Facies Belts Research Articles

Linking diagenetic history to depositional attributes in a high-frequency sequence stratigraphic framework: A case from upper Jurassic Arab formation in the central Persian Gulf

The carbonate Arab Formation (Kimmeridgian-Tithonian) is one of the most prolific hydrocarbon reservoirs in the world. Petrographic studies of this unit in the central Persian Gulf led to identification of eleven sedimentary facies, and facies are grouped into 5 shore-parallel facies belts (supratidal, tidal flat, lagoon, shoal and mid-ramp). Evidences such as the absence of barrier reefs and the low diversity of facies reveals that these facies formed in a ramp-like platform. Postdepositional processes (diagenetic evolution) considerably influenced reservoir properties of sedimentary facies. Depositional and diagenetic features in this formation are strongly controlled by the interplay of glacio-eustasy and climate leading to sea-level fluctuations during the Late Jurassic. Based on the petrographic analyses and previous studies, four third-order sequences are recognized and all sequence boundaries are capped by evaporites. The main diagenetic processes impacting the Arab Formation occurred under marine and hypersaline conditions and burial realms. The hypersaline conditions correspond to sea-level fall and development of brines and are characterized by dolomitization and moldic dissolution in the Upper Arab Formation. Vuggy dissolution is another major reservoir improving factor. Generally, vuggy dissolution along with anhydrite cementation, occurred extensively in the Upper Arab Formation, while in the Lower Arab Formation, these processes are limited to just beneath of the sequence boundary (nodular anhydrite form). Diagenetic features (eogenetic and mesogenetic environments) appear to correlate to depositional environments in the Arab Formation and are controlled by sea-level changes. Consequently, diagenetic processes and reservoir properties can be predicted within a sequence stratigraphic framework.

Reservoir rock typing using integrating geological and petrophysical properties for the Asmari Formation in the Gachsaran oil field, Zagros basin

The Gachsaran oilfield is one of the largest oilfields in the Zagros basin and the Asmari Formation is a primary reservoir rock in it. In this study, the reservoir rock typing has been carried out by a combination of microscopic (microfacies and diagenesis) and petrophysical (electrofacies and hydraulic flow units) studies. Microscopic studies of 478 thin sections from core samples led to identification of 11 microfacies belonging to 4 facies belts including tidal flat, lagoon, shoal, and open marine as well as diagenetic phenomena such as micritization, bioturbation, neomorphism, compaction, dissolution, cementation, dolomitization, and anhydrzation. In order to determine the rock types, the effects of diagenesis on distribution of porosity-permeability data in the petrophysical diagram were investigated and eventually 12 geological rock types introduced. Concepts of electrofacies and hydraulic flow units have been used to determine the petrophysical rock types and then their propagation in non-cored depths and wells. In this context, by means of the well logging data cluster analysis in the Paradigm ™ Geolog 6.7.1 software, 4 electrofacies were determined and modeled. The validity of these electrofacies was examined through their correlation with 12 geological rock types, core porosity-permeability data, and capillary pressure curves. Furthermore, by using core porosity-permeability data, the flow zone indicators were calculated and accordingly 6 flow units identified in the framework of discrete rock types. Subsequently we tried to validate these flow units by using estimation of permeability for each of them and their comparison with core permeability, and also correlating flow units with capillary pressure curves. Finally, in order to predict the flow zone indicators and propagation of the flow units in the non-cored depths and wells, modeling of flow zone indicators was carried out by establishing a quantitative relation between calculated flow zone indicators and well logging data in the software. The results of this research can be useful for hydrocarbon development and production in the future of this oilfield and similar fields.

