Large Cave Research Articles

Cave-roosting specialisation of an oceanic island endemic microbat elevates extinction risks as caves face multiple threats

Bats (Chiroptera) constitute one-fifth of the global mammalian diversity, with over 1,450 species. Yet, 16.5 % of them are threatened with extinction, a situation exacerbated on islands, underscoring an urgent need for conservation efforts there. However, effective conservation action can only be as sound as the understanding of the basic ecology of such insular species. Mauritius harbours one endemic Endangered cave-roosting insectivorous bat, in recent decline, whose ecology is poorly known, hampering its conservation. We used Mormopterus acetabulosus as a model species to inform insectivorous island bat conservation in highly transformed habitats. We compared the species’ historical and current known roost distribution and sizes and estimated any change in its Area of Occupancy (AOO) and Extent of Occurrence (EOO) using GeoCAT. We also compiled the physical characteristics, environmental and anthropogenic variables of caves that are occupied and unoccupied by the bats to identify potential factors influencing roost occupancy. We found roosts in 17 of the recorded 130 lava cave tunnels, up from the nine previously recorded, including 11 permanent and six seasonal roosts, mostly (∼71 %) on privately owned land. Compared to the last surveys in 2017, the EOO remained stable at 818.9 km2, while the AOO increased by 33.3 %. This expansion may reflect previously undetected roosts or recent colonisation events or both. Cave dimensions in terms of entrance size and height as well as the number of chambers, relative humidity and distance to water best explained roost occupancy by M. acetabulosus. Caves with larger entrances may be preferred for roosting as this characteristic eases manoeuvrability and predator avoidance, and larger caves may be preferred for enhanced microclimatic stability. Occupied and unoccupied caves were found to differ in terms of physical characteristics, although cave microclimate did not vary notably. Based on their characteristics and their potential contributions to increasing the species’ EOO and AOO, six currently unoccupied caves were identified as priority sites for future conservation efforts. Legal protection of all caves, regardless of current bat occupancy is necessary to mitigate extinction risks and preserve their associated ecological functions.

Preliminary Data on the Species Composition and Distribution of Land Snail (Mollusca: Gastropoda) in Son Doong Cave, Phongnha - Kebang National Park

Son Doong Cave it is the largest cave on the planet and also the most majestic cave in Vietnam. Son Doong Cave is divided into habitats: Dark cave (covered by the cave ceiling) and light cave (not covered by the cave ceiling), including doline 1 and doline 2.Surveying and collecting land snail samples in Son Doong cave was carried out in January 2023. For the first time, the list of terrestrial snail species in Son Doong Cave was published. This includes 30 species (11 phenotyps are just only identified to genus) belonging to 24 genera, 11 families, 3 orders, 3 subclasses; of which the presence of 5 species (Pincerna mouhoti, Leptopoma perlucidum, Scabrina patera, Chloritis khammouanensis, Amphidromus haematostoma) are newly recorded and added to the list of terrestrial gastropod fauna of Vietnam. In Son Doong cave, land snails are only distributed in the light part of the cave (Doline 1 and Doline 2), not in the dark part of the cave, which accounts for most of the cave area. Doline 2 has a higher species diversity (27 species belonging to 22 genera, 8 families, 2 orders) than Doline 1 (13 species belonging to 11 genera, 6 families, 3 orders). The diversity of land snail species in Son Doong cave is very unique and highly distinctive, but the species populations here is very small, requiring recommendations for tourists and the management board to protect vegetation to maintain and develop populations of this species.

