Reef Margin Research Articles

Sequence stratigraphic analysis of the late Permian Changhsingian platform marginal reef, Western Hubei, South China

This study presents a comprehensive analysis of the late Permian platform marginal reefs in the Sichuan Basin, focusing on reefal lithofacies and sequence stratigraphic patterns. Field outcrop observations and rock sample analyses from the Jiantianba reef were conducted to establish an evolution model of sponge reef development and spatial distribution. Four stages of marginal carbonate platform were documented: open platform, gently sloping reef, steeply sloping reef, and reef bank system. Distinct lithofacies were identified in these stages, reflecting different depositional environments and growth rates. The gently sloping reef was composed of filled skeleton framestone, filled skeleton bafflestone, and micrite organism limestone, indicating limited reef-building capacity. In contrast, the lithofacies of steeply sloping reefs were composed of open skeleton framestone, open skeleton bafflestone, binding skeleton bafflestone, and benthic organism bindstone, indicating stronger reef-building ability. Based on depositional features and carbon isotopic trends, the reef strata were divided into two sequences. Sequence 1 corresponds to the formation of unit 1, and sequence 2 can be further divided into units 2 and 3. In unit 1, reefs developed in a relatively deeper-water setting. It was characterized by rich micrite limestone, forming a gentle margin. Unit 2 witnessed reef development in shallower waters. Early marine cementation and microbial clots were prevalent, contributing to form a steep margin. In the early stage of unit 3, reefs primarily developed in a tidal-controlled environment. Subsequently, reef strata experienced a transition to a wave-influenced environment, leading to the formation of a reef bank system. In general, sequence 1 mainly formed in a heterozoan-dominated factory, and reefs contributed to a relatively gently sloping platform margin. In contrast, sequence 2 formed in a photozoan-dominated factory, and reefs contributed to a relatively steeply sloping platform margin.

Bacterial and Symbiodiniaceae communities’ variation in corals with distinct traits and geographical distribution

Coral microbiomes play crucial roles in holobiont homeostasis and adaptation. The host’s ability to populate broad ecological niches and to cope with environmental changes seems to be related to the flexibility of the coral microbiome. By means of high-throughput DNA sequencing we characterized simultaneously both bacterial (16S rRNA) and Symbiodiniaceae (ITS2) communities of four reef-building coral species (Mussismilia braziliensis, Mussismilia harttii, Montastraea cavernosa, and Favia gravida) that differ in geographic distribution and niche specificity. Samples were collected in a marginal reef system (Abrolhos, Brazil) in four sites of contrasting irradiance and turbidity. Biological filters governed by the host are important in shaping corals’ microbiome structure. More structured associated microbial communities by reef site tend to occur in coral species with broader geographic and depth ranges, especially for Symbiodiniaceae, whereas the endemic and habitat-specialist host, M. braziliensis, has relatively more homogenous bacterial communities with more exclusive members. Our findings lend credence to the hypothesis that higher microbiome flexibility renders corals more adaptable to diverse environments, a trend that should be investigated in more hosts and reef areas.

Microplastic pollution in tropical coral reef ecosystems from the coastal South China Sea and their impacts on corals in situ

