Lithofacies Changes Research Articles

Middle Ordovician shallow-water gastropods from southern Xizang (Tibet), China

Many new specimens (36 in total) of gastropods were collected from the Ordovician Alai Formation of the Chiatsun Group at Jiacun village, Nyalam County, southern Xizang (Tibet). These gastropods comprise five species assigned to two genera, including Maclurites parviumbilicatus Yu, M. cf. nyalamensis Yu, M. cf. xizangensis Yu, M. cf. subconicus Yu, and “Hormotoma” ordosensis Yu, forming a Maclurites-“Hormotoma” association. All the species are described in detail herein, and some of which are redefined. This fauna is of middle to late Darriwilian age, and palaeoecologically characterised by species of shallow-water within the photic zone. A comparison of the Maclurites community with the contemporary gastropod fauna from the Zhuozishan Formation exposed along the west margin of the North China Block shows a close palaeogeographical affinity. The occurrences of Ordovician gastropods from China are analyzed biogeographically and the results reveal that the peri-Gondwana gastropod faunal turnover event was initiated during the Middle to Late Ordovician transition. The coincident faunal turnover and the lithofacies changes in the Himalaya and western margin of North China suggest that tectonic activities and relative sea-level changes may account for the biogeographical affinities of some specific fossil groups in peri-Gondwanan regions.

The genetic relationship between paleoenvironment, mineral compositions and lithofacies in the Ordovician–Silurian Wufeng–Longmaxi sedimentary succession in the Sichuan Basin, SW China

• Five intervals (I–V) showing changes in minerals, lithofacies and paleoenvironment were identified. • High productivity and low detrital influx lead to the enrichment of nondetrital quartz in siliceous shale in Intervals I-II. • The enrichment of carbonate in calcareous shale in Interval III was due to high detrital influx. • High detrital influx and strong chemical weathering lead to an abundance of silt and clay minerals in silty–argillaceous shale and argillaceous shale in Intervals IV-V. Changes of sedimentary environments recorded in the Late Ordovician–Early Silurian sedimentary succession can be reflected in mineral composition and lithofacies of shales. Outcrops and cores from this set of shales in the Sichuan Basin, China, are analyzed. The results show that quartz, clay minerals, and carbonate are the main minerals. Five major lithofacies are developed. Based on the provenance proxies (Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc ratios), chemical index of alteration (CIA), paleoproductivity proxies (Cu/Al and Ni/Al ratios), redox condition proxies (U/Th, V/Cr, and Ni/Co ratios), proxies of detrital influx (Ti and Al contents), and sedimentation rate (SR), the paleoenvironment is analyzed. The source rocks are dominated by felsic igneous rock. Comparison between five lithofacies revealed that the relatively low degree of chemical weathering, anoxic conditions, maximum paleoproductivity probably associated with the high frequency of volcanic activities, and minimum detrital influx and SR (0.60–5 m/Ma) led to the enrichment of nondetrital quartz and organic matter, which eventually formed siliceous shale. The moderate degree of chemical weathering, dysoxic–oxic conditions, decreasing paleoproductivity, and increasing detrital influx and SR (10–16.67 m/Ma) resulted in the increase in detrital quartz, clay minerals, and carbonate content, and the development of mixed shale and calcareous shale. Relatively strong chemical weathering, oxic conditions, minimum paleoproductivity, and maximum detrital influx and SR (100.74 m/Ma) favored the enrichment of silt and clay minerals, forming silty–argillaceous shale and argillaceous shale. This study provides an insight into the relationship between the environmental changes and shale mineral composition and lithofacies.

Miocene Stratigraphy of Abu-Jir Fault Zone, Al-Baghdadi Sub-Basin, Western Iraq.

