Lime Mudstone Research Articles

Reservoir characterization and hydrocarbon accumulation in late Cenozoic lacustrine mixed carbonate-siliciclastic fine-grained deposits of the northwestern Qaidam basin, NW China

Abstract Late Cenozoic lacustrine mixed carbonate-siliciclastic sequences in the northwestern Qaidam basin (NW China) are mainly composed of high porosity and low permeability fine-grained rocks but have big potentials of oil-gas commercial production. However, how these fine-grained rocks generate high-quality reservoirs and accumulate hydrocarbons, remains poorly known. This study focuses on shallow-buried Shang Youshashan Formation (Middle–Late Miocene, N22) and Shizigou Formation (Late Miocene–Pliocene, N23) reservoirs and hydrocarbons in Xiaoliangshan area of the northwestern Qaidam basin. We conduct mineralogy, petrography and porosity and permeability analysis for reservoir rocks and biomarker analysis for hydrocarbons and potential source rocks. The results indicate that the N22 high-quality reservoirs are dominated by fractured marlstones and lime mudstones and constitute structural traps, whereas the N23 hydrocarbon reservoirs are mainly composed of siliciclastic-dominantly rocks and form stratigraphic-lithologic traps. Similar biomarker ratios between crude oil samples and Paleogene organic matter-rich rocks reveal that hydrocarbons in the shallow system were likely derived from deep-buried source rocks, rather than late Cenozoic lacustrine fine-grained rocks themselves. In this case, large-scale vertical fractures created by kink band structures in the northwestern Qaidam basin are supposed to facilitate hydrocarbon migration and accumulation. Preliminary production data and exploration well oil-gas test results demonstrate highest yields and thus large hydrocarbon potentials in the N22 marlstone and lime mudstone reservoirs. Collectively, we suggest that relatively high-permeability, fractured and carbonate-rich reservoirs on structural highs are favorable targets for future oil-gas exploration in the northwestern Qaidam basin. This petroleum system provides a typical and significant example for hydrocarbon exploration and development in mixed carbonate-siliciclastic fine-grained rocks of nonmarine petroliferous basins.

Western Tethyan Epeiric Ramp Setting in the Early Triassic: An Example from the Central Dinarides (Croatia)

In the central part of the External Dinarides in Plavno, Croatia, near Knin, a remarkably thick (927.5 m) Early Triassic depositional sequence was investigated. The Plavno sequence starts in the EarlyGriesbachian and ends with a continuous transition into the Anisian strata. A complete 13C isotope curve has been achieved and combined with conodonts, bivalves and ammonoids to establish and correlate stage and substage boundaries. The δ13C curve is consistent with former studies. It displays a general increase from the Griesbachian to a prominent maximum beyond the +8‰ amplitude around the Dienerian-Smithian boundary (DSB), followed by a steep and continuous decline to low, negative values in the Smithian. Around the Smithian-Spathian boundary (SSB) a steep rise to a second maximum occurred. It is followed by a saw-tooth shaped decline in the Spathian and a similar increase to a peak at the Spathian-Anisian boundary (SAB). Sedimentologically, the Plavno sequence is interpreted as having been deposited on an extensive epeiric ramp under long-term transgressive conditions, sharing depositional characteristics of both the epeiric platform and the carbonate ramp. The entire Plavno sequence was deposited above the storm-wave base and was storm influenced. Three informal members are differentiated: 1) the dolostone member (Early Griesbachian); 2) the siliciclastic member (red-coloured shale, siltstone, sandstone with oolitic/bioclastic grainstone intercalations), which can be further divided into lower, middle and upper intervals (Late Griesbachian, Dienerian and Smithian); and 3) the mudstone member (grey lime mudstones, marls and calcisiltites with common ammonoids and gastropods-Spathian). The Plavno sequence is compared with other western Tethyan sections. Observed differences stem from local controls on deposition in the overall shallow marine environment.

