Karst Reservoirs Research Articles

Characteristics and exploration prospects of Middle Permian reservoirs in the Sichuan Basin

In recent years, encouraging progress has been made in the Middle Permian natural gas exploration in the Sichuan Basin – numerous wells have obtained high-yield industrial gas flows in the dolomite porous reservoir sections in the Middle Permian Maokou Formation and the Qixia Formation, making the Middle Permian one of the most realistic replacement strata at present. In order to clarify the further exploration prospect of the basin, the Middle Permian depositional settings and prospective reservoir types were analyzed. The results show that: (1) the depositional environment was predominately open, shallow water; the paleogeomorphology was high in the west and low in the east, with bioclastic flat reservoirs more extensive in the middle-west part than the east part; crustal extension made Middle Permian deposits have good conditions for thermal water deposition and hydrothermal alteration; and (2) the prospect reservoirs for exploration were shoal facies dolomite porous reservoir of the Qixia Formation, thermal water dolomite porous reservoir and karst reservoir of the Maokou Formation, thus, dolomite porous reservoir was the most perspective target in the Middle Permian. It is concluded that Middle Permian limestone source rocks have large hydrocarbon generation intensity in the northern part of Western and Central Sichuan Basin and have the material basis for the formation of large-medium gas fields; the thick dolomite reservoir of the Qixia Formation in the Western Sichuan Basin is the most prospective reservoir in the Middle Permian; three types of reservoirs are distributed in the Central Sichuan Basin, and they are superimposed vertically with large exploration potential; Jiange–Nanchong–Fengdu region is located in the thermal sub-basin zone, where the thermal water dolomite reservoir of the Maokou formation is pervasive and superimposed on the karst reservoir of the Maokou Formation, showing good natural gas exploration prospects.

Major factors controlling the development of marine carbonate reservoirs

Abstract Through case study of reef-shoal, karst and dolostone reservoirs in the Tarim and Sichuan Basins, the material base and the mechanisms of porosity creation, modification and preservation of carbonate reservoirs were examined carefully in order to figure out the major factors controlling the development of various types of carbonate reservoirs. Three factors control the development of carbonate reservoirs: (1) carbonate sediments of reef-shoal facies are the material base for not only reef-shoal reservoirs, but also dolostone and karst reservoirs; (2) epigenetic environment is crucial for porosity creation, i.e. reservoir space in ancient carbonates includes primary depositional porosity and/or secondary dissolution pores caused by epigenetic, freshwater dissolution; and (3) burial environment is the setting for porosity modification and preservation. There is no carbonate reservoirs of solely one origin, carbonate reservoirs are all the products of the combined effect of the above three factors. The four main controlling factors, reef-shoal sediments, epigenetic karstification, evaporation facies belt and burial-thermal fluid dissolution give rise to the reef-shoal, karst, sedimentary dolostone and burial-hydrothermal alteration dolostone reservoirs respectively.

Unconformities and their influence on lower Paleozoic petroleum reservoir development in the Tarim Basin

Abstract Ten regional unconformities punctuate the Lower Paleozoic carbonate succession of Tarim Basin. They can be divided into three orders and include four unconformity types. According to their extents and intensities of development, they can be categorized into three orders, a first-order unconformity of great lateral extent and depth of erosion, and second-order and third-order unconformities of lesser extent and erosional intensity. From the viewpoint of their origins, these unconformities can be classified into four types, structural unconformities, long-term exposure unconformities, synchronous sedimentary exposure unconformities and superimposed unconformities that include combinations of the previous three types. Through analyses of the geographic distribution of unconformity types across the geographic extents of the 10 unconformities and their temporal evolution, it was determined that unconformities of various orders have different effects on the development of carbonate reservoir beneath these unconformities. First-order and second-order unconformities are prominent in the development of karst reservoir of the Tarim Basin. Periodic exposure of carbonate rocks which occurs during erosion that generated third-order unconformities contributed to the development of multiple karst intervals. In addition, the abundance and size of karst intervals in progradational strata increased during sea level falls. Origins of unconformities control scales and types of karst reservoirs. Tectonic (angular) unconformities developed during tectonic uplift and denudation, which controlled the development of weathering crust karst reservoirs within zones containing buried-hill (e.g. low paleo-uplift). Long-term exposure (parallel) unconformities formed during long-term sea level fall which controlled the development of regional interlayer karst reservoirs within the platform. Synchronous sedimentary exposure unconformities formed due to intermittent exposure caused by sedimentary basinal infilling and subdued sea level fluctuations which controlled the development of penecontemporaneous karst reservoirs of smaller geographic extent along platform margins. Superimposed unconformities formed due to multiple superposed tectonic movements, which caused the superposition of multiple weathering crusts as individual karst reservoirs, or the stacking of karst reservoirs developed beneath different unconformity types.