Coral reefs possess extremely high ecological value in tropical and subtropical waters worldwide. Microplastics as emerging and pervasive pollutants pose a great threat to the health of coral ecosystems. However, in situ studies on microplastics pollution and its impacts in coral ecosystems globally are limited. The occurrence characteristics of microplastics in the environment mediums and reef-dwelling organisms were investigated in coral reef areas from the southern Hainan Island, and the impacts of microplastics on corals in situ were evaluated in this study. Average microplastics abundance was 9.48 items L-1 in seawater, 190.00 items kg-1 in sediment, 0.36 items g-1 in coral, 1.50 items g-1 in shellfish, 0.48 items g-1 in fish gill, and 1.71 items g-1 in fish gastrointestinal tract. The prevalent microplastics in the above samples were characterized as being less than 1000 µm in size, fibrous, and transparent, with predominant polymer types as polyethylene terephthalate, polypropylene, polyethylene, and rayon. The microplastic enrichment capacity of different corals varied (Pocillopora >Acropora >Sinularia). Notably, microplastics were more abundant on the surface of corals compared to their interiors, with distinct characteristics observed, including larger-sized (>500 µm) and fiber-shaped polyethylene terephthalate microplastics on the surface and smaller-sized (20–200 µm) fragmented polyethylene microplastics within coral interiors. Furthermore, the investigation showed species-specific impacts of microplastics on corals in situ, including photosynthetic activity of photosymbionts and antioxidant and immune activities of corals. Furthermore, the ecological risks of microplastics were minor across most environmental media in the studied areas, with exceptions in the bottom seawater and surface sediment of YLW, which exhibited extreme and medium risk levels, respectively. Coral risk levels were generally medium, except for dangerous levels in DDH and high levels in LHT. The potential sources of microplastics in the marginal reefs of southern Hainan Island were primarily tourism, residential, and fishing activities.

Revisiting 150 years of coral studies to assess changes in species records, distribution, and functional structure of corals in the Brazilian Province

Historical information serves as a crucial foundation for understanding past natural ecosystems, enabling us to track changes in biodiversity. It plays an indispensable role in both biodiversity conservation and restoration ecology. In the Brazilian Province, there is still an absence of historical accounts relative to the coral assemblage diversity. We compiled historical and modern information on coral species and mapped coral diversity in the Brazilian Province over the past 150 years. Using abundance data, we contrasted the functional structure of coral assemblages sampled 40 years apart (1960–2000) in the Abrolhos Bank, the largest reef complex of the Southwestern Atlantic Ocean. The number of species records increased over time, especially after the 1960's, with the Abrolhos region amassing numerous historical accounts and harboring the greatest coral richness. This increase in species records reflects the recent accumulation of knowledge about these marginal reefs. Historical declines of two major reef building species, Millepora alcicornis and Mussismilia braziliensis, resulted in temporal changes in their relative contributions to the functional structure of coral assemblages in the Abrolhos Bank. The erosion of such assemblages within the past 40 years highlights the importance of considering historical information to adjust environmental baselines, setting proper conservation and restoration targets.

Reefal ostracod assemblages from the Zanzibar Archipelago (Tanzania)

Abstract. Tropical reefs encompass tremendous biodiversity yet are imperiled by increasing natural and anthropogenic disturbances worldwide. Meiobenthic biota on coral reefs, for example, ostracods, may experience substantial diversity loss and compositional changes even before being examined. In this study, we investigated the reefal ostracod assemblages from the highly diverse and productive ecosystem in the Zanzibar Archipelago (Pemba, Zanzibar, and Mafia islands), Tanzania, to understand how their diversity and faunal structure vary in response to water depth, benthic community type, and human impacts. We characterized four distinct ostracod faunas associated with different benthic habitats, which were deep fore reefs, shallow fringing reefs, degraded fringing reefs, and algae-covered intertidal flats. We identified typical ostracod associations, i.e., Bairdiidae versus Loxoconchidae–Xestoleberididae, that showed affinities towards hard corals or algae on the reef platforms, respectively. The highest diversity was found on shallow fringing reefs where coral-affined and algae-affined taxa exhibited maximum overlap of their distributional ranges, while the sand flats, mangrove, and marginal reefs within the intertidal zone had much lower diversity with a high dominance of euryhaline taxa. Along the western coast of Zanzibar Island, coastal development likely resulted in a unique faunal composition and comparatively low diversity of ostracod assemblages among those in reefal habitats, in conjunction with overall reef ecosystem degradation. This study represents the first large-scale assessment of shallow-marine ostracods in the Zanzibar Archipelago. It lays a solid foundation for future research into the ecological significance of ostracods on coral reefs.