Exploring microbial diversity and functional gene dynamics associated with the microbiome of Sof Umer cave, Ethiopia

Sof Umer Cave is the largest cave in East Africa and located in southeastern Ethiopia, is an unexplored extreme environment harboring novel microbes and potential genetic resources. However, the microbial ecology and their potential genetic resource, remain undiscovered. The purpose of this study was to employ high-throughput shotgun sequencing to explore the microbial diversity and functional gene dynamics within the microbiomes of Sof Umer Cave. High-molecular-weight DNA was extracted from homogenized sample using the GeneAll DNA Soil Mini Kit and 1% CTAB-SDS method. Purified environmental DNA was sequenced using a NovaSeq PE150. Microbial gene analysis revealed that Sof Umer Cave is primarily inhabited by Protobacteria, Actinobacteria, Bacteroidota, Verrucomicrobiota, Acidobacteiota, and Cyanobacteria, according to the Micro-RN database. The functional genes identified through the KEGG, eggNOG, and CAZy databases included 44,780 genes involved in metabolism and the biosynthesis of bioactive compounds. However, 34,716 genes related to metabolic processes remained unidentified, suggesting the presence of potentially novel genes for the discovery of natural bioactive compounds. Additionally, functional gene modeling using the FBA-built metabolic model, Model seed and MS2-prokaryotic metabolic model revealed 1,742 reactions, 1,542 compounds, and the addition of 302 new reactions during gap filling. Finally, the results revealed that the Sof Umer Cave is a reservoir for novel microbes and diverse functional genes, offering potential for the discovery of natural bioactive compounds.

New distribution and breeding record of Wroughton’s free-tailed bat (Otomops wroughtoni) (Chiroptera: Molossidae) from Meghalaya, India

Abstract The Wroughton’s free-tailed bat (Otomops wroughtoni) is classified as Data Deficient under IUCN Red List due to inadequate information on its distribution, population, and ecology. The species was previously recorded from Belgaum district of Karnataka and from East Khasi Hills and Jaintia Hills districts of Meghalaya and from Cambodia, represented by a single specimen. Through our study on Chiroptera taxa across 19 Community Reserves between October 2021 and January 2022 using physical search, mist netting and acoustic study we have identified a new colony of O. wroughtoni from the Mikadogre village in South Garo Hills District of Meghalaya, which extends the current distribution to 135 km from the nearest known distribution site. Interestingly, unlike other known roosts of O. wroughtoni which are in large caves with high roofs and deep crevices or cupolas, this roost was in the open limestone cliff located next to a stream. In addition to this, we have also documented the breeding colony of the species from a previously known site at Pynurkba. This new finding emphasises that there are more potential roosts across the Northeast region of India with similar habitats and altitudes waiting to be identified.

Estimating transmissivity and storativity in a karst aquifer by direct and indirect methods

AbstractThe karst aquifer located in the Environmental Protection Area of Lagoa Santa, Minas Gerais, Brazil, exhibits the characteristic morphological features of an evolved karst environment, including large caves, sinkholes, and copious springs. Understanding the hydrogeological characteristics of karst aquifers is crucial for effective management of public resources and prevention of contamination, as these aquifers are highly vulnerable, making it essential to identify and mitigate potential risks. Given the ongoing economic and population growth in this region, the assessment of the hydrogeological parameters that control the dynamics of local flow becomes relevant for the proper evaluation of hydrogeological models and the management of water resources. Four classical pumping test analysis methods were compared to quantify the flow and storativity of the aquifer: Theis (1935), Neuman (1972), Moench (1984), and Agarwal (1980). In addition, indirect estimation methods based on the identification of capture zones and the analysis of recession curves of water courses were employed, since these approaches are cheaper and easier to apply in areas with low information. The statistical analysis of the results suggests that classical solutions can be applied to the karst aquifer to estimate hydrogeological parameters, apart from the Storage Coefficient obtained using the Neuman method. In general, the indirect estimation methods tend to produce values that are consistent with those obtained through direct methods, albeit with some reservations. The transmissivity values obtained from the pumping tests were approximately 3.0 × 10−4 m2/s, while the storage coefficient is estimated to be around 3.7 × 10−3. The indirect estimation methods yield transmissivity values of about 1.8 × 10−4 m2/s using the capture zone method, and storage coefficient values of 4.0 × 10−3 and 3.9 × 10−2 using recession curve analysis.