Preglacial palaeoenvironmental evolution of the Ediacaran Loma Negra Formation, far southwestern Gondwana, Argentina

The Ediacaran Loma Negra Formation may be deposited in a platform environment related to the Clymene Ocean. This unit is exposed in the Olavarría and Barker areas within the Tandilia Basin, and composed entirely of lime mudstones (∼40 m thick). The age of the Loma Negra Formation (LNF), the uppermost unit of the Sierras Bayas Group, has been debated, but it should be older than 560 Ma (age of the overlying succession) and younger than 600 Ma (age of the underlying iron-rich mudstone of the Colombo Formation). This extended carbonate platform (>100 km) is divided into two sedimentary facies: one related to the basal section, in which lime mud originated by suspension fall-out in an open-marine ramp (reddish or greenish limestones), and the other facies represented by black to dark grey limestones from the middle and upper sections. The LNF limestones studied, obtained from five sections near the Olavarría and Barker areas, reveal constant curves of positive δ13C values (∼+2 to +4%), similar to those recorded in other middle-Ediacaran marine proxies worldwide. Our sedimentological and geochemical study of the LNF sections shows extraordinarily consistent carbon and oxygen isotope trends that may be useful for detailed intrabasinal correlations. Samples of the LNF fabric-retentive micrite preserve an unusually narrow range of oxygen isotope compositions, which is consistent with potential climatic variation, considering the multi-proxies retained. A subsequent decrease in carbonate primary productivity and an increase in siliciclastic supply are associated with the negative δ13C trend in the overlying unit, at the base of the La Providencia Group. This negative excursion in δ13C from +2.8 to −3.6% is also regarded as a response to the cooling and sea-level fall probably linked to the Gaskiers glaciation. Furthermore, such data are consistent with late-Ediacaran carbonates from other Neoproterozoic successions.Strontium isotope ratios in limestones (0.7060–0.7070) should reflect the near-primary strontium isotope composition of the original seawater, particularly regarding the strontium content (∼300 ppm) and the Rb/Sr ratios (≤0.001) of primary micrite, consistent with a seawater origin. Strontium isotope data reaches 0.7082 with an Rb/Sr ratio of 0.001–0.006.

Petrography and geochemical constrain on dolostones of the Shahbazan Formation in Lorestan (Iran)

The Shahbazan Formation (Fm.) was deposited in the Lorestan subzone of the Zagros Basin during middle to late Eocene, which presently is part of the Zagros fold and thrust belt. The Shahbazan Fm. consists of limestone and dolostone beds, which hosts some considerable hydrocarbon occurrences in the Zagros basin. In the present study, the reference Mamulan and Murani sections in the Lorestan province were studied. A total of 330 samples were collected. Petrographic examination of stained sections (alizarin red S and potassium ferricyanide) allowed differentiating five types of dolomites within the Shahbazan Fm. Within the Mamulan section, three distinct types where defined including dolomicrite (D1), associated by evaporites, peloides, intraclast, and algal laminites; dolomicrosparite (D2), with well-preserved algal laminites; and limpid dolosparite (D3) locally embedded within poikilotopic anhydrite. The Murani section comprises fabric-destructive dolosparite (D4), pore-filling dolomite (D5), as well as D1. Some fractures of the Shahbazan Fm. were occluded by evaporites and calcite spars in the Mamulan and Murani sections, respectively. The pore-filling (D5) dolomite displays a fine zonation pattern under CL, which may reflect successive growth stages during shallow to relatively burial diagenesis. According to microfacies analysis, seven microfacies including dolomudstone (F1), dolomitized bioclast–intraclast wackestone to packstone (F2), dolomitized lime mudstone (F3), interbedded laminated lime mudstone and evaporites (F4); dolomitized peloid packstone to wackestone (F5); dolomitized intraclast bioturbated wackestone (F6); bioclast miliolid wackestone to packstone (F7), as well as various anhydrites (e.g., nodular and laminated) were recognized which suggest intertidal–supratidal depositional setting for the Shahbazan Formation. ICP-MS analysis on some selected samples of the D1, revealed their chemical composition with an average for Fe of 1291 and 185 ppm, for Na of 644 and 650 ppm, for Sr of 125 and 185 ppm, and a Mg/Ca ratio of 0.57 and 0.505, for Mn of 23 ppm (only for Mamulan section) in the Mamulan and Murani sections, respectively. Relatively high Mg/Ca ratio suggests that these dolomites might be originated from salt waters. The Sr/Ca vs Mn suggests a relatively semi-closed diagenetic system. Variable Fe and Mn contents are probably due to reaction with meteoric water.