Water resources assessment and management of karst aquifer systems in Greece

Groundwater resources of karst aquifers within Circum-Mediterranean are a vital natural resource, which have been exploited since the dawn of the Mediterranean civilization. In Greece, carbonate formations are thought to be one of the most important sources for various water uses, that are vital for the country’s economy and development, that is agriculture and water supply. In most cases, they are considered as integral parts of some of the most strategic hydrosystems in Greece, containing surface water and groundwater resources that secure water supply for several anthropogenic activities. This paper provides a review of different aquifer types in Greece (granular and karstic), focusing on karst hydrosystems. It presents their spatial distribution throughout the entire Greek territory, and describes the most important environmental stresses upon them, while it analyzes their importance and role to the country’s water supply. Finally, this study analyzes the river basin of (Central Greece), used as a reference hydrosystem, to prove the ample importance of karst reservoirs in Greece.

Paleofluid restoration and its application in studies of reservoir forming: A case study of the Ordovician in Tarim Basin, NW China

Abstract Karst reservoir paleofluid types of the Ordovician formation in the Tarim Basin are restored based on the analysis of element boron of filled mud, the test of fluid inclusions homogenization temperature and salinity, liquid anion and hydroxyl isotope of filled calcite in vug-fractures, and regional geologic background. The origin of the karst reservoirs are analyzed on this basis. The element boron contents of mud filled in vug-fractures are less than 80 μg/g generally; fluid inclusions have different homogenization temperatures in different regions, and the salinities are in a large range. The HCO 3 − contents are high, and the Cl − and SO 4 2− contents are dispersive in the liquid component of the fluid inclusions. The hydroxyl isotope contents are relatively dispersive, with relatively negative δD value and positive δ 18 O value. This evidence shows that the paleofluid of the Ordovician was from supergene atmospheric freshwater, buried fresh-brackish mixed water, seawater or concentrated seawater, and buried brine from underlying dolomite or evaporate rock of the Cambrian formation. The main constructive diagenesis for the formation of vug-cave type reservoirs is erosion and dissolution caused by atmospheric freshwater, and the reservoirs have been subjected to thermal fluid reformation from underlying evaporate rock of the Cambrian during burial stage.

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.

Predicting potential exploration targets based on key factors controlling hydrocarbon accumulation—a case study from the Lower Ordovician Yingshan Formation in the Tazhong area, Tarim Basin, NW China

