Carbonate Evaporite Research Articles

The Main Controlling Factors of the Cambrian Ultra-Deep Dolomite Reservoir in the Tarim Basin

The genesis of deep-to-ultra-deep dolomite reservoirs in the Tarim Basin is crucial for exploration and development. The Cambrian subsalt dolomite reservoirs in the Tarim Basin are widely distributed, marking significant prospects for ultra-deep reservoir exploration. Based on big data methodologies, this study collects and analyzes porosity and permeability data of carbonate reservoirs in the western Tarim Basin, specifically targeting the Cambrian deep-oil and gas-reservoir research. Through an examination of the sedimentary evolution and distribution of carbonate–evaporite sequences, and considering sedimentary facies, stratigraphic sediment thickness, fault zone distribution, and source-reservoir assemblages as primary reference factors, the study explores the macro-distribution patterns of porosity and permeability, categorizing three favorable reservoir zones. The controlling factors for the development of Cambrian carbonate reservoirs on the western part of the Tarim Basin are analyzed from the perspectives of sedimentary and diagenetic periods. Factors such as tectonic activity, depositional environment, microbial activity, and pressure dissolution are analyzed to understand the main causes of differences in porosity and permeability distribution. Comprehensive analysis reveals that the porosity and permeability of the Series2 carbonate reservoirs are notably high, with extensive distribution areas, particularly in the Bachu–Tazhong and Keping regions. The geological pattern of “Three Paleo-uplifts and Two Depressions” facilitated the formation of inner-ramp and intra-platform shoals, creating conducive conditions for the emergence of high-porosity reservoirs. The characteristics of reservoir development are predominantly influenced by diagenetic and tectonic activities. The Miaolingian is chiefly affected by diagenesis, featuring high permeability but lower porosity and smaller distribution range; dolomitization, dissolution, and filling processes under a dry and hot paleoclimate significantly contribute to the formation and preservation of reservoir spaces. In the Furongian, the Keping and Bachu areas display elevated porosity and permeability levels, along with substantial sedimentary thickness. The conservation and development of porosity within thick dolomite sequences are mainly governed by high-energy-particulate shallow-shoal sedimentary facies and various dissolution actions during diagenesis, potentially indicating larger reserves.

Hydrocarbon accumulation in deep ancient carbonate–evaporite assemblages

The Ediacaran–Ordovician strata within three major marine basins (Tarim, Sichuan, and Ordos) in China are analyzed. Based on previous studies focusing on the characteristics of the Neoproterozoic–Cambrian strata within the three major basins (East Siberian, Oman, and Officer in Australia) overseas, the carbonate–evaporite assemblages in the target interval are divided into three types: intercalated carbonate and gypsum salt, interbedded carbonate and gypsum salt, and coexisted carbonate, gypsum salt and clastic rock. Moreover, the concept and definition of the carbonate–evaporite assemblage are clarified. The results indicate that the oil and gas in the carbonate–evaporite assemblage are originated from two types of source rocks: shale and argillaceous carbonate, and confirmed the capability of gypsum salt in the saline environment to drive the source rock hydrocarbon generation. The dolomite reservoirs are classified in two types: gypseous dolomite flat, and grain shoal & microbial mound. This study clarifies that the penecontemporaneous or epigenic leaching of atmospheric fresh water mainly controlled the large-scale development of reservoirs. Afterwards, burial dissolution transformed and reworked the reservoirs. The hydrocarbon accumulation in carbonate-evaporite assemblage can be categorized into eight sub-models under three models (sub-evaporite hydrocarbon accumulation, supra-evaporite hydrocarbon accumulation, and inter-evaporite hydrocarbon accumulation). As a result, the Cambrian strata in the Tazhong Uplift North Slope, Maigaiti Slope and Mazatag Front Uplift Zone of the Tarim Basin, the Cambrian strata in the eastern-southern area of the Sichuan Basin, and the inter-evaporite Ma-4 Member of Ordovician in the Ordos Basin, China, are defined as favorable targets for future exploration.