Paleogeography and shale development characteristics of the Late Permian Longtan Formation in southeastern Sichuan Basin, China

Abstract The Late Permian Longtan Formation is extensively developed within the southeastern Sichuan basin which lies on the western margin of the Yangtze Plate in southwestern China. The stratigraphy and sequence boundaries of the Longtan Formation were interpreted from a combined study of field data, well log data and various data analysis in order to determine the extent and thickness of shale sequences. This interpretation defined the broad sequence stratigraphic framework and separated the Longtan Formation into three 3rd-order sequences. In addition, the formation was divided into five sedimentary facies using sedimentary, paleontological, geophysical and other data. The facies identified were fluvial, low energy shore, tidal flat, melanged accumulated shelf and platform basin. Three paleogeographic maps were created based on the 3rd-order sequences: SQ1, SQ2 and SQ3. The study shows that SQ1- SQ3 sedimentary environment of the study area, which extends from the southwest to northeast, was developed followed by fluvial, shore, tide, shallow water melanged accumulation shelf, deep water melanged accumulation shelf and platform basin. The mud shale rock types of the Longtan Formation that were identified are carbonaceous mud shale, silty mudstone and lime mudstone. According to the divided standard of mud shale effective thickness, there are three mud shale assemblages present in the sequences SQ1, SQ2, SQ3, namely, mud shale with sandstone, mud shale with limestone, and mud shale. SQ1 has the thickest mud shale identified on well logs. Mud shale with sandstone was well developed in the southwest of the study area and was interpreted to have been deposited in a shore or tidal flat sedimentary environment. This rock assemblage has a large effective thickness and extensive lateral distribution. In contrast, the assemblage of mud shale with limestone was mainly developed in the northeast of the study area and was interpreted to have formed in a melanged accumulation shelf sedimentary environment. The effective thickness of the mud shale with limestone is small and the distribution is limited. There is developed mud shale in above two type's sedimentary environment, but the distribution is also limited. The Longtan Formation's mud shale reached stage B diagenesis and is principally composed of quartz and clay minerals. The average quartz content is 43.40% and the average clay mineral content, which is mainly composed of illite, mixed illite/smectite, chlorite and kaolinite, is 39.58%. The sedimentary environment has controlled the distribution of minerals within the sequences. The clay mineral content gradually decreases as the sedimentary environment changes from shore-tidal flat in the southwest to deep water melanged accumulation shelf in the northeast. Therefore this comprehensive analysis shows that the southwest of the study area is the key area for shale gas exploration in the Longtan Formation.

Petrographical and Sedimentological Study of Balambo Formation at Northeastern Iraq

This study was carried out for Balambo Formation (Valanginian - Turonian). Area of study alocated within Azmir anticline at Sulaimaniya area at northeastern Iraq. From petrography study, it was found that the radiolaria forming the main skeletal such as; Novixitus mclaughlini, Archaeodictyomitra simplex, Xitus spinosus, Paronaella ewingi, Cryptamphorella conara, Rhopalosyringium hispidum, Crucella messinae and Vitorfus morini . While the planktonic foraminifera such as; Globigerinelloides bentonensis, Globigerinelloides ultramicus, Planoheterohelix globulosal, Hedbergella (Asterohedbergela), Planomalina buxtorfi, Ticinella praeticinensis and Calcispheres in second order. The ground mass formed of gray to brown micrite. The rocks effected by diagenetic processes with many types like silicification, recrystallization, cementation, and dissolution. According to Microfacies analysis study, the recognized main facies are tow: lime mudstone and lime wackstone. These facies divided into four submicrofacies: radiolarian lime mudstone submicrofacies, calcispheratic lime mudstone submicrofacies, radiolarian lime wackestone submicrofacies and Globigerinelloids lime wackestone submicrofacies. Based on facies and petrographic studies, its deposited within deep sea (Bathyal Zone) .

A lithology identification method for continental shale oil reservoir based on BP neural network