Operational Mapping of Submarine Groundwater Discharge into Coral Reefs: Application to West Hawai‘i Island

Submarine groundwater discharge (SGD) is a recognized contributor to the hydrological and biogeochemical functioning of coral reef ecosystems located along coastlines. However, the distribution, size, and thermal properties of SGD remain poorly understood at most land–reef margins. We developed, deployed, and demonstrated an operational method for airborne detection and mapping of SGD using the 200 km coastline of western Hawai‘i Island as a testing and analysis environment. Airborne high spatial resolution (1 m) thermal imaging produced relative sea surface temperature (SST) maps that aligned geospatially with boat-based transects of SGD presence–absence. Boat-based SST anomaly measurements were highly correlated with airborne SST anomaly measurements (R2 = 0.85; RMSE = 0.04 °C). Resulting maps of the relative difference in SST inside and outside of SGD plumes, called delta-SST, revealed 749 SGD plumes in 200 km of coastline, with nearly half of the SGD plumes smaller than 0.1 ha in size. Only 9% of SGD plumes were ≥1 ha in size, and just 1% were larger than 10 ha. Our findings indicate that small SGD is omnipresent in the nearshore environment. Furthermore, we found that the infrequent, large SGD plumes (>10 ha) displayed the weakest delta-SST values, suggesting that large discharge plumes are not likely to provide cooling refugia to warming coral reefs. Our operational approach can be applied frequently over time to generate SGD information relative to terrestrial substrate, topography, and pollutants. This operational approach will yield new insights into the role that land-to-reef interactions have on the composition and condition of coral reefs along coastlines.

Genomic data reveals habitat partitioning in massive Porites on Guam, Micronesia

Corals in marginal reef habitats generally exhibit less bleaching and associated mortality compared to nearby corals in more pristine reef environments. It is unclear, however, if these differences are due to environmental differences, including turbidity, or genomic differences between the coral hosts in these different environments. One particularly interesting case is in the coral genus Porites, which contains numerous morphologically similar massive Porites species inhabiting a wide range of reef habitats, from turbid river deltas and stagnant back reefs to high-energy fore reefs. Here, we generate ddRAD data for 172 Porites corals from river delta and adjacent (<0.5 km) fore reef populations on Guam to assess the extent of genetic differentiation among massive Porites corals in these two contrasting environments and throughout the island. Phylogenetic and population genomic analyses consistently identify seven different clades of massive Porites, with the two largest clades predominantly inhabiting either river deltas or fore reefs, respectively. No population structure was detected in the two largest clades, and Cladocopium was the dominant symbiont genus in all clades and environments. The perceived bleaching resilience of corals in marginal reefs may therefore be attributed to interspecific differences between morphologically similar species, in addition to potentially mediating environmental differences. Marginal reef environments may therefore not provide a suitable refuge for many reef corals in a heating world, but instead host additional cryptic coral diversity.

Local adaptation of trophic strategy determined the tolerance of coral Galaxea fascicularis to environmental fluctuations

The escalation of global change has resulted in heightened frequencies and intensities of environmental fluctuations within coral reef ecosystems. Corals originating from marginal reefs have potentially enhanced their adaptive capabilities in response to these environmental variations through processes of local adaptation. However, the intricate mechanisms driving this phenomenon remain a subject of limited investigation. This study aimed to investigate how corals in Luhuitou reef, a representative relatively high-latitude reef in China, adapt to seasonal fluctuations in seawater temperature and light availability. We conducted a 190-day plantation experiment with the widespread species, Galaxea fascicularis, in Luhuitou local, and from Meiji reef, a typical offshore tropical reef, to Luhuitou as comparison. Drawing upon insights from physiological adaptations, we focused on fatty acid (FA) profiles to unravel the trophic strategies of G. fascicularis to cope with environmental fluctuations from two origins. Our main findings are threefold: 1) Native corals exhibited a stronger physiological resilience compared to those transplanted from Meiji. 2) Corals from both origins consumed large quantities of energy reserves in winter, during which FA profiles of local corals altered, while the change of FA profiles of corals from Meiji was probably due to the excessive consumption of saturated fatty acid (SFA). 3) The better resilience of native corals is related to high levels of functional polyunsaturated fatty acid (PUFA), while insufficient nutrient reserves, possibly due to weak heterotrophic ability, result in the obstruction of the synthesis pathway of PUFA for corals from Meiji, leading to their intolerance to environmental changes. Consequently, we suggest that the tolerance of G. fascicularis to environmental fluctuations is determined by their local adapted trophic strategies. Furthermore, our findings underscore the notion that the rapid adaptation of relatively high-latitude corals to seasonal environmental fluctuations might not be readily attainable for their tropical counterparts within a brief timeframe.

