Karst Reservoirs Research Articles

Characteristics and development model of karst reservoirs in the fourth member of Sinian Dengying Formation in central Sichuan Basin, SW China

The reservoir space, types and distribution characteristics of karst carbonate gas reservoirs in the fourth member of Sinian Dengying Formation (Deng 4 Member) in central Sichuan Basin are analyzed based on the drilling, logging and seismic data. A development model of karst reservoirs is constructed to support the high-efficiency development of gas pools. The research shows that the reservoirs in Deng 4 Member have mainly small-scale karst vugs and fractures as storage space, and can be divided into three types, fracture-vug, pore-vug and pore types. The development patterns of the karst reservoirs are determined. On the plane, the karst layers increase from 65 m to 170 m in thickness from the karst platform to the karst slope, and the high-quality reservoirs increase from 25.0 m to 42.2 m in thickness; vertically, the reservoirs at the top of Deng 4 Member appear in multiple layers, and show along-bedding and along fracture dissolution characteristics. The reservoirs at the bottom are characterized by the dissolution parallel to the water level during the karstification period, and have 3–5 large-scale fracture-cave systems. Based on the reservoir development characteristics and the genetic mechanism, three types of reservoir development models of karst reservoir are established, i.e., bed-dissolved body, fracture- dissolved body and paleohorizon-dissolved body. The construction of karst reservoir development models and seismic response characteristics of the three types of reservoirs can provide parameter for well placement and trajectory design, and substantially improve productivity and development indices of individual wells and gas reservoirs. The designed production capacity of the gas reservoir has enhanced from the initial 3.6 billion to 6 billion cubic meters, making the profit of the reservoir development increase noticeably.

Structural Characteristics and their Significance for Hydrocarbon Accumulation in the Northern Slope of the Central Sichuan Paleo‐uplift

AbstractBased on 2D and 3D seismic data, the latest drilling data and field outcrop data of the northern slope of the Central Sichuan paleo‐uplift, the structural analysis method is used to analyze unconformity development characteristics and fault characteristics during the key structural transformation period, discussing the influence of the structural characteristics on the hydrocarbon accumulation of deep carbonate rocks. The results show that: (1) The two key unconformities of the Tongwan and Caledonian periods were primarily developed in deep carbonate rocks. Firstly, Tongwan's unconformities are characterized by regional disconformities between the second and third members of the Dengying Formation, the top formation of the Sinian and the lower Cambrian, strips of which zigzag through the north and south sides of the study area. Secondly, the Caledonian unconformity is characterized by a regional unconformable contact between the lower Permian and the ower Paleozoic strata. From NE to SW, the age of the strata, which were subject to erosion, changes from new to old, the denudation distribution showing as a nose‐shaped structure which inclines towards the ENE. (2) Boundary fault and transtensional strike‐slip faults developed in the Sinian to Paleozoic strata. In profile, there are three types of structural styles: steep and erect, flower structures, ‘Y’ and reversed ‘Y’ type faults. In plane view, the Sinian developed extensional boundary faults extending in an almost NS direction, strike‐slip faults developing and extending linearly in approximately EW, WNW and NE strikes in the Cambrian, with characteristically more in the south and less in the north. (3) The faults in the northern slope show obvious zonal deformations in transverse view as well as significant stages and stratified activity in a longitudinal direction. Among them, the activity of faults in the Sinian was the strongest, followed by the activity in the Cambrian period, the activity intensity of faults in the Permian period being the weakest. This fault activity can be divided into four periods: Sinian, Cambrian‐Permian, the early Indosinian period and the late Indosinian‐Himalayan period, the transtensional strike‐slip faults being the products of oblique extensions of pre‐existing weak zones in the Xingkai and Emei taphrogenesis, with a particular inheritance in the main faults. (4) Combined with hydrocarbon accumulation factors, it is considered that the epigenetic karstification of the Tongwan and Caledonian unconformities in the northern slope controlled the formation and distribution of carbonate karst reservoirs over a large area, also acting as a good pathway for oil and gas migration. The extensional faults developed at the margin of the NS trending rift, controlling the sag‐platform sedimentary pattern in the Dengying Formation of the Sinian. Strike‐slip faults in NE, WNW and ENE directions may control the micro‐geomorphological pattern inside the platform and intensify the differential distribution of grain beach facies. The multi‐stage hereditary activity of strike‐slip faults not only improved the porosity and permeability of the reservoirs, but also acted as the main channel of oil and gas migration, providing favorable conditions for the development of the current multi‐layer gas‐bearing scenario in the northern slope of the Central Sichuan Basin.

