Evaporite Sequence Research Articles

Characterization of brines and evaporite deposits for their lithium contents in the northern part of the Danakil Depression and in some selected areas of the Main Ethiopian Rift lakes

Abstrat Lithium is a metal with increasing demand on the global market mainly for its application in lithium ion batteries for electric vehicles. Ethiopia has potential for lithium resources related to pegmatites and to rift-bound brine deposits. The current research focused on known brine lakes and hot springs in the Danakil Depression and the Main Ethiopian Rift. Lithium contents in water samples from rift-bound lakes like Abijata, Shala, Chamo, Abaya, and Chew Bahir show low concentrations consistent with background values for surface waters. Lithium concentrations are elevated in brine lakes, brine ponds, and hot springs of the Danakil Depression. Hot springs in the Dallol contain up to 20 ppm lithium. Lithium enrichment was also noted in residual brines from multi-stage solar evaporation pans during soda-ash production. However, brines produced during precipitation of trona and halite contain about 0.009 ppm lithium, which is well below economically extractable concentrations. Core samples from the Danakil Depression potash exploration reported up to 20–40 ppm in the carnallite and bischofite layers of the evaporite sequence. At least three potential sources for lithium are noted in the Danakil Depression. For example, lithium is leached and mobilized during water rock interactions by deeply circulating and cycling hot meteoric and possibly juvenile geothermal fluids related to the intense volcano-tectonic processes. The current hydrological condition also suggest the possibility of lithium mobilization during surface run off from the basement and Mesozoic sedimentary rocks exposed along the western rift margin of the Danakil Depression. Moreover, variable lithium contents in the different layers of the evaporite sequence suggests, that lithium originated from sea water evaporation before the Danakil Depression became a closed basin. The current research strongly indicates that more detailed investigation is required to evaluate the lithium resources as a primary product and/or as a secondary byproduct during potash mining.

Architecture of the evaporite accumulation and salt structures dynamics in Tiddlybanken Basin, southeastern Norwegian Barents Sea

AbstractAn extensive, reprocessed two‐dimensional (2D) seismic data set was utilized together with available well data to study the Tiddlybanken Basin in the southeastern Norwegian Barents Sea, which is revealed to be an excellent example of base salt rift structures, evaporite accumulations and evolution of salt structures. Late Devonian–early Carboniferous NE‐SW regional extensional stress affected the study area and gave rise to three half‐grabens that are separated by a NW‐SE to NNW‐SSE trending horst and an affiliated interference transfer zone. The arcuate nature of the horst is believed to be the effect of pre‐existing Timanian basement grain, whereas the interference zone formed due to the combined effect of a Timanian (basement) lineament and the geometrical arrangement of the opposing master faults. The interference transfer zone acted as a physical barrier, controlling the facies distribution and sedimentary thickness of three‐layered evaporitic sequences (LES). During the late Triassic, the northwestern part of a salt wall was developed due to passive diapirism and its evolution was influenced by halite lithology between the three‐LES. The central and southeastern parts of the salt wall did not progress beyond the pedestal stage due to lack of halite in the deepest evaporitic sequence. During the Triassic–Jurassic transition, far‐field stresses from the Novaya Zemlya fold‐and‐thrust belt reactivated the pre‐salt Carboniferous rift structures. The reactivation led to the development of the Signalhorn Dome, rejuvenated the northwestern part of the salt wall and affected the sedimentation rates in the southeastern broad basin. The salt wall together with the Signalhorn Dome and the Carboniferous pre‐salt structures were again reactivated during post‐Early Cretaceous, in response to regional compressional stresses. During this main tectonic inversion phase, the northwestern and southeastern parts of the salt wall were rejuvenated; however, salt reactivation was minimized towards the interference transfer zone beneath the centre of the salt wall.