Stress precursors of the Messinian salinity crisis as recorded by calcareous plankton and geochemistry in the Eastern Mediterranean: The Upper Metochia section of the Gavdos Island (Greece)

The road to the Messinian salinity crisis was paved with increasing environmental stress in the Mediterranean marine realm. The nature and origin of the crisis of the marine biota is still matter of debate because the crisis itself hinders univocal paleoenvironmental reconstructions by selecting the most resilient taxa, which are generally adapted to a wide range of stresses. The commonly accepted hypothesis, recently reiterated, is that increasing salinity was the main factor eliminating the marine biota from the geological record and eventually leading to the deposition of evaporitic carbonates in the Eastern Mediterranean just before the onset of the crisis dated at 5.97 Ma. We suggest that stratification and increasing restriction (continental water influence), leading to strong variability of surface water conditions, better explain the 87Sr/86Sr excursion below the global ocean value and the pattern of calcareous plankton events in the uppermost portion of the Metochia section (Gavdos Island, Greece), from 6.24 to 5.95 Ma. The early reduction of foraminifera at 6.07 Ma, typical of relatively deep successions lacking primary gypsum during the first phase of the crisis, precedes a sequence of calcareous nannofossil events such as the influx of Braarudosphaera bigelowi at 6.168 Ma, the acme of Reticulofenestra perplexa at 6.058–6.018 Ma, and the peaks of Sphenolitus spp. and Helicosphaera carteri at 5.99 and 5.973 Ma, respectively. The peak of such opportunistic calcareous nannofossil and the foraminifer disappearance, commonly observed in both deep and shelf successions, closely approximate the onset of the Messinian salinity crisis, regardless the occurrence of the Primary Lower Gypsum unit, and is interpreted as the final restriction step, which led to a 600 ky environmental perturbation culminating with the deposition of the Mediterranean salt giant.

Heterogeneity evaluation of pore types based on dipole shear sonic imager logs by means of statistical parameters, the central Persian Gulf

AbstractHeterogeneity analysis in carbonate reservoirs is of great importance for the interpretation of their characteristics. This study focuses on the evaluation of pore‐type heterogeneity by means of the sonic wave velocities in the carbonate–evaporite series of Permian–Triassic sequences in the Persian Gulf. Thin section petrography and laboratory measured the porosity and permeability of 1576 samples, as well as routine wire‐line logs, were employed. To manage heterogeneity, six electrofacies (EF) were determined based on an integrated analysis of compressional, shear and Stoneley waves as well as three routine logs (gamma, neutron and density). The frequency of pore types in each electrofacies was calculated. Then, the heterogeneity of pore‐type distribution in each electrofacies was measured by standard error and coefficient of variation. Sonic velocity has been compared with the geological properties of electrofacies. Results indicated that velocity is affected by factors other than the amount of porosity, facies or lithology. The standard error values in all electrofacies were close to zero. The coefficient of variation values ranges from 1.08 to 4.60. The lowest amount of coefficient of variation was observed in EF6 in the Khuff (K) 4 zone for moldic porosity. This value for intercrystalline porosity distribution in EF1 in the K1 zone was low, too. These results indicate the lowest standard deviation and the highest homogeneity in the distribution of intercrystalline and moldic porosities in K1 and K4. Also, comparing the mean values of sonic logs in each electrofacies with the total mean value in the whole sequence indicated that there is a relationship between sonic velocity logs and the type of porosity. High values of sonic velocity correspond to intercrystalline porosity in K1 and low values of velocity correspond to moldic porosity in K4. Results show that integrated pore‐type studies should be carried out to understand the sonic velocities in carbonate reservoir rocks.

Stratigraphic architecture and depositional–diagenetic evolution of Oligocene–Miocene carbonate–evaporite platform in the southern margin of the Neo-Tethys Ocean, Lurestan zone of Zagros, Iran

The Oligocene–Miocene (OM) depositional sequences (Asmari Formation) are marked by remarkable changes in thickness, lithology, depositional facies characteristics, and diagenesis history throughout the Zagros area, as the southern margin of the Neo-Tethys Ocean. An integrated sedimentological, geochemical and sequence stratigraphic analysis of these sequences is presented in two surface and subsurface sections located in the Lurestan zone. Macroscopic (field-scale) descriptions and microscopic petrographic studies represent 12 microfacies deposited on a ramp-like carbonate platform. Complex diagenesis history of the Asmari Formation encompasses hypersaline, marine, and meteoric, to shallow- and deep burial realms. Cathodoluminescence microscopy and stable isotopic (carbon and oxygen) compositions of whole-rock, micrite, and carbonate cements of different generations are used for the precise interpretation of depositional–diagenetic processes. Regional stratigraphic architecture of these strata is defined by the correlation of studied sections throughout the Zagros area, in a sequence stratigraphic framework. Development of restricted lagoons or salterns with deposition of evaporites, and hypersaline diagenetic overprints point to a completely different depositional-diagenetic evolution. The Oligocene–Miocene transition (OMT) event is correlated with global records. A 3 to 4 ‰ increase in δ18O is considered as the signature of a cooling event at the OM boundary. Variable geochemical features of the OM sequences in the Zagros area can be due to the tectonic activities and changes of paleoenvironmental conditions. Results of this study provide new insights about the Oligocene–Miocene depositional systems in the Zagros area and, in a larger scale, on the southern margin of the Neo-Tethys Ocean.