The Tazhong area, at the centre of the Tarim Basin, northwestern China, contains abundant hydrocarbon resources and features complex geological setting, principally in the Lower Ordovician carbonate reservoirs. Despite years of exploration and development, the hydrocarbon accumulation mechanisms and the key factors controlling hydrocarbon accumulation are still unclear. This lack of knowledge is the critical factor restricting hydrocarbon exploration in the Lower Ordovician Yingshan Formation. An analysis of the geological setting and reservoir distribution characteristics, the accumulation mechanisms and the key factors are investigated to predict the target exploration area. Our results show that the hydrocarbons, which experienced four stages of accumulation, originated from mixed sources including Cambrian–Lower Ordovician and Middle–Upper Ordovician source rocks within the Tazhong area and beyond. Hydrocarbons, by point charging, migrated into weathering crust karst reservoirs along 10 intersections of northeast and northwest fault sets, and then laterally migrated, principally in the NW–SE direction. The Yingshan Formation carbonate reservoirs are typically lithologically defined and primarily controlled by differential capillary forces. The accumulation and distribution of hydrocarbons in the Yingshan Formation were controlled by excellent reservoirs and hydrocarbon charge energy. The physical properties of excellent reservoirs control the petroliferous features and the hydrocarbon accumulation threshold: these reservoirs have a threshold with the minimum porosity and permeability, approximately 1.8% and 0.1 mD, and have a threshold with the maximum capillary forces, approximately 2.5 MPa. Hydrocarbon charge energy controls the hydrocarbon enrichment range: hydrocarbons enriched in areas near injection points and the hydrocarbon charge distance threshold of 20 km. Excellent reservoirs and hydrocarbon charge energy coupled index (RSI) controls hydrocarbon accumulation and distribution. Accumulation does not occur when the value of RSI is <0.5 but is favoured when values are higher. The coupled index is used to predict that lithological reservoirs in the Yingshan Formation are mainly distributed in the northeastern area of the ZG17–TZ72 well field, the area between the ZG17 well field and ZG22 well field, and the southwestern area of the ZG44–ZG53 well field.

Accumulation conditions and exploration directions of natural gas in deep subsalt Sinian-Cambrian System in the eastern Sichuan Basin, SW China

Abstract Based on comprehensive analysis of outcrops and drilling data, and interpretation of more than 6×10 4 km 2 seismic data, this paper delineates the regional distribution of the gypsum-salt layers in Middle-Lower Cambrian in eastern Sichuan Basin, and discusses accumulation conditions for natural gas in deep subsalt Sinian-Cambrian System in eastern Sichuan Basin, and points out favorable exploration directions. There are two sets of regional gypsum-salt layers in the Middle-Lower Cambrian in eastern Sichuan Basin, which are regional detachment layers, with fault-related folds occurring above and broad anticlines occurring below. There are 6-7 rows of NNE and NE trending large traps developed under the salt. Good space configuration of source rocks, reservoirs and cap rocks provide favorable accumulation conditions for natural gas in the deep subsalt in this region: three sets of hydrocarbon source rocks including Sinian Doushantuo Formation, the third Member of Sinian Dengying Formation, and dominant Cambrian Shuijingtuo Formation; three sets of weathering crust karst reservoirs and grain beach reservoirs including the second and fourth Members of Sinian Dengying Formation, and Cambrian Shilongdong Formation; and the regional cap of gypsum-salt in the Middle-Lower Cambrian and shale in Shuijingtuo Formation. The top target in deep subsalt exploration in eastern Sichuan Basin should be the fourth Member of Dengying Formation, followed by Shilongdong Formation and the second Member of Dengying Formation, and areas on migration direction with appropriate burial depth and being far away from the edge of the basin should be the first choices in exploration. Therefore, the favorable hydrocarbon exploration areas in the near future are the Honghuadian-North Huaying Mountain, Liangshuijing-Pubao Mountain-Leiyinpu, and Datianchi structural belts.

Carbonate reservoir characterization using seismic diffraction imaging

Although extremely prolific worldwide, carbonate reservoirs are challenging to characterize using traditional seismic reflection imaging techniques. We used computational experiments with synthetic models to demonstrate the possibility that seismic diffraction imaging has of overcoming common obstacles associated with seismic reflection imaging and aiding interpreters of carbonate systems. Diffraction imaging improved the horizontal resolution of individual voids in a karst reservoir model and identification of heterogeneous regions below the resolution of reflections in a reservoir-scale model.