Millennial calcification trends in Porites corals: Resilient skeletal density and vulnerable linear extension in response to environmental change

Global coral reefs are currently facing widespread declines in calcification rates, a trend that underscores the need to understand variations between calcification in contemporary and ancient corals. This study investigates the changes in coral calcification over the last millennium by analyzing 21 subfossil and 11 modern coral samples from a marginal reef in the northern South China Sea. Our results reveal that although coral skeletal density has remained relatively stable, significant differences are evident in linear extension rates between the Medieval Climate Anomaly (MCA, 900–1300 CE) and the Little Ice Age (LIA, 1550–1850 CE), as well as between the pre- and post-industrial periods. This finding suggests that inshore Porites corals have greater resilience in their skeletal density compared to linear extension rates when responding to environmental changes. Our analysis indicates that sea surface temperature (SST) is the primary environmental driver influencing coral calcification, with a positive correlation observed between SST and both modern coral linear extension and calcification rates. In contrast, factors such as seawater pH, salinity, and total solar irradiance appear to have minimal impact on coral skeletal growth. Therefore, the lower linear extension and calcification rates observed in the LIA corals, as compared to the MCA corals, are likely due to the cooler temperatures during the LIA. We also hypothesize that the extended period of cold SST from the LIA to the early 20th century may have led to the reduced linear extension and calcification rates in modern corals, with recent warming insufficient to facilitate a return to the optimal skeletal growth observed during the MCA.

The growth and demise of a Middle Miocene carbonate platform in Central Luconia, offshore Malaysia

The evolution and facies distribution of relatively small carbonate platforms, ∼30 km2, are not well documented, even though they are common in ancient and modern carbonate systems. This study investigates depositional facies and evolution of a Miocene carbonate platform in Central Luconia, offshore Malaysia. An integrated approach, including core-described lithofacies, well-log correlation, and seismic analysis, allowed for the mapping of facies through time. The platform was divided into five stratigraphic zones (1 [top], 2 Upper, 2 Lower, 3 Upper, 3 Lower [bottom]) separated by low porosity intervals. Five depositional facies groups were recognized in core samples: (1) bioclastic wacke-packstone facies deposited in a platform interior, (2) bioclastic packstone facies deposited in backreef environments, (3) coral boundstone facies deposited at the reef margin, (4) interbedded shale and carbonate facies deposited on the slope, and (5) argillaceous carbonate facies deposited across the platform as flooding layers. Well D penetrated almost the entire platform, and facies in that well changed upward from mainly bioclastic wacke-packstone at the bottom to mainly bioclastic packstone in the middle to coral boundstone near the top before being covered by interbedded shale and carbonate. Six seismic facies were identified and combined with well data to map the five facies groups through time. The platform shows repeated backstepping of facies at flooding layers followed by aggradation of facies belts up to exposure surfaces, resulting in a long-term backstepping of depositional environments until the platform drowned. Sr-isotopes dating near the top of the platform yielded an age of approximately 12 Ma. This suggests that the platform drowned earlier than other carbonate platforms in Central Luconia (8–10 Ma). More tectonic subsidence in this area during the Middle Miocene is proposed as the main cause of the early demise of this carbonate platform compared to other Luconia platforms.