Role of mineralogical, structural and hydrodynamic rock properties in conduits formation in three distinct carbonate rock types

Adopting sustainable strategies to manage water resources of karst reservoirs requires to understand the carbonate rock reactivity that is responsible for the formation of these reservoirs, as well as the role of the rock mineralogy in the conduit formation. To this end, three carbonate rocks (chalk, crinoidal limestone and dolomite) with different mineralogy and internal structure are submitted to laboratory dissolution experiments by injecting an acid solution under atmospheric conditions and various hydrodynamic conditions with a homemade experimental device. The core samples are characterized by petrophysical investigations with laboratory and imagery techniques before and after the experiments, and the changes in chemical and hydraulic properties are recorded during the experiment. The resulting carbonate dissolution leads to the formation of preferential conduits and the increase in porosity and permeability of the samples. For the three rock types, the dissolution rate mainly depends on the mineral composition, the flow conditions and the initial structural properties. In addition, we observe that (i) for each rock type, the lower Péclet conditions applied to the samples, the lower global dissolution rate, (ii) for each hydrodynamic condition, chalk and crinoidal limestone have the highest and smallest dissolution rates, respectively, (iii) for a similar amount of injected acid, the dissolution is lower in dolomite than in calcite, and (iv) high rock heterogeneities in the chalk samples are responsible for high renewal rates that induce high dissolution rates.

Risk assessment of non-point source pollution in karst reservoirs based on ‘source–sink’ landscape theory

Abstract The quantitative identification of areas at risk for such pollution is conducive to allocating limited government funds to critical areas and the efficient and economical management of water environments. Here, the Baihua Lake watershed, an important drinking water source for Guiyang City, was taken as the study area. The location-weighted landscape contrast index (LCI) and non-point source pollution risk index (NSPRI) were developed based on the ‘source–sink’ landscape theory. The method takes into account the risk of pollution source formation and pollutant transport. A total of 348 natural sub-watersheds were used as assessment units by estimating the nitrogen and phosphorus pollution emission (absorption) potentials of different landscape types in the sub-watersheds and considering the influence of vegetation cover, distance from the reservoir, and slope in the transmission process, a quantitative assessment of Baihua Lake's pollution was carried out; the reliability of the method was verified by comparing the assessment results with measured water quality data and field surveys. The results indicate (1) 132 sub-watersheds (37.93%) dominated by source effects, mainly distributed in Yanshanhong Township, Yeya Township, and the Qinglong Subdistrict, with construction land and farmland as the main landscape types, and 216 sub-watersheds (62.07%) dominated by sink effects, mainly distributed in Zhanjie and Baihuahu Townships, with forests as the primary landscape type. (2) Additionally, 17 sub-watersheds (4.89%) show extremely high risk for non-point source pollution; these watersheds are mainly distributed in the Qinglong Subdistrict and mainly consist of urban residential areas and schools. These sub-watersheds discharge a large volume of sewage, which threatens the water quality of the upper reaches of Baihua Lake and must be managed. (3) The rivers corresponding to relatively high-risk, high-risk, and extremely high-risk sub-watersheds include the Dongmenqiao, Limu, Changchong, and Maixi Rivers.