Sedimentary architecture during Carboniferous rift initiation – the arid Billefjorden Trough, Svalbard

Analysis of the sedimentary architecture of the early Billefjorden Trough, an exceptionally well-exposed Carboniferous half-graben located in Svalbard, Arctic Norway, allows detailed reconstruction of fault development and basin geometry. Sedimentary facies distribution reveals facies variability in an early synrift fill, when an array of meso-scale intra-basinal faults segmented the basin into rotated blocks with differential subsidence. Growth of these faults into larger systems impacted the distribution of accommodation space and the drainage pattern expressed in fluvial and deltaic sandstone-dominated units. An axially back-stepping character of the early synrift facies associations (Stages 1–3) is reflected in aggrading homogenous facies types of continental depositional Stage 1, which evolved into back-stepping facies belts of deltaic to nearshore depositional environment, Stages 2 and 3, due to the opening of a connection to the sea. The drainage pattern and accommodation distribution in the early synrift deposits reflect a relatively symmetrical basin that predates a later prominent half-graben configuration of the basin. Supplementary material: Figures presenting lithostratigraphic columns of the well data (Supplementary material 1), location of logs in Ragnardalen (Supplementary material 2) and geological map used in Figure 2b (Supplementary material 3) are available at https://doi.org/10.6084/m9.figshare.c.4293077

Gas accumulation conditions and key technologies for exploration & development of Sinian and Cambrian gas reservoirs in Anyue gasfield

Exploration of the Sinian-Lower Paleozoic formations in the Sichuan Basin starts from the 1950s, and is tortuous and complex. To summarize the exploration experiences is of great significance of reference for exploration of oil and gas in deep carbonate rocks of the Sichuan Basin. Gas accumulation in the Anyue oversize gasfield is chiefly controlled by the Deyang-Anyue intra-platform rift and central Sichuan paleouplift. The intra-platform rift controls the scale hydrocarbon generation center and effective hydrocarbon plays. The Anyue gasfield is characterized by near-source hydrocarbon accumulation. The marginal zone of intra-platform rift and high part of the paleouplift control the scale distribution of favorable facies belts of reservoir development, and overlapped by paleokarstic reformation, the reservoirs are widely distributed. The intra-platform rift and paleouplift jointly control formation of the scale traps and hydrocarbon enrichment. Gas reservoir features and major controlling factors for hydrocarbon accumulation have some differences in the main play strata, such as Longwangmiao Formation, and Member 4 and Member 2 of Dengying Formation. The gas reservoir in the Longwangmiao Formation is a lithologic gas reservoir under the tectonic setting, and the inherited paleouplift is a key for reservoir formation and hydrocarbon accumulation. The gas reservoir in the Member 4 of Dengying Formation is mainly controlled by large structural-stratigraphic composite traps, and the gas reservoir in the Member 2 of Dengying Formation is the structural trap. Distribution of the large gasfields in the Dengying Formation is controlled by the intra-platform rift, near-source hydrocarbon-reservoir configuration controlled by the ancient erosion plane and stratigraphic sealing in the updip direction. For exploration and development of the Anyue gasfield, four series of technologies are developed, namely, reservoir seismic fine description technology, well logging fine evaluation technology of gas layers, fast drilling technology of complex strata, and stimulation technology for high temperature and high pressure gas layers. Technical progresses provide powerful guarantee for efficient exploration and development of the oversize gasfields.

Integrated approach for zonation of a mid-Cenomanian carbonate reservoir in a sequence stratigraphic framework

The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bearing reservoir horizons in the Sirri Oil Fields of the Persian Gulf. Due to the general heterogeneity of carbonate reservoirs, the use of an integrated approach is helpful for investigating porosity and permeability distribution along with recognizing controlling pore system factors in the reservoir. Thus, for the reservoir characterization of the Mishrif Fm., an integrated approach including facies analysis, diagenetic history and sequence stratigraphic analysis is considered. Detailed petrographic studies showed a total of eight microfacies and seven facies belts, related to inner ramp to the basin of a homoclinal carbonate ramp. Humid climatic condition and tectonic activity, associated with eustatic sea-level fluctuations during the mid-Cretaceous, led to meteoric diagenesis of the Mishrif carbonates during subaerial exposures (mid-Cenomanian and Cenomanian-Turonian disconformities). General diagenetic overprints and modifications include micritization, cementation, dissolution, compaction, dolomitization, pyritization and fracturing. Considering this reservoir in the sequence stratigraphic framework reveals that the reservoir zones development is basically related to the Cenomanian–Turonian sequence boundary, recognized in the three studied wells, and also to the mid-Cenomanian boundary, identified only in one well. In addition, pore system properties were inspected by differentiation of Hydraulic Flow Units (HFUs) within the reservoir. The identified flow units, based on their capability for fluid flow, can be classified into four main rock types with very high- (HFUD), high- (HFUC), medium- (HFUB) and low-quality (HFUA). Accordingly, this study shows that the main part of the Mishrif Reservoir is affected by diagenetic processes related to subaerial exposures, resulting in zones with higher storage capacity and fluid flow rates. So, the study of depositional and diagenetic characteristics of the Mishrif carbonates in the sequence stratigraphy framework is essential to unravel the reservoir heterogeneity, and to describe the reservoir zones and their distribution in the field and regional scale. In addition, observed changes in the thickness of hydrocarbon column are attributed to the different location of the studied wells on the anticline structures, which show a tilted oil-water contact with a slope to the North.