Global patterns of herbivorous reef fish productivity: the role of Prionurus laticlavius in the Galápagos

Herbivorous fishes play important roles on coral reefs, acting as key trophic conduits of primary productivity. Whilst these roles are widely appreciated on tropical reefs, the relative contribution of fishes which inhabit marginal reefs, such as Prionurus surgeonfishes, is not well understood. Here, we examine the extent to which herbivorous fish productivity varies amongst global ecoregions, specifically considering the relative contribution of Prionurus. We also compare the productivity of Prionurus to that of other herbivorous fishes in relation to water temperatures. Our analysis revealed that the Eastern Galápagos Islands support the highest levels of herbivorous fish productivity recorded to date, with Prionurus laticlavius accounting for over 94% of that productivity. Moreover, Prionurus productivity peaked at relatively cool water temperatures (~ 22–25 °C), although patterns were driven by P. laticlavius. These results highlight the exceptional herbivorous fish productivity in the Eastern Galápagos Islands and the disproportionate contribution of P. laticlavius in this locality.

The aesthetic value of Brazilian reefs: from species to seascape

The marine wildlife and seascape components of reef environments are part of peoples' senses and perceptions of nature and and ecosystem services. While people worldwide perceive and value reef components differently, such aesthetics evaluations often overlook marginal reefs, limiting our comprehensive understanding of their cultural significance. We interviewed 320 different users (e.g., recreational divers, researchers, recreational and commercial fishers, and beach tourists) using an online questionnaire with embedded pictures of 82 fish species and 65 seascapes. To assess people's perception about reefs, we related the aesthetic value (i.e., the preference rate) of each picture to body size, body shape and colour for bony and cartilaginous fish, and to the presence of predators, fish schools, benthic organisms, reef patches and area for the underwater and out-of-the-water seascape. We then created an index composed by site-level species aesthetic values, species relative abundances, under and out-of-water aesthetics and measuredthe overall aesthetic value of nine reef sites along the Brazilian province. The different users attributed similar aesthetic values to fish species, under and out-of-the-water components, yet preferences differed among them. Divers preferred large parrotfishes, while tourists preferred coloured fish. The most valued fish traits were the body shape of manta rays and colorful bony species. Out-of-the-water seascapes with emerging reef patches had the highest aesthetic values. When combining the different aesthetic value components in the index, we found that marginal biogenic reefs in Northeastern Brazil had the highest aesthetic value among the evaluated sites. These reefs hold immense value for national tourism, significantly contributing to the revenue of these states. Also, they harbour threatened species, which possess both high aesthetic value and great ecological importance. Therefore, incorporating aesthetic value in conservation initiatives has the potential to improve our ability to maintain and develop cultural reef services.

New branched Porolithon species (Corallinales, Rhodophyta) from the Great Barrier Reef, Coral Sea, andLord Howe Island.

Porolithon is one of the most ecologically important genera of tropical and subtropical crustose (non-geniculate) coralline algae growing abundantly along the shallow margins of coral reefs and functioning to cement reef frameworks. Thalli of branched, fruticose Porolithon specimens from the Indo-Pacific Ocean traditionally have been called P. gardineri, while massive, columnar forms have been called P. craspedium. Sequence comparisons of the rbcL gene both from type specimens of P. gardineri and P. craspedium and from field-collected specimens demonstrate that neither species is present in east Australia and instead resolve into four unique genetic lineages. Porolithon howensis sp. nov. forms columnar protuberances and loosely attached margins and occurs predominantly at Lord Howe Island; P. lobulatum sp. nov. has fruticose to clavate forms and free margins that are lobed and occurs in the Coral Sea and on the Great Barrier Reef (GBR); P. parvulum sp. nov. has short (<2 cm), unbranched protuberances and attached margins and is restricted to the central and southern GBR; and P. pinnaculum sp. nov. has a mountain-like, columnar morphology and occurs on oceanic Coral Sea reefs. A rbcL gene sequence of the isotype of P. castellum demonstrates it is a different species from other columnar species. In addition to the diagnostic rbcL and psbA marker sequences, the four new species may be distinguished by a combination of features including thallus growth form, margin shape (attached or unattached), and medullary system (coaxial or plumose). Porolithon species, because of their ecological importance and sensitivity to ocean acidification, need urgent documentation of their taxonomic diversity.