The Dongying Depression and Jiyang Depression of the Bohai Bay Basin consist of continental sedimentary facies with a variable sedimentary environment and the shale layer system has a variety of lithologies and strong heterogeneity. It is difficult to accurately identify the lithologies with traditional lithology identification methods. The back propagation (BP) neural network was used to predict the lithology of continental shale oil reservoirs. Based on the rock slice identification, x-ray diffraction bulk rock mineral analysis, scanning electron microscope analysis, and the data of well logging and logging, the lithology was divided with carbonate, clay and felsic as end-member minerals. According to the core-electrical relationship, the frequency histogram was then used to calculate the logging response range of each lithology. The lithology-sensitive curves selected from 23 logging curves (GR, AC, CNL, DEN, etc) were chosen as the input variables. Finally, the BP neural network training model was established to predict the lithology. The lithology in the study area can be divided into four types: mudstone, lime mudstone, lime oil-mudstone, and lime argillaceous oil-shale. The logging responses of lithology were complicated and characterized by the low values of four indicators and medium values of two indicators. By comparing the number of hidden nodes and the number of training times, we found that the number of 15 hidden nodes and 1000 times of training yielded the best training results. The optimal neural network training model was established based on the above results. The lithology prediction results of BP neural network of well XX-1 showed that the accuracy rate was over 80%, indicating that the method was suitable for lithology identification of continental shale stratigraphy. The study provided the basis for the reservoir quality and oily evaluation of continental shale reservoirs and was of great significance to shale oil and gas exploration.

Palaeoenvironment and palaeogeography of Middle and Upper Devonian strata from the Loei fold belt, Indochina terrane (northeast Thailand)

Limestone and chert sections from the Loei fold belt in Loei Province, northeastern Thailand, have yielded Givetian to Famennian conodonts and Late Devonian radiolarian faunas. The mid-Devonian siliciclastic section containing an abundant benthic assemblage is interbedded with volcaniclastics and is replaced laterally, in places, by pillow basalts. This siliciclastic section is conformably overlain by argillaceous limestones, divided into two microfacies. The lime mudstone (MF1) and spiculite wackestone (MF2) which contains Polygnathus linguiformis linguiformis, Belodella resima and Icriodus arkonensis, indicating the lower Givetian, deposited in a quiet subtidal to deeper water environment. This argillaceous limestone is succeeded by upper Givetian bioherm containing a diverse stromatoporoid–coral fauna. Microfacies of bioherm consists mainly of crinoidal wackestone/packstone (MF3) deposited in a flank and stromatoporoid–coral framestone/bafflestone/rudstone (MF4) forming a core of bioherm. Continuing relative sea-level rise led to drowning of the bioherms and the deposition of condensed section of non-oceanic, continental margin oozes containing the radiolarians Trilonche chiangdaoensis, Trilonche echinata, Trilonche elegans, Trilonche hindea, Trilonche minax and others and the Famennian conodonts Palmatolepis triangularis, Palmatolepis werneri, Palmatolepis minuta minuta and Palmatolepis wolskae. The transgressive Middle to Upper Devonian strata ranging from fossiliferous sandstones through argillaceous limestone and bioherm to radiolarian cherts are broadly similar to sections in South China and in the Rhenohercynian belt of Germany. Deposition in a continental margin basin, possibly on thinned continental crust, such as a back-arc basin, is more likely than previous models that suggested deposition in and around seamounts within a major ocean basin.

Large-scale mud diapirism as a record of late-stage Laramide tectonism within the Eocene Green River Formation, Wyoming

The Gruene and Verde mud diapirs, which range in size from 70 to 95m in diameter, are exposed within the Wilkins Peak Member of the Eocene Green River Formation near Flaming Gorge Reservoir, southwest Wyoming. These diapirs are sourced from the lower unit of the Wilkins Peak Member (lower, middle, and upper units can be defined based on dominant lithology), and were emplaced into the upper unit of the Wilkins Peak Member, cross-cutting hundreds of meters of strata. The structures contain brecciated dolomicritic and lime mudstones with minor amounts of siliciclastic clays. Pervasive quartz veining is present throughout both diapirs. Though heavily disturbed, original bedding can be seen in both localities, either dipping radially outward towards the margins of the diapir as is dominant at the Gruene diapir or tilted but near horizontal as is the dominant expression at the smaller Verde diapir. The Gruene diapir can be seen in contact with several fluvial sandstone bodies within the upper Wilkins Peak strata in outcrop, and these fluvial sandstones transition from regional, near-horizontal dips to steeply dipping beds and bedsets within 50m of the diapir. These sands show no signs of brittle deformation, and dip ranges from 20 to 50° radially away from the center of the diapir to the north, west, and east where sands are observed. These are interpreted here to have been deformed pre-lithification, at or near the surface, by the rising mud diapir, offering an important control on the timing of diapirism near the end of the early Eocene. Deformation structures such as clastic dikes, brecciation, and convoluted bedding have been documented by others at or near this stratigraphic interval in Utah, Wyoming, and Colorado, and these smaller structures have been interpreted as seismites. We suggest that these mud diapirs are a larger-scale expression of this same seismic activity.

