Karst Highland Research Articles

Epigenetic karst in carbonate buried hills and its influence on reservoir development: A case study of the Carboniferous Weixi’nan Sag, Beibuwan Basin, China

Karst is widely researched in the field of carbonate reservoirs. This study proposes a new formation mechanism and development model, specifically epigenetic karst, and asserts that high-quality reservoirs can be developed in the karst highlands and karst slopes of ancient landforms under the epigenetic karst with facies-controlled characteristics. The results indicate that there are three stages of karst in the study area. The first period is syngenetic karst, characterized by molds, intragranular dissolution pores, and micritization. Isotopic data reveal that the range of variation of carbonate rock 13C and 18O is similar to that of cement. The second period is epigenetic karst, the most significant karst in the study area. It undergoes a prolonged development time and features non-selective fabric dissolution. Oxygen and carbon isotopes exhibit negative values (18O ranges from −10.033‰ to −13.767‰ and 13C ranges from −2.56‰ to −0.52‰), which are significantly altered by atmospheric precipitation. The third period is buried karst, characterized by suture structure and asphaltene filling. The negative value of 18O in calcite veins is significantly <−15‰. The dissolved pores and caves formed by the epigenetic karst become the primary reservoir space for oil and gas in carbonate rocks. These exhibit facies-control characteristics, indicating that bioclastic limestone karst is well-developed and micritic limestone karst is underdeveloped. The study establishes a development model for karst reservoirs in the study area, selecting the highlands and slopes of ancient landforms as favorable exploration sites.

Karst architecture characterization of deep carbonate reservoir using image logs in Tarim Basin, West China

This paper has conducted a high‐resolution characterization of a deeply buried carbonate karst in the Tarim Basin of China by utilizing the FMI (Full‐bore Formation Micro Image) method integrated with seismic data, drilling cores and thin sections. It was found that the type and scale of wellbore karst architecture are mainly controlled by karst paleogeomorphology and vertical faults. The karst highlands and slopes, respectively, develop vadose karst and phreatic karst, with different compositions of karst elements. In terms of the karst thickness in the wellbore, the vertical fault development zone is almost twice that of the non‐development zone, both for the vadose karst and phreatic karst. Furthermore, the vertical fault is also able to promote the development of mixing karst. The mixing zone is located 55 m below the water table, with a strata thickness of approximately 25 m, a net reservoir thickness of 5–15 m, and an average porosity of 4%–8%, which serves as an effective hydrocarbon reservoir in the Yingshan Formation. The results will provide insights into the vertical division of karst reservoir and help predict reservoirs using well logs.

Restoration of palaeogeomorphology and its petroleum-geological significance: Transitional-facies deposition during the early Permian in the southeastern Ordos Basin, North China

The identification of favourable shale-gas areas in marine-continental transitional shales are challenging because of the multiple thin producing layers and abrupt lateral pinch-outs. Using data obtained from drilling, logging, and 3D seismic and natural gamma spectroscopic logging, this paper investigates the control of early Permian palaeogeomorphology on the distribution of organic-rich shale in the southeastern Ordos Basin. Using the impression method and de-compaction correction of strata thickness based on seismic velocity reconstruction, we restore the karst palaeogeomorphology at the top of the Taiyuan Formation for the early Permian. Three geomorphic units including karst highland, karst slope, and karst depression are identified, and constitute a geomorphological pattern of interjacent depressions and highlands. In addition, based on the multi-lithological eigenvalue method, the spatial distribution of transitional shales in the Shan 2 lower sub-member of the lower Permian Shanxi Formation was inferred using post-stack seismic parameter inversion. Our results show that organic-rich shale is distributed mainly in the low section of Shan 2 lower sub-member, which is controlled by karst palaeogeomorphology mainly distributed in karst depressions. Organic-rich shale distributed in karst depressions are characterised by large cumulative thickness, high TOC, moderate thermal maturity, strong hydrocarbon generation intensity, and relatively high gas content, indicating that shale-gas sweet spots should be distributed mainly in karst depressions rather than in highlands or slope areas. Meanwhile, Th/U and Th/K ratios, which indicate paleo-environment during the period of shale deposition, show that the strength of reducing conditions was greatest in karst depressions, moderate in karst slopes, and lowest in karst highlands. Therefore, it is inferred that the distribution of shale-gas sweet spots may be related to the weakly reducing environment, which favoured the preservation of organic-rich sediments during sedimentation. The upper section of Shan 2 lower sub-member was dominated by a high-energy transitional sedimentary environment, and the paleo-redox conditions tended to be oxic, disfavouring the preservation of shale organic matter, and the organic-rich shale in the upper section is relatively poorly developed. This study shows that palaeogeomorphology and paleo-redox conditions affect the deposition and preservation of organic-rich sediments in lower section, the relationship between palaeogeomorphology and the distribution of organic-rich shale can provide reference for shale gas sweet spots prediction and promote the rapid exploitation of shale gas in eastern Ordos Basin.