The Triassic of Traill Ø and Geographical Society Ø, East Greenland: Implications for North Atlantic palaeogeography

The continental Triassic succession north of 72° in East Greenland has seen little investigation but is key in understanding how facies belts vary towards the East Greenland Shelf, and more widely in the North Atlantic region, through this period. This study presents sedimentological analysis of exposures in northern Traill Ø (the Mols Bjerge) and further north in northern Geographical Society Ø (Laplace Bjerg). These sections are correlated with the more widely studied succession which lies to the south, in Jameson Land and broad scale palaeogeographical reconstructions of the North Atlantic are presented. A largely continental Triassic succession of 1445 m was recorded from the Mols Bjerge. However, a 125 m thick clean sand unit (the Vega Sund Member), which includes in its upper part the Gråklint Beds, is re‐interpreted as of shallow marine origin. Over 1200 m of Triassic strata were recognized on Laplace Bjerg. Within this succession 300 m of clean, cross‐bedded sandstones are recorded and are correlated with the Vega Sund Member recorded in the Mols Bjerge. The northward increase in thickness of marine strata developed at this time provides evidence for a more extensive marine influence in the north. This work has important implications for regional palaeogeographies and the character of the Triassic succession in adjacent basins, in particular, constraining the southern extent of the Boreal Ocean during the Mid to Late Triassic. Furthermore, the correlations made places bitumen staining, reported from Laplace Bjerg, within Triassic strata, suggesting the viability of a Triassic play at depth in adjacent offshore basins.

Facies, depositional environments, and sequence stratigraphy analysis of the upper Barremian-lower Aptian carbonates in the northeast Kelardasht, N Iran

The carbonates accumulated within a time interval spanning the late Barremian-early Aptian, comprise the Member 2 of the Chalous Formation. The member cropped out in the northern Central Alborz Mountains Range, northern Iran. Detailed field surveys, lithofacies and facies analyses was carried out to investigate the depositional environments and sequence stratigraphy analysis at five outcrop sections. Based on petrographic investigation and texture, sixteen facies and microfacies were defined and assigned to the back-barrier lagoon, shelf margin/shoal, and shallow open marine facies belts. Vertical and lateral facies changes, lack of several geological evidence including siliciclastics, bioherms, calciturbidites, clinoforms, together with lateral thickness changes as well as comparison to recent and ancient carbonate platforms, suggest deposition on a non-rimmed isolated carbonate platform. The upper Barremian to lower Aptian deposits of the study areas comprises three third-order depositional sequences.We infer that relative sea-level changes and climate played a deeply influential role in the platform evolution and its geometry on the base of facies analysis, vertical transition of fauna, diagenetic overprints, accompanied by stacking patterns and geometries, and correlation of the depositional sequences.

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

AbstractThe Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO2/(CO2+H2O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.

Sequence stratigraphic evolution of the syn-rift Early Cretaceous sediments, West Beni Suef Basin, the Western Desert of Egypt with remarks on its hydrocarbon accumulations