Inclusion Characteristics of the Fracture-Cave Calcite of Ordovician Yingshan Formation and Its Indication to the Formation of Paleokarst Reservoir in the Northern Slope of Tarim Basin Center Area, China

A karst fracture-cavity was developed in the Ordovician Yingshan Formation in the northern slope of the Tarim Basin center area (Tazhong area); however, the area experienced multistage karst and diagenesis in the later period, and the reservoir was heavily reformed, characterized by complex distribution. Based on the testing of the inclusions in the karst fractures and fracture-cave calcite, this study summarizes the physical and chemical characteristics of the inclusions in the northern slope of Tazhong. The major inclusions were liquid and gas–liquid phases, while the minor ones were hydrocarbon phases. Based on the salinity–homogenization temperature diagram, the gas–liquid inclusions could be divided into seven types: low-temperature low-salinity inclusions (the lowest salinity was 0.88%, which is lower than that of present sea water; the lowest temperature of the inclusions was 49°C); low-temperature high-salinity inclusions; medium-temperature low-salinity inclusions; medium-temperature medium-salinity inclusions; medium-temperature high-salinity inclusions; high-temperature medium-salinity inclusions; and high-temperature high-salinity inclusions. The development of inclusions was characterized by high temperature and high salinity or low temperature and low salinity. In addition, the low-temperature and low-salinity inclusions near the unconformity surface were well-developed. The high-temperature and higher-salinity inclusions far from the unconformity surface were also well-developed. A burial history analysis showed three stages of karstification or filling in the area: exposed karstification in the Middle Caledonian, shallow burial filling in the Middle Hercynian, and burial filling in the Late Hercynian. By determining the inclusions in calcite associated with mud in the karst caves and combined with previous studies, the Middle Caledonian was found to be the period associated with the eogenetic karstification of meteoric water and seawater. This is the key factor in the formation of karst reservoirs in the Tazhong area. The large-scale calcite growth in the Middle Hercynian and Late Hercynian burial periods led to further filling of the early karst reservoir, reduced reservoir space, and enhanced reservoir heterogeneity.

Restoration of formation processes of dolomite reservoirs based on laser U-Pb dating: A case study of Ordovician Majiagou Formation, Ordos Basin, NW China

To address the issue of non-unique interpretation of dolomite reservoir diagenetic and porosity evolution in the previous qualitative or semi-quantitative studies, we investigate two dolomite reservoir types, i.e. weathering-crust karstic reservoirs and mound-beach reservoirs, in the Ordovician Majiagou Formation, Ordos Basin by using in-situ laser ablation U-Pb dating as well as carbon and oxygen isotopic composition analysis. The results show that: (1) According to the dating of 8 reservoir samples, the Majiagou Formation experienced 5 diagenetic stages (Stage 1: deposition of matrix dolomite or penecontemporaneous dolomitization, in 444.0–494.0 Ma; Stage 2: dogtooth- or blade-shaped dolomite cementation, in 440.0–467.0 Ma; Stage 3: dolomitic silt filling, in 316.5–381.0 Ma; Stage 4: crystalline dolomite filling, in 354.0 Ma; Stage 5: crystalline calcite filling, in 292.7–319.0 Ma). (2) Supra-salt weathering-crust karstic dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, weathering-crust karstification, packing, and rupturing in succession. Gypsum mold pores formed in the phase of hypergenic karstification and were filled with such minerals as dolomitic silts and calcites, and thus the porosity decreased from 10%–40% to 3%–8%. (3) Sub-salt mound-beach dolomite reservoirs went through the diagenetic processes including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, packing, and rupturing. The porosity of reservoirs was originally 10%–30%, decreased to 0–6% due to seawater cementation, rose back to 5%–15% owing to penecontemporaneous corrosion, and finally declined to 2%–6% as a result of crystalline dolomites and calcites packing. The above methodology for the restoration of dolomitization and porosity evolution may be helpful for the restoration of porosity evolution in other basins or series of strata.