Facies variability and depositional settings of Laguna Salada de Chiprana, an Iberian hypersaline lake

AbstractHypersaline lakes are unique depositional environments that are highly sensitive to climatic changes and anthropogenic pressures. Strong physicochemical gradients and the interplay of physical, chemical and biological depositional processes coupled to variable preservation potential associated with these environments generate highly complex sedimentary sequences. This study aimed to investigate spatio‐temporal variations throughout the last three centuries in the sedimentary sequence of Laguna Salada de Chiprana, a small, permanent hypersaline lake situated in Northeast Spain. Short sediment cores reveal a complex sedimentary sequence influenced by detrital, chemically‐induced and biogenically‐influenced depositional processes. Nine facies associated with six sedimentological units represent six periods of environmental change beginning from approximately pre‐1800 ad to the present day. They include: (i) a shallow, permanent saline lake that experienced brief periods of anoxia and drawdown between pre‐1800 ad and ca 1850 ad. This was succeeded by: (ii) increased lake levels and detrital input to the lake from ca 1850 to 1900 ad; and (iii) a fall in lake levels and the establishment of microbial mats within bottom waters fluctuating between oxic and anoxic conditions from ca 1900 to 1950 ad. (iv) From ca 1950/1960 ad, a minor increase in lake levels and colonization of the littoral and sub‐littoral zones by macrophytic vegetation occurred; followed by (v) falling lake levels and increasing salinity between 1960 and 1970 ad leading to widespread encrustation of macrophytes in the littoral zones by carbonate–evaporite precipitates. Finally (vi), from ca 1970 ad to the present day, falling lake levels, elevated salinity and widespread colonization of the lake by microbial mats characterize the sedimentary environment. Climate reconstructions and documentary evidence indicate that depositional evolution was mostly controlled by changes in farming practices and irrigation policies. Understanding Laguna Salada de Chiprana’s depositional evolution contributes to future definitions of high‐resolution palaeoenvironmental models, not only within the Iberian Peninsula but also in other locations where saline–hypersaline lakes act as a key source of palaeoenvironmental information.

Reservoir facies classification based on random forest and geostatistics methods in an offshore oilfield

Machine learning methods are increasingly employed in various seismic and petrophysical methods for parameter estimation, interpretation, prediction, and classification. Reservoir facies classification assists the interpretation of seismic data as an important step in petroleum exploration and production monitoring. In this study, we estimate a reservoir facies model by integrating random forest (RF) algorithms and geostatistics modeling. The Surmeh Formation with the Jurassic age is known as one of the most important hydrocarbon reservoirs in the Middle East. The upper part of the Surmeh Formation is equivalent to the Arab Formation, which includes sequences of evaporitic carbonate facies in the study area. Well log data including DT, GR, RHOB, and PHI are used in the RF method for reservoir facies classification. Cross-validation verifies the high accuracy of our classification, with an average accuracy of 95%. The predicted reservoir facies consistently describe the carbonate and evaporitic facies with the geological information of this formation. The decision tree diagrams of the RF algorithm give valuable information on decision limitations and how to select features for efficient computation. We use the classification results for facies modeling. The comparison between facies models and drilling core data shows that the APE value of the sequential indicator simulation model is less than that of the indicator kriging model.

Diagenetic evolution and its impact on reservoir porosity in Lower Triassic Jialingjiang Formation in eastern Sichuan Basin, SW China: Evidence from petrography, trace elements, and Isotopic Studies

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13CVPDB values of −3.4–+7.3 ‰ and δ18OVPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm)N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34SV-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13CVPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18OVPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34SV-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity.