岩溶缝洞储层地震识别及预测—以元坝地区雷四II亚段为例

受印支运动影响，元坝地区雷四II亚段发育古岩溶风化壳白云岩储层，通过地震响应特征分析，明确雷四II亚段岩溶缝洞储层发育的地震响应特征为：须家河组底界面地震同相轴能量变弱，出现复波。针对岩溶缝洞储层发育特点及地震响应特征，开展了岩溶缝洞储层识别及检测的地球物理方法研究。明确多子波分解与重构技术能够识别岩溶缝洞储层的地震响应特征；相干加权能量变化属性能够有效反映雷四II亚段白云岩储层分布范围；体曲率技术能够刻画岩溶缝洞储层分布范围，并预测元坝地区雷四II亚段可能存在5个岩溶缝洞发育群，预测结果为元坝地区雷口坡组勘探提供了基础材料。 Based on Indosinian movement, there deposited karst reservoir in Lei42 of Leikoupo Formation. Via seismic study, the karst reservoir’s seismic characteristics were determined, which were of low seismic energy and muti-wave reflection. According to geology and seismic reflection characteristic study, the seismic prediction method on karst reservoirs was developed. It concluded that seismic wavelets decomposition and reconstruction technology could recognize the karst reservoir; coherent weight energy variation attribute could predict the area of dolomite reservoirs; seismic cube curvature could illustrate the karst reservoir area. There were five weathering crust zones predicted, which provided objective basement data for exploration of Leikoupo formation.

Mechanism and development model of karsts in Ordovician buried hills in western Lungu area, Tarim Basin

Abstract In the western Lungu area in the Tarim Basin, dissolution pores, holes and fractures served as reservoir space in the Ordovician carbonate rocks. The karst reservoirs had a strong heterogeneity, and were classified into four types, including fracture, hole, fracture - hole, and cave. In different landscape units, the karsted fracture and hole system was vertically zoned. The mechanism of karst was studied with sheet analysis, and a developmentmodel of the karst reservoir in the Ordovician western Lungu carbonate rocks was established. The Lungu7 block in the east was the highest in the study area, and recharged the buried hills in Lunnan. The karst platform in the western Lungu area recharged the western Lungu area. Meteoric water infiltrated, forming dissolution pores and fractures, which were filled by clays and calcites. On the karst slope, groundwater migrated horizontally, and a karst river/pipeline system developed, filled by karst breccias and clays. The karst basin drainage area determined the scale of karst development.

The Origin of Paleokarst in the Huanglong Formation of the Eastern Sichuan Basin: Evidence from δ13C, δ18O and87Sr/86Sr

Karst rocks from the Huanglong Formation exposed at the margin of the Eastern Sichuan Basin can be divided into four types: slightly corroded, moderately corroded porous, intensely corroded brecciated and intensely corroded and replaced secondary calcic karstic rocks. The carbon, oxygen and strontium isotope compositions of the various karst rocks are analyzed systematically and compared to rocks without karst corrosion. The results indicate that (1) the Huanglong Formation in the eastern Sichuan Basin was a restricted bay supplied and controlled by freshwater in which mud-micrite and mud-dolomicrite exhibit low δ13C and δ18O values and high 87Sr/86Sr ratios; (2) all types of karstic rocks in the paleokarst reservoirs of the Huanglong Formation in the research area are affected by atmospheric freshwater with the δ13C and δ18O values and 87Sr/86Sr ratios in the original formation approaching those of atmospheric freshwater, which reflects ancient hydrological conditions, fluid properties, isotopic source and the fractionation effect; (3) the intensely corroded and replaced secondary limestone is affected by a variety of diagenetic fluids, often reflected by δ13C and δ18O values, while the 87Sr/86Sr ratios exhibit the strong degree of the corrosion; (4) after comparing the 87Sr/86Sr ratios of each type of karst rock, the diagenetic fluids are determined to be mainly atmospheric freshwater, and depending on the strength of corrosion, and the low 87Sr/86Sr ratio fluids in the layer will participate in the karst process. The carbon, oxygen, and strontium isotopes of different karstic reservoirs can provide meaningful geochemical information for forecasting and evaluating the development and distribution rules of the Huanglong Formation at the margin of the eastern Sichuan Basin in time and space.