Sedimentary characteristics and origins of Late Pennsylvanian–Early Permian carbonate mud-mounds at the Shangdan section, Inner Mongolia

Late Pennsylvania–Early Permian carbonate mud-mounds exposed in the Shangdan section, Inner Mongolia, are commonly massive, showing depositional relief, and are distinct from the thin- to medium-bedded limestones of intra-mounds. Based on the various proportions of lithofacies, four types of mud-mounds are recognized: MM-A (type A carbonate mud-mound) and B (type B carbonate mud-mound) consist of massive lime mudstone and massive wackestone (Subfacies B), respectively; MM-C (type C carbonate mud-mound) are composed of massive wackestone (Subfacies B) in the lower parts, analogous to MM-B, and massive wackestone (Subfacies A) and boundstone in the upper parts, similar to MM-D (type D carbonate mud-mound). Carbonate mud-mounds are deposited in low-energy settings on a ramp, likely below fair-weather wave base, where the depositional settings of MM-A, B, C and D have a shallowing-upward trend. Carbonate mud-mounds have diverse origins of lime mud: allochthonous lime mud in MM-A; lime mud produced by the disintegration of phylloid algae in MM-B, C and D; lime mud linked to cyanobacterial activity in MM-D and the upper parts of MM-C. Different from MM-A and B, which were formed only by mechanical accumulation, MM-C and D resulted from the combinations of mechanical accumulation and microbial binding and solidifying. MM-A, B, C and D significantly expand the global spectrum of Late Pennsylvania–Early Permian carbonate mounds.

A reservoir prediction method based on sensitivity curve in the background of lime mudstone: a case study on middle Es 3 in Gubei subsag

A reservoir prediction method based on sensitivity curve in the background of lime mudstone: a case study on middle Es 3 in Gubei subsag

Seismogenic rotational slumps and translational glides in pelagic deep-water carbonates. Upper Tithonian-Berriasian of Southern Tethyan margin (W Sicily, Italy)

Soft-sediment deformation structures (SSDSs), which reflect sediment mobilization processes, are helpful to identify punctual events of paleoenvironmental stresses. In the upper Tithonian-Berriasian calpionellid pelagic limestone of the Lattimusa Fm. outcropping in the Barracù section (W Sicily), paleoenvironmental restoration reveals the occurrence of a deep-water flat basin, characterised by undeformed planar bedding, laterally passing to a gentle slope where the deformed horizons alternate with undeformed beds. Here, two types of gravity slides have been differentiated on the basis of different kinds of SSDSs, brittle deformation, involved lithofacies, geometry and morphology. Type 1 slump sheets, involving marl-limestone couplets, are characterised by chaotic stratification, truncated beds, recumbent folds, tension gashes, clastic dikes and small-scale listric normal faults and thrusts. They moved downslope as a rotational slump, displaying the typical wedge-shaped morphology, becoming thicker toe-wards. Type 2 slump sheets, involving bed packages of lime mudstone, are characterised by arcuate slump scars developing downwards as listric normal faults, associated with rotational beds and bulging. Displaying tabular geometries, they moved coherently along an overall bedding parallel detachment surface as a translational glide characterised by sediment creep.Regional geology indicates that the basin (Sicanian), bordered by a stepped carbonate platform, was affected by synsedimentary tectonics throughout the Mesozoic. The synsedimentary extensional tectonics that affect the Upper Triassic-Jurassic deposits with steeped normal faults, causing tilt-block and instability of the seafloor through fluidization processes, triggered the rotational slumps of the lower portion of the section. Seismic shocks, induced by outside sector tectonics, like those recorded in the adjacent Busambra stepped carbonate platform margin, triggered the rotational slumps that are not related to local-scale faults. To explain the translational glides of the upper portion of the section, the aforementioned outside sector earthquakes produced instability of the sea-floor through thixotropy of marls. In this case, the driving forces (i.e., gravitational) were favoured by the uplifting trend that is well recorded in the Southern Tethyan continental margin.We suggest that the various early deformations represent rotational slump and sediment creep that may be characteristic for slump structures in pelagic carbonates, depending on the involved lithologies (i.e., paleoenvironmental setting) and the intensity/distance of the trigger mechanism (i.e., earthquake epicentre).