High‐latitude marginal reefs support fewer but bigger corals than their tropical counterparts

Anthropogenic impacts are typically detrimental to tropical coral reefs, but the effect of increasing environmental stress and variability on the size structure of coral communities remains poorly understood. This limits our ability to effectively conserve coral reef ecosystems because size specific dynamics are rarely incorporated. Our aim is to quantify variation in the size structure of coral populations across 20 sites along a tropical‐to‐subtropical environmental gradient on the east coast of Australia (~ 23 to 30°S), to determine how size structure changes with a gradient of sea surface temperature, turbidity, productivity and light levels. We use two approaches: 1) linear regression with summary statistics (such as median size) as response variables, a method frequently favoured by ecologists and 2) compositional functional regression, a novel method using entire size–frequency distributions as response variables. We then predict coral population size structure with increasing environmental stress and variability. Together, we find fewer but larger coral colonies in marginal reefs, where conditions are typically more variable and stressful, than in tropical reefs. Our model predicts that coral populations may become gradually dominated by larger colonies (&gt; 148 cm2) with increasing environmental stress. Fewer but bigger corals suggest low survival of smaller corals, slow growth, and/or poor recruitment. This finding is concerning for the future of coral reefs, as it implies that current marginal populations, or future reefs in increasingly stressful environmental conditions may have low recovery potential. We highlight the importance of continuously monitoring changes to population structure over biogeographic scales.

Contribución de los heterótrofos a la calcificación secundaria en arrecifes marginales del Pacífico mexicano

Contribución de los heterótrofos a la calcificación secundaria en arrecifes marginales del Pacífico mexicano

Adaptation strategies of relatively high-latitude marginal reef corals in response to severe temperature fluctuations

The large seasonal temperature fluctuations caused by global warming and frequent marine heatwaves pose new challenges to survival of relatively high-latitude marginal reef corals. However, the adaptation strategies of high-latitude marginal corals are not fully understood. We employed integrated approach to investigate the response mechanism of hosts, Symbiodiniaceae, and symbiotic bacteria of marginal reef corals Acropora pruinosa and Pavona decussate in response to large seasonal temperature fluctuations. The coral holobiont maintained a high level of immunity to adapt to seasonal pressure by increasing Symbiodiniaceae energy supply. The symbiotic Symbiodiniaceae of two coral was dominated by C1 subgroup, and was stable across seasons. The α-diversity of symbiotic bacteria P. decussata and A. pruinosa in summer was higher than that in winter. The symbiotic bacterial community of two coral reorganized during different seasons. Scleractinian corals improve adaptability to seasonal stress by increasing energy supply to maintain high levels of immunity, increasing symbiotic bacterial α-diversity, and changing dominant bacteria. This study demonstrates the adaptation strategies of marginal reef corals to seasonal temperature fluctuations and provides novel insights into the study of the adaptation of corals and relatively high-latitude coral refuges in the context of global warming and intensified marine heatwaves.

Characterizing canopy complexity of natural and restored intertidal oyster reefs (Crassostrea virginica) with a novel laser‐scanning method

The structural complexity of oyster reef canopy plays a major role in promoting biodiversity, balancing the sediment budget, and modulating hydrodynamics in estuarine systems. Although oyster canopy structure is both spatially and temporally heterogeneous, oyster canopies are generally characterized using simple first‐order quantities, like oyster density, which may lack the ability to sufficiently parameterize reef roughness. In this study, a novel laser‐scan approach was used to map the surface of intact reference and restored reefs (restoration age: 1–4 years) during low tide, when the oyster canopy was fully exposed. Measurements were used to estimate hydrodynamically relevant roughness characteristics over the entire reef surface (&gt;140 m2; 0.50 m resolution), providing estimates of the canopy height (hc), standard deviation (), rugosity index (R), and fractal dimension (D). Average canopy heights ranged from 3.6 to 4.9 cm, with canopy height standard deviations between 1.4 and 2.0 cm. Mean rugosity indices and fractal dimensions were relatively low on the youngest (1 year) restored reef (R = 1.28;D = 2.67), with substantial increases observed for more mature reef canopies (4 years:R = 1.56;D = 2.71). Structural complexity was consistently greater on reef margins than in reef interiors. Increases in complexity were linked to restoration age, with older reefs exhibiting more complex oyster canopies. The highest fractal dimension was observed on the intact reference reef, highlighting the importance of sustained reef growth for maintaining higher‐order structural complexity. Results provide spatially explicit surface roughness characterizations for healthy, intertidal oyster reefs, with applications in both restoration science and natural and nature‐based feature design.