Characteristics and controlling factors of beach-controlled karst reservoirs in Cambrian Longwangmiao Formation, Moxi-Gaoshiti area, Sichuan Basin, NW China

Abstract Based on data from boreholes, cores and lab analysis, the characteristics, genesis and controlling factors of different types of reservoirs in Cambrian Longwangmiao Formation, Moxi-Gaoshiti area, Sichuan Basin, are examined, and the distribution of favorable reservoir zones is predicted. The reservoirs can be subdivided into four types according to the different types of reservoir space and their combination with the “piebald” karst system: pinhole, “piebald” pinhole, cave, and “piebald” cave. Among them, the “piebald” cave reservoir is the best in quality, followed by the cave and “piebald” pinhole reservoirs, and the pinhole reservoir is the worst in quality. The genesis and controlling factors of Longwangmiao Formation reservoir are that the regional shoal deposition gave rise to a large area of grain dolomite, the layers with intergranular pores and small amount of intragranular dissolution pores of shoal facies provide a material base for later karst reformation. During the Caledonian period, the karst water flowing and corroding along the porous bed formed previously played a key role in the formation of premium reservoirs. During the period of the Caledonian-Hercynian, the tectonic paleogeomorphology controlled the fluid potential of karst water, which in turn decided the development of reservoirs. Karst is most developed on the slope of the paleotopography (along the well line of Moxi 201-Moxi 9-Moxi 12), where “piebald” cave or cave reservoirs usually occur, which are the most favorable reservoir zones.

Petroleum geological features and exploration prospect of deep marine carbonate rocks in China onshore: A further discussion

Deep marine carbonate rocks have become one of the key targets of onshore oil and gas exploration and development for reserves replacement in China. Further geological researches of such rocks may practically facilitate the sustainable, steady and smooth development of the petroleum industry in the country. Therefore, through a deep investigation into the fundamental geological conditions of deep marine carbonate reservoirs, we found higher-than-expected resource potential therein, which may uncover large oil or gas fields. The findings were reflected in four aspects. Firstly, there are two kinds of hydrocarbon kitchens which were respectively formed by conventional source rocks and liquid hydrocarbons cracking that were detained in source rocks, and both of them can provide large-scale hydrocarbons. Secondly, as controlled by the bedding and interstratal karstification, as well as the burial and hydrothermal dolomitization, effective carbonate reservoirs may be extensively developed in the deep and ultra-deep strata. Thirdly, under the coupling action of progressive burial and annealing heating, some marine source rocks could form hydrocarbon accumulations spanning important tectonic phases, and large quantity of liquid hydrocarbons could be kept in late stage, contributing to rich oil and gas in such deep marine strata. Fourthly, large-scale uplifts were formed by the stacking of multi-episodic tectonism and oil and gas could be accumulated in three modes (i.e., stratoid large-area reservoir-forming mode of karst reservoirs in the slope area of uplift, back-flow type large-area reservoir-forming mode of buried hill weathered crust karst reservoirs, and wide-range reservoir-forming mode of reef-shoal reservoirs); groups of stratigraphic and lithologic traps were widely developed in the areas of periclinal structures of paleohighs and continental margins. In conclusion, deep marine carbonate strata in China onshore contain the conditions for widely and intensively preserving hydrocarbons, so large oil or gas fields are expected.

Prospects of and challenges to natural gas industry development in China

An unprecedented breakthrough has been made over the past decades in natural gas industry, which helps improve energy mix and promote the low-carbon economy in China. With such abundant hydrocarbon resources, China owns two intensive oil and gas producing blocks in the Ordos Basin and Xinjiang province and two other concentrated gas producing blocks in Sichuan and Western South Sea. In addition, arterial gas lines have been connected as a gas grid all over China and natural gas market has become more and more mature and expanded. Thus, a natural gas industry system has come into being. However, with natural gas unevenly scattering all across China, the remnant resources mainly are distributed in the stratigraphic strata, deep strata in superimposed basins or in mature exploration zones, foreland basin thrust belts, marine gas fields, and so on. In reality, the future gas exploration should focus on such domains as the weathered crust karst reservoirs or carbonate and stratigraphic traps, deep clastic gas layers, and unconventional oil and gas plays. Achievements have been made in marine shale gas exploration, CBM gas steady development, and other unconventional natural gas resources. From the perspective of exploration potential, more giant oil and gas fields will be possibly discovered in deep strata or deep sea water, and stratigraphic hydrocarbon reservoirs and tight oil and gas reservoirs will also be the exploration focus. With the increase of exploration depth and degree, the overall oil and gas exploration cost will be significantly rising in general. New discoveries or reserves increase in natural gas exploration will highly depend upon research theory and technology progress, and such development technologies as 3D seismic survey, horizontal drilling and fracturing treatment will be more highlighted. Through enhancing the cost in natural gas exploration and development and strengthening the research of core technologies, natural gas industry will keep the trend of rapid growth in near future in China.