The Beni Suef Basin is a petroliferous rift basin straddling the River Nile containing a thick Mesozoic–Paleogene succession. The Kharita Formation is formed in the syn-rift phase of the basin formation and is subdivided into the Lower and Upper Kharita members. These two members are regarded as two third-order depositional sequences (DSQ-1 and DSQ-2). The lowstand systems tract (LST-1) of the DSQ-1 is represented by thick amalgamated sandstone bodies deposited by active braided channels. Mid-Albian tectonic subsidence led to a short-lived marine invasion which produced coastal marine and inner-shelf facies belts during an ensuing transgressive systems tract (TST-1). At the end of the mid-Albian, a phase of tectonic uplift gradually rose the continent creating a fall in relative sea level, resulting in deposition of shallow marine and estuarine facies belts during a highstand systems tract (HST-1). During the Late Albian, a new phase of land-rejuvenation commenced, with a prolonged phase of fluvial depositional. Fluvial deposits consisted of belts of amalgamated, vertically aggraded sandstones interpreted as braided and moderately sinuous channels, in the lower part of the Upper Kharita Member lowstand stage (LST-2). The continuous basin filling, coupled with significant lowering in the surrounding highlands changed the drainage regime into a wide belt of meandering river depositing the transgressive stage (TST-2). The history of the Kharita Formation finalized with a Cenomanian marine transgressive phase. Economically, the TST-1 and HST-1 play a significant role as source rocks for hydrocarbon accumulations, whereas LST-2 act as good reservoir rocks in the Early Cretaceous in the Basin.

Western Canada Sedimentary Basin petroleum systems: A working and evolving paradigm

Western Canada Sedimentary Basin (WCSB) crude oil source rocks accumulated typically in “starved” depositional settings of Sloss outer detrital facies belts and lesser stratigraphic cycles. These produced petroleum from marine type II organic matter in response to burial by commonly westward-thickening overlying successions. Oil occurs commonly within the “Sloss” sequence containing its source rock, often up dip from the “petroleum kitchen.” Migration pathways cross stratal contacts, unconformities and structures, and much oil migrated into adjacent sequences, especially into Lower Cretaceous Mannville Group reservoirs. Anaerobic biodegradation affects oil quality and generates secondary biogenic gas. The WCSB oil system paradigm predates the recognition of anaerobic biodegradation. Biodegradation in post-Mannville reservoirs remains underappreciated. Natural gases originate by thermogenic and biogenic mechanisms from kerogens, coals, and crude oils. Gases are variably altered: physically, microbially, and inorganically. Few oil studies addressed solution and associated primary thermogenic or secondary biogenic gas. Gas studies are independent of oil studies and none recognize secondary biogenic gas even in association with biodegraded oils. We hypothesize that secondary biogenic gas occurs commonly, often mixed with other gas, to produce hydrocarbon isotope ratios and variations distinctive from primary biogenic and thermogenic gases. Where Mannville oil pools have sources in underlying marine rocks, Mannville gases are attributed largely to nonmarine sources. Currently, cross-stratal migration is inferred less commonly for gas than for oil. The inference of gas stratigraphic immobility is problematic for biodegradation studies that infer large secondary biogenic gas fluxes into soil and atmospheric sinks, the migration pathways of which pass through Cretaceous strata. In some unconventional plays, gas isotopic “rollover” and “reversal” due to thermal cracking has implications for reservoir performance. Efforts to understand Cordilleran petroleum systems merit investigation to extend unconventional resource plays westward from Interior Platform.

Microfacies, depositional environments and meter-scale cycles of the middle jurassic Tuwaiq Mountain formation, central Saudi Arabia

The Jurassic succession in Saudi Arabia represents the most prolific succession in the world. However, a detailed microfacies and paleoenvironmental model description, high-frequency cycles (HFC) analysis, and paleoenvironmental reconstruction of the main source rock interval for the Jurassic reservoirs, the Callovian Tuwaiq Mountain Formation (TMF) are poorly documented. This article reports on a study of the TMF, and discusses the succession of the microfacies, paleoenvironments, HFC, and depositional sequences, and paleoenvironmental constrains. On the basis of detailed field and lab work, 8 microfacies were recognized and interpreted to be deposited in four facies belts representing the inner part of a rimmed carbonate platform including: deep lagoon, shallow lagoon, shoal and peritidal and tidal flat. Within the deep and shallow lagoonal settings, massive platy and domal coral colonies form patch reefs. These patch reefs increase in size from deep to shallow lagoon settings under response to changing in environmental conditions; grater water clarity and harder substrate promoted larger patch reef size in shallow lagoon. Patch reefs decrease in size and disappear in shoal settings. The siliciclastic input was minute and could have been derived from tidal channels and/or windblown dust from the Arabian Shield. The stacking pattern of the microfacies revealed that the TMF represents a regressive cycle (3rd order depositional sequence), which is formed by repeated aggrading to prograding medium and small scale cycles. The HFC analysis gave an estimated duration of a small cycle about 45 Kyr, and an average rate of carbonate accumulation of 0.0455 m/Kyr. Therefore, medium and small scale cycles possibly represent 4th and 5th order depositional cycles, respectively, which are orbitally driven by high frequency eustatic fluctuations (Milankovitch cycles “obliquity”). The paleoenvironmental reconstruction revealed that the deposition occurred under the influence of occasional storm events with normal marine euhaline salinity and becomes more restricted towards the proximal part (lagoon to tidal flat).

The pre-Sinian rift in central Sichuan Basin and its control on hydrocarbon accumulation in the overlying strata

Abstract Based on the latest seismic data, resistivity profile, outcrop evidence and logging data, the structural features of basement in Sichuan Basin and its control on the hydrocarbon accumulation in the Sinian-Cambrian strata was discussed. It was found that a NE striking pre-Sinian rift was developed across the whole basin. Controlled by a series of rift-parallel normal faults, horst-graben structures were developed inside the rift, large horst-graben structures and later activity of their boundary faults controlled the distribution of beach facies of the overlying strata. The horst-graben structures induced the formation of local highs of ancient landform and controlled the successive development of overlapped bioherm beach facies in long-term marine setting from the Sinian period to the Permian period, and as a result a widely distributed favorable sedimentary facies belt was developed. The pre-Sinian rift and later activities of related normal faults controlled the development of the grain beach and karst reservoirs and the deposition of high quality source rock, which form structural-lithologic traps. Through comprehensive evaluation, two large structural-lithologic composite trap favorable exploration areas in the south and north of the Gaoshiti-Moxi area, were selected.

Types, distribution and play targets of Lower Cretaceous tight oil in Jiuquan Basin, NW China

Abstract Based on drilling and laboratory data, the formation conditions of tight oil reservoirs in the Jiuquan basin were comprehensively analyzed and the exploration domains were sorted out. The Jiuquan basin underwent three cycles of lake level fluctuation in early Cretaceous, leaving three sets of high-quality source rocks, the Zhonggou, Xiagou and Chijinbao Formations in the Lower Cretaceous. There are two types of reservoir assemblages, source-reservoir in one type and source below reservoir type, and two types of tight reservoirs, argillaceous dolomite and conglomerate. The “sweet spots” control the enrichment of oil and gas. Argillaceous dolomite tight oil reservoirs have the characteristic of “integrated source-reservoir”, with fractures connecting the matrix micro-pores, pore-fracture type and fracture-pore type “sweet spots” distributed in large scale. The sandy conglomerate tight oil reservoirs were formed by source-reservoir lateral connection, and can be divided into source below reservoir type, source-reservoir side by side type and “sandwich” type. The overlapping areas of the favorable facies belts of fan-delta front and the secondary pore developing belts are the “sweet spot” sites. The favorable areas for seeking conglomerate tight oil are fan-delta front deposits around the Qingxi, Ying'er and Huahai sags, with an exploration area of 550 km2; while the area to seek argillaceous dolomite tight oil is the NW fracture developed belt in Qingxi sag, with an exploration area of 100 km2.

Main controlling factors and distribution of high-quality deep dolomite reservoirs in typical cratonic basins in China

Deep dolomite in cratonic basins is an essential target for future natural gas exploration in China. Statistical analyses of major cratonic basins in China, such as the Sichuan, Tarim, and Ordos basins, indicate that the major types of deep dolomite (including granular dolomite, microbial dolomite, and crystalline dolomite) where fracture-vug and fracture-pore dominate the pore types. It is proposed that the favorable sedimentary microfacies is the base, whereas early dissolution is a necessary condition to form pores. Early dolomitization in penecontemporaneous and shallow burial periods is helpful to preserving the pores that developed during the penecontemporaneous period. Additionally, supergene karstification and structural fractures improve reservoir physical properties. It is pointed that sedimentary facies still control the referred high-quality deep dolomite reservoirs; they inherited more than being transformed in structure. The early sedimentary cycles, rather than depth, control their vertical distribution. Meanwhile, their lateral distribution is controlled by high-energy sedimentary facies belt rather than karstification. It is suggested that future exploration of high-quality deep dolomite reservoirs should target “both sides of an intra-platform rift” and low-middle parts of carbonate slopes.