Stratigraphy of Late Oligocene to Middle Miocene age, suggested the carbonates evaporate succession in the area between Heet and Haditha cities within Abu-Jir fault zone has distinguished as “Al-Baghdadi sub-basin” after Al-Baghdadi city. It has NW–SE trending align the western bank of the Euphrates. Al-Baghdadi sub-basin is a local basin, which forms a part of the western marginal of the large foreland Tertiary basin (Zagros Basin) that was created on the Arabian shelf as a result of the collide of Arabian and Eurasian plates. The impact of the Abu-Jir fault on the formations on both regions of the Euphrates has been determined by observing the changes in thickness and lithofacies of the Euphrates, Jeribe and Fat’ha formations. On the western side, the Euphrates and Fat’ha formations were deposited with a relatively a little thickness, with absence of the Jeribe Formation between them and the top of the Euphrates Formation is above sea level. On the eastern side of the Euphrates River, the three formations were deposited with large thickness as compared to the western side, and the top of the Euphrates Formation is below sea level. This is enough to indicate that the western side was uplifting locally due to the activity of the Abu-Jir Fault as a positive flower structure during Miocene, while the eastern side was subsiding. Abu-Jir fault zone behaves as the basin bounding fault that acted like a growth fault. There are also variations in lithofacies between outcrops within Al-Baghdadi sub-basin as revealed from lateral missing or thinning of some layers.

Lithofacies Characteristics, Depositional Environment and Sequence Stratigraphic Framework in the Saline Lacustrine Basin-A Case Study of the Eocene Low Member of Xingouzui Formation, Jianghan Basin, China

The Jianghan Basin is a lacustrine basin in central China developing multiple salt bearing deposits in the Eocene with the success of shale oil exploration in the Qianjiang Formation. The lower member of the Xingouzui Formation in the Chentuokou Depression has become another exploration target. However, rapid changes in lithofacies and strong sedimentary heterogeneity limit the exploration progress. This study aimed to explore the sequence division, lithofacies characteristics, and sedimentary environment using sedimentological, X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. The sequence stratigraphic analysis indicates that the low member of the Xingouzui Formation is divided into two third-order sequences, namely SQ1 and SQ2, and four system tracts, including highstand systems tract (HST), lake expanding system tract (EST), early highstand system tract (EHST), and late highstand system tract (LHST). Moreover, a total of nine major lithofacies and five lithofacies associations (LA1-5) were identified. The organic geochemical data show that the laminated argillaceous dolomite in EST and EHST developed the best oil content with an average TOC of 1.18% and S1 of 3.18 mg/g, The laminated argillaceous dolomite deposited in anoxic conditions with a humid climate, moderate salinity, and stratified deep waterbody is a favorable exploration facies for shale oil.

Litho- and biostratigraphy of a late Oligocene–Early Miocene succession in the Weber area, southern Hawke’s Bay, and implications for early Hikurangi subduction-margin evolution

ABSTRACT A deep-marine sedimentary succession of Whaingaroan to Altonian age (Early Oligocene–Early Miocene), cropping out near Weber, southern Hawke’s Bay, records abrupt changes in depositional paleoenvironments and sedimentary lithofacies. Highly calcareous early Waitakian (latest Oligocene) Weber Formation (Mangatu Group/Waka Supergroup) is unconformably overlain by terrigenous-dominated late Waitakian–Otaian (Early Miocene) Coast Road or Whakataki formations (Tolaga Group/Māui Supergroup). Most notably, in some localities, lowermost parts of the Coast Road Formation (Mangapuku Mudstone Member) contain prominent matrix-supported extra-formational olistoliths of centimetre to decametre scale derived from the underlying Weber Formation. Deposition of these blocks is attributed to deep-water collapse of parts of rapidly growing, over-steepened, reverse-fault-controlled thrust ridge(s). The onset of Coast Road and Whakataki formation deposition and associated abrupt changes in sedimentary lithofacies correlate to prominent Early Miocene lithological, mineralogical, and paleoenvironmental changes elsewhere along the Hikurangi Margin. We attribute these bio- and lithostratigraphic changes as evidence for significant shortening and upper-plate reverse faulting marking the onset of subduction beneath eastern North Island during the mid-Waitakian (c. 23 Ma). Proposed lithostratigraphic revisions presented herein help clarify depositional events and correlative sedimentary packages within the Miocene Hikurangi margin.