Multidisciplinary investigations of a karst reservoir for managed aquifer recharge applications on the island of Vis (Croatia)

Managed aquifer recharge (MAR) refers to a suite of methods by which excess surface water or non-conventional water is stored underground for subsequent recovery or environmental purposes. MAR solutions have been largely used in unconsolidated aquifers, while their application in karst aquifers is rare. This research presents the first results of a MAR viability study on the island of Vis, a small karstic island in the Adriatic Sea. Favorable geological and hydrogeological conditions enable the formation of karst aquifers, making the island autonomous in terms of water supply. The island’s main aquifer, exploited in the Korita well field, is protected from seawater intrusion by several hydrogeological barriers. However, climate change and high seasonal pressures related to tourism pose a threat to the future availability of freshwater. Multidisciplinary field and laboratory investigations were carried out to detail the geological and hydrogeological setting of the island and its groundwater resource. Field analyses consisted of groundwater monitoring and sampling, geophysical investigations (i.e., electrical resistivity tomography), and structural measurements. Laboratory analyses included measurements of principal cations and anions and tritium activity. Despite low precipitation during the observation period (September 2019 - December 2020), the groundwater resource at the Korita site showed stable trends of physico-chemical parameters with a good storage potential and a long-term reserve. Geophysical investigations evidenced a relatively homogeneous sequence of the rock mass at a larger scale, while structural analyses indicated the occurrence of E-W karstified and open fractures that could represent a preferential flow path in the carbonate aquifer. A MAR solution for the Vis island was proposed combining an infiltration pond scheme with the direct injection of the accumulated waters into the aquifer using available wells. The potential water source could be represented by the runoff collected in an old artificial channel and the associated pond system in Korita.

Niche and interspecific associations of Pseudoanabaena limnetica – Exploring the influencing factors of its succession stage

In aquatic ecosystems, the dynamics of ecological parameters play an important role in community organization. To understand the changes in the phytoplankton community structure in karstic reservoirs, the Hongfeng reservoir in Guizhou plateau was monitored monthly from 2016 to 2019. The results showed that the dominant organisms in phytoplankton comprised five phyla and twelve species during 4 years, among which the proportion of abundance of Pseudoanabaena limnetica begin to rise in 2018 and reached 46.57% in 2019 (the highest during the study period). The niche breadth (0.259–0.925) and overlap value (0–0.991) of P. limnetica have an obvious seasonal trend and are significantly affected by environmental factors such as water temperature, nutrients, and interspecific competition. This is primarily reflected in the high adaptability of the dominant species to changes in the N:P concentration ratio in water. Variance Partitioning Analysis (VPN) results show that the diffusion degree of P. limnetica is the result of the interaction between internal (i.e. interspecific competition, with a contribution rate of 13.4%) and external (i.e. environmental factors, with a contribution rate of 86.6%). The greater the external influence, the higher the diffusion degree. The results of niche-based theories and neutral-based theories showed that the stable community structure (variance rate (VR > 1)) and the excessive fast diffusion rate of dominant species (migration rate (m = 0.84)) may lead to the risk of P. limnetica and other toxic filamentous cyanobacteria blooms. The results provide basic data for the study of early warnings regarding phytoplankton community succession and cyanobacteria blooms in large reservoirs in the karst areas.