Massive dolomites in the Messinian evaporitic sequence (Sicily, Italy): multi-analytical characterization and implications for the dolomitization processes

A combined geochemical, mineralogical, petrographic and geological approach has allowed to constrain the dolomitization process responsible for the formation of massive dolomitic body within the Messinian evaporitic series. We focused our investigations on an example of massive dolomite body, outcropping over the marginal area of the evaporitic basin in the Peloritani Mountains (NE area of Sicily region, Italy). Field observations allowed recognizing several sedimentary structures, such as erosion surfaces, lenses of conglomerates and ripples highlighting a clastic origin. The petrographic study revealed fine-grained and massive structure, scarce porosity, light to pink color and absence of micro-fossils. The chondrite-normalized trace element pattern shows a general enrichment in LREE and depletion in HREE. The isotopic signatures of the studied dolomites (δ18O = – 4.38–1.24‰, V-PDB; δ13C = – 1.48–1.94‰, V-PDB) are comparable with those of type-3 “Calcare di Base” which is the common reference to record the Messinian salinity crisis. The positive co-variant relationship between the δ18O and δ13C values suggests a dolomitization process in the seawater/freshwater mixing zone. The occurrence of the studied dolomites in the evaporitic sequence coupled with the results we gained point towards an origin from “early diagenetic” processes. As a consequence, a model explaining the genetic mechanism for the studied dolomitic rocks can be essentially summarized as: (1) the massive dolostones were originally deposited as evaporitic limestones under rather high-salinity conditions; (2) the dolomitization process occurred in the seawater/freshwater mixing zone that provided Mg-rich fluids over uplifted submarine masses.

Evaporite-bearing orogenic belts produce ligand-rich and diverse metamorphic fluids

Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO4-rich compositions in upper amphibolite facies rocks makes it unavoidable that the regional metamorphic fluids were locally highly saline and oxidised, and that high salinities persisted throughout metamorphism. Electron microprobe analyses and chemical maps of individual scapolite grains show zoning in Cl and S, likely to reflect buffering of the metamorphic fluid by scapolite during progressive metamorphism. The zoning in Cl and S demonstrates that scapolite has the potential to record changes in fluid chemistry during metamorphism. The variation in scapolite composition between samples, in combination with whole rock geochemistry, shows that different layers within this heterogenous rock package generated fluids of different chemistries. Interaction between scapolite-bearing rocks and externally-derived magmatic or metamorphic fluids that are out of equilibrium drives scapolite breakdown, releasing Cl to the fluid. In the Mount Isa region, metamorphic fluid production was enhanced by periods of magmatism, which promoted development of a regionally extensive and unusually saline fluid system that was active at multiple stages over a 250 million-year period. The highly saline and oxidised fluids formed through interaction with scapolite are well suited to transporting a broad range of metals, and may explain the diverse range of syn-orogenic mineral deposits in the Mount Isa Inlier. Metamorphic belts with large volumes of evaporitic material are ideal for generating a broad spectrum of syn-orogenic hydrothermal ore deposit types - including Fe oxide Cu-Au, Fe sulphide Cu-Au, Mo-Re and U-REE, but lacking the Au-only deposits found in typical orogenic belts. Unlike regions hosting traditional orogenic gold deposits, belts containing evaporitic sequences can preserve Cl-rich minerals such as scapolite in the metamorphosed source region, allowing them to remain active as ore forming systems through relatively high-grade metamorphism and multiple stages of tectonism. Periods of supercontinent breakup, such as the Mesoproterozoic, may have resulted in the formation of large, intracontinental basins well suited to the development of widespread evaporitic sequences. This, in combination with overprinting orogenesis and high temperature magmatism, may have provided the ingredients for widespread ore deposit formation at a global scale.

Numerical study of tectonic structure evolution in a multi-layer evaporite sequence

Field studies carried out in the Kłodawa Salt Structure (KSS), Poland, reveal that within a carnallite-halite-kieserite sequence, finger-like structures developed during an early, pre-diapiric stage of the KSS evolution. We used 99 numerical simulations to model finger development and investigate mechanical properties of carnallite, halite and kieserite during deformation. Models used inverse density stratification of a three-layer set with varying 1) layer viscosities, 2) thicknesses and 3) density difference ratios. Finger shapes in the models were categorized into single and multiwavelength structures. Among the single-wavelength shapes, bulb, mushroom, thumb and bloated (i.e. without a regular shape) structures were distinguished. We show two transitions between the single and multiwavelength structures, which correspond to the cases of either the lower or upper layer having viscosity more than an order of magnitude larger than the two other layers. Through comparison of finger shapes developed in the models and in the natural structures, we estimate viscosities of >1017 Pa s for halite, <1017 Pa s for carnallite and <1016 Pa s for kieserite. The estimated low viscosity of kieserite can be attributed to the small grain size of the rock, the presence of fluids during deformation or to different mineral composition of the layer than presently observed.

The characteristics, formation and exploration progress of the potash deposits on the Khorat Plateau, Thailand and Laos, Southeast Asia

ABSTRACT The giant potash deposit on the Khorat Plateau is one of the most promising targets for exploitation of potassium salts. So far, many researches and geologic survey have been conducted on the giant potash deposits. Hence, it is necessary to make an overall review on the potash deposits. The potash deposit on the Khorat Plateau was formed during the Middle to Late Cretaceous, during which seawater was enriched in Ca2+ and depleted in SO42- compared with those of modem seawater. In addition to seawater, continental water and hydrothermal fluids could have affected the evaporite basins. The seawater was probably derived from Tethys ocean, and the brine should have evaporated to some extent before entering into the basin systems based on the evidence of absence of carbonates and unproportionate sulphate compared with chloride salts. The paleo-climate during Middle to Late Cretaceous was characterized as high temperature and extremely arid environment, which is favourable for deposition of potassium-magnesium saline minerals. The major saline minerals are of anhydrite, halite, camallite, sylvite and, tachyhydrite, with trace amounts of borates. The resources of the potash deposit on the Khorat Plateau could be approximately as much as 400×109 t of camallite and 7×109 t of sylvite. The evaporite sequences have been deformed and altered by postdepositinal processes, including tectonic movements and chemical alteration. Salt domes were formed in the postdepositional processes. Based on the analyses of geophysical surveys and drilling projects, high-quality sylvinite ores are commonly found at the flanks of those salt domes due to incongruent dissolution of camallite. The future potential prospecting areas for the high-quality sylvinite ores would be on the edges of the Khorat Plateau.

Chapter 4 Depositional geometry at selected locations around the basin

Abstract This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.

The Cavendish Field, Block 43/19, UK North Sea

Abstract The Cavendish Field is located in UK Continental Shelf Block 43/19a on the northern margin of the Outer Silverpit Basin of the Southern North Sea, 87 miles (140 km) NE of the Lincolnshire coast in a water depth of 62 ft (18.9 m). The Cavendish Field is a gas field in the upper Carboniferous Namurian C (Millstone Grit Formation) and Westphalian A (Caister Coal Formation) strata. It was discovered in 1989 by Britoil-operated well 43/19-1. Production started in 2007 and ceased in 2018. Gas initially in place was 184 bcf and at end of field life 98 bcf had been produced. The field was developed by three wells drilled through the normally unmanned platform into fluvio-deltaic sandstone intervals that had sufficiently good reservoir quality to be effective reservoirs. The majority of the formation within closure comprises mudstones, siltstones and low permeability, non-reservoir-quality feldspathic sandstones. The quality of the reservoir is variable and is controlled by grain size, feldspar content and diagenesis. The field is a structural trap, sealed by a combination of intra-Carboniferous mudstones and a thick sequence of Permian mudstones and evaporites.

Chapter 2 Structural development of the Arabian Intrashelf Basin region

Abstract This chapter summarizes the tectonic events that have affected the region of the Arabian Intrashelf Basin and the development of the intrashelf basin. Precambrian–Infracambrian fault systems provided a structural framework, which was later reactivated during the Late Paleozoic. Further development along the structural trends continued in the Triassic and Early Jurassic with the development of an Early Jurassic tectonically controlled intrashelf basin. The accommodation space was filled with Dhruma Formation carbonates by the early Bathonian, resulting in a broad and fairly flat platform. A crucial factor is how suppressed tectonism was during the Mid- and Late Jurassic. The intrashelf basin developed on the broad, tectonically stable Tethyan passive margin continental shelf 200--300 km distant from the Tethyan outer shelf edge. During the Mid- and Late Jurassic, this tectonic stability provided the foundation for a broad, stable Tethyan continental shelf region at least 1000 × 1200 km in area, far removed from siliciclastic sources, within which the huge Arabian Intrashelf Basin formed and the sequences of carbonate rocks and evaporites that created the Jurassic hydrocarbon system were deposited. Tectonism during the Mid- to Late Jurassic evolution of the Arabian Intrashelf Basin initially included only moderate subsidence. There was subtle uplift along some of the structural trends in the Late Jurassic and uplift and westwards structural tilt along the Tethys oceanic margin. Relatively stable tectonism continued after the Jurassic and was a major factor in the accumulation of the vast reserves of Jurassic sourced oil. Tectonic stability prevented major faulting and structural compartmentalization of the basin features, which provided large areas for hydrocarbon migration as the large and broad anticlines developed into the huge structural traps. The lack of major faulting limited the movement of burial fluids, preserving the early formed porosity and the regionally extensive seals. The present day structures primarily developed from the Late Cretaceous to Miocene as the Tethys Ocean closed.

Hydrothermal alteration associated with magnetite mineralization in the Bafq iron deposits, Iran

Bafq Mining District (BMD) is an iron mineralization province that contains more than 2 Gt of iron ore located in the early Cambrian volcano-plutonic arc known as the Kashmar-Kerman zone. The host rocks of the Fe mineralization are granitic and microgranitic rocks and their extrusive equivalents. Field observations and petrographic studies showed that both sodic and calcic alteration are related to iron mineralization. Other alteration styles, including silicic, chlorite, epidote, and sericite alteration are less important and post-dated the mineralization. Sodic alteration is associated with the formation of chessboard albite, whereas the calcic alteration is featured by a mineral assemblage of actinolite–tremolite- magnetite- apatite. Available evidence suggests that the involved metasomatic fluids could be mainly of evaporitic sources, with a minor contribution from magmatic fluids. We suggest that mineralization formed at the end of the Precambrian, in pull-apart basins in a magmatic back-arc region associated with subduction of the Proto-Tethys oceanic crust under Central Iranian Plate. Iron ore mineralization may be related to the interaction of granite-derived hydrothermal fluids and brine-derived fluids sourced from the latest Precambrian Rizu and Dezu evaporitic sequences.

Synrift evaporite deposition and structural characterization of the onshore Alagoas subbasin

The Sergipe-Alagoas Basin, situated in the north-east Brazilian margin, has a long tradition of oil and gas production and the presence and distribution of evaporites play an important role in petroleum systems in the basin. However, little research has focused on the structural evolution of the older, synrift evaporitic sections of the basin. We have focused explicitly in the detailed subsurface structural characterization of the rift in the Alagoas subbasin and the distribution of the Early Aptian evaporites. To accomplish this objective, we interpreted selected 2D and 3D seismic and well data located in two areas known as the Varela Low (VL) and Fazenda Guindaste Low (FGL). We identified diverse deformation styles in those two basin depocenters. Our interpretation indicates that VL consists of a half-graben with a significant rollover structure, controlled by two listric northeast–southwest border faults. The deformation in the hanging wall is also accommodated by release faults and minor antithetic faults. In this depocenter, we mapped in the seismic and the well data an older evaporitic sequence within the Coqueiro Seco Fm., known as Horizonte Salt. This evaporitic section occurs in the internal part of the VL half graben, where it is limited by release and antithetic faults. Significant salt strata growing toward the antithetic fault is observed. Whereas, the FGL represents a graben elongated along the north-east direction and is controlled by several types of structures. We recognized normal synthetic and antithetic faults, transfer zones, release faults, and rollover anticlines in the seismic throughout this depocenter. We mapped an evaporitic section within the Maceió Fm., known as Paripueira Salt, which consists of disconnected salt bodies, restricted to the hanging walls of synrift faults.

Biomarker records and mineral compositions of the Messinian halite and K\u2013Mg salts from Sicily

The evaporites of the Realmonte salt mine (Sicily, Italy) are important archives recording the most extreme conditions of the Messinian Salinity Crisis (MSC). However, geochemical approach on these evaporitic sequences is scarce and little is known on the response of the biological community to drastically elevating salinity. In the present work, we investigated the depositional environments and the biological community of the shale–anhydrite–halite triplets and the K–Mg salt layer deposited during the peak of the MSC. Both hopanes and steranes are detected in the shale–anhydrite–halite triplets, suggesting the presence of eukaryotes and bacteria throughout their deposition. The K–Mg salt layer is composed of primary halites, diagenetic leonite, and primary and/or secondary kainite, which are interpreted to have precipitated from density-stratified water column with the halite-precipitating brine at the surface and the brine-precipitating K–Mg salts at the bottom. The presence of hopanes and a trace amount of steranes implicates that eukaryotes and bacteria were able to survive in the surface halite-precipitating brine even during the most extreme condition of the MSC.Graphical abstract

Deformation of intrasalt competent layers in different modes of salt tectonics

Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Enterolithic folds in evaporites as microbially induced sedimentary structures: New model of formation and interpretation in the geological record

AbstractEnterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars.

An Ionic Limit to Life in the Deep Subsurface.

The physical and chemical factors that can limit or prevent microbial growth in the deep subsurface are not well defined. Brines from an evaporite sequence were sampled in the Boulby Mine, United Kingdom between 800 and 1300 m depth. Ionic, hydrogen and oxygen isotopic composition were used to identify two brine sources, an aquifer situated in strata overlying the mine, and another ambiguous source distinct from the regional groundwater. The ability of the brines to support microbial replication was tested with culturing experiments using a diversity of inocula. The examined brines were found to be permissive for growth, except one. Testing this brine’s physicochemical properties showed it to have low water activity and to be chaotropic, which we attribute to the high concentration of magnesium and chloride ions. Metagenomic sequencing of the brines that supported growth showed their microbial communities to be similar to each other and comparable to those found in other hypersaline environments. These data show that solutions high in dissolved ions can shape the microbial diversity of the continental deep subsurface biosphere. Furthermore, under certain circumstances, complex brines can establish a hard limit to microbial replication in the deep biosphere, highlighting the potential for subsurface uninhabitable aqueous environments at depths far shallower than a geothermally-defined limit to life.

Continuous-Circulation Technique Drills Narrow-Margin Deepwater Wells

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 192749, “Deepwater High-Pressure/High-Temperature Drilling Through an Ultranarrow Pore-Pressure Fracture Gradient Window: A Case Study,” by Enrico Squintani, Andrea Uslenghi, Susanna Ferrari, and Luca Affede, Eni, prepared for the 2018 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 12–15 November. The paper has not been peer reviewed. Ultradeepwater wells are commonly characterized by a narrow margin between pore and fracture gradients. In these wells, even a small variation in pressure profile may lead to severe operational issues. This paper describes the drilling of high-pressure/high-temperature (HP/HT) deepwater wells through an ultranarrow pore-pressure fracture gradient (PPFG) window by means of technology application and strict procedural control. Introduction An exploratory well with a narrow drilling margin can be realized successfully by use of a customized design concept. Such a concept may not follow standard procedures strictly as ordinarily defined but still can fulfill the guidelines and concepts behind conventional approaches. To apply such a customized concept, a multidisciplinary team was created to design the well through all the possible expected scenarios. An integrated assessment was prepared, evaluating how real-time monitoring could mitigate the operating risk of the exploration activity. The operator’s guidelines used for standard parameters such as kick tolerance and choke margin were reviewed and re-interpreted for this specific application, because the introduction of new technologies could allow different design factors to control the bottomhole pressure (BHP) without affecting the margin of risk. Well A Planning. This was the first exploration well drilled in a Mediterranean block with the use of a dynamically positioned rig with continuous-circulation technology installed. The estimated water depth of the area was greater than 2000 m. The distance of the nearest reference well from Well A was more than 90 km. Because of this long distance, the degree of uncertainty of the predicted PPFG was very high. The well was planned to explore sands targets at a total depth (TD) of approximately 6000 m. From a well-design viewpoint, attention was paid mainly to identify the casing points; otherwise, because the narrow margin between pore and fracture gradients was so tight, the targets could require reaching the TD in 6-in. hole instead of the planned 8½-in. hole. This development could compromise data collection. Because of these factors, it was decided to use the operator’s proprietary device and continuous-circulation system to add an additional safety margin to the geopressure fluctuation. Continuous circulation was planned to be used from the drilling of the intermediate hole size until the well reached TD and also while running the casing/liner. This project saw the first installation of the system on a drilling ship. Uncertainties were expected to be managed primarily with standard operational precautions; in fact, the ability to place the casing shoes inside the right formation was very important and the correct depth had to be determined while drilling the well. The salt was supposed to be drilled with a 17½-in. bottomhole assembly. The casing point of this section was planned at approximately 4000 m measured depth, approximately 100 m above the bottom of the theoretical evaporitic sequence but before entering the possible overpressured zone.

Unique hydrocarbon-generating mechanism of Oligocene evaporites of Qaidam Basin, West China

The principle hydrocarbon source in West Qaidam Basin, China, is the Xiaganchaigou formation, which charges a series of Cenozoic large reservoirs. We identified a series of organic-rich intervals within a sequence of Oligocene-age Xiaganchaigou evaporites in southwest Qaidam. By analyzing 2D and 3D seismic tomography and gamma ray well logs, we defined the distribution and extent of these source rocks. We also characterized their composition with trace element analysis and carbon and oxygen isotope measurements. Our data demonstrated distinctive characteristics of these source rocks with high salinity, abundant organic material, high hydrocarbon generation efficiency, and low maturity, which are associated with relatively rapid and productive hydrocarbon generation. Using high-pressure and high temperature experiments to simulate pyrolysis in samples of the source rocks, we investigated what enabled these young salt-rich source rocks to generate such an abundance of hydrocarbons. Our results highlighted the key role of saline and radioactive minerals in catalyzing accelerated pyrolysis of hydrocarbons during diagenesis and catagenesis. Salt catalysis clearly played an important role in hydrocarbon generation within lower maturity formations and also significantly improved and redefined the source rocks distribution and the extent of probable reserves in the southwest Qaidam.

Hydrocarbon Generation, In-Source Conversion of Oil to Gas and Expulsion: Petroleum System Modeling of the Duwi Formation, Gulf of Suez, Egypt

A multiple forward 1D modeling approach on four wells in the Abu Rudeis-Sidri oil field has been performed in accordance with the plate tectonics and crustal development of the Gulf of Suez, Egypt. The current work intends to simulate the hydrocarbon generating capability, in-source conversion of oil to gas, expulsion and adsorption (retention) of marine sulfur-rich Duwi kerogen. The integration of log responses and organic geochemistry indicate mature organic-rich intervals that have been confirmed by the 1D model. The rifting phases and its associated thermal cooling have a prominent contribution on the thermal maturation, particularly the Mio-Pliocene event. The elevated basal heat flow associated with the lithosphere thinning due to different rifting phases accelerates the thermal maturation of the high sulfur content of Duwi organic-rich interval. The pre-rift sequences are thermal insulators in contrast to the post-rift sequences of evaporites that can cool the underlying strata. Hydrocarbon generation, gas secondary cracking and expulsion are influenced by the post-rift thermal subsidence (the first and second phases). The kerogen has attained a thermal maturity level to generate liquid hydrocarbon since the Messinian (~ 5.8 Ma) and thermogenic gas secondary cracking since Zanclean (~ 3.57 Ma). The hydrocarbon generation (Early Pliocene) is related to the combination of basin burial (accompanied the first phase post-rift thermal subsidence) and the rift renewal through the Pliocene (Messinian Time Event). The gas generation is related to the second phase post-rift thermal subsidence that is accompanied by the deposition of the Post-Zeit Formation. Most of the hydrocarbons attained peak bulk generation during Pliocene (5.8 Ma), which dominated until expulsion commenced (2.52 Ma). The expulsion onset (Late Pliocene) attained subsequent to gas generation and after rift structural trap formation in Late Oligocene–Early Miocene. The expulsion onset (2.52 Ma) related mainly due to a high transformation ratio of kerogen, compaction and partly because of higher initial values of TOC (4.8%) as compared with its present-day values (3.2%). Applying the sensitivity analysis inferred that the source rock properties (HI and TOC) are not assigned as a controlling factor in the maturation process. The eroded thickness has small influence on the maturation process. In contrast, the excellent correlation (Pearson correlation coefficient = 1) supports that heat flow superimposed on the burial-related maturation.

3-D seismic travel-time tomography validation of a detailed subsurface model: a case study of the Záncara river basin (Cuenca, Spain)

Abstract. A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image in the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m×500 m area of the shallow subsurface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and fault or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline geophysical measurements. The analysis of the well log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. This study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow subsurface. In this methodological validation test, travel-time tomography demonstrated to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.

Source and possible leaching process of ore metals in the Uragen sandstone-hosted Zn-Pb deposit, Xinjiang, China: Constraints from lead isotopes and rare earth elements geochemistry

The Uragen world-class Zn-Pb deposit is located in the Ulugqat basin in Xinjiang, China. The deposit is characterized by low grade (Zn + Pb ∼3 wt%) and large tonnage of ∼4.84 Mt metals in the southern ore zone and potentially 10 Mt metals for the whole deposit. The deposit is mainly hosted in sandstone and conglomerate of the fifth member of the Lower Cretaceous Kezilesu Group. To provide better constraints on the source of ore metals, systematic analyses of the Pb isotopes and REE were conducted on sulfides and possible source terranes. The sulfides have homogeneous Pb isotopic ratios for 206Pb/204Pb of 18.528–18.690, for 207Pb/204Pb of 15.545–15.743, and for 208Pb/204Pb of 38.429–39.082. These ratios are different from those of the Upper Silurian, Upper Devonian Tangaitaer and Paleocene Aertashi formations, the fourth member of the Lower Cretaceous Kezilesu Group, and the Permian granite. Despite partly overlapping with the values of sulfides, the ratios of the Proterozoic Aksu metamorphic rocks are different in extent and variability. However, the clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group have almost the same Pb isotopic compositions as the sulfides if the preferential leaching of radiogenic Pb is taken into account. The sulfides in the disseminated ores show LREE-enriched patterns with high LREE/HREE ratios (3.50–12.06), variable changes of Ce anomalies (0.67–1.22) and Eu anomalies (0.52–1.33) that are also similar to those of the clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group, but are significantly different from those of the basement rocks of the Proterozoic Aksu Group and the evaporite sequences of the Paleocene Aertashi Formation in the Ce and Eu anomalies. Petrographic observations indicate that the clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group have undergone sedimentation, diagenetic reddening and bleaching. Reddening is related to the in-situ oxidation of mafic minerals such as biotite to produce ferric iron oxide and hydroxide (IOH) that accompanied the liberation of ore metals from the silicate phase. During subsequent crystallization of IOH to hematite, preferential adsorption onto hematite could have been the main mechanism for separating Zn-Pb from Cu. Bleaching resulted from the dissolution of IOH during the oil-gas infilling stage, and, based on mass-balance calculations, substantial amounts of leachable Fe-Zn-Pb were extracted from the red clastic rocks by the ore-forming fluid. Therefore, we propose that the red clastic rocks of the fifth member of the Lower Cretaceous Kezilesu Group may be the predominant source of ore metals in the Uragen deposit, and that the leaching may be possibly related to the bleaching process.