Hydrothermal fluid alteration of the Ordovician epigenetic karstification reservoir in the Tahe Oilfield, Tarim Basin, NW China

In recent years, the Ordovician carbonate reservoirs in the Tarim Basin have received attention due to continuous advancements in of deep strata exploration for oil and gas. The Ordovician carbonates in the Tahe Oilfield have experienced multiple phases of tectonic movement and ancient karst action to form reservoirs consisting of multi-scale spaces such as large caves, fractures, and dissolution pores. This study identifies indicators of atmospheric freshwater karst and hydrothermal karst in the Ordovician carbonate rocks of the Tahe Oilfield by comparing and analyzing lithological observation, geochemical data, fluid inclusions, logging interpretation, and seismic data. The spatial and temporal distribution of karst reservoirs are summarized, and the results show that the stage of tectonic movement and pulsating uplift of strata occurred in the early Caledonian and Hercynian orogenies, and the carbonate rocks were uplifted to the surface and large-scale atmospheric freshwater karstification.In the deeper strata, the thermal convection of fluids caused by volcanic activity accelerated the thermochemical sulfate reduction (TSR), and the generated H2S gas accompanied the upward transport of hydrothermal fluids, which further dissolved and modified the original karst system, increasing the storage space. However, away from the heat source, calcite (positive Eu anomaly, higher 87Sr/86Sr ratio, lower δ18O value,) reprecipitated due to the temperature reduction, so the role of hydrothermal activity in reservoir reconstruction is limited. The two karst action patterns indicated that epigenetic karstification is an important process for forming carbonate reservoirs in the Tahe Oilfield and the basis of hydrothermal karst reservoir formation. This study demonstrates the research and exploration value of karstification for the geo-energy field, which could benefit sustainable development in the Tarim Basin.

Caves, Karst Features and Speleological Heritage in Chapada Diamantina, Bahia, Brazil

Chapada Diamantina is among the best known and most visited landscapes in Brazil. Located in the state of Bahia, Northeast region of the country, it is characterized by mountains and plateaus that developed on Proterozoic sedimentary and metasedimentary rocks. Much of its territory is covered by carbonate and siliciclastic rocks, where relevant karst systems develop, marked by the occurrence of sinking streams, sinkholes, and caves with a great diversity of morphologies, speleothems, subterranean fauna, and paleontological and archaeological records. Relevant carbonate systems occur in Iraquara, such as the Lapa Doce, Torrinha, and Pratinha caves, which represent important tourist attractions. Also in these rocks, stand out the Brejões Cave, with a 106-m high entrance, and the Toca da Boa Vista, the largest cave in South America, extending across 114 km. Cultural manifestations are present in the prehistoric cave paintings at Santa Marta Shelter and recent religious pilgrimages at Mangabeiras Cave, in Ituaçu. In turn, siliciclastic karst systems are mainly in Serra do Sincorá. The Lapão and Castelo caves have expressive speleogens and speleothems, as well as the Torras Cave, in the Igatu region, ranked as the second largest in Brazil considering siliciclastic rocks.

Cooling of hydrothermal fluids rich in carbon dioxide can create large karst cave systems in carbonate rocks

Karst systems, comprising interconnected voids and caves, are ubiquitous in carbonate formations and play a pivotal role in the global water supply. Accumulating evidence suggests that a significant portion of the global karst is hypogenic, formed via rock dissolution by groundwater ascending from depth (rather than by infiltration from the surface), yet the exact formation mechanism remains unclear. Here we show that cooling of carbon dioxide-rich geothermal fluids, which turns them into highly corrosive agents due to their retrograde solubility, can dissolve and sculpt large caves on short geological timescales. A conceptual hydro-thermo-geochemical scenario is numerically simulated, showing cave formation by rising hot water discharging into a confined layer. Our models predict field observations characteristic of hypogenic caves, including enigmatic locations of the largest passages and intricate maze-like networks. Finally, we suggest that deep-seated carbon dioxide consumption during karst formation may constitute a link to the global carbon cycle.