Causes of a premature thermal breakthrough of a hydrothermal project in Germany

Sustainable heat extraction is of utmost importance for the economic life of geothermal production systems. Especially in hydrothermal sedimentary basins, a temperature decrease at the production well prompts a financial loss. A premature thermal breakthrough occurred in one of the geothermal projects in the Greater Munich Area, a well-known region for deep, low enthalpy geothermal development in the Upper Jurassic carbonate reservoir (syn. Malm) beneath the North Alpine Foreland Basin. Accordingly, questions are raised regarding the geologic controls on the reservoir´s permeability, the occurring type of heat and mass transport mechanism, and the fitting reservoir model type. 3D seismic data interpretation delineates seismic scale structures potentially influencing the reservoir permeability. Geophysical borehole data characterize sub-seismic scale elements (fractures, karst, and lithofacies), where we correlate them with the inflow zones and the interpreted seismic scale structures. The integration of both scales delineates the reservoir heterogeneity controlled by a lateral change in the carbonate lithofacies overprinted by faults and fractures. A NNW-SSE oriented seismic scale structure matches the orientation of hydraulically active sub-seismic scale fractures, where they are aligned with the maximum horizontal stress orientation. This structure induces additional anisotropic permeability, providing a preferential hydraulic pathway between the wells of the project. This study recommends considering fractured reservoir models to simulate the mass and heat flow in the highly heterogeneous Malm carbonates instead of composite reservoir models of equivalent hydraulic properties. Finally, we present a hydrogeological model describing the reservoir's structural and matrix permeability distribution to act as a basic concept for future thermal-hydraulic numerical modeling. The applied methods in this study are applicable in similar geothermal systems with adequate seismic and geophysical borehole data for future geothermal reservoir characterization, modeling, and management.

Lacustrine Sediments Bacterial Community Structure Vertical Succession of the Linxia Basin, NE Tibetan Plateau: Significance for Paleoenvironment Reconstruction

The study on sediments in the marginal basins of the Tibetan Plateau is of great significance for global climate change. The geological information of the Linxia Basin has been intensely investigated; however, the profiles of the microbial communities in this basin remain largely unknown. Here, based on the 16S rRNA high-throughput sequencing method, the bacterial community structure vertical succession is studied with different thicknesses of sedimentary samples. The bacterial community with a total of 1,729,658 paired reads distributed within 1,042 phylogenetic amplicon sequence variants (ASVs) from twenty sediments, and three surrounding soil samples were sequenced. First, high-throughput sequencing results highlight the surrounding soil sample bacterial community structures were significantly different from those recovered from the sediment samples. In addition, as observed in the PCoA and PERMANOVA, there is a dramatic change shift event of the community structure at M311. Our data suggest that shifts in relative abundances of the abundant taxa (˃1%) and the significant variations in the diversity of bacterial community implied community structure responses to changes in different sedimentary layers. Predicted community function changes demonstrate that the sediment bacterial community aerobic chemoheterotrophy has been significantly increased, and we believe that the possible influence of the lithofacies changes from the anaerobic system to the aerobic environment, possibly accompanied by the significant uplift of the plateau that has previously been associated with enhanced aridity in Central Asia at ∼8 Ma. Taken together, these results illustrate the potential for the microbial community as a biological indicator in sediment ecosystems to reconstruct paleoenvironments.

Capture the variation of acoustic impedance property in the Jaisalmer Formation due to structural deformation based on post-stack seismic inversion study: a case study from Jaisalmer sub-basin, India