Characteristics, main controlling factors, and formation of Ordovician karst reservoirs in the western slope of the Shulu Sag, North China

Characteristics, main controlling factors, and formation of Ordovician karst reservoirs in the western slope of the Shulu Sag, North China

Impact of hydro-chemical conditions on structural and hydro-mechanical properties of chalk samples during dissolution experiments

The importance of karst reservoirs for water resources, and their complexity in terms of structural properties and hydraulic responses, require a better understanding of the formation and location of conduits in these systems, in particular for chalk reservoirs. For this purpose, we conduct laboratory experiments of acid solutions injection in chalk core samples using a homemade experimental device. Three different flow rates and two acid concentrations are applied on six samples at atmospheric conditions with Peclet numbers ranging from 0.1 to 1.7 and Damköhler numbers from 10−6 to 10−3. Hydraulic and chemical measurements are carried out during the experiments, while petrophysical characterization is conducted on the samples before and after the reactive percolation experiments. The analysis of these data shows an increase in permeability of the rock samples due to calcite dissolution, which is associated with the creation of preferential conduits. We also observe that the initial structural properties control the dissolution regime with the following main conclusions. For homogeneous systems, the overall dissolution rate and percolation time are directly linked to the considered flow rate and injected acid concentration. For heterogeneous systems, (i) pre-existing large-scale structural heterogeneities control the location of newly created paths while reducing the value of the dissolution rate, and (ii) micro-scale heterogeneities result in highly heterogeneous general structures, which are responsible for channels formation while applying low Damköhler numbers.

Indosinian Supergene Karsts in the Qaidam Basin, NW China: Evidence from Paleo‐tectonics, Paleoclimates and Paleo‐fluids

AbstractIn the Qaidam basin, Northwest China, karstic fissures and caves are developed in many places of the Lower Carboniferous Chengqianggou Fm. and Huaitoutala Fm. and the Upper Carboniferous Keluke Fm., in which are found typical karstic marks. Several suites of transgressive carbonate rocks formed during the Late Paleozoic epoch are preserved as reservoir properties in the Qaidam basin, with paleokarst‐related cave‐fractured diagenetic structures, which provided the basis for cave‐fractured development. Carbonate and clastic samples covering or infilling caves and fractures in Carboniferous strata were collected in the eastern Qaidam basin to focus on petrological and geochemical analyses of karst and infilling materials. Based on this study of the Caledonian diagenetic sequence, and bitumen‐infilled inclusion temperature and burial history, there existed a continuous atmospheric freshwater leaching process, which played a critical role in the construction of the Carboniferous supergene paleokarsts. Investigation and survey of the tectonic setting, plate paleolatitude and river development intensity also proves that there was abundant rainfall during the Indosinian period, after which the strata undergo a shallow burial process. Because Indosinian period is earlier than the key timing of hydrocarbon‐generation of the Carboniferous source rocks, this kind of karst reservoir has potential significance for oil‐gas resources in this area.

Connectivity, permeability and flow channelization in fractured karst reservoirs: A numerical investigation based on a two-dimensional discrete fracture-cave network model

Fractured karst reservoirs play a significant role in hydrocarbon reserves and groundwater storage. They often exhibit complex multiscale heterogeneities involving pores, fractures and caves, whose length scales range from microns to tens or hundreds of meters. Thus, the study of fractured karst reservoirs is faced with a significant unresolved challenge in quantitatively characterizing the geometrical connectivity and hydraulic conductivity as well as their interrelationships in such strongly heterogeneous, multicomponent systems. In this paper, we propose an analytical formulation to characterize the connectivity of discrete fracture-cave networks building upon the excluded area concept of the percolation theory. By implementing a state-of-the-art computational model solving coupled Navier-Stokes (free flow) and Darcy (porous media flow) equations, we numerically derive the permeability of a fractured and karstified porous media, such that the relationship between the connectivity and permeability is further explored. The high-fidelity numerical model also permits us to elucidate the process of flow channelization within the fracture-cave network. The results show that the fracture-cave network connectivity correlates to the permeability via a power law scaling for connected systems. Significant flow channeling occurs around the percolation threshold where the flow is dominated by a limited number of preferential pathways. Caves play a crucial role in the flow due to the fact that caves could globally enhance the network connectivity and locally serve as hotspots for high fluid velocity. Finally, a semi-analytical permeability model for fractured karst reservoirs is developed. The results of our research and insights obtained have important implications for understanding the subsurface fluid flow in fractured karst reservoirs.