CONTENT OF <sup>13</sup>С AND <sup>15</sup>N ISOTOPS IN BONE COLLAGEN OF GEOGRAPHICAL, AGE AND SEX GROUPS OF THE URAL CAVE BEAR (MAMMALIA, CARNIVORA, URSIDAE, URSUS (SPELAEARCTOS) KANIVETZ VERESTCHAGIN, 1973)

Data on the content of δ13C and δ15N isotopes in the collagen of bones of the Ural cave bear (Ursus (S.) kanivetz Verestchagin, 1973) from the Northern and Middle Urals were analyzed. The bones date from the first half of MIS 3. The bones of newborn individuals, individuals at the age of 1 year, males and females at the age of 2 years, 3 years, 4 years and older than 4 years were studied. Differences in δ13С values between age, sex, and geographical samples are not significant. With age, the value of δ15N significantly decreases, which is associated with the transition from milk nutrition to independent nutrition. The proportion of meat food in the diet of adult bears in the Middle Urals was higher than in the diet of adult bears in the Northern Urals. There are no noticeable differences in isotope signatures between males and females of different ages. The large cave bears of the Urals and Europe had a similar type of diet.

Inferring the territoriality of Upper Palaeolithic hunter-gatherer’s groups settled at Cueva del Gato 2 (Épila, Zaragoza)

In recent decades, the development of different analytical procedures applied to the study of archaeological lithic remains has allowed us to approach the territoriality of past societies. The application of geochemical tools has improved the study of lithic raw materials, allowing direct connections between archaeological samples and specific geological formations. In a similar way, the incorporation of GIS tools to the study of past mobility and territoriality has allowed to define which could have been the most probable routes used by past groups to stock up on rocks. In this paper, we present the results obtained after the geochemical study by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) of lithic cherts found at Upper Palaeolithic human occupations at Cueva del Gato 2 (Épila, Zaragoza, Spain) as well as the least cost path routes obtained after GIS analyses. The Cueva del Gato 2 archaeological site is a large cave placed in the first foothills of the Iberian System, close to the Middle Ebro Basin, in Eastern Iberia. The archaeological work has revealed a long sequence of occupation with archaeological levels from the Bronze Age, the Ancient Neolithic and the Upper Palaeolithic. In this paper, we have focused on the study of level II, which has been dated in the range 21,000–23,000 cal BP. Results have demonstrated that past human groups from the Last Glacial Maximum knew and frequented the Sierra de Lanaja – Montes de Castejón lacustrine chert sources, outcropping in the Middle Ebro Basin.

Coastal Tourism Enhancement and Waste Management Initiative in Negeri Ureng, Central Maluku

This study assesses the implementation of a community-led tourism exploration and beach cleaning initiative in Negeri Ureng, Central Maluku, particularly in Island 3 Nusa Ela. Our goal was twofold: firstly, to explore and identify new tourism sites and, secondly, to foster a greater understanding and action towards coastal waste management. The adopted method incorporated community engagement, direct observation, and active participation in waste cleanup activities. Findings highlighted picturesque sites around Island 3 Nusa Ela, such as the vicinity near the villa at the island's end, and a unique fresh water source in a large cave. However, the cleanup activities revealed a significant amount of poorly managed waste, predominantly plastic, along the coastline, thus necessitating immediate waste management strategies. As a result, signage was installed to educate both locals and visitors about the adverse impacts of improper waste disposal on the ecosystem and coastal aesthetics. The study underscores the vital role of community involvement in preserving and enhancing coastal areas while showcasing its potential for sustainable tourism. This initiative bears significant implications for other coastal communities worldwide striving to strike a balance between tourism development and environmental conservation.Highlights:
 
 Tourism Locations: The study identified scenic tourism sites in Island 3 Nusa Ela.
 Waste Management: The cleanup revealed significant plastic waste along the coastline, necessitating immediate intervention.
 Environmental Education: Signage was used to educate locals and visitors on the importance of proper waste disposal.
 
 Keywords: Coastal Tourism, Waste Management, Negeri Ureng, Community Initiative, Environmental Preservation.