The Rajasthan basin situates in the western part of India. The basin architecture comprises three significant sub-basins such as Barmer-Sanchor, Bikaner-Nagaur and Jaisalmer. Barmer-Sanchor and Bikaner-Nagaur sub-basins are intracratonic categories, whereas the Jaisalmer sub-basin comes under intracratonic nature. The current study was conducted in the Jaisalmer sub-basin. The study was conducted in two regions in the Jaisalmer sub-basin through a comparative quantitative interpretation study with the help of two vintages seismic surveys. Ghotaru and Bandha are two adjacent areas in the Jaisalmer sub-basin where Ghotaru has seen few hydrocarbon discoveries; however, no such discoveries are encountered in the Bandha area. The current study was concentrated on the Jaisalmer limestone formation in the Jurassic age. The sub-basin and its related study area have been structurally deformed due to various tectonic activities. Structural deformation was played a crucial role in changing the rock property of limestone facies. A post-stack seismic inversion was carried out to capture the rock property changes in the limestone reservoir based on P-impedance values. Development of high P-impedance was observed in the Ghotaru region compared to the Bandha region from this study. A frequency changes of the limestone lithofacies with structural components was also captured in this study. The high impedance limestone lithofacies is a probable hydrocarbon-bearing reservoir unit in the Jaisalmer Formation of the Ghotaru region.

Electrical Imaging of a Shallow Free-Phase Stray Gas Plume from an Abandoned Exploration Well

Geophysical imaging of free-phase gas (FPG) within aquifers is an emerging method for understanding the mechanisms controlling stray gas migration from oil and gas wells. Crystal Geyser is an unsealed and partially cased well that transports stray CO2 gas to the shallow subsurface. Accumulations of subsurface CO2 FPG near Crystal Geyser have been inferred, but the actual location and dimensions remained unclear. Here, the subsurface FPG distribution surrounding Crystal Geyser was characterized by interpreting 2D electrical resistivity images with previous drilling records and field mapping. An approximately 70-metre-wide FPG plume was located laterally between Crystal Geyser’s conduit and the Little Grand Wash Fault. The FPG plume spanned the vertical extent of approximately 20 to 55 metres below the ground surface, located within the Slick Rock Member sandstone with the relatively low permeability Earthy Member silty sandstone acting as a caprock. The FPG plume was identified from an anomalously high resistivity zone within the Slick Rock Member that was not caused by lateral lithofacies changes or fault displacement. The conceptual FPG migration pathways beneath Crystal Geyser are presented, based on the interpreted FPG distribution from the electrical resistivity images combined with previous site characterization and the principles of buoyant FPG migration. FPG accumulates within the Slick Rock Member by buoyant up-dip migration beneath siltstone capillary barriers of the Earthy Member. FPG leaks to the ground surface within high permeability preferential pathways along the Little Grand Wash Fault and the conduit of Crystal Geyser.

The properties of reservoir rocks of fault traps in Yevlakh-Aghjabedi depression

The structures formed in Yevlakh-Aghjabedi depression, as well as the changes of lithofacies of reservoir rocks of fault traps have been studied based on the geophysical survey. The distribution and reservoir properties of Cretaceous, Eocene and Miocene sediments, which are essential objects for prospectivity of whole depression, have been researched as well. Alongside existing fields and prospective structures such as Muradkhanly and Zardab, non-anticline traps and the characteristics of their reservoir rocks have been studied in the areas identical for some features. Commercial recovery obtained from the upper eroded parts of cracked-porous reservoirs of volcanic sediments of Upper Cretaceous in Muradkhanly field and formation of the deposits of both carbonated and volcanogenic-sedimentary rocks in Zardab field allows supposing that separate structures formed in the same conditions along the whole studied territory may have high prospectivity. In the areas, where fault traps are associated with Paleogene sediments, which were formed by covering raised residues of the landscape or discordantly embed on the denuded surface of Upper Cretaceous, there are favorable conditions for formation of hydrocarbon deposits in carbonate-cracked or volcanogenic sediments of Cretaceous Age.

Sedimentological and provenance analysis of the Cretaceous Moro formation Rakhi Gorge, Eastern Sulaiman Range, Pakistan