Carbonate karst reservoirs of the Tarim Basin, northwest China: Types, features, origins, and implications for hydrocarbon exploration

Karst reservoirs are important exploration targets in marine carbonates of hydrocarbon-bearing basins around the world. In the Tarim Basin of northwest China, four types of carbonate karst reservoirs were delineated using data from more than 200 wells, including cores, thin sections, wireline logs, drilling logs, and production data; these were supplemented with seismic and other data. The four types of karst reservoirs described were all formed by meteoric water circulation but not limited in the vadose or phreatic zones, including buried tower karst, interlayer karst, overlying-aquiclude confining karst, and fault-related karst. Buried tower karst occurred in the carbonate successions underneath a regional unconformity (buried tower area), resembling the active karsting in the Guilin Region of South China. Interlayer karst developed during a relatively shorter exposure period compared with buried tower karst but is also situated on a paleouplift. Overlying-aquiclude confining karst is essentially a by-product and on the fringes of buried tower karst, and though it appears to be distributed evenly below a disconformity, it is not related to the disconformity in origin. Fault-related karst occurred in the carbonate succession of a high steep anticline with faults cutting through aquicludes and into the carbonates; dissolution is concentrated along faults and the crest of the anticline. We studied three previously undocumented types of karst and proved that karst reservoirs in the Tarim Basin occur not only in the carbonate succession under a regional unconformity, but also where no significant unconformity is recognized. Similar examples have been found in other basins of China and are expected to occur in other basins worldwide, particularly in areas of complex geologic history.

Paleo-geomorphology formed during Tongwan tectonization in Sichuan Basin and its significance for hydrocarbon accumulation

Abstract Through analyzing the nature and periods of Tongwan tectonization, by using seismic data, drilling data and outcrop data, the pre-Qiongzhusi Formation eroded paleo-geomorphology was described with remaining thickness and moldic methods, and the hydrocarbon accumulation conditions were analyzed. The Sichuan Basin and its adjacent areas experienced Tongwan tectonization from late Sinian to early Cambrian which include three episodes that all manifested as regional uplift and erosion, forming three disconformities. Affected by that movement, a large scale Deyang-Luzhou eroded valley in N-S direction was formed in Central-Southern Sichuan Basin. In the valley, thick Maidiping Formation and Qiongzhusi Formation are Cambrian important source kitchens; they form good source-reservoir combinations with two sets of weathering karst reservoirs in Deng 2 and Deng 4 Member in Dengying Formation, favorable for the formation of large gas field with huge resource potential. The analysis of forming mechanism and evolution history of the eroded valley shows the thrusting-fold uplifting and erosion in the second episode of Tongwan tectonization in late Sinian period, and the extensional movement in the background of rapid marine transgression in early Cambrian are key to the formation of the eroded valley.