Marginal Reef Systems: Resilience in a Rapidly Changing World

Marginal coral reefs live under sub-optimal environmental conditions (e [...]

Prediction of Permian karst reservoirs in the Yuanba gas field, northern Sichuan Basin, China

Karst reservoir has great hydrocarbon potential, however karst reservoir prediction is inhibited by the strong lateral and vertical heterogeneity of karstification which in turn results in recognition difficulty when geological and geophysical methods are used in isolation. Combined geological and geophysical methods, including core observation, thin section analysis, well log interpretation, seismic attributes and seismic inversion are applied to understand the depositional environment, types of karstification, the geophysical response and distribution of karst reservoir and the controls over and evolution model of karst reservoir in Permian Yuanba gas field, northern Sichuan Basin. The results show that there are four types of major seismic facies recognized in the study area, which correspond to platform margin reef, carbonate platform, platform margin inter-bay and platform margin slope. Karstification can be divided into three zones: the supergene karst zone, the vertical seepage zone and the horizontal underflow zone, among which the supergene karst zone have the strongest karstification. Karstification is highlighted with low seismic impedance, low Poisson's ratio, and high seismic attenuation on seismic inversion as well as micro scale paleo geomorphology recognized by trend surface analysis. By comparing the distribution of karstification with paleo geomorphology, fault distribution and gas contents, it can be observed that the karstification have a good matching relationship with these factors. The dominant type of karstification is epigenic karst, which is strongly influenced by depositional facies and paleo geomorphology. Fracture networks have contributed to karstification but are not the dominant factor of karst formation. The integration of geological and geophysical methods can predict karst reservoir with high accuracy within large area and can be applied to karst reservoir hydrocarbon exploration of similar geological setting.

Types and Distribution Characteristics of Callovian-Oxfordian Reservoir on the Right Bank of Amu Darya River in Turkmenistan

Given the characteristics of significant or enormous resources, a wide range, many reservoir types, and challenging exploration on the right bank of the Amu Darya River, we systematically studied the characteristics of its reservoirs. Based on the core description, thin sections in combination with logging and seismic facies characteristics as well as regional tectonic-sedimentary background, the Callovian-Oxfordian platform and the gentle slope sedimentary system in the Amu Darya Basin of Turkmenistan were summarized. Sedimentary subfacies such as evaporation platform, restricted platform, open platform, and platform margin reef are developed in block A and its west. In contrast, the upper slope, lower slope, and basin sedimentary subfacies are developed in the east of block A. On this basis, the main reservoir types on the right bank of the Amu Darya River are summarized, namely, porous reservoir, vuggy reservoir, fractured-porous reservoir, and fractured-vuggy reservoir. After describing the characteristics of various reservoirs in detail, the main controlling factors, development patterns, and distribution rules of the development of different reservoirs are summarized. Specifically, based on the platform reef, the porous reservoir and vuggy reservoirs are developed mainly on the concealed palaeouplift in the study area and are greatly influenced by atmospheric freshwater leaching and buried dissolution. Based on the dominant sedimentary facies, the fractured-porous reservoirs are mainly developed in the central areas on the right bank of the Amu Darya River. In the later period, hydrothermal fluid and hydrocarbon-generating acidic fluid can dissolve the reservoir through strike-slip faults and their associated fractures. Diagenetic fluid enters the reef through faults and associated fractures to form dissolved reservoir bodies of a certain scale. The fractured-vuggy reservoirs are mainly controlled by faults and dissolution and are mainly developed near the eastern thrust fault on the right bank of the Amu Darya River, effectively guiding the direction for further exploration and development in this area.