Characteristics, Main Controlling Factors and Prediction of Complex Fracture–Cavity Reservoirs of Buried Carbonate Hills in the Weixinan Depression, Western South China Sea

Buried hill oil reservoirs have become a key area for offshore oil and gas exploration. In this paper, a typical oil field in the western South China Sea is used as the research object, and a study on the characterization, cause of formation and prediction of the fracture–cavity reservoir distribution is carried out. The reservoir in the study area is a complex fracture–cavity reservoir that developed due to weathering and leaching, tectonic movement and dissolution reconstruction on the limestone skeleton. The reservoir spaces are composed of karst caves, fractures and pores. The main controlling factors include lithological changes, karst landforms, tectonic deformation and faulting. To address the controlling mechanisms of the lithological changes on the formation of fracture–cavity reservoirs, a new parameter, the lithology standard deviation, to evaluate lithological changes is proposed based on the characteristics of the lithological changes, and the distribution of these lithological changes is portrayed in combination with the seismic attributes. The tectonic deformation principal curvature inversion algorithm is used to simulate the distribution of the tectonic principal curvature at the top of the Carboniferous. The larger the tectonic principal curvature is, the stronger the deformation of the rock formation and the more favorable the conditions are for fracture formation. The karst geomorphology controls the overall reservoir distribution, and the karst highlands and karst slope areas are the zones with the most–developed secondary pore space (or fractures and karst caves). The faulting control area is the fracture and dissolution pore development area, major faults control the distribution of the karst cave reservoirs, and secondary faults influence the formation of fractures in the faulted area. The study predicts and evaluates the distribution of fracture–cavity reservoirs from the perspective of fracture–cavity genesis quantification, and by gridding and normalizing the four major genesis quantification evaluation parameters and fusing the geological factors that control the formation of fractures and karst caves by using Back–Propagation neural network deep learning algorithms, a method for predicting the distribution of fracture–cavity reservoirs constrained by geological genesis analysis is developed.

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.

The Middle Permian Maokou reservoir (southern Sichuan Basin, China): Controls on karst development and distribution

The Middle Permian Maokou limestone is a major natural gas producing reservoir in the southern Sichuan Basin, China. While matrix porosity is low, bit drops, losses while drilling, blowouts, and extremely high well testing flow rates indicate the presence of a dual permeability system in the form of paleocaves, which were formed by karstification in the late Maokouan Age. A combination of geological, paleogeomorphological and karstic analyses reveal that the development and distribution of the karst features within the reservoir are primarily regulated by shoal facies, paleotopography and syndepositional faults. In our conceptual paleogeomorphological model, the potential cave-bearing zones are located along the rim and outer edge of the Luzhou Paleohigh, which was the highland during the karst. Outside the highland were the karst slopes, and the poljes were on the flanks of the study area. Reservoir performance data suggest, there are two types of sweet spots in the study area. The first corresponds to the syndepositional fault zones along the rim of the karst highland and on the karst slope, where the caves appear to distribute along the faults, impacting the reservoir's vertical permeability. The second corresponds to the shoals of the Mao 2 and Mao 3 members on the lower western karst slope and the upper eastern karst slope, where the caves are widespread, enhancing the horizontal permeability and connectivity of the reservoir. However, the caves are rare in the central karst highland, lower karst slopes, and poljes, which is consistent with the limited gas discoveries.