Moon Diver: Exploring a pit's exposed strata to understand lunar volcanism

Natural pits on the Moon expose deep cross-sections of the lunar maria, enabling direct investigation of the Moon's volcanic history and providing potential access to subsurface lava tubes. The Moon Diver mission concept seeks to explore the Mare Tranquillitatis pit, which exposes the largest wall of bedrock of the mare pits (∼65 m). The concept is enabled by two innovative capabilities: pinpoint landing near the pit and robotic access to its near-vertical wall with an instrument package to examine the elemental chemistry, mineralogy, and morphology of these bedrock layers. Pinpoint landing uses closed-loop guidance with terrain-relative navigation (TRN), which was advanced by Perseverance landing on Mars, to deliver the lander within a 100-m ellipse. The Axel robotic explorer, which remains tethered to the lander, would egress onto the lunar surface and traverse the relatively flat terrain to the pit's funnel entrance. The lander, which is the data link to Earth, also serves as an anchor and provides power and communication to the rover through its tether. The rover is a novel two-wheeled platform with a trailing boom and a spool that pays out the tether as the rover traverses toward the pit. The 300-m long tether is well margined for the rover to scale the pit wall. The rover carries a surface preparation tool and three additional instrument types: (a) three high-resolution cameras for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray spectrometer (APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for mineralogy. The surface-preparation tool removes dust and patina from the rock wall by grinding a small area. This tool, the MMI, and the APXS would be deployed from an instrument bay inside the wheel wells. The rover would independently point each instrument at the same target on the wall with millimeter repeatability. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar daytime (14 Earth days). Beyond the primary mission, the rover would be capable of descending from the overhang and peering into the void that may open to a large cave or lava tube. Lunar pits provide an exciting new target for exploration using innovative robotic capabilities that have been tested with integrated science instruments at multiple terrestrial analogue sites including a pit with basaltic layers in Arizona.

Subterranean Fauna of the Lukina Jama–Trojama Cave System in Croatia: The Deepest Cave in the Dinaric Karst

The Dinaric Karst is a global hotspot for subterranean diversity, with two distinct peaks of species richness in the northwest and southeast, and an area of a lower species richness in the central part. In this article, we present a species list and describe the ecological conditions of the Lukina jama–Trojama cave system, located in the central part of the Dinaric Karst. This cave system is the deepest and one of the most logistically challenging cave systems sampled so far in the Dinaric Karst. Repeated sampling resulted in a list of 45 species, including 25 troglobionts, 3 troglophiles, 16 stygobionts, and 1 stygophile. Most of the recorded species are endemic to the Velebit Mountain, while three species are endemic to the Lukina jama–Trojama cave system. Within the system, species richness peaks in the deepest third of the cave, most likely reflecting the harsh ecological conditions in the upper parts, including ice, cold winds, and occasional waterfalls. Milder and more stable deeper parts of the cave contain a rich subterranean species community, part of which is associated with two very distinct aquatic habitats, the cave hygropetric and the phreatic zone. The newly recognized hotspot of subterranean biodiversity in the central Dinaric Karst, which has emerged between the two known centers of biodiversity, further highlights the species richness in large cave systems, but also challenges the diversity patterns in the Dinaric Karst overall.

Unmanned aerial vehicle navigation in underground structure inspection: A review

Many years after construction, a number of existing old tunnels and underground structures are deteriorating with time as evidenced by cracks, large deformations, water leakage and so forth, which usually require regular site inspections to record their structural deterioration by taking high‐pixel, high‐overlap images along miles of a tunnel network. For complex underground structures (e.g., long tunnels and large caves), unmanned aerial vehicles (UAVs) may be adaptive in acquiring images at multiple heights and angles with low operational costs. So far, UAV underground structural health monitoring has become mature for open‐air surveying with rapid developments in robotic software and hardware. However, the UAV image acquisition for underground working conditions still faces a number of key challenges. This paper aims to provide an overview of UAV navigation techniques in confined dark spaces for geotechnical engineers, geologists, drone developers and other interdisciplinary researchers & professionals in the structural health monitoring field. It specifies the challenges for UAV application in underground space, mainly including lack of Global Navigation Satellite System (GNSS) signals, poor lighting conditions, weak features and obstacle avoidance and then followed by strategic solutions. For example, in light of poor GNSS signals, the fusion of multi‐sensors (e.g., laser imaging, detection and ranging (LiDAR) and multi‐cameras) can enhance localization accuracy in low‐luminance underground conditions. To address obstacle avoidance, computer vision (CV)‐based navigation algorithms (e.g., deep reinforced learning [DRL]) enable effective navigation in complex 3D spaces, but their adaptability is limited by arithmetic power and pre‐training needs. The review of relevant previous studies concludes that further development for UAVs in underground space inspection may focus on operation in large‐scale geometric inspection environments, obstacle avoidance, features and semantic recognition.