Genesis and pore evolution of dolomite reservoir in the Majiagou Formation, Ordos Basin, China

In gypsum–carbonate rock assemblages, multistage and complex fluids control the formation of dolomite reservoirs that are a focus of hydrocarbon exploration. It is difficult to determine the types of dolomite reservoirs and their formation mechanisms due to the diverse rock assemblages and multiple stages of diagenesis. In this study, we investigated the petrology, reservoir physical properties, and geochemistry of the 6th sub-member of member five of the Majiagou Formation (i.e. Ma56) in the Ordos Basin, China. These data were used to determine the nature and types of gypsum–carbonate rocks, and constrain their reservoir characteristics and diagenetic history, and fluid-related mechanisms that led to dolomite reservoir development and preservation. The Ma56 was deposited on a restricted evaporatic platform in the North China Craton, and contains three main types of dolomite reservoirs with variable types of reservoir space. Dolomite reservoir formation was closely related to penecontemporaneous dolomitization, karstification, and differential cementation. Early large-scale dolomitization produced dolomitized carbonate sediments that were resistant to compaction and dissolution, which was conducive to the preservation of primary and secondary pores. The intermittent exposure and dissolution of mound–shoal facies sediments, due to high-frequency sea-level fluctuations, was the dominant mechanism for formation of secondary dissolved pores and high-quality reservoirs. During burial, differential cementation occurred due to interaction between fluids and pore size, which determined the extent of reservoir preservation. In general, the studied dolomite reservoirs have undergone multistage diagenesis and alteration, which led to complex and multistage development of the reservoir porosity. However, the reservoir lithology and pore space developed mostly in the depositional to penecontemporaneous stages. Our results provide new insights into the origins of deeply buried dolomite reservoirs in carbonate–evaporite successions.

Saline fluids drive Cu mineralization in Precambrian metasediments: Evidence from the Trans-North China Orogen

Epigenetic hydrothermal Cu-(Co) deposits are widely distributed in the Zhongtiao Mountains within the Paleoproterozoic Trans-North China Orogen in the North China Craton. Here we investigate the Henglingguan Cu deposit hosted in metasediments to gain insights on the fluid evolution characteristics associated with the ore formation. This Cu deposit is characterized by early disseminated mineralization and late quartz-vein type mineralization. The negative sulfur isotope values (about −10‰) of the vein sulfides suggests a sedimentary rock source for sulfur associated with the hydrothermal Cu mineralization. The coexistence of CO2-rich carbonic inclusions and halite-bearing inclusions within a single fluid inclusion assemblage suggests fluid immiscibility in the H2O-NaCl-CO2 system. Based on the fluid phase analysis, our data show that the parent ore fluid was characterized by X(NaCl) of 10 to 20 mol.% (salinity of around 23 to 41 wt% NaCl) and X(CO2) of 10 to 15 mol.% before fluid immiscibility. The estimated temperature–pressure conditions of 320 to 340 °C and 3.5 to 4.0 kbar based on the carbonic fluid inclusions is suggested to be close to the conditions of fluid immiscibility. In-situ composition analyses of fluid inclusions by LA-ICP-MS show that the ore fluids contained Na, Ca, Fe and Mn, and evolved towards Sr-Ba enrichment. The data are consistent with a metamorphic brine derived from the sedimentary rocks. The brine-rich metamorphic fluids were likely derived from the dehydration and decarbonation of evaporitic carbonate rocks developed on the basin margin or a passive continental margin. The decompression during post-collisional exhumation triggered fluid immiscibility resulting in the quartz vein-type Cu mineralization.