Reconstruction and petroleum geological significance of lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in northwestern Sichuan Basin, SW China

Based on analysis of outcrop, drilling, logging and seismic data, and geotectonic background, the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed, and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed. The Maokou Formation is divided into 3 long-term cycles, namely LSC1, LSC2 and LSC3, which correspond to the Member 1, Member 2 and Member 3 of the Maokou Formation, respectively. Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting, the area had tectonic differentiation in NWW and NE, and sedimentary basement took on episodic settlement from north to south, as a result, the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf. According to the residual thickness, paleokarst geomorphologic units such as karst highland, karst slope and karst depression at different stages were reconstructed. The karst geomorphological units were developed successively on the basis of sedimentary geomorphology. Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation. The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations: high energy grain shoal and karst monadnock, platform margin slope and karst slope. Based on this understanding, the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology. The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.

Reconstructing large‐scale karst paleogeomorphology at the top of the Ordovician in the Ordos Basin, China: Control on natural gas accumulation and paleogeographic implications

AbstractKarstification in carbonate successions has an important influence on hydrocarbon accumulation. Taking the Ordos Basin, currently the largest petroliferous basin in China, as an example, this study examines the large‐scale, long‐term (~120 Myr) paleokarst at the top of the Ordovician. The objectives of the study are to characterize the karst paleogeomorphology of this area, to explain the inconsistency between existing understandings of karst paleogeomorphology and exploration in the eastern Ordos Basin, and to reveal the control of paleokarst on natural gas accumulation and its paleogeographic significance. A total of 860 exploration wells were used for detailed stratigraphic correlation and analysis, along with core observations, well‐logging analyses, physical property characterization, and isotope analyses. Results of residual thickness and moldic thickness reconstruction reveal variation in karst paleogeomorphology between north and south in the eastern Ordos Basin, differing from the traditionally recognized E‐W variation. Two geomorphic units are classified as follows: the karst highland and the karst slope from north to south, with the karst slope being subdivided into northern and southern slope areas. The karst highland area has negligible reservoir capacity and hydrocarbon accumulation owing to the enhanced denudation that occurred there. In contrast, the northern karst slope shows favorable reservoir properties and has abundant gas wells according to well‐logging interpretations, whereas the southern karst slope is of poor reservoir quality and hosts mainly water wells. Differences in dissolution‐filling effects controlled by the surface paleodrainage system are suggested to be the main contributor to differential reservoir space preservation, which, together with the variable width and depth of source rocks in the grooves (thereby variably exposing source rock), further promoted differential gas accumulation. The Ordos Basin and its periphery in the southwestern North China Craton (NCC) show inheritance of sedimentary‐tectonic patterns from the Middle Ordovician to the Late Carboniferous. These results should provide a reference for hydrocarbon exploration in the Ordovician of other basins in the NCC in which karst occurs and karst basins worldwide, and deepens understanding of the paleogeographic framework in the context of regional uplift of the North China Platform.

Paleokarstic water tables and their control on reservoirs in Ordovician Yingshan Formation, Tazhong Area, Tarim Basin, NW China

Abstract Based on cores, well logging and karstic paleogeomorphology, the identification method of multi-period paleokarstic water tables and their distribution and control on reservoirs were researched of Yingshan Formation in Tazhong Area, Tarim Basin. Three periods' water tables numbered 1 to 3 have been identified from bottom to top of Yingshan Formation in Tazhong Area. The corresponding typical horizontal undercurrent zones are characterized by single karst fabrics such as sack-like dissolved vugs or subhorizontal channels, combination of various karst fabrics like pre-existing high-angle fractures intersected by small caves and subhorizontal channels, and combination of multi-layered caves and different karst fabrics of large pores. On the plane, periodical rising sea levels caused the development of No.1, No.1 and 2, No.1, 2 and 3 water tables in karst depressions, slopes and highlands respectively. As the result of the nearly horizontal occurrence of Yingshan Formation, submembers of Yingshan Formation had tiny angles with fracture-cave bodies of each period's water table, so the karst reservoirs are characterized by “crossed strata regionally and developed along the stratum locally”. The distance between water table and unconformity, thickness of horizontal undercurrent zone and average thickness of reservoir all tended to increase with the rise of karst landforms. The karst highland and slope had 2-3 periods' water tables and thicker horizontal undercurrent zones, so they had thicker and better quality reservoirs, making them favorable exploration areas.