The Cretaceous Moro Formation from the Rakhi Nala section Dera Ghazi Khan has been studied in detail to investigate the Sedimentology and provenance. This paper describes the litho-facies changes, depositional environment, and provenance analysis of the Cretaceous Moro Formation from the Rakhi Nala section, eastern Sulaiman Range. The studied Formation is 110-140 meters thick and consists mainly of fine to coarse-grained sandstone, with minor-siltstone, mudstone (claystone, shale), and limestone. The uppermost beds of the Moro Formation are consist of sandstone with iron types of cement. Twelve lithofacies have been identified based on a petrographic investigation related to the depositional environment of the Moro Formation ranging from deltaic to marine setting (Delta Plain-Delta front). Petrographic analysis of sandstone reveals the presence of quartz both, mono-crystalline and poly-crystalline, less feldspar; heavy minerals like hematite and magnetite, and glauconite were found in negligible amounts. Detrital mineral composition shows that in Moro Formation, the sandstone shows a litharenite. Modal composition of the sandstone from the QFL diagram was Q 66% F 0.3% L 33.7% and that of the QmFLt diagram was QM, 57% F 0.23% L 43.77%. The overall average composition is Q 61.5% F 0.27% L 38.7%. A total of 37 thin-sections are studied for provenance analysis, out of which twenty-seven samples are considered as Litharenite (this shows recycled, or craton interior origin), eight Quartz arenite categories are identified and two samples are fall in the sublitharenites category (Quarts recycled source area).

Relationship between tectonism and desertification inferred from provenance and lithofacies changes in the Cenozoic terrestrial sequence of the southwestern Tarim Basin

The modern-day Tarim Basin is covered almost entirely by the Taklimakan sand desert and is one of the most arid regions in the world. Unraveling the aridification history of the desert is important for understanding global climate changes during the Cenozoic, yet the timing and mechanisms driving its formation remain controversial. One of the leading hypotheses is that the uplift of the Pamir, located to the west of the Tarim Basin, blocked the intrusion of moist air and induced the aridification in the Tarim Basin. In this study, we explore the linkage between the uplift of the Pamir and the desertification in the Tarim Basin from the late Eocene to the middle Miocene in the Aertashi section, which is located at the southwestern edge of the Tarim Basin and offers the longest sedimentary record with a reliable age model. Provenance changes in fluvial deposits along the Aertashi section were examined using electron spin resonance (ESR) signal intensity and crystallinity index (CI) of quartz in the sand fraction of fluvial sandstones and clast counting based on the identification of clast types by thin section observation to identify timings of tectonic events in the Pamir, from which clastic materials were supplied to the Aertashi section by rivers. Our results suggest that major provenance changes in the drainage of the paleo-Yarkand river delivering clasts to the Aertashi section occurred at ca. 27, 20, and 15 Ma. These timings are mostly consistent with those observed in previous provenance studies in the Aertashi section and probably reflect tectonic events in the Pamir. On the other hand, according to the previous studies, the first occurrence of sand dune deposits indicates that the Tarim Basin was relatively arid after ca. 34 Ma. Hence, our result does not support the hypothesis that the initial aridification in the Tarim Basin was triggered by the uplift of the Pamir and the resultant blocking of moisture supply from the Paratethys Sea, although the subsequent intensification of tectonic events at ca. 27 Ma in the Pamir might have caused aridification indicated by the initiation of loess deposition.

Late Jurassic syn-extensional sedimentary deposition and Cenozoic basin inversion as recorded in The Girón Formation, northern Andes of Colombia

The Yariguíes Anticlinorium, a regional structure located at the western flank of the Eastern Cordillera of Colombia, includes the thickest record of continental sedimentary rocks accumulated near to the Jurassic-Cretaceous boundary. The sedimentary rocks are lithoarenites and feldspathic arenites, grouped in the Girón Formation, and deposited in a Late Jurassic extensional basin interpreted in this work as a rift basin. We analysed the sedimentologic and compositional characteristics of two sections that accumulated in a complex rift system. We identified important thickness variations, from 3,350 m in the type section to at least 525 m in a reference section in the Zapatoca area, as well as petrographic and lithofacies changes. This led us to confirm that the Girón Formation encompasses all the continental facies, whose source rock correspond mainly to the exhumed blocks of the Santander Massif during the Late Jurassic. The synrift successions were segmented by transverse structures and regional longitudinal faults of the rift-shoulder, as the Suárez Fault. The tectonic frame of the study area shows the relevance of the W-E compressional regimes, explaining the local kinematics as a heritage of the former configuration and tectonic inversion of the basins. However, clockwise rotation of the stress field was detected from the stress tensor analysis. The latest orientation of the stress tensors and shear joints are related to the effect of the transpressional Bucaramanga and Lebrija faults along the study area.