Restoration of karst paleogeomorphology and its significance in petroleum geology—Using the top of the Middle Triassic Leikoupo Formation in the northwestern Sichuan Basin as an example

The influence of karstification on gas reservoir formation is an area of interest in petroleum geology, and karst paleogeomorphology restoration is a topic of reservoir research. However, most previous studies of karst paleogeomorphology have focused on karst reservoirs, ignoring the influence of the karst paleogeomorphology on the overlying strata and gas accumulations. Therefore, the value of the petroleum geology information associated with karst paleogeomorphology is not maximized. The carbonate strata of the Leikoupo Formation were uplifted and exposed by the Indosinian movement, leading to the development of a weathered karst stratum (Lei 4 3 sub-member). Many commercial gas wells have been developed in the Lei 4 3 sub-member. Based on the latest drilling, logging, and two- and three-dimensional seismic data for the northwestern Sichuan Basin, we selected the residual thickness and the impression methods to restore the karst paleogeomorphology of the Lei 4 3 sub-member. The karst paleogeomorphology was generally high in the southeast and low in the northwest. The area can be successively divided into three secondary karst geomorphological units, namely, karst highlands, karst slopes, and karst depressions, and several tertiary geomorphological units. The karst geomorphology had the function of controlling reservoir, hydrocarbon source, and gas accumulation distribution. The karst processes were different in the various geomorphological units, further controlling the distribution and development of the karst reservoirs. The inherited geomorphology also controlled the sedimentation of the clastic rocks in the overlying formation. Due to the paleogeomorphological setting of higher in the east and lower in the west, the hydrocarbon generation intensity of the early Xujiahe Formation increases from the karst highland to the karst depression. Due to the cutting and lateral sealing of the formation by faults and karst valleys, there are no unified gas-water boundaries in the Lei 4 3 sub-member. As a result of the overall tectonic setting of high in the east and low in the west, the natural gas is enriched in the karst monadnocks. • Karst palaeogeomorphology at the top of the Leikoupo Formation was restored. • The karst geomorphology controlled the reservoir, source rocks and gas accumulation. • The gas accumulations have characteristics of each karst monadnock is a gas accumulation.

СТРОЕНИЕ ВОДОНОСНЫХ ГОРИЗОНТОВ ПО АНОМАЛИЯМ МАГНИТНОГО ПОЛЯ И СИЛЫ ТЯЖЕСТИ

The results of the study of the main characteristics and structural features of the aquifer systems of cold and thermal waters on the example of well-known sources in the Middle East are presented. The study of groundwater aquifers was carried out on the basis of the technology of geophysical data complex interpretation, taking into account the features of the aquifers geological structure. Aquifers and underground reservoirs are visualized as zones of reduced magnetization and density of rocks on deep petrophysical sections calculated from magnetic field and gravity anomalies. Horizontal subterranean water conduits can be clearly traced on the areal distributions of magnetization and density along the stretches of the lineaments with reduced values. Vertical subterranean water conduits stand out vividly on magnetic and density sections in the form of ascending tracks. As a result of the study of deep sections through the zones of known thermal springs, an estimate of the dependence of the source temperature on the depth of the feeding underground reservoirs was obtained. The article examines the potential of underground resources aquifers of the arid territories of Israel, the Kingdom of Saudi Arabia and the United Arab Emirates, for which the relevance of the water problem, apparently, will not decrease in the coming decades. The forecast of the location of groundwater karst reservoirs can be carried out by magnetic and density sections, taking into account two-dimensional models of the distribution of magnetization and density of rocks. The methodology of the study of aquifer systems is applicable to the analysis of underground resources of Africa and the Middle East arid regions.

Residual Carbonate Karst Reservoir Reconstructed by Karst Planation: A Case Study of the Ordovician Paleokarst Reservoir in the Ordos Basin

Residual Carbonate Karst Reservoir Reconstructed by Karst Planation: A Case Study of the Ordovician Paleokarst Reservoir in the Ordos Basin

Prediction of Permian Karst Reservoirs in the Yuanba Gas Field, Northern Sichuan Basin, China

Prediction of Permian Karst Reservoirs in the Yuanba Gas Field, Northern Sichuan Basin, China

Numerical Optimization of Geothermal Energy Extraction from Deep Karst Reservoir in North China

Numerical Optimization of Geothermal Energy Extraction from Deep Karst Reservoir in North China

Petrographic, mineralogical and geochemical constraints on the fluid origin and multistage karstification of the Middle-Lower Ordovician carbonate reservoir, NW Tarim Basin, China

The Middle-Lower Ordovician carbonate formation is the most important hydrocarbon-bearing unit in the Tarim Basin, China. Previous studies have predominately attributed the fracture-cave carbonate reservoirs to epigene karstification, and few involved polygenic karstification. Until now, mechanism and evolution model of the fracture-cave reservoirs has not been very robust. Based on detailed petrographic, mineralogical, karstological and geochemical analysis at the outcrop, this research provides new insights into the characteristics of the karst reservoir, origin of diagenetic fluids and multistage karstification in the Middle-Lower Ordovician. The results indicate that matrix porosity and permeability of the reservoir rock are low, ranging from 0.03% to 1.35% (mean 0.46%) and from 0.000864 mD to 9.15 mD (mean 0.33 mD), respectively, and that karst cave and fracture systems account for the dominant secondary reservoir porosity in the Middle-Lower Ordovician. Meteoric water and hydrothermal fluids are the two most important diagenetic fluids in the Middle-Lower Ordovician, and two stages of dissolution are responsible for the development of the fracture-cave reservoirs in the region. Extensive dissolution by meteoric water resulted in the formation of large volumes of fracture-cave reservoirs during the Middle Caledonian. Hydrothermal dissolution induced by the hydrothermal event during the Early Permian formed additional fracture-cave reservoirs in the Middle-Lower Ordovician carbonate as well. The meteoric dissolution is primarily controlled by the karst unconformity, and the fracture-cave system resulting from meteoric dissolution has a stratiform distribution below the unconformity. The fracture-cave system formed by hydrothermal fluids is mainly controlled by the deep-seated faults and the previous permeable layers, which determine that its distribution is closely related to faults. Moreover, hydrothermal dissolution may occur at high temperature and at great depths, while mineral precipitation may be the dominate process at relatively low temperature and at shallow depths. In this study, a reservoir development model of multistage karstification was established that could further enhance the knowledge of karstification process and assist in petroleum exploration in deep carbonate strata. • Two types of diagenetic fluids are revealed by integration of Geochemical, Petrographic and Mineralogical analysis. • The effects of multistage karstification on the carbonate reservoir are determined. • Hydrothermal karstification plays a positive role in the deeply burial environment. • A development model of the carbonate reservoir covered by the overlying impermeable strata is established.

Characteristics of collapsed subsurface paleokarst systems and controlling factors of subsurface paleokarst development in the Lianglitage Formation, Halahatang oilfield, Tarim Basin, NW China