Paleogeomorphy evolution of the Ordovician weathering crust and its implication for reservoir development, eastern Ordos Basin

Gas reservoir development of the Ordovician weathering crust in the Ordos Basin is closely controlled by the pre-Carboniferous paleogeomorphy. Previous studies show that the paleogeomorphy is high in the west and low in the east, and the karst highland, karst slope and karst basin are developed from west to east. With further exploration in recent years, many karst breccia that represent strong karstification, are found in the east area which previously is considered to be the Ordovician karst basin. Thus, it is necessary to revaluate controlling factors of karst paleogeomorphy development from a viewpoint of the dynamic paleogeomorphy evolution, to investigate the paleogeomorphy evolution of the Ordovician weathering crust in geological history and guide further research and prediction of development law of reservoir spaces. In order to reconstruct the paleogeomorphy of the weathering crust in the top of Ordovician in the east Ordos Basin, paleogeography, thickness of residual strata and paleokarst characteristics are well studied. The result shows that a wide range of paleokarst highland occurred in the central to east part of Ordos Basin in the early period, and the karst reservoir spaces were formed; but in the late period, the east part of the basin gradually evolved into the paleokarst basin, the current pre-Carboniferous paleogeomorphy was thus formed, and the dissolution reservoir spaces formed in the early period were mostly filled, accordingly the reservoirs were tight. However fracture networks formed by cave collapse connect intercrystalline pores of dolomite, therefore, the reservoirs can still be well preserved locally and are worthy targets for hydrocarbon exploration.

Characteristics of paleogeomorphology and paleokarstification and the impact on natural gas accumulation: a case study of upper assemblage of Majiagou formation in central Sulige gas field, Ordos Basin, China

The Lower Paleozoic gas reservoirs in central Sulige gas field are mainly accumulated in Ordovician ancient weathering crust karst carbonate reservoirs, and they are especially rich in the upper assemblage of Ordovician Majiagou formation (Ma51–Ma54 sub-members). However, the spatial distribution of gas is unclear yet because of the different denudation degree and uneven development of paleokarstification. This paper mainly focuses on the characteristics of paleogeomorphology and paleokarstification and their impact on natural gas accumulation. Geological stratifying has been done based on the observation of cores and thin sections, and the analysis of drilling, logging and seismic data. The paleogeologic map of pre-carboniferous was depicted according to the ancient weathering crust outcropped strata. Paleogeomorphology reconstruction is based on a comprehensive method combining the moulage method with the residual thickness method. Result indicates that the paleogeomorphology can be divided into western karst highland and eastern karst slope in the area investigated, and the denudation degree of ancient weathering crust in the west is more serious than that in the east. The karst highland can be divided into remnant hill and depression while the karst slope can be divided into gentle hill and shallow depression in detail. Paleokarst section can be vertically divided into epikarstic zone, vertical vadose zone, horizontal phreatic zone and deep slow flow zone based on the analysis of paleokarst identification mark and vertical dissection of single well. By analyzing the relationship between paleokarst characteristics and production data, the study shows that paleogeomorphology and paleokarstification have a significant control on nature gas accumulation and distribution. Gentle hills on karst slope are expected to be the most favorable areas for reservoirs horizontally, horizontal phreatic zone is more advantageous vertically, and second the vertical vadose zone.