Middle Jurassic lacustrine source rocks controlled by an aridification event: A case study in the northern Qaidam Basin (NW China)

A change from a humid to an arid climate occurred during the Middle Jurassic. Information about this transitional phase is stored in two oil shales: the lower and upper units of the shale member of the Shimengou Formation (J2s) in the northern Qaidam Basin (NQB), Northwest China. The drastic changes in lithofacies are also reflected in the organic and inorganic inventory of the two oil shales, and suggests the existence of this aridification event. Thus, the two units varied greatly in organic matter source, productivities and depositional environments. The lower unit is interpreted as lacustrine sediment deposited under a humid climate with a high terrigenous organic matter portion besides algal biomass. Furthermore, the high sedimentation rate and large organic matter size prevented organic matter degradation. In contrast, deposition of the upper unit was coincident with progressive aridification, which resulted in minor terrigenous organic matter input, and enhanced algal biomass deposition. Its preservation was favored by an enhanced salinity and stratification of water column with an underlying anoxic bottom water condition. Today, the sediments of the upper unit are characterized by a high total organic carbon (TOC) content and type I kerogen.In summary, two depositional scenarios of source rock formation under different climatic conditions are introduced to highlight the effect of progressive aridification on the evolution of source rocks. Moderate aridification in the transition period (early stage of aridification) is suggested to have promoted the formation of lacustrine source rocks with the potential to generate waxy oil. The occurrence of similar lacustrine source rocks is also predictable when deposited while similar aridification events, particularly in the Jurassic with its frequent climate changes.

Palynofacies as a palaeoenvironment and hydrocarbon source potential assessment tool: An example from the Cretaceous of north Western Desert, Egypt

Optical examination employing transmitted light and UV-fluorescence microscopy of palynological preparations of eighteen cutting samples representing the Alam El Bueib Member (Hautervian-Barremian), Kharita/lower Bahariya (Cenomanian), and Abu Roash (Turonian-Santonian) formations collected from the Faghur Hj5-1 well, north Western Desert, Egypt, allows the identification of three different palynological assemblages from the studied rock units. These assemblages are mainly non-marine but apparently marine at the base of the Alam El Bueib Member, as evidenced by dinocyst occurrence. In addition, the presence of the Pediastrum and chlorophycean algae ecozone, recognised in previous works, is a good datum for the Abu Roash Formation in the north Western Desert of Egypt. Three associations of palynofacies linked to lithofacies changes are recognised and employed in identification of depositional environments. The Alam El Bueib samples yielded mixed kerogen assemblages of non-marine and marine organic facies. The Kharita/lower Bahariya interval is mostly barren, possibly due to prevailing sandstone lithofacies, except for one sample at its upper part which contains a diverse palynological assemblage. The overlying Abu Roash Formation has a homogeneous kerogen composition comprising mainly granular fluorescent AOM and algae as well as rare palynomorphs. Qualitative as well as quantitative variations of palynofacies allow the reconstruction of the depositional environment. The obtained data have the potential for discriminating spatial and redox status differences and providing also information about terrestrial/freshwater influxes. Results support the model that the Alam El Bueib Member was deposited in a marginal dysoxic-anoxic to distal suboxic-anoxic basin. The Kharita/lower Bahariya unit in the studied well was deposited under marginal dysoxic-anoxic conditions whereas the overlying Abu Roash Formation in a distal suboxic-anoxic basin. Palynofacies results also show that the studied material comprises two distinct facies of kerogen. First, Type II > I kerogen (AOM-rich) is overwhelmingly dominant in the Abu Roash Formation and a few samples from the Alam El Bueib Member which are presumed highly oil-prone, whereas Type III kerogen (phytoclast-rich) is particularly common in the Alam El Bueib Member and Kharita/lower Bahariya unit which are considered gas-prone. Thermal maturity determinations obtained from colour changes of smooth-walled palynomorphs reveal that Alam El Bueib samples belong to immature to mature stages; however, Kharita/lower Bahariya and Abu Roash samples are within the immature phase.