Prospect of Ultra-Deep Petroleum Onshore China

A series of breakthroughs have been achieved in the exploration of ultra-deep formations onshore China as geological understanding and technologies progress, such as Tarim Basin, Sichuan Basin, Bohai Bay Basin. Some new understandings are put forward suggesting that ultra-deep petroleum potential should be beyond expectation: hydrocarbons are controlled by rift valleys in the pre-Cambrian in China, conventional marine carbonate hydrocarbon source kitchens and scattered liquid hydrocarbons in source rocks are cracked to provide a large scale of gas, clastics in ultra-deep formations develop excellent source rocks, the low limit of gas generation is extended and the upper temperature limit for ultra-deep oil cracking without catalysis is beyond 230°C, the low pore radius limit is different for different types of reservoirs and hydrocarbon accumulating mechanisms. The preservation of three types of carbonate karst reservoirs and three types of clastic reservoirs as well as the physical properties of volcanics are slightly affected by burial depth, and it is predicted that favorable reservoirs are widely spread in ultra-deep formations. Carbonate karst, source rock and clastic reservoir interbeds are associated effectively in large ancient uplifts and slopes close to source rocks, faults control hydrocarbons and volcanics form accumulations nearby sources. This decides that ultra-deep carbonates, clastics and volcanics can all form large accumulations, but the abundance of reserve is relatively low. In Chinese ultra-deep formations, there are remaining petroleum resources of 162.9×108t, gas of 16.3×1012m3, nine practical exploratory areas, 14 relay exploratory areas, and a great potential of exploration. In future, ultra-deep carbonates are an important relay area for oil and gas, ultra-deep clastics are secondarily important, and ultra-deep volcanics are and important area for further study.

Prediction of weathering paleokarst reservoirs by combining paleokarst landform with unconformity: A case study of Sinian Dengying Formation in Leshan-Longnüsi paleo-uplift, Sichuan Basin

Abstract Based on the research of unconformity type and distribution, and restoration of structural feature of paleo-uplift in the late sedimentary period of Sinian Dengying Formation, the evolution of paleokarst landform and karstic hydrodynamic field during depositional hiatus between the Cambrian and Sinian of Leshan-Longnusi paleo-uplift is revealed to predict weathering paleokarst reservoir of Dengying Formation dynamically and effectively. The restoration of ancient structures shows that salient structures were developed continuously in the west of Weiyuan-Renshou, forming concurrent erosional-overlap unconformity, but in the east of Weiyuan- Renshou line, the palaeostructure high shifted from east of Zizhong-Yingshan to the west, forming inversion erosional-overlap unconformity. On this basis, the ancient karstic hydrodynamic field change caused by the change in paleogeomorphology was analyzed to find out the modes and strength of dissolution for different paleokarst landforms. The study indicates that areas along Hechuan-Dazu, Peng'an-Guang'an-Anyue-Neijiang and north of Renshou experienced strong dissolution, developing multiple sets of karst layers; Dazu-Tongnan-Hechuan area underwent fairly strong dissolution, resulting in fairly developed karst reservoirs; the two ends of the area west of Weiyuan-Lezhi-Xichong-Yilong also experienced certain degree of dissolution, with fairly developed karst reservoirs, whereas the middle of this area experienced weak dissolution, resulting in poor development of karst reservoirs.

Geological features and hydrocarbon accumulation in the Xinken oil field, Tarim Basin

Recently, a large-scale Ordovician oil and gas pool has been discovered in Xinken, north of the Tarim Basin, and it has strongly heterogeneous reservoirs and complicated fluid distribution. Through analyzing oil, gas and water samples collected from this area, in combination of field production and testing data, this study reveals that the Xinken oil pool contains dominantly normal oil characterized by low viscosity, low sulphur, and high wax. It has a low content of natural gas, which presents typical characteristics of wet gas and is oil-associated gas. Oil-gas correlation shows that the oil was from the Middle-Upper Ordovician source rocks and the charge and accumulation of oil and gas took place in the Late Hercynian Period. Controlled by paleogeomorphology, sedimentary facies, ancient karstification, faults, and other factors in the Caledonian Period, the Ordovician carbonate reservoirs are dominated by dissolved pores, cavities and fractures. The reservoir body of fractures and cavities is distributed in the shape of strip and laminate along faults and excellent reservoirs are mostly in the range of 70–100 m below the top of the Yijianfang Formation. As a whole, the Xinken oil pool is a fracture-cavity pool controlled by interbedded karst reservoirs and is a complex composed of stacked karst fractures and cavities of various sizes. It contains oil in large areas horizontally, which is quasi-laminated in distribution, and the accumulation of oil and gas is controlled by the distribution and development degree of the Ordovician carbonate reservoirs. The study shows that this area has abundant resources and a great exploration potential for oil and gas.