Retraction Note to: Depositional environments and porosity distribution in regressive limestone reservoirs of the Mishrif Formation, Southern Iraq

Eight subsurface sections and a large number of thin sections of the Mishrif Limestone were studied to unravel the depositional facies and environments. The allochems in the Mishrif Formation are dominated by bioclasts, whereas peloids, ooids, and intraclasts are less abundant. The sedimentary microfacies of the Mishrif Formation includes mudstone, wackestone, packstone, grainstone, floatstone, and rudstone, which have been deposited in basinal, outer shelf, slop followed by shoal reef and lagoonal environments. The formation displays various extents of dolomitization and is cemented by calcite and dolomite. The formation has gradational contact with the underlying Rumaila Formation but is unconformably overlain by the Khasib Formation. The unconformity is recognized because the skeletal grains are dominated by Chaophyta (algae), which denotes the change of environment from fully marine to lacustrine environment. Thus, the vertical bioclast analysis indicates that the Mishrif Formation is characterized by two regressive cycles, which control the distribution of reservoir quality as well as the patterns of calcite and dolomite cement distribution. Mishrif Formation gradationally overlies Rumaila Formation. This was indicated by the presence of the green parts of Chaophyta (algae) as main skeletal grains at the uppermost part of well Zb-47, which refer to lacustrine or fresh water environment. Petrographical study shows that the fossils, peloids, oolitis, and intraclasts represent the main allochem. Calcite and dolomite (as diagenetic products) are the predominant mineral components of Mishrif Formation. Fossils were studied as an environmental age and facial boundaries indicators, which are located in a chart using personal computer programs depending on their distributions on the first appearance of species. Fifteen principal sedimentary microfacies have been identified in the Mishrif Formation, which includes lime mudstone, mudstone–wackestone, wackestone, wackestone–packstone, packstone, packstone–grainstone, grainstone–floatstone, packstone–floatstone, packstone–rudstone, and wackestone–floatstone. Markov chain analysis has been used to study the transitional pattern of different microfacies types vertically in each well and laterally in all wells as a composite section. The vertical analysis indicates that the Mishrif Formation characterized by two regressive cycles, the main one started with basinal or outer shelf environment, slop environment followed by shoal or reefal environment, and ended with a lagoonal environment. The lateral analysis shows the same regressive cycle, and by using the lithofacies association concepts, we built the depositional model of the Mishrif Formation environment.

Early labechiid stromatoporoids of the Yeongheung Formation (Middle Ordovician), Yeongwol Group, mideastern Korean Peninsula: Part I. Environmental distribution

This study examines depositional facies and environmental significance of early labechiid stromatoporoids in the Yeongheung Formation (late Middle Ordovician), Yeongwol Group in the mideastern part of the Korean Peninsula. The formation is composed of five depositional facies: lime mudstone to wackestone (LM/W), peloidal, intraclastic and bioclastic packstone to grainstone (P/G), laminated dolomitic lime mudstone (LMdl), algal laminite (La), and oolitic packstone to grainstone (P/Go). Three types of shallowing upward cycles in the succession are identified, including subtidal (LM/W–P/G), subtidal to peritidal (LM/W–P/G–LMdl–La), and peritidal cycles (LMdl–La). The stromatoporoids occur exclusively in the packstone to grainstone (P/G) facies of the subtidal cycle. These early labechiids inhabited on a grainy substrate under moderate- to high-energy conditions, conforming to the previous notion that stromatoporoids primarily occupied level-bottom or reefs, surrounded by grainy calcareous sediments during the Middle Ordovician.

Tidal rhythmites in the southern Bouse Formation as evidence for post-Miocene uplift of the lower Colorado River corridor

Abstract Uncertainty over depositional paleoenvironments of the upper Miocene to lower Pliocene Bouse Formation obscures our understanding of the timing and magnitude of regional uplift as well as the conditions and processes that were active during integration and early evolution of the Colorado River (western United States). This paper presents new sedimentologic and quantitative evidence for a strong tidal influence on deposition of the basal carbonate member of the southern Bouse Formation. Lithofacies with tidal sedimentary structures include lime mudstone with reed and grass imprints; heterolithic facies with horizontal, flaser, wavy, and lenticular bedding; and cross-bedded grainstone with sigmoidal bundles, reactivation surfaces, and complex bedding. Systematic alternation of relatively thick and thin layers records a predominantly semidiurnal mixed-tidal depositional environment, and Fourier analysis highlights the cyclic, non-random nature of layer-thickness trends. These observations provide evidence that the southern Bouse Formation accumulated in a pre–Colorado River tide-dominated marine setting at the north end of the Gulf of California. Deposition at sea level requires ∼330 m of uplift in the lower Colorado River and western Colorado Plateau region during the past ∼5.0 m.y.