Cenomanian–Turonian facies, sequence stratigraphy and sea–level changes from east–central Sinai (Egypt)

The widely exposed Cenomanian–Turonian successions in east–central Sinai display well–marked vertical and lateral changes in lithology, biofacies and depositional sequences. Three sections were studied (from northeast to southwest: Gebel Themed, Sheikh Attiya and Gebel Shiti, respectively) to document sequence stratigraphy. Twenty–four microfacies types grouped into nine lithofacies associations ranging from peritidal to mid– to outer ramp settings were identified. These microfacies reveal a general shallowing trend towards the southwest (Gebel Shiti section) in a mixed siliciclastic/carbonate ramp, whereas the northwest (Gebel Themed section) is marked by a deeper carbonate–dominated regime. The Cenomanian–Turonian succession is divided into six third–order depositional sequences (SD1 to SD6) that are bracketed by basin–wide sequence boundaries (SB AIB to SB Tu3) of varied time span and marked by vertical changes in lithofacies, microfacies and faunal content. Within these depositional sequences, six maximum flooding surfaces are also recorded. In most depositional sequences, the absence of lowstand and falling systems tract are marked, possibly as a result of the lack of accommodation space due to the proximal position of the studied sections. The identified sequence boundaries are also compared with those identified in Egypt, Jordan, Tunisia and the Arabian Plate. The present sequence boundaries match with those documented from the Arabian and African plates as also with the eustatic sea–level falls. However, for those noted across the Cenomanian/Turonian boundary and during the early–middle Turonian transition, there is discordance, possibly due to the influence of regional tectonics, that caused major hiatuses and amalgamated several of them. The chronostratigraphic assignment followed in the present study is based on ammonites, enabling the recognition of five zones, upper Cenomanian Neolobites vibrayeanus and Vascoceras cauvini, lower Turonian Choffaticeras (Choffaticeras) segne and Wrightoceras munier and the middle–upper Turonian Coilopoceras requienianum Zone.

Late Permian–Early Triassic depositional history in the southern Bonaparte Basin: new biostratigraphic insights into reservoir heterogeneity

The upper Permian to Lower Triassic sedimentary succession in the southern Bonaparte Basin represents a marginal marine depositional system that hosts several gas accumulations. Of these, the Blacktip gas field has been in production since 2009, while additional gas resources are under consideration for development. The sedimentary succession extends across the Permian–Triassic stratigraphic boundary, and shows a change in lithofacies from the carbonate dominated Dombey Formation to the siliciclastic dominated Tern and Penguin formations. The timing, duration, distribution and depositional environments of these formations in the Petrel Sub-basin and Londonderry High is the focus of this study. The sedimentary succession extending from the Dombey to the Penguin formations is interpreted to represent marginal marine facies which accumulated during a long-lasting marine transgression that extended over previous coastal and alluvial plain sediments of the Cape Hay Formation. The overlying Mairmull Formation represents the transition to fully marine deposition in the Early Triassic. Regional scale well correlations and an assessment of biostratigraphic data indicate that marginal marine depositional systems were initiated outboard before the end-Permian extinction event, and migrated inboard at about the Permian–Triassic stratigraphic boundary. Marginal marine deposition across the southern Bonaparte Basin continued through the faunal and floral recovery phase as Triassic species became established. The depositional history of the basin is translated to a chronostratigraphic framework which has implications for predicting the character and distribution of petroleum system elements in the Petrel Sub-basin and Londonderry High.