Late Smithian microbial deposits and their lateral marine fossiliferous limestones (Early Triassic, Hurricane Cliffs, Utah, USA)

Recurrent microbialite proliferations during the Early Triassic are usually explained by ecological relaxation and abnormal oceanic conditions. Most Early Triassic microbialites are described as single or multiple lithological units without detailed ecological information about lateral and coeval fossiliferous deposits. Exposed rocks along Workman Wash in the Hurricane Cliffs (southwestern Utah, USA) provide an opportunity to reconstruct the spatial relationships of late Smithian microbialites with adjacent and contemporaneous fossiliferous sediments. Microbialites deposited in an intertidal to subtidal interior platform are intercalated between inner tidal flat dolosiltstones and subtidal bioturbated fossiliferous limestones. Facies variations along these fossiliferous deposits and microbialites can be traced laterally over a few hundreds of meters. Preserved organisms reflect a moderately diversified assemblage, contemporaneous to the microbialite formation. The presence of such a fauna, including some stenohaline organisms (echinoderms), indicates that the development of these late Smithian microbial deposits occurred in normal-marine waters as a simple facies belt subject to relative sea-level changes. Based on this case study, the proliferation of microbialites cannot be considered as direct evidence for presumed harsh environmental conditions.

Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.

The Carbonate Platform Model and Reservoirs’ Origins of the Callovian-Oxfordian Stage in the Amu Darya Basin, Turkmenistan

The Calloviane-Oxfordian carbonates in the northeastern Amu Darya Basin of southeastern Turkmenistan are composed of medium- to thick-bedded, mostly grainy limestones with various skeletal (bivalves, brachiopods, echinoderms, foraminifera, corals, and sponge) and non-skeletal grains (intraclasts, ooids and peloids). Two facies zones, six standard facies belts and some microfacies types were recognized, and sedimentary model “carbonate ramp-rimmed platform” was proposed and established that can be compared with the classical carbonate sedimentary models. In this model, favorable reservoirs not only developed in the intraplatform shoal of open platform, or reef and shoal on the platform margin, but also in the patch reefs, shoal and mound facies on the upper slope. The reservoir’s pore space is dominated by intergranular and intragranular pores and fissure-pore reservoirs exist with medium porosity and medium to low permeability. Sedimentary facies and diagenetic dissolution are the key controlling factors for the development of high-quality reservoirs.

Implications of sea-level rise in a modern carbonate ramp setting

This study addresses a gap in our understanding of the effects of sea-level rise on the sedimentary systems and morphological development of recent and ancient carbonate ramp settings. Many ancient carbonate sequences are interpreted as having been deposited in carbonate ramp settings. These settings are poorly-represented in the Recent. The study documents the present-day transgressive flooding of the Abu Dhabi coastline at the southern shoreline of the Arabian/Persian Gulf, a carbonate ramp depositional system that is widely employed as a Recent analogue for numerous ancient carbonate systems. Fourteen years of field-based observations are integrated with historical and recent high-resolution satellite imagery in order to document and assess the onset of flooding. Predicted rates of transgression (i.e. landward movement of the shoreline) of 2.5myr−1 (±0.2myr−1) based on global sea-level rise alone were far exceeded by the flooding rate calculated from the back-stepping of coastal features (10–29myr−1). This discrepancy results from the dynamic nature of the flooding with increased water depth exposing the coastline to increased erosion and, thereby, enhancing back-stepping. A non-accretionary transgressive shoreline trajectory results from relatively rapid sea-level rise coupled with a low-angle ramp geometry and a paucity of sediments. The flooding is represented by the landward migration of facies belts, a range of erosive features and the onset of bioturbation. Employing Intergovernmental Panel on Climate Change (Church et al., 2013) predictions for 21st century sea-level rise, and allowing for the post-flooding lag time that is typical for the start-up of carbonate factories, it is calculated that the coastline will continue to retrograde for the foreseeable future. Total passive flooding (without considering feedback in the modification of the shoreline) by the year 2100 is calculated to likely be between 340 and 571m with a flooding rate of 3.40–8.64myr−1. However, adopting the observation that global sea-level rise only accounts for 15% of the recorded shoreline retreat, this figure rises dramatically to a total likely dynamic flooding (considering modifications to the shoreline) of between 2.3 and 3.8km. Loss of microbial and mangal habitats will subject the exposed shoreline to increasing erosion. Shoreline retreat will threaten existing coastal infrastructure.

