO-FeO-MgO triangular diagram, the samples fall clearly on the four areas. The carbon and oxygen isotope data on the carbonate cements show that δ13C are from −6.44 to +4.79, δ18O are from −4.25 to −15.21, focused on three areas in the map. The paleosalinity Z from 107 to 133 calculated by the carbon and oxygen isotope value reflects the characteristics that salinity changes are caused by fresh water filling lake water. The average temperature of calcite precipitation calculated by the oxygen isotope value is 650°C, while dolomite precipitation is 70.6°C. The precipitation temperature changes greatly and shows negative correlation with temperature and salinity, which reflects the characteristics of lake sediments.

Origin of carbonate cements in Cretaceous sandstones from lower Benue Trough, Nigeria: Evidence from petrography and stable isotope composition

Beds of authigenic carbonates were identified from three Cretaceous lithostratigraphic units of the Lower Benue Trough, Nigeria. Three carbonate lithologies were recognized by petrographic analysis in the study area. Carbonate-cemented sandstones are dominated by ferroan calcite cements with subordinate amount of siderite and dolomite/ankerite cements formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtian proto-Niger Delta sequences. δ18O for the cements range from -4.22 to -6.91‰(PDB) in Asu River Group; -5.23 to 12.66‰(PDB) in Eze-Aku and -4.45 to 6.89‰(PDB) in Campano- Maastrichtian proto-Niger Delta sandstones. δ13C values averaged -5.23‰ PDB for Asu River Group; -11.03‰ PDB and -5.88‰ PDB for Eze-Aku and Campano-Maastrichtian proto-Niger Delta sandstones. Petrography and geochemical data suggest that the mixing of meteoric and marine waters in the sediments caused dolomitization in Asu River group and Eze-Aku Group sediments while deep burial diagenesis under marine depositional environment precipitated calcite cements. Both authigenic and biogenic carbonate cements with low and high δ13C values exits in the sandstones analyzed. The low δ13C values of Asu River Group, Eze-Aku and Campano-Maastrichtian cements that range from 0 to 2.98 ‰ suggest that they are shallow water carbonates. Biogenic carbonate cements were also identified in some of the sandstones. The carbonate cements were formed under the influence of both meteoric- and marine diagenetic conditions which changed in time and space. The dolomitized rocks occur just below erosion surfaces on the continental sediments and on which shallow marine facies are developed.

Reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the Jiyang Super-depression, Bohai Bay Basin, China

A great difference exists between the hydrocarbon charging characteristics of different Tertiary lacustrine turbidites in the Jiyang Super-depression of the Bohai Bay Basin, east China. Based on wireline log data, core observation and thin-section analyses, this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs. The lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0.5–10 km 2, with a thickness of 5–20 m. The sandstones of the Tertiary turbidites in the Jiyang Super-depression have been strongly altered diagenetically by mechanical compaction, cementation and mineral dissolution. The effect of compaction caused the porosity to decrease drastically with the burial depths, especially during the early diagenesis when the porosity was reduced by over 15%. The effect of cementation and mineral dissolution during the late-stage diagenesis is dominated by carbonate cementation in sandstones. High carbonate cement content is usually associated with low porosity and permeability. Carbonate dissolution (secondary porosity zone) and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone. Two stages of carbonate cementation were identified: the precipitation from pore-water during sedimentation and secondary precipitation in sandstones from the organic acid-dissolved carbonate minerals from source rocks. Petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones: high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs. It is also noticed that interstitial matter content affects the oil-bearing property to some degree. There are three essential elements for high oil-bearing turbidite reservoirs: excellent pore types, low carbonate cement (<5%) and good petrophysical properties with average porosity >15% and average permeability >10 mD.

Chemosymbiotic bivalves and stable carbon isotopes indicate hydrocarbon seepage at four unusual Cenozoic fossil localities

Four unusual Cenozoic fossil localities are identified here as ancient hydrocarbon seep sites using palaeontological, petrological, and stable carbon isotope data. Late Eocene carbonate-cemented sandstone bodies in the Wagonwheel Mountains in California were previously suspected to represent ancient hydrocarbon seep sites, but the relatively high δ13Ccarbonate values (−12.2 to −5.0‰) suggest oil rather than methane seepage. The Oligocene fauna of the Elmira asphalt mine, Cuba, was previously interpreted as a mix of freshwater and marine taxa, but all species are here identified as belonging to marine groups, including the bivalve families Lucinidae and Vesicomyidae, whose extant members live largely in symbiosis with chemoautotrophic endosymbionts. A carbonate concretion from this site showed δ13Ccarbonate values as low as −32.2‰, which most likely indicates methane seepage. A previously unpublished Oligocene fossil locality in Atlantico, northern Colombia, is dominated by large solemyid, mytilid, lucinid, and vesicomyid bivalves, which most likely lived with chemotrophic endosymbionts. Seepage of biogenic methane without a significant contribution of thermogenic methane is indicated by δ13Ccarbonate values as low as −51.3‰. We confirm that the Pleurophopsis-dominated sites of the Heath Shale and Lomitos Chert in northern Peru are ancient seep sites, although the previous identification of Pleurophopsis peruviana as a vesicomyid is doubtful.