Structural and facies architecture of a diapir-related carbonate minibasin (lower and middle Jurassic, High Atlas, Morocco)

We report the structural geometry and facies architecture of a small diapir-related carbonate-dominated basin from the Jurassic rift of the Moroccan High Atlas. The Azag minibasin is a lozenge-shaped depocenter completely enclosed by tectonic boundaries that we interpret as welds after former salt anticlines or salt walls. The exposed ca. 3000 m-thick infill of the Azag minibasin is asymmetric; layers are tilted to the W defining a rollover geometry. Areally-restricted sedimentary discontinuities and wedges of growth strata near the basin margins indicate sedimentation contemporaneous with diapiric rise of a Triassic ductile layer. Facies evolution through the basin reflects local accommodation by salt withdrawal and regional events in the High Atlas rift. The early basin infill in the Sinemurian and Pliensbachian shows thickness variations indicative of low-amplitude halokinetic movements, with reduced exposed thicknesses compared to surrounding areas. The exposed Toarcian and Aalenian deposits are also reduced in thickness compared to areas outside the basin. Subsidence increased dramatically in the Bajocian-early Bathonian (?), the main phase of downbuilding, when over 2600 m of carbonates and shales accumulated at a rate > 0.5 mm/a in the depocentral area of the minibasin governed by W-directed salt expulsion. The stratigraphic units distinguished often show maximum thicknesses and deeper facies in the depocentral area, and rapidly change to shallower facies at the basin margins. The Bajocian carbonate facies assemblage of the minibasin include: reservoir facies as microbialite-coral reefs in the basin margins (formed during periods of strong diapir inflation and bathymetric relief), basin-expansive oolite bars (formed during episodes of subdued relief), and organic-rich, dark lime mudstones and shales that show source-rock characteristics. The Azag basin is a good analog for the exploration of salt-related carbonate plays in rifts and continental margins where source-rock and reservoir can form in a same minibasin.

Half-year results of Johnson Matthey for the six months ended 30 Sep 2016

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 m thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone.The cyclic limestone is Olenekian in age, 164 m thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2–7.4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences.The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo–Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic.

Crouching shells, hidden sponges: Unusual Late Ordovician cavities containing sponges

Marine cavities harbouring cryptic organisms have been ubiquitous throughout the Phanerozoic. However, our knowledge of early cryptic communities is as yet insufficient, and how metazoans began to utilize such habitats remains unknown. In this study, we document demosponge remains within intraskeletal cavities embedded in the micritic succession of a shallow carbonate platform in the Upper Ordovician (Katian) Xiazhen Formation of South China. Molluscs (gastropods, bivalves, and nautiloids) and corals (the solitary rugosan Tryplasma and colonial agetolitids) within the succession commonly contain patches of “spicular” demosponge remains (11%; n=45/415), mainly occupying intraskeletal spaces with areas of 1–30mm2 in thin-section. Sponge occurrence varies according to sedimentary facies: within lime mudstone facies, sponges commonly occur both inside and outside intraskeletal cavities, suggesting that sponges would have inhabited and become preserved within any available space in this environment. In contrast, when other sessile organisms co-occur in wackestone to packstone facies, there are fewer sponge occurrences both inside and outside cavities, possibly due to competition in open habitats and/or their poorer preservation in such environments. Overall, this result suggests that sponges would have exploited cryptic habitats by normal expansion of the open-surface biota. In addition, compared with coeval reef and hardground crypts, the Xiazhen intraskeletal cryptic biota is monotonous in composition, suggesting “decoupled” occupation of cryptic habitats in different environments.