Vendian Hydrogeochemistry of the Siberian Platform

Abstract —The Vendian deposits of the Siberian Platform are represented by four regional horizons (from bottom to top): Nepa, Vilyuchan, Tira, and Danilovka. The TDS of brines in the Nepa and Vilyuchan horizons varies from 170.3 to 470.1 g/L, and the TDS in the overlying Tira and Danilovka horizons, from 84.6 to 583.1 g/L. Brines of Cl Na, Cl Na–Ca, Cl Ca–Na, Cl Ca, and Cl Ca–Mg types have been identified. The pH values of the brines vary from 3.0 to 7.0; acidic and weakly acidic brines (pH = 3.0–5.5) prevail. The redox potential Eh varies from –440 mV (highly reducing conditions) to +130 mV (oxidizing conditions), averaging –176 mV. Areal regional hydrogeochemical zoning is controlled by the outer (on the periphery of the Siberian Platform, where infiltration of meteoric waters took place) and inner feeding areas. The outer feeding areas bear infiltrogenic brines of different ages (group I), mostly of Cl Na composition. They are characterized by TDS ≤ 280 g/L, rNa/rCl ≤ 1.55, Cl/Br ≤ 2512, and average Ca/Cl = 0.3. The average values of the integrated brine metamorphization index S (according to S.L. Shvartsev) range from 50 to 200. In the intraplatform feeding areas, there are several phases of brines that underwent gravitational sinking into the Vendian horizons in the periods when Cambrian salt-bearing basins existed on the platform. These brines were metamorphosed during filtration along the fracture and fault zones. The intrusion of traps in the late Permian–early Triassic was accompanied by hydrothermal activity. The waters of the enclosing Cambrian dolomite horizons were heated to 800–1000 °C; their subsequent interaction with carbonate rocks and evaporites (in particular, chlorides) resulted in extrastrong saturated brines. These brines were also subjected to gravitational sinking into the Vendian reservoirs, which was the main cause of the inverse vertical hydrogeochemical zoning in the hydrogeologic subsalt formation. The brine metamorphism processes and temperature changes in the rock horizons during the intrusion of traps and their subsequent cooling led to a significant alteration of the enclosing rocks under interaction with brines and to salinization of the reservoirs, an increase in the total salinity of the brines, and catagenetic changes in their composition. The brines closest to the maximum saturation with halite were found in reservoir zones with the highest salinization and near faults and boundaries of intrusive bodies. During the pre-Vilyuchan, pre-Nepa, pre-Tira, and pre-Danilovka sedimentation breaks, infiltrogenic waters penetrated into the Vendian sediments. Geochemically, the sedimentogenic, infiltrogenic, and metamorphic strong brines (group II) have a predominantly Cl Ca or Cl Ca–Mg composition with TDS > 350 g/L. They are characterized by low rNa/rCl ratios (on average, 0.3), Cl/Br ≤ 100, high Ca/Cl values (on average, 0.4), and S > 250.

Evaporitic carbonates in the pre salt of Santos Basin: Genesis and tectonic implications – A reply

Pietzsch (2021) commented about the clumped isotope and strontium isotopic ratios published in Farias et al. (2019), suggesting in their interpretation that clumped isotopes of the Barra Velha Formation may require consideration of non-equilibrium isotopic effects or isotopic reordering, and that 87Sr/86Sr data do not support leaching predominantly basalts or originating from interaction with the mantle as in the geochemical model proposed in our paper.In our reply to Pietzsch's comment, we emphasize that we agree with other possibilities of interpretation based on the clumped isotope data, while we clarify our geochemical model and correct an erroneous interpretation included in Pietzsch (2021), who suggested the influence of mantle serpentinization fluids in the model. Lastly, we propose here an alternative interpretation of radiogenic 87Sr/86Sr data in pre-salt carbonates, not previously considered by Pietzsch (2021), indicating the increasing strontium isotopic ratio in fluids as a result of the infiltration of marine waters in continental aquifers.

Paleozoic sequence stratigraphy, depositional systems, and hydrocarbon habitats across the Arabian plate

The Paleozoic sedimentary section of the Arabian plate recorded the effects of numerous fluctuations in sea level and gentle epeirogenic movements. Eustatic changes in sea level over the Arabian plate resulted in transgressions and regressions and created many local and regional unconformity surfaces, identified by missing section and nondeposition. The distant deformation of the Caledonian tectonic orogeny and mountain building of the North Atlantic region, and the later Hercynian tectonic event, are partially responsible for eustatic sea-level changes. Additionally, detailed evidence of Paleozoic glacial erosion and glacial eustasy in the regressive sedimentary section over uplifted paleohighs comes from deep wells penetrating local exotic tectonic blocks. The resulting surfaces mark the sea-level falls associated with the glacial events in the Ordovician, Late Devonian, and Carboniferous–early Permian sediments. The depositional settings of the Cambrian–upper Permian rocks include deltaic and glacial settings and are expressed as alluvial–fluvial fans that grade into braided plains and shallow-marine carbonates flanked by distal shales. Late Permian sediment fill accompanied the development of widespread accommodation, resulting from a major sea-level rise, coinciding with a major period of carbonate–evaporite deposition, in which clastic sediments were only a minor factor. The regional sedimentary facies have good reservoir quality and are juxtaposed to regional source rocks and seals associated with both structural and minor stratigraphic traps. Production occurs in Precambrian through Permian strata in which mature clastic and shallow-water carbonate reservoirs occur, commonly sealed by shales, and sourced by Silurian hot shales or older source rocks in the section.

Comment on “Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications”

Based on petrography, mineralogy, facies analysis and geochemical data Farias et al. (2019) suggested that syn-depositional hydrothermal fluids generated in part by mantle serpentinisation and seepage of brines through the Walvis Ridge–Rio Grande Rise sourced the South Atlantic basins, which resulted in the deposition of the Barra Velha carbonates in the Santos Basin, focus of their study. However, they seek support for this hypothesis in clumped isotope data, which does not unequivocally provide evidence for this interpretation. Furthermore, they presented significantly radiogenic 87Sr/86Sr data, which in turn do not give support for the inference of significant mantle-derived fluid sources. Therefore, their hypotheses should be carefully reconsidered.

Sedimentological and Lithostratigraphic Study of the Senonian Series in Timstiguite Region, Central High Atals, Morocco

In this paper, we present the sedimentological, stratigraphic and paleogeographic aspect of the Upper Cretaceous red formations (Senonian series) of Timstiguite region (Central High Atlas of Morocco). The formations were measured bed by bed and sampled in a very detailed way. An exoscopic analysis of the quartz grains was carried out by a systematic prospecting of the grains relief and theirs surface characteristic, this led us to characterize their origin and consequently to determine the different paleoenvironnements. The clay sediments of these formations seem to be a favorable prospect in point of view: evolution of the clay and non-clay sediments in this central basin of the High Atlas will depend mainly on the conjugation of tectonic, eustatic, climatic and other influences. The senonian thickness is more important than other sub basins (Albban and Imider) where it shows a very reduced thickness. The Senonian of Timstiguite corresponds to a succession of detrital and evaporitic carbonate facies, which confirms a coastal depositional environment with passages of some periods of marine emersions.

Early silicification of the Cyrenaican chert, Libya: The importance of moganite as a transitional silicon dioxide phase

AbstractThe Messinian lagoonal carbonate–evaporite sequence of Cyrenaica, north‐east Libya, hosts: (i) opaline nodules in gypsiferous microbial‐rich mudstone; (ii) nodules replacing poikilotopic gypsum cementing bioclastic carbonates; and (iii) bedded porcelanite with large lenticular gypsum pseudomorphs intercalated with recrystallized microbial mats. Optical microscopy, scanning electron microscopy, backscattered electron images – secondary electron images, X‐ray fluorescence, X‐ray diffraction and cathodoluminescence techniques were employed to elucidate on the early stages of chert diagenesis and demonstrate the role of moganite in the formation of varieties of chalcedony during silicification. The opaline nodules composed of opal‐A, exhibit shrinkage cracks lined by isopachous fibrous quartzine underlain by a thin birefringent zone. This suggests that quartzine may develop by crystallization and conversion of opal‐A through a transitional moganite. The quartzine is followed by amorphous silica gel that converts into length‐fast chalcedony. By contrast, lutecite evolved by silica replacement of gypsum through a transitional moganite phase, which forms an amorphous zone in apparent optical continuity with the fibrous lutecite; as moganite advances by pseudo‐crystalline terminations controlled by the gypsum cleavage. In gypsified oyster shells, intercrystalline organic matter enveloping pseudomorphed shell microstructures enabled moganite nucleation that merges syntaxially around the gypsum crystal rim. The associated lutecite occurs as radial‐fibrous beekite aggregates that are consistent with early shell silicification. The bedded porcelanite consists of opal‐CT groundmass undergoing recrystallization and enclosing micro‐cavities lined by quartzine. The associated pseudomorphs display phases of gypsum replacement by moganite and lutecite at the outer margin, followed by dissolution and cavity filling by quartz varieties in response to fluid flushing. In all cases, organic matter and microbes played a vital role in the silicification process.

Oldest preserved sodium carbonate evaporite: Late Paleozoic Fengcheng Formation, Junggar Basin, NW China