A novel semi-analytical model for transient pressure behavior in fracture-cave carbonate reservoirs

Fracture-cave carbonate reservoirs represent a significant amount of oil and gas resources worldwide, while their intrinsic complex pore network, large caves and tectonic fractures bring challenges to reservoir characterizations and productions. Many models have been proposed to solve the pressure transient analysis (PTA) solutions for such reservoirs. With recent explorations, the position of fractures and caves can be determined by seismic data. However, models using the position information with the coexistence of discrete fractures and caves were not reported in the literature. This paper proposes a novel semi-analytical model based on the Boundary Element Method (BEM), to describe the transient pressure behavior of the fracture-cave carbonate reservoirs. Basically, the proposed model treats the cave edge as an inner boundary and includes the fracture-cave fluid interchanges. As a results, the model's solution is proved to be flexible for arbitrary cave and reservoir shape. A typical system consisting of one fracture and one case is discussed in detail. The result indicates the well location is the key factor to the pressure response, where the pressure response is mostly affected by the cave volume and fracture conductivity when the well is on the cave and fracture, respectively. The sensitivities of three major parameters on the pressure response are analyzed. In addition, the proposed model is applied in two field cases. The result shows the proposed model is reliable and accurate.

Caves as wildlife refuges in degraded landscapes in the Brazilian Amazon

Cross-habitat spillover may be the outcome of a process of habitat loss or degradation where the receiving habitat serves as a refuge for organisms. Once surface habitats are lost or degraded, animals can find underground refuge in caves. This paper is focused on testing whether taxonomic order richness inside caves is positively affected by the loss of the native vegetation cover surrounding caves; whether degradation of native vegetation cover predicts cave community composition; and whether there is a pattern of cave community clusters delimited by similarity in the effects of habitat degradation on animal communities. We gathered a comprehensive speleological dataset consisting of occurrence data of thousands of invertebrates and vertebrates sampled in 864 iron caves in the Amazon, to test the effects of both variables measured inside caves and surrounding landscapes on spatial variation in richness and composition of animal communities. We show that caves can work as refuges for the fauna in landscapes where the native vegetation cover surrounding them was degraded, which was evidenced by landcover change increasing the richness of cave communities and clustering caves by similarity in community composition. Therefore, habitat degradation on the surface should be a key variable when characterizing cave ecosystems for conservation prioritization and offset planning. Habitat degradation causing a cross-habitat spillover effect highlights the importance of maintaining the connection between caves by the surface, especially large caves. Our study can help guide industry and stakeholders working on the complex conciliation between land use and biodiversity conservation.

Comprehensive Experimental Design of Polymer Crosslinked Gel Plugging for Drilling

In order to further improve the comprehensive experimental skills of undergraduates and penetrate the concept of green chemistry education, a comprehensive experiment was designed to prepare polymer crosslinked gel plugging agent with polymer as the main agent and loofah pulp as the toughening material, and evaluate its effectiveness in simulating downhole leakage layer. This experiment applies organic/inorganic compound chemistry knowledge to the field of oilfield chemistry, and combines experimental skills with field work (TaHe Oilfield) to improve students' professional skills and oilfield field adaptability. TaHe Oilfield is the main position for the construction of tens of millions of tons of Northwest Oil Field. During the "Fourteenth Five-Year Plan" period, the large caves were basically under full control. How to develop small and medium-sized fractures and improve the use of reserves and realize the stable production of Tahe Oilfield is facing greater challenges. When adopting the "one well multiple control" method to develop small and medium-sized fractures and cavities, the key technical problem is the low success rate of plugging in carbonate fracture-cavity reservoirs, which requires plugging operations. Through this experiment, students can understand the mechanism of plugging agent in the leakage channel, and master the preparation and performance evaluation methods of gel plugging agent. Let students understand the site construction technology.