The Loppio Oolitic Limestone (Early Jurassic, Southern Alps): A prograding oolitic body with high original porosity originated by a carbonate platform crisis and recovery

The Loppio Oolitic Limestone is a lithostratigraphic unit of the Early Jurassic Trento Platform in the Southern Alps, Northern Italy, which deposited over an area of ca. 3500 km2. It appears as a roughly tabular or wedge-shaped sedimentary body with thickness gradually increasing from 0 to ca. 100 m toward the western platform margin. We investigated the sedimentology, petrography and bulk carbonate carbon isotope geochemistry of the Loppio Oolitic Limestone in order to shed light on its depositional setting and origin. The Loppio Oolitic Limestone is made almost exclusively of oolitic grainstone, and can be subdivided in two parts. In the lower part, ooids are poorly sorted and sedimentary structures are scarce or absent. In the upper part, sorting becomes good and sedimentary structures are common. The vertical succession of sedimentary structures and the upward increase in sorting suggest a shallowing upward trend within the oolite. A reddened surface, meteoric cements and dinosaur footprints occur at the top of the unit, testifying for a subaerial exposure which is also confirmed by carbon and oxygen stable isotopic data. In terms of sequence stratigraphy, the Loppio Oolitic Limestone represents a Highstand Systems Tract, bounded at the top by a subaerial exposure surface. Bulk carbonate stable carbon isotope curves across the Loppio Oolitic Limestone from 7 stratigraphic sections could be correlated over distances of tens of km on the whole Trento Platform. This correlation suggests that the deposition of ooids was nearly synchronous across the platform. A negative excursion of carbon isotopes with magnitude of ca. 1‰ VPDB was identified within a lime mudstone unit (“Nodular lithozone” of the Monte Zugna Formation) immediately below the Loppio Oolitic Limestone, which can be correlated to a global perturbation of the carbon cycle in the mid-Sinemurian. The flooding of a wide area of formerly peritidal carbonate platform below the wave base was interpreted as due to an ecological crisis that caused a drop of carbonate production. We suggest that the subsequent recovery of carbonate production is marked by the shallowing upward succession of the Loppio Oolitic Limestone, which quickly occupied the accommodation space formed in consequence of the crisis, thus preventing the platform drowning. The Loppio Oolitic Limestone deposited as an initially highly porous oolitic sand that was then topped by a clayey interval (base of the Rotzo Formation), giving origin to a structural and stratigraphic configuration that could be favourable for the accumulation of hydrocarbons in the subsurface. The recurrence of similar facies superpositions, formed in consequence of perturbations of the carbon cycle with documented climatic effects, is discussed with regard to the Tethysian record of Mesozoic carbonate platforms.

Geobody architecture, genesis and petrophysical characteristics of the Budakalász travertines, Buda Hills (Hungary)

Subsurface Pre-salt Lower Cretaceous reservoir systems encountered offshore Brazil and Angola that consist of porous continental carbonates are difficult to access because of the limited core material that is under concession. Therefore, it is important to study comparable continental carbonate analogues, such as travertines and lacustrine carbonates, that potentially possess similar porosity and permeability characteristics. The Budakalasz Pleistocene travertines (Buda Hills, Hungary) have been studied from a sedimentological, geochemical and reservoir properties point of view in order to understand their formation, petrophysical properties and 3D reservoir architecture. The three-dimensional geobody architecture of the Budakalasz travertines illustrates the potential complexity of kilometre scale continental carbonate systems. Heterogeneous low angle sloping travertine lobes consisting of terrace slope, smooth slope and reed facies, are covered by a lime mudstone dominated, gastropod- and charophyte-rich marsh-pool facies. The observed facies represent a change from a subaerial travertine to a palustrine to lacustrine system, reflecting the ceasing of the system due to tectonic uplift. The dominant paleo-flow direction of the sloping lobes was towards the Danube depression, with several subordinate flow systems identified. The SrCO isotope geochemical signatures reflect travertine precipitation at ambient to superambient temperatures from meteoric derived fluids with a mixed CO2 origin of the marine carbonate substrate rocks, a potential atmospheric and a soil-derived component. Although the uniform signatures of the stable oxygen and carbon isotopes, in addition to petrographic observations suggest partial diagenetic isotopic resetting, the primary isotope trends seem to have been preserved. A reservoir-oriented facies evaluation in the three-dimensional geobody frame of the Budakalasz carbonates based on porosity and permeability core plug analyses reveals their heterogeneous nature. Terrace, smooth slope and marsh-pool facies have considerable porosities of 5–15%, while the porosity of the reed facies may reach up to 27%. Permeability of all facies is strongly anisotropic, mainly controlled by a better connectivity along layer-parallel primary pores. The reconstruction of the 3D geobody architecture of the Budakalasz lobe complex coupled to its geochemical and petrophysical properties provides a unique dataset that contributes in understanding fossil travertine systems.