Paleogeomorphological reconstruction and geological implications of the weathered-crust karst on the top of the Middle Triassic Leikoupo Formation in the Longgang area, Sichuan Basin, China

Weathered-crust karst carbonate is developed in the third submember of the fourth member of the Middle Triassic Leikoupo Formation (Lei43 submember) in the Longgang area of Sichuan Basin, China, and acts as an important oil and gas reservoir. To reconstruct the paleogeomorphology of the weathered-crust karst, we analyzed the seismic thickness of the interval from the erosional unconformity surface on the top of the Leikoupo Formation to the karst at the bottom of the third member of the Xujiahe Formation (Xu3 member), based on existing three-dimensional seismic and drilling data, and the sedimentary characteristics of the Leikoupo and Xujiahe formations. There are three secondary types of geomorphological unit, namely the karst highlands, the karst transitional zone, and the karst basin, and these have distinct karst water development patterns and hydrological conditions that determine the intensity of karstification and reservoir quality. Among them, the karst highlands have a poor reservoir capacity due to their long-term exposure above the water table, which resulted in severe denudation. The karst transitional zone features the superimposition of multiple periods of strong karstification, and this has resulted in high-quality reservoir conditions in the karst monadnocks but poor reservoir conditions in the karren. The karst basin represents a drainage area that experienced weak karstification, and the reservoir capacity is generally poor, although some good reservoirs were developed in shoals. Paleogeomorphological maps provide excellent guides to finding karst monadnocks in karst transitional zones, and monadnocks should serve as the main targets of exploration in the weathered-crust karst reservoir on top of the Lei43 submember.

Paleokarst reservoir features and their influence on production in the Tahe Oilfield, Tarim basin, China

This study analyzed the characteristics of cave heights and fillings, influences on cave distribution by unconformity and geomorphology, and production control trend for geological factors. The heights and numbers of caves in different geomorphologies are basically similar. The cave heights are greater near the unconformity surface. On an average, there are higher degrees of cave development and larger cave heights in the karst slopes than in the karst highlands and karst depression. Based on core observations, four types of reservoirs were identified: unfilled caves, filled caves, open fractures and filled fractures. The formation of caves is mainly controlled by faults and underground water surfaces. Faults works obviously in the karst highland, while underground water surfaces obviously in the karst slope. Caves controlled by faults are mostly unfilled, while controlled by underground water surfaces are mostly filled with clastic sediments. According to the results from both first-day and cumulative production, production is not highly correlated with cave heights, but rather storage and connectivity of caves and fractures. Generally, wells with unfilled caves or open fractures have higher first-day and cumulative production than filled caves or fractures. The distribution of high first-day and cumulative production is consistent overall.

Paleogeomorphology restoration and the controlling effects of paleogeomorphology on karst reservoirs: a case study of an ordovician-aged section in Tahe oilfield, Tarim Basin, China

The Tarim Basin is the largest petroliferous basin in northwestern China and contains the Tahe oilfield, the highest hydrocarbon enrichment in the entire basin. Paleogeomorphology has important controlling effects on carbonate reservoir development. Using core data, thin sections and drilling, logging and seismic data, paleokarst was identified and analyzed. The paleogeomorphology of the top low-middle Ordovician was recovered following a comprehensive utilization of the impression and residual thickness methods in combination with unconformity identification. Based on the filling thickness and residual thickness, the karst paleogeomorphic units were classified, and their controlling effects on reservoirs analyzed. The second-order paleogeomorphic units primarily include four types: karst highland, karst gentle⁃slope, karst steep⁃slope and karst basin. The karst gentle⁃slope and karst steep⁃slope are further classified quantitatively into third-order paleogeomorphic units (monadnock, tableland, karst terrace and trench). The results show that the development of the karst reservoir differed in different geomorphic units. In the karst gentle slope, the favorable areas for karst reservoir development are the karst terrace, tableland, monadnock, and the ends and flanks of trench. In the karst steep⁃slope, the karst terrace and tableland are favorable areas. In the vertical vadose and phreatic zones of the gentle-slope region, a large number of caves and underground river systems developed, but in the steep-slope region, only a small number of shafts and isolated caves are observed. This study plays an important role in oil and gas exploration and development.