Book review Caves of Assynt (3rd edition), edited by TJ Lawson & PNF Dowswell. Grampian Speleological Group, Edinburgh, 2022. £20.00, 211 pp., ISBN 978-1-739735-0-3

The Assynt District in the Northwest Highlands is a veritable treasure trove of geological features, the most famous – amongst geologists – being the Moine Thrust. Perhaps less well known is that Assynt also hosts the most extensive cave systems in Scotland, developed in the Cambro-Ordovician Durness limestone. These two features are, of course, linked: Caledonian thrusting and folding resulted in thickening and fracturing of the limestone strata, allowing/enhancing the development of large cave systems.

Microfacies and Reservoir Space Characteristics of Carbonate Reservoir of the Ordovician Yingshan Formation in Northern Slope of Tazhong Uplift, Tarim Basin

As an important stratum of exploration in the Tarim Basin, which is the largest petroleum basin in China, Yingshan Formation has great potential. The relationship between the fine division of microfacies and reservoir space characteristics has not been systematically studied which restricts further exploration. On the basis of the analysis of core, thin section, logging data, imaging logging data (FMI), and field outcrop, the microfacies types and reservoir space characteristics of the Ordovician Yingshan Formation are systemically studied. The results show that (1) nine microfacies (Mf1-Mf9) and four microfacies associations (MA1-MA4) were identified within the Ordovician Yingshan Formation. MA1-MA2 represent a medium- to high-energy sedimentary background, and MA3-MA4 represent a medium- to low-energy sedimentary background. (2) Four reservoir space types were observed in the Yingshan Formation, which are dominated by fracture-dissolution pore type, followed by small dissolution vug type, seldom fracture type, and large-scale karst-cavity type. (3) High-energy microfacies are the material basis of favorable reservoirs. The high-energy microfacies associations are prone to small dissolution vug and fracture-dissolution pore types of reservoirs; large cave reservoirs can sometimes be developed in the high part of the sequence and form favorable reservoirs in the Yingshan Formation. The low-energy microfacies associations mostly appear as original strata or interlayer, but after the superposition of structural cracks, buried karsts, or epigenetic karsts, low-energy microfacies associations may form relatively small-scale fractured and fracture-dissolution pore types of reservoirs in this area.

3D Modeling of Fracture-Cave Reservoir from a Strike-Slip Fault-Controlled Carbonate Oilfield in Northwestern China

A giant strike-slip fault-controlled Fuman Oilfield has been found in the Ordovician fractured carbonates of the Tarim Basin. However, conventional seismic methods are hardly able to distinguish the fractured reservoir and its connectivity in the ultra-depth (>7000 m) carbonate fault zones. We propose thin-likelihood and tensor-thickness process methods to describe the fracture network and large cave reservoir, respectively. Together with the two methods for 3D visualization of fracture-cave reservoirs, we had an application in the ultra-deep well deployment in Fuman Oilfield. The results show that the fracture network and cave reservoir can be 3D-imaged more clearly than conventional methods. The fracture network and cave reservoir show distinct segmentation by the fault assemblage in Fuman Oilfield. Furthermore, 3D modeling is favorable for the reservoir connectivity description along the carbonate fault zones. There are three distinct reservoir models: fault core-, fault damage zone- and overlap zone-controlling fractured reservoirs along the fault zones. This revealed variable fractured reservoirs that are related to fault maturity and segmentation. The method has been widely used in fracture-cave reservoir description and subsequent well optimization, suggesting a favorable method for economic oil exploitation in the ultra-depth reservoirs. This case study is not only useful for the complicated reservoir 3D description and modeling but also helpful for well employment to provide support for the target evaluation and optimization in ultra-depth fractured reservoirs.