Integrated outcrop and subsurface geomodeling of the Turonian Wall Creek Member of the Frontier Formation, Powder River Basin, Wyoming, USA

Inter-well heterogeneities influencing fluid migration in deltaic reservoirs are controlled by lateral lithofacies changes and vertical complexities such as low permeability thin-beds (i.e. mudstones). Subsurface tools cannot adequately predict the spatial and stratigraphic organization of these architectural elements, nor their influence on effective reservoir properties and connectivity. Our outcrop-based geomodeling study of the Turonian Wall Creek Member of the Frontier Formation in the Powder River Basin, Wyoming, USA, integrates subsurface production and flow simulation data to quantify the impact of multi-scale stratigraphic heterogeneity on analogous reservoir behavior and horizontal well design. The upscaled representation of thin-bed complexity in our 500 m × 715 m x 15 m geomodel is derived through flow simulation of internally nested, facies specific, centimeter-scale models (basic depositional element models or BDEMs). These BDEMs are populated with quantitative data of mudstone thin-bed geometries directly derived from digital outcrop measurements and measurements in the field, and aided by subsurface (core) petrophysical properties. Reservoir simulations are performed using various landing zone and completion configurations, and under both single-phase and multi-phase flow conditions history matched to subsurface production data.Results from our workflow demonstrate that, in heterolithic settings, the permeability of mudstone thin-beds impart the most significant impacts toward upscaled reservoir behavior, and deliver unique Kv/Kh flow anisotropies dependent on the governing depositional environment. Thin, laterally continuous and planar mudstone beds in the wave-and river-dominated distal delta front setting have the greatest negative effect on Kv and a minimal impact on Kh, promoting laterally continuous and vertically baffled flow units. Conversely, the more curvilinear and intersecting architectures of mudstone beds in the tidal dune facies maintain better vertical connectivity but decreased Kh compared to the wave-influenced facies. Reservoir flow is only sensitive to changes in engineering design (landing zone, well orientation, completion size) where thin-beds act as complete barriers rather than baffles to flow. Layer cake subsurface models failing to account for high-resolution stratigraphic architectures are likely to overestimate reservoir production where thin-beds act as barriers capable of appreciable reservoir compartmentalization, which in our models is more characteristic of the distal delta front setting. Synthesized results of this study are immediately applicable toward development planning in similar tight sandstone reservoirs.

Environmental response to volcanic activity and its effect on organic matter enrichment in the Permian Lucaogou Formation of the Malang Sag, Santanghu Basin, Northwest China

Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l2) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ13C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ13C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.

Interacción entre vulcanismo, tectónica y sedimentación en una plataforma carbonatada somera: Ejemplo en el Tethys occidental (Jurásico Medio, sureste de la Cordillera Ibérica)

En el sureste de la Cordillera Ibérica (España) existen una serie de afloramientos de rocas volcánicas origi­nadas por erupciones submarinas, principalmente de tipo volcanoclástico, interestratificadas en las sucesiones carbonatadas del Jurásico Inferior y Medio. La cartografía geológica de los afloramientos de rocas volcánicas revela que se encuentran alineados a lo largo de tres elementos de debilidad cortical bien definidos (zonas de falla de Teruel, de Caudiel y de Alcublas), originados probablemente por fracturas relacionadas con la apertura y expansión del Tethys Ligur y del Tethys Alpino. En este trabajo se presentan los resultados del estudio de la evolución tectonosedimentaria de un ambiente submarino somero afectado por la presencia de una acumulación volcánica que tuvo lugar en el Jurásico Medio (límite Aaleniense‒Bajociense). Se trata del afloramiento volcánico de Caudiel, que es el más suroriental de todos los afloramientos localizados en uno de los tres elementos de debilidad estructural mencionados: la Zona de Falla de Caudiel. La presencia de un relieve positivo en el fondo de la plataforma, ya de por sí bastante somera, condiciona la sedimentación carbonatada, muy sensible a la pro­fundidad, hasta el punto de llegar a inducir cambios significativos de facies con respecto a otras áreas cercanas o de impedir la sedimentación. Los movimientos episódicos del substrato generaron una topografía irregular y cambiante a lo largo del tiempo, con tasas variables de hundimiento y levantamiento en diferentes áreas. El aflo­ramiento de Caudiel constituye un buen ejemplo fósil de interacción entre varios factores (vulcanismo, tectónica y sedimentación) en plataformas carbonatadas someras, en las que los cambios de acomodación y las tasas de sedimentación parecen estar subordinadas a perturbaciones tectónicas de carácter local.