Abstract Sodium carbonate evaporites can provide important paleoclimatic constraints and economic resources but are relatively rare; most known examples are limited to the Cenozoic. The first known late Paleozoic occurrence was recently reported from the Fengcheng Formation in the northwestern Junggar Basin, NW China. However, its mineralogy, age, and geologic setting have been sparsely documented. To help establish a broader understanding of the formation of these important deposits, this paper presents a comprehensive new data set that includes drill core, well logs, petrographic analyses, X-ray diffraction data, organic geochemistry (biomarkers), and U-Pb zircon age data. Evaporite deposition is estimated to have occurred between ca. 305 Ma and 296 Ma in a series of extensional grabens that hosted a stratified, hypersaline lake (or lakes). The lake deposits are closely associated with coeval volcanic rocks, suggesting that subsurface interaction of these with CO2-enriched hydrothermal fluids may have contributed to the excess alkalinity required to precipitate Na-carbonate. Trona (NaHCO3 · Na2CO3 · 2H2O) in the Fengcheng Formation was likely formed syndepositionally and then partly replaced by wegscheiderite (Na2CO3 · 3NaHCO3). Other evaporite minerals most likely formed during diagenesis. The Fengcheng Formation also contains nahcolite (NaHCO3), which requires pCO2 >680 ppm to form, although no textural evidence was found to demonstrate a primary origin. The presence of extensive lacustrine evaporites nonetheless reflects a relatively warm and arid climate at ∼40–50°N that apparently coincided with the late Paleozoic icehouse.

Middle Triassic tectono‐sedimentary development of Sichuan Basin: Insights into the cratonic differentiation

The Middle Triassic succession of the Sichuan Basin contains key information about the Early Indosinian Movement, during which it has changed from extensional to compressional. In order to clarify the relationship between the evolution of the Sichuan Basin during the Indosinian Movement, the Middle Triassic tectonostratigraphic environment should be determined. Detailed stratigraphy and facies analysis integrated with structural and basin analysis, allowed a full reconstruction of the tectonic and depositional environments for the Middle Triassic strata of the Sichuan Basin. Six facies associations from two rock systems of: (a) carbonate–evaporite system and (b) a mixed carbonate–siliciclastic system, were characterized. Four depositional stages highlighted by the lithostratigraphic members of the Leikoupo (Lei) and Badong (Ba) formations were divided: (a) stable subsidence and deposition of the Lei‐1/Ba‐1 Member, leading to an embryonic ‘uplifts and sags’ configuration; (b) inherited deposition of the Lei‐2/Ba‐2 Member, with the platform margin belt extended; (c) rapid deposition of the Lei‐3/Ba‐3 Member, characterized by an eastward shift of the depositional centre and formation of the central anhydrite‐rich lagoon; and (d) intensification of cratonic tectono‐sedimentary differentiation during the Lei‐4/Ba‐4 deposition, that resulted in subaerial exposure along the Luzhou–Kaijiang uplift zone and a hydrodynamically restricted depression along the western platform. The Middle Triassic tectonic framework of the Sichuan Basin was dominated by compression in the east and extension in the west. The western margin was a passive continental margin, inheriting an early rifting event, while the eastern area within the craton showed an ‘uplift‐sag’ pattern, which is a response to intracontinental tectonic movement of the Xuefengshan uplift belt. The basin evolved from incipient low‐relief uplifts and sags within the intracratonic depression to subsequent isolated foreland and restricted depression during the Middle Triassic.

An integrated approach to distribute carbonate reservoir properties, using lithofacies and seismic attributes: a case study from SW of Iran

Complexity and heterogeneity of carbonate deposits make the understanding of the reservoir quality challenging. This study is a contribution to the understanding of the distribution of carbonate–evaporite reservoir property of the Permo–Triassic Kangan–Dalan succession in one of the giant gas fields in the southwest of Iran. To capture and describe the complexity and heterogeneity of the carbonate reservoir, we defined different rock properties approach to the management of quantitative data on carbonate reservoir which is based on well and seismic data with an integrated sedimentological investigation, petrophysical evaluation, and seismic analysis to determine the new reservoir lithofacies classification. A lithofacies-based reservoir characterization study was conducted carbonate reservoir to build a numerical 3-dimensional (3D) geological model including reservoir property prediction for reservoir flow simulation. To do this, full suite of logs from six wells, core from four wells in the studied interval and 3D seismic volume across the studied interval have been involved and investigated. Five lithofacies were identified from detailed analysis, and the relationship between lithofacies and reservoir property was considered and geostatistically propagated in the studied area. This study shows that an integrated study using seismic data and well logs can be used to basically divide the reservoir into relatively homogeneous zones based on core and petrophysical analysis, and then evaluate each zone to distinguish high-quality zones. This approach provides useful insight to Permo–Triassic carbonate reservoir and other similar reservoirs and can also be effectively used in areas which lack well or outcrop data.

The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida, Kenya Rift Valley

AbstractSaline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.