Potentials of low permeability gas in intracratonic basin: Insights from sedimentary facies of the Shan1 Member in the Su6‐Zhao42 Block of the Sulige gas field, Ordos Basin

The Sulige gas field is a typical low permeability gas field with multi‐thin layer of pay zones. The upper member of the Shanxi Formation (Shan1) is the primary productive layer of the Su6‐Zhao42 area, located in the north‐west of the Shanbei Slope of the Sulige gas field. The Shan1 Member could be divided into 3 tiny sequences by coring and well logging data. On the basis of sedimentary sequence analysis, the sedimentary environment and facies are studied combined with lithological information, logging data, and sampling. This paper summarizes the characteristics and distribution of the sedimentary facies of the Shan1 Member in the Su6‐Zhao42 area. During the deposition of the Shan1 Member, this area developed a meandering river sedimentary facies, which can be subdivided into meandering riverbed, river floodplain, and abandoned channel subfacies. The sedimentary facies of meandering river in the Shan1 Member are characterized by point bars, crevasse splays, floodplains, flood swamps, and abandoned channels. The sedimentary facies bands extend roughly east‐west parallel to the meandering river. It is indicated that the sand bodies deposited in river channel have better physical conditions and larger thickness for gas accumulation based on the analysis of physical characteristics and seismic attributes. The effective reservoir, being controlled by the sedimentary facies belt, distributes along a NE–SW trending belt where is the favourable area for the gas reservoir.

Seismic stratigraphy of Dinantian carbonates in the southern Netherlands and northern Belgium

AbstractDue to their potential as a petroleum or geothermal system, the Dinantian carbonates of the Netherlands have recently attracted renewed interest because of the identified presence of excellent reservoir properties. This notion contrasts with the general assumption that these carbonates are tight. Therefore, in order to give the current knowledge state, this paper re-examines the sparse publicly available well and seismic data and literature to assess the distribution and reservoir properties of the Dinantian carbonates.Dinantian carbonate deposition occurred throughout the study area (southern onshore and offshore of the Netherlands and northern Belgium), which is situated on the northern margin of the London–Brabant Massif, progressively onlapping the latter structure. This study confirms the presence of three carbonate facies types in the study area: a Tournaisian low-gradient carbonate ramp system, succeeded by a succession in which the carbonate ramp system evolved to a rimmed shelf setting. Subsidence of the northern margin of the London–Brabant Massif resulted in a landward shift of the shallow-marine facies belts, while the formation of normal faults resulted in a ‘staircase’-shaped shallow-water platform–slope–basin profile, associated with large-scale resedimentation processes. After deposition, the limestone deposits were frequently exhumed and reburied. A first period of regional exhumation occurred at the end of the Dinantian, which seems to be associated with porosity-enhancing meteoric karstification, possibly limited to the palaeo-shelf edge. The most intense alterations seem to be present as a deep leached horizon below the Cretaceous unconformity at the top of the Dinantian sequences. In addition, clear evidence for hydrothermal fluid migration is found locally, enhancing reservoir properties at some places while occluding porosity at others. The timing of these phases of hydrothermal fluid circulation is poorly understood.Whereas in the United Kingdom hydrocarbons have been produced from karstified Dinantian carbonates, this petroleum play has received little attention in the Netherlands. This paper shows that, also for the Netherlands, a karstic reservoir probably existed before the start of hydrocarbon generation from the onlapping basal Namurian shales. The hydrocarbon prospectivity of these sediments, however, is primarily controlled by the presence of both a karst-related reservoir and migration routes from a decent-quality source rock. Two geothermal projects producing from this reservoir in the southern onshore Netherlands have shown the potential of the Dinantian carbonates for ultra-deep geothermal projects. To conclude, the findings presented herein are relevant for studies of the hydrocarbon prospectivity and studies of the geothermal potential of Dinantian carbonates in the Dutch on- and offshore.