Diversification of Tourism Offer as a Means of Socio-economic Development of Peripheral Regions – Case Study of Kupres

Many modern countries have significant problems with disparities in development between core and periphery. Municipality of Kupres in Bosnia and Herzegovina is one typical example of peripheral area, which is located within the sparsely populated region of Dinaric Karst Highland. It is taken for the case study in this paper, because it shows certain signs of economic activation in the field of tourism. However, its mainstream tourism offer is of seasonal character, and it is not sufficient for sustainable economic growth and significant progress in socio-economic sense. That is why tourism offer need to become more diversified. Methodology in this paper is focused on analysis of all potentials and motives that wider area of Kupres possesses, which can be valorized through systematic approach of institutions and local community. It is intended to emphasize a significance of geographical expertise in these kinds of problematic. Field research and GIS analysis are used for exploration of study area.

Characteristics and genetic models of Lower Ordovician carbonate reservoirs in southwest Tarim Basin, NW China

Lower Ordovician carbonate successions in the Taxinan area are intensively cemented and filled. Matrix porosity and permeability are low in the Lower Ordovician carbonate reservoirs of the Taxinan area, and reservoir space is composed of karst vugs and fractures. The reservoir space is dominated by four types of karst vug, including vesicular vugs, enlarged dissolved vugs along fractures, tabular vugs and isolated vugs, and three types of fractures, including medium-high-angle fractures, low-angle and horizontal fractures as well as irregular fracture networks. The quality and scale of reservoirs in different zones of the Taxinan area are obviously different, and the best reservoir rock is developed in the eastern part of the Taxinan area. These karstic reservoirs can be divided into three types according to the reservoir formation mechanism, including buried-hill vug-fracture reservoirs, vug reservoirs below unconformity surfaces and deep bedding underflow karst vug reservoirs. Different types of reservoir developed in significantly different successions and areas and are controlled by different factors. Buried-hill vug-fracture reservoirs are mainly developed in the Yingshan Formation and distributed in the fault-uplift zones of the eastern part of the Taxinan area. Their formation is controlled by paleogeomorphology and tectonic movement. Vug reservoirs beneath unconformity surfaces are developed in the Yingshan Formation and distributed in the gentle zone in the central-western Taxinan area. Their formation is controlled by the scale and erosion intensity of the unconformity. Deep bedding underflow karst vug reservoirs are mainly developed in the Penglaiba Formation and distributed in the fault-sag zones between the NE-trending fault-uplift zones in the eastern part of the Taxinan area. Their formation is controlled by paleogeomorphology, rifting and unconformity. Guided by this geological model of reservoir development, combined with drilling, well logging and seismic data, the distribution of favourable high-quality reservoirs in the Taxinan area is predicted. The results showed that the flanks of the fault-uplift and fault-sag zones in the eastern part of the Taxinan area and the haystack hills and karst highlands in the central-western parts of the Taxinan area are favourable reservoir exploration areas.

The Detailed Palaeogeomorphologic Restoration of the Ordovician Weathering Crust in the Northern Jingbian Gas Field: Methods and Applications into Reservoir Properties and Gas Accumulation

Palaeogeomorphology plays a crucial role in the development of sedimentary facies and the reservoir distribution in the late period of basin evolution, and to a certain extent, controls the assemblage of the late hydrocarbon reservoirs and caprock formations. Therefore, it has a function in controlling the enrichment of oil and gas. Caledonian orogeny uplifted the Ordovician Majiagou Formation in the Ordos basin and thus led to the formation of the weathering crust reservoirs in the process of weathering, erosion and leaching for over 130 Ma. Such palaeogeomorphology represents the background of the palaeokarst, which has an essential influence on the formation of reservoir rocks. Based on Pre-carboniferous paleogeologic map plotted by 305 wells, and the thickness of bauxite in 272 wells, methods of residual thickness and moulage are applied to reconstruct the palaeogeomorphology in the studied area. In combination with the thickness of horizon to the weathering crust, the criteria for the second order palaeogeomorphologic units were proposed. They are karst highland, karst slope and karst basin, which can be further subdivided into the third order palaeogeomorphologic units of remnant hill, depression, gentle hill, shallow depression and erosional grooves. According to the criteria and refined identify results of ancient grooves using geological data of 272 wells, palaeogeomorphology of the Ordovician weathering crust in the northern Jingbian gas field has been restored accurately. The results show that karst highland developed in the northwest part of the studied area, with remnant hill and depression occurring locally. Karst slope was present in the central and east, predominatly in the form of gentle hills and shallow depressions, with 5 nearly EW-trending ancient erosional grooves developed locally. Finally, integrated data of reservoir property with gas production test implied that the remnant hill in the karst highland, the gentle hill in the karst slope and the two sides of the erosional grooves were favorable positions for gas accumulation.