The deeply buried Ordovician Lianglitage Formation (>6500 m) in the Halahatang area, Tarim Basin, is characterized by a paleokarst topography and subsurface, collapsed paleokarstsystems. Based on core and thin-section observation and wireline-log interpretation, this study analyzed basic features of collapsed karst systems and mapped the pattern of collapsed and original karst systems through root mean square (RMS) amplitude and variance attribute fusion. Employment of production data revealed the general rule of these ultra-deep, buried, collapsed systems as reservoirs for oil and gas. This study showed that epigenic subsurface karst systems in the Lianglitage Formation experienced full collapse, filling, and subsequent strong compaction. Passages of subsurface karst systems have a dendritic pattern These ultra-deep and strongly compacted collapsed systems are not favorable for oil and gas production owing to poor reservoir qualities. Factors controlling original subsurface karst development in this area are extensive subaerial exposure during the Falling-Stage Systems Tract (FSST) and Lowstand Systems Tract (LST) periods following deposition of the Lianglitage Formation, faults and fractures formed before or during karstification, and surface drainage systems on top of the Lianglitage unconformity. This study provides a method to comprehensively characterize the features, distribution and mechanism of the ultra-deep buried epigenic karst reservoirs and clarified their low potential as reservoirs for oil/gas exploration and development. It also helps clarify that the true oil pay reservoirs in this reservoir resulted from hypogene dissolution rather than epigenic karst.

Key technologies for the efficient development of ultra-deep ancient dolomite karst gas reservoirs: A case study of the Sinian Dengying Formation gas reservoir in the Anyue gas field of the Sichuan Basin

The Sinian Dengying Formation gas reservoir in the Anyue gas field of the Sichuan Basin is an ultra-deep ancient dolomite karst gas reservoir, whose development is of high difficulty, for the reservoir is thin and scattered with strong heterogeneity and is overall characterized by low porosity and low permeability. In order to provide support for the large-scale productivity construction of the gas reservoir of the fourth member of Dengying Formation (Deng 4 Member) in the marginal platform belt of the Anyue gas field, this paper carries out multi-discipline comprehensive researches on the technical bottlenecks based on the geological characteristics of the gas reservoir. In addition, the targeted key technologies for the efficient development of gas reservoirs were developed in the mode of “simultaneous research, application and improvement”. And the following research results are obtained. First, based on the modeling technology of fractured–vuggy dolomite karst reservoirs, accurate modeling of karst reservoirs is realized. Late actual drilling results show that the model's coincidence rate is increased from 63.0% to 90.7% and the “transparency” of gas reservoirs is achieved. Second, based on the reserve mobility evaluation technology of low-porosity and highly-heterogeneous karst reservoirs, the recovery factors of different reservoirs are determined. And consequently, the lower limit of well-controlled reserves and radius for the efficient development of different reservoirs are obtained and type I and II reservoirs are selected as the main development targets. Third, based on the development optimization design technology of highly-heterogeneous karst gas reservoirs, it is recommended to apply highly-deviated wells (the maximum deviation angle of 75°–84°) in the situations with multiple reservoirs crossed and superimposed. If type I and II reservoirs are developed intensively, it is recommended to adopt horizontal well drilling with a horizontal section of 800–1100 m. What's more, the reasonable well spacing in type I and II reservoirs shall be 1.6–3.3 km and 0.9–1.4 km, respectively. Fourth, based on the distinguished drilling technology of highly-deviated wells/horizontal wells in deep carbonate rocks, the fault complexity ratio is decreased from 20.7% to 4.9%, so safe and fast drilling and completion of highly-deviated wells/horizontal wells is realized and well control risks are diminished. What's more, the drilling rates of type I and II reservoirs are increased from 24.2% to 87.3%, indicating an effective improvement of reservoir drilling rates. Fifth, based on the open-hole segmented accurate acid fracturing technology of highly-deviated and horizontal wells, the difficulty in the highly-heterogeneous reservoir stimulation by horizontal well drilling is solved by conducting the differentiated acid fracturing process of “one segment, one strategy”. Based on the application of these innovative development technologies in the Deng 4 Member gas reservoir in the marginal platform belt of the Anyue gas field, the annual natural gas production capacity and cumulative natural gas production by the end of December 2020 are 60 × 108 m3 and 103 × 108 m3, respectively. In this way, the Sinian Dengying Formation gas reservoir of the Anyue gas field, the economic benefit of which is close to the marginal benefit, becomes the main force of conventional natural gas production increase.