Restoration of paleokarst geomorphology of Sinian Dengying Formation in Sichuan Basin and its significance, SW China

Abstract Based on newly drilled well data in the Gaoshiti area in the central Sichuan Basin, profile data of more than 150 field outcrops of the regional geological survey, and stratigraphic division and correlation of more than 30 wells in the Sichuan Basin and its adjacent areas, combined with regional seismic data, moldic methods are comprehensively used to restore the karst paleogeomorphy of the Dengying Formation, and thus studying the paleogeographic pattern and the significance of oil and gas exploration. The Sichuan Basin was surrounded by paleo-lands/underwater highlands in the late Sininan Dengying period, including Kangdian paleo-land in the west, Songpan paleo-land in the northwest, Hannan paleo-land in the north, Qianjiang-Zheng'an, Zhenba and Wuxi-Jianshi underwater highlands in the southeast and northeast. The Sichuan Basin was adjacent to Jiangnan Basin southeastwards and Qinling paleo-ocean northeastwards respectively. Affected by the separation of NS-striking Zitong-Junlian Aulacogen, NE-striking Langzhong-Tongjiang and Chongqing-Kaixian depressions in this basin, the Sichuan Basin presents the NS-trending framework of “three uplifts (Zhenba, Chuanzhong and Qianjiang-Zheng'an) and two depressions (Langzhong-Tongjiang and Chongqing-Kaixian)”, and is divided into two relatively isolated EW-trending paleo-uplift systems (NS-striking Mianyang-Leshan-Xichang paleo-uplift and nearly NE-striking Chuanzhong paleo-uplift). Controlled by karst paleogeomorphy of the Sinian Dengying Formation, the pattern of karst landscape consists of five secondary geomorphic units, such as karst highland, karst platform, karst slope, karst depression and karst basin, of which the karst platform and karst slope are the favorable zones for the development of karst reservoirs, providing advantages for the formation of large gas fields.

Landforms of mountainous karst in the middle latitudes: reflections, trends and research problems

Research into the landforms of karst highlands in the middle latitudes began in the “Classical Karst” in the 19th century and has been marked by alternating periods of progress and stability since. In the beginning, the elementary morpho-types were recognized, in particular the circular closed depression defined as the “karst doline”. Later the mid-latitude karst forms were compared with those of other climatic zones, especially tropical humid karst. In recent decades many papers have been published on the morphometry, the fillings, and models of the evolution of the surface karst landforms. It has been emphasized that in the mid-latitude karsts the morphogenesis is the result not only of karst solution but also of several other weathering processes such as cryoclastic (frost) action and other periglacial processes. Examination of the main scientific journals dealing with karst and other geomorphology shows that present day research in karst areas is mostly concerned with very specific subjects, such as speleogenesis, hydrogeology, sedimentology, geochemistry, mineralogy, cave biology, unusual landforms: only a minority of papers focus on karst surface landforms, and among these only a few attempt organic and comprehensive studies of the entire assemblage of relief forms in a karst morpho-unit. In reality, it is evident that the surface landform complex in a karst morpho-unit has to be considered in its entirety and that only such an integrated approach to this complex entity may bring significant progress in understanding. The landforms may be considered from different point of views, but always as a part of a holistic approach, in particular: a) in the geodynamic context of the evolution of the particular mountain group, b) as part of a complex geo-ecosystem, c) in the framework of models based on modern geomorphological concepts. Such comprehensive studies carried out by multi- and inter-disciplinary research teams applying the most modern techniques and methodologies will present stimulating challenges not only for progress in karstology but also in all the environmental sciences.

The Early Hercynian paleo-karstification in the Block 12 of Tahe oilfield, northern Tarim Basin, China

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower–Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower–Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage.