2012 Vol. 34, No. 3

Display Method:
2012, 34(3)
Abstract(3438) PDF-CN(3026)
Abstract:
Application of basin-orogeny coupling in study of abnormally high pressured basin fluids
Wu Genyao, Liang Jiangping, Yang Jianguo, Zhu Defeng, Chen Junliang, Shen Jianian
2012, 34(3): 223-233. doi: 10.11781/sysydz201203223
Abstract(4043) PDF-CN(1752)
Abstract:
The basin-orogeny coupling theory was applied to study the abnormally high pressure in basin for the first time in this paper. The formation and distribution of abnormally high pressure were controlled by regional stress fields. In compressed basins, the key contributing factors for abnormally high pressure were compressive stress and charging of natural gas (or oil-gas), where the abnormally high pressure distributed zonaly or planarly. In extended and pull-apart basins, the abnormally high pressure was mainly resulted from thermal activities. In extended basins, the abnormally high pressure distributed in the shape of scattered points or restricted planes, and it distributed pointedly or in the shape of scattered points in sheared basins. The reverse, normal and strike-slip faults had different occurrences, which decided the oblique or vertical conductions of abnormally high pre-ssure. There were different ways for pressure reservation or releasing in compression or reversion. From the dynamic evolution of basin to the dynamic evolution of reservoir and fluid, it was an important step for the application of basin-orogeny coupling theory.
Current status and progress in research of hydrocarbon cap rocks
Zhou Yan, Jin Zhijun, Zhu Dongya, Yuan Yusong, Li Shuangjian
2012, 34(3): 234-245. doi: 10.11781/sysydz201203234
Abstract(5115) PDF-CN(2738)
Abstract:
It was reviewed in this paper the history and current status of the research of hydrocarbon cap rocks. The research progress in both mud and evaporite cap rocks was summarized, and the current research status of dynamic sealing mechanism was analyzed. There are 5 stages in the research development of cap rocks, and the focus of research has shifted from rock physics to deformation process and sealing effectiveness during hydrocarbon accumulation, such as study in high quality seals, mechanical properties of cap rocks, matching between sealing capacity and accumulation conditions, and so on. Extensive literature review suggests that evaporites are very important hydrocarbon seals all over the world and karstification in later stages would affect the sealing capacity; sedimentary texture, structure, and composition (especially clay content) of the mud rocks show fairly important influencees on sealing capacity of mud rock seals; deeply buried mud rocks which have been through high diagenetic evolution still show excellent sealing capacity. Capillary physical sealing, overpressure sealing, capillary multiphase sealing and several other types of sealing mechanism have been discovered and studied, but the capillary physical sealing is the most fundamental sealing mechanism. Counties that are active in cap rock research include the United States of America, Norway, Austrialia, Britain, Italy, Germany, and so on. Institutions involved are mainly colleges and universities, goverment institutions, and petroleum companies.
Favorable targets of breakthrough and built-up of shale gas in continental facies in Lower Jurassic, Sichuan Basin
He Faqi, Zhu Tong
2012, 34(3): 246-251. doi: 10.11781/sysydz201203246
Abstract(4459) PDF-CN(2550)
Abstract:
Compared with the successful exploration of shale gas in the North America, the shale gas in the main hydrocarbon-bearing basins in our country is hosted in continental deposition. The mineral composition and sedimentary structure of shale are different between continental and marine facies. The characteristics of fracturing related to rock mechanics are the keys for the effective development of continental shale gas. The shale gas in continental facies mainly develops in mud-shale stones of lacustrine facies in the Lower Jurassic, the Sichuan Basin. Through a lot of old wells reviewed and reevaluated, we have acquired industrial shale gas in Jiannan and Yuanba areas in the east of Sichuan, showing a good potential. The controlling factors for the generation and accumulation of shale gas in the Lower Jurassic of the Sichuan Basin have been analyzed, indicating that the lacustrine facies mud-shale stones are widespread in the Lower Jurassic and are outstanding for in-situ creation and in-situ accumulation. The TOC of mud-shale stones, the thickness of entrapped hydrocarbon and favorable preservation conditions all control the accumulation of shale gas in the Lower Jurassic. The contents of brittle mineral provide good conditions for massive hydraulic fracturing. Reasonable extract mode is the key to acquire industrial and long-term productions. According to shale gas evaluation and fracturing test, the shale gas in the Lower Jurassic of the Sichuan Basin is a favorable target for the breakthrough and built-up of shale gas in continental facies.
Petroleum exploration potential and targets in fault depressions, southern Songliao Basin
Hu Chunxin, Hong Xue, Zhao Hongwei, Liu Yuhua, Zhou Jing
2012, 34(3): 252-256. doi: 10.11781/sysydz201203252
Abstract(4196) PDF-CN(1816)
Abstract:
The Songnan fault depressions locate in the south of the Songliao Basin. According to the development ages and late reforming characteristics of the fault depressions, with the Xar Moron River fault as the northern boundary and the Chifeng-Kaiyuan fault as the southern boundary, the Songnan fault depressions can be divided into three belts from north to south:the Changtu-Manghan fault depression belt (the north), the Zhangwu-Dongsheng fault depression belt (the middle) and the Heishan Jurassic residual basin (the south). The Zhangwu-Dongsheng fault depression belt has good structure and is rich in oil all over the belt; hence explorations should be focused on the source rocks with continuous layered reflection, and the structural and subtle reservoirs should be targeted. In the Changtu-Manghan fault depression belt, the source rocks of the Jiufotang Formation has the characteristics of big thickness and fine geochemical index, so explorations should be carried out close to or in the source faults, and oil and gas mainly distribute in traps in the fault or close to it.
Analysis of hydrocarbon resource potential in Awati Depression of Tarim Basin
Gu Yi, Zhao Yongqiang, Jia Cunshan, He Guangyu, Luo Yu, Wang Bin, Lu Qinghua
2012, 34(3): 257-266. doi: 10.11781/sysydz201203257
Abstract(4100) PDF-CN(2067)
Abstract:
The degree of petroleum prospecting in the Awati Depression is the lowest comparing with that in platform depression areas of the Tarim Basin. Some oil and gas shows such as the Wuluqiao oil show and the Silurian asphaltic sandstone were found in the depression and its surroundings. Based on the studies of regional tectonic setting and its evolution, depositional environments and geochemical characteristics of the main source rocks, two different kinds of principle source rocks with different sedimentary backgrounds and geochemical features, deve-loped in lagoonal facies and continental shelf and slope facies respectively, were determined. Source rocks deve-loped in the Lower Cambrian Yuertusi Formation, the Middle and Upper Ordovician Saergan Formation and Yingan Formation were dominated by slope facies and showed extra-high organic content which was related to ascending current. And the Middle and Lower Cambrian source rocks with relative high organic content were developed in lagoonal facies. The sedimentary models for the formation of quality source rocks and the identification marks for depositional environments were proposed. According to the studies of sequence stratigraphy, seismic facies and depositional facies, the identification and prediction of source rocks confined by seismic and depositional facies under sequence stratigraphic framework were carried out. On the basis of dissecting of typical reservoir and synthetic analysis of hydrocarbon accumulation, the potential targets for petroleum exploration were indicated in the Awati Depression and its surrounding areas.
2012, 34(3): 266-266. doi: 10.11781/sysydz201203266
Abstract:
Controlling effect of channel strike-slip fault basin on deposition and accumulation:a case study in Yitong Basin
Jiang Tao, Qiu Yuchao, Deng Xiaoguo, Wang Huihui, Jie Junxiao, Cheng Liyuan
2012, 34(3): 267-271. doi: 10.11781/sysydz201203267
Abstract(3694) PDF-CN(1726)
Abstract:
Formed in Neozoic, the Yitong Basin is a channel strike-slip fault basin in the north of the Tancheng-Lujiang Fault Belt. It locates in Jilin province and has undergone regional structural impacts which originate from the Tancheng-Lujiang Fault Belt and the West Pacific Plate. The tectonic evolution of the basin can be divided into 3 stages including early inclined stretch, middle continuous strike slip, and late structural reversion, resulting in important controlling effects on petroleum accumulation. The influences of structure on deposition and accumulation have been studied based on the tectonic style and evolution process of the basin. It has been concluded that the early inclined stretch of the channel strike-slip fault basin is favorable for the generation of source rock and reservoir while the late structural reversion is favorable for the accumulation of oil and gas pool.
Reservoir-forming characteristics of inner buried hills in Jizhong Depression, Bohai Bay Basin
Tian Shifeng, Gao Changhai, Zha Ming
2012, 34(3): 272-276. doi: 10.11781/sysydz201203272
Abstract(4173) PDF-CN(1873)
Abstract:
The reservoir-forming characteristics of inner buried hills in the Jizhong Depression of the Bohai Bay Basin were studied based on seismic, well drilling and logging data as well as exploration practice. The results show that source rocks, reservoir-cap rock association and hydrocarbon migration pathways together controlled the formation of inner buried hill reservoirs. Two reservoir-forming patterns (related to unconformity and fault, respectively) and six reservoir types (layers along slope, blocks along slope, layers against slope, blocks in residual hill, blocks along fault wall and layers along fault wall) generated in the study area. Explorations should be focused on the deep inner traps in the central uplift belt, and the inner traps of the eastern slope belt and the western fault-step belt.
Petroleum exploration potential in abyssal zone of Qiongdongnan Basin, South China Sea
Zhu Tingxiang, Duan Tiejun
2012, 34(3): 277-280. doi: 10.11781/sysydz201203277
Abstract(4547) PDF-CN(1835)
Abstract:
The evolution of the Qiongdongnan Basin in the South China Sea can be divided into three stages including intracontinental rift during Eocene, rift during Oligocene, and passive continental margin depression ever since Miocene. Multiple sets of petroleum generating materials of different facies were deposited in abyssal zone. For example, lacustrine facies in the first stage, marine-continental and continental facies in the second stage, and marine facies in the third stage. Three high thermal fluid events took place during the three evolution stages, contributing to the maturation of source rocks. Multiple types of reservoir and multiple sets of reservoir-cap assemblage provided abundant room for the enrichment of petroleum. Various types of trap formed during stages of tectonic movements were favorable for accumulation.
Causes for complex oil and water distribution in parts of Chang 8 reservoir, Huaqing area, Ordos Basin
Chen Shijia, Zhang Jizhi, Yao Jingli, Lu Jungang, Wen Yanchun, Qi Juan, Zhang Jing
2012, 34(3): 281-284. doi: 10.11781/sysydz201203281
Abstract(4060) PDF-CN(1761)
Abstract:
Huaqing area locates at the development area of Chang 7 source rocks, where oil supplies are sufficient and reservoir formation conditions are favorable. Many petroliferous zones have been found; however, the distribution of oil in the area is unbalanced. In some local regions with high porosity and permeability, a large amount of water has been found in well testing, for example, in well Bai 241, etc. Based in the data of well drilling, logging, testing and reservoir fluorescence, it is analyzed in this paper the causes for the complex oil and water distribution in parts of Chang 8 reservoir, Huaqing area, the Ordos Basin. It is pointed out that mudstone layers between the Chang 7 source rocks and the Chang 8 reservoir block the migration of oil generated from the Chang 7 source rocks into the Chang 8 reservoir. The carrier bed between source rocks and reservoir controls the potentials of Chang 8 reservoir. These conclusions provide geological basis for further exploration and evaluation in the study area.
Reformation effect of atmospheric water on volcanic clastic rocks:a case study in Tamtsag Basin, Mongolia
Qu Xiyu, Liu Li, Meng Qi'an, Yu Miao, Zhang Ge, Wu Wenbo
2012, 34(3): 285-290. doi: 10.11781/sysydz201203285
Abstract(3841) PDF-CN(1616)
Abstract:
The volcanic clastic rocks from the Tamtsag Basin were studied. The experiments of atmospheric water and volcanic clastic rock reaction were conducted under different temperatures. It has been concluded that the soluble minerals in volcanic clastic rocks, such as feldspar and calcite, can be dissolved by atmospheric water, and the dissolution intensity will be enhanced with the increasing of temperature. Tuff in volcanic clastic rocks could be dissolved easily and is the main dissolution mineral. The researches of reservoir properties in the Southern Tamtsag Sag showed that within 30 m beneath the unconformity surface of the Tongbomiao Formation, the physical properties of reservoir were improved obviously by the eluviations of atmospheric water. Porosities increased by 5%-14% and the secondary porosities accounted for 88.48%-95.00%. There was no effect away from the scope of 80 m. The eluviations of atmospheric water play the main contribution to the secondary porosity formed in the Tongbomiao Formation of the Southern Tamtsag Sag, and the lowest limit is 30 to 80 m below unconformity surface.
Hydrocarbon accumulation pattern and exploration direction in Baiyinchagan Sag of Erlian Basin
Wang Xi, e, Wang Youqi, Liu Shuquan, Yang Faqun
2012, 34(3): 291-295. doi: 10.11781/sysydz201203291
Abstract(4271) PDF-CN(1792)
Abstract:
From the geological setting and structural situation of the Baiyinchagan Sag, through typical reservoir research, several different hydrocarbon accumulation patterns were built. Four of them were distinguished, namely fault-nose, fault block, lithologic type and onlap stratigraphic type. Three sets of source rock and two sets of reservoir were developed in the Bayanhua Group of the Lower Cretaceous in the Baiyinchagan Sag. Geochemical research shows that the main source rocks in the second member of the Aershan Formation and the Tenggeer Formation are already in hydrocarbon-generating peak. The source rocks and reservoirs are connected and superimposed, which provides perfect conditions for oil and gas migration. The fundamental condition of hydrocarbon accumulation and basic reservoir distribution were analyzed. The results suggest that nearshore subaqueous fans are the prior exploration field.
Petroleum geology and exploration potential of Ucayali Basin, Peru
Wang Qing, Wang Jianjun, Wang Ping, Zhu Zhiqiang, Zhao Xu
2012, 34(3): 296-302. doi: 10.11781/sysydz201203296
Abstract(5272) PDF-CN(1871)
Abstract:
The Ucayali Basin is one of the sub-Andean foreland basins with several types of pays. There are two sets of main source rocks including the Pucara Formation (T-J) and the Ene Formation (P). Multiple sets of reservoirs generated, such as the Chonta, Vivian, Cushabatay, Agua Caliente and Raya Formations of Cretaceous and the Ene Formation of Permian. And the seal factors for the reservoirs are excellent. There are two types of thrust deformations in the Ucayali Basin. Basement-involved structures mainly occur in the north, while thin-skinned thrusting structures exist in the south. Traps of the discovered fields in the basin are mainly anticlines related to thrust faults. In the Northern Ucayali sub-Basin, hydrocarbons from the Pucara Formation move through unconformities and faults towards Cretaceous formations, and re-accumulate after the Quechua tectonic movement. However, in the Southern Ucayali sub-Basin, natural gas generate from the over mature shales of the Ene Formation, and migrate into the Permian and Cretaceous reservoirs. Four types of exploration leads exist in the basin:traps in thrustfold-foredeep belt, Carbonate reservoir in the Pucara Formation, gas prospects of Permian in the northern basin, and Cretaceous stratigraphic traps in the east of the basin.
Comparison between comprehensive 2d gas chromatography and conventional gas chromatography in geochemical analysis of petroleum geology
Jiang Qigui, Ma Yuanyuan, Qian Menhui, Song Xiaoying
2012, 34(3): 303-308. doi: 10.11781/sysydz201203303
Abstract(3969) PDF-CN(2165)
Abstract:
Geochemical analysis of petroleum geology has been made by using comprehensive 2d gas chromatography (GC×GC) and conventional gas chromatography. The parameters of light hydrocarbons and isoprenoids by oil direct injection and by saturated hydrocarbons or aromatics injection are comparable according to a comparative study of geochemistry, whereas part of the aromatics such as naphthalene series and benzfhiore tri-fluorene are characterized with obvious differences. Comparing with aromatics injection, the dimethylnaphthalene in crude oil direct injection is in dominant position and dibenzofurans is significant higher, which shows that the separation of compound-grouped fractions in pretreatment of sample changes the original appearance of aromatics. Therefore the establishment of the benzfhiore tri-fluorene chart which reflects organic deposition environment should not be applied to the analysis results of oil by GC×GC. Oil direct injection can provide more real information because there is no need for pretreatment.
Geochemical characteristics of lacustrine carbonate rocks in lower part of first member of Shahejie Formation in Qijiawu area of Huanghua Depression, Bohai Bay Basin
Gao Shengli, Wang lianmin, Wu Xi, Gao Pengpeng
2012, 34(3): 309-313. doi: 10.11781/sysydz201203309
Abstract(3798) PDF-CN(1309)
Abstract:
Carbonate reservoirs dominate in the lower part of the first member of the Shahejie Formation in Qijiawu area of the Huanghua Depression, the Bohai Bay Basin. Through analyzing the chemical composition of rock, carbon and oxygen isotope and trace element of the core samples from over 10 wells, the results show that the contents of SiO2, Al2O3 and CaO are relatively higher, while that of MgO varies with the differences of lithology. Meanwhile, SiO2 is positively correlated with Al2O3 and K2O + Na2O. SiO2 is negatively associated with MgO. The enrichment of SiO2 shows that the sedimentary environment in the study area is lake carbonate facies. Carbon and oxygen isotope tests show that δ18OPDB varies from -4.73‰ to -9.46‰ and δ13CPDB varies from -0.08‰ to 7.71‰, which is in line with the carbon and oxygen isotope characteristics of limited lake, reflecting the salinization and dilution effects happened in the study area. Trace tests show that the composition and content of trace element are so complex, that is to say, the content of Sr is higher than 1 000×10-6 in the limestone-based rock, but the content of Sr is less than 1 000×10-6 in the limestone with higher argillaceous. The content of B mostly distributes in (7.77-122)×10-6. For mud samples, the B/Ga ratio is greater than 4, and the Sr/Ba ratio is greater than 1. For carbonate samples, the Sr/Ba ratio ranges from 2.3 to 20.33, up to 33.98, and the B/Ga ratio is less than 10, mainly from 2 to 5, reflecting the alteration between freshwater and brackish sedimentary environments.
Simulation of source rock evolution in Carboniferous, eastern Junggar Basin
Cao Qian, Jiang Xingge, Zhu Jianhui, Chen Yongfeng
2012, 34(3): 314-318. doi: 10.11781/sysydz201203314
Abstract(4048) PDF-CN(1888)
Abstract:
The histories of bury, thermal evolution and hydrocarbon generation of Carboniferous source rocks in the east of the Junggar Basin were studied by means of the TSM basin stimulation software. It was concluded that the depocenter of the study area transferred from the south to the north and then back to the south from Jurassic to Cenozoic. In the Jimsar Sag in the south of the basin, the source rocks in the Batamayineishan Formation gave birth to hydrocarbon during 2 key periods:1) the end of Jurassic when the Ro of source rocks ranged from 0.6% to 1.3%; 2) after the middle and late Cenozoic when the Ro of source rocks ranged from 1% to 2%. In the Wucaiwan Sag in the north of the basin, the source rocks in the Dishuiquan Formation generated hydrocarbon mainly at the end of Cretaceous when Ro ranged from 0.7% to 1.5%, indicating a large amount of hydrocarbon. In the Shiqiantan Sag in the north, the source rocks in the Shiqiantan Formation entered the threshold for hydrocarbon generation at the end of Cretaceous and the evolution degree was low. The hydrocarbon intensities of the Dishuiquan and Bashan Formations were higher, amounting to 200 mg/g, while the highest value of the Shiqiantan Formation was only 40 mg/g.
Research on thermal evolution and hydrocarbon expulsion history of source rocks in low-exploration basins:a case study on Jiaojiang Sag, East China Sea Basin
Tong Zhigang, Zhao Zhigang, Yang Shuchun, Xi Xiaoying, Hao Jianrong, Yu Yingmei
2012, 34(3): 319-324. doi: 10.11781/sysydz201203319
Abstract(3931) PDF-CN(1812)
Abstract:
In low-exploration basins, especially offshore ones, wells are rare and usually locate in the higher positions. The source rocks in basin centers are buried deeply and are not drilled. Therefore, it is the key for basin/sag evaluation to study the thermal evolution and hydrocarbon expulsion history of source rocks in basin centers. Case studies have been carried out in the Jiaojiang Sag of the East China Sea Basin. Based on the measured temperature and Ro data collected from wells locating in the sag edge or the higher positions, by means of forward modeling method, the lithosphere structure and thermal history of the Jiaojiang Sag have been recovered, and the thermal history of Paleocene source rocks in the whole sag have been modeled. The results show that the maximum Ro of Paleocene source rocks in the sag is 2.8%, indicating over-mature state. The total amount of oil expulsion is a little bit more than that of gas expulsion, and the amount of hydrocarbon expulsion is 33×108 m3 oil equivalent. Hydrocarbon expulsion mainly took place before Miocene. In the future exploration, the prospects with good traps and were formed before Miocene in the central uplifts should be focused on.
An improved research method on fault sealing capacity:taking Longmenshan thrust belt as an example
L? Zhizhou, Li Min, Tian Jun, Li Qing
2012, 34(3): 325-329. doi: 10.11781/sysydz201203325
Abstract(3824) PDF-CN(1624)
Abstract:
Preservation condition is a key element of oil and gas accumulation in foreland thrust belt, and fault sealing capacity plays a vital function. Due to the difference of lithologic interface of fault plates in three-dimensional space, the sealing capacity of the identical fault is different along different orientations or at different depths. The improved Allen section was applied in this paper to study the lateral sealing capacity of fault. The vertical sealing capacity of fault was evaluated based on the pressure of fault section and the net-to-gross ratio of displaced layers. Semi-quantitative comprehensive evaluation was made by means of the studies of vertical and lateral sealing capacity of fault. This method might be used to judge the fault sealing capacity initially in the places with low exploration degree, especially in those places without wells. And the places with high exploration degree, we could develop the results of evaluation from semi-quantitation to quantification based on exploration data.
Research on carbonate reservoir matrix model building based on porosity cycles
Tang Hong, Liao Mingguang, Hu Jun, Jin Song, Hu Changlong
2012, 34(3): 330-333. doi: 10.11781/sysydz201203330
Abstract(3629) PDF-CN(1651)
Abstract:
Reservoir modeling refers to many technologies, and the improvements on any of which will affect model accuracy. Despite that digenesis and tectonic factors have made carbonate reservoirs become complicated and heterogeneous, for pore or cave layer carbonate reservoirs connected by extensional fractures, the distribution of pore or cave layers has regularity and cyclicity. Taking a gas reservoir in the Sichuan Basin as an example, porosity cycle is put forward as a model unit to build up carbonate reservoir matrix geological model. Porosity cycles are divided based on the change law of porosity curves. Then with these porosity cycles, combining deterministic modeling with stochastic simulation, carbonate reservoir matrix geological model is developed to increase reservoir modeling precision.
Parameter optimization of tight reservoir porosity determination
Tian Hua, Zhang Shuichang, Liu Shaobo, Ma Xingzhi, Zhang Hong
2012, 34(3): 334-339. doi: 10.11781/sysydz201203334
Abstract(4175) PDF-CN(1883)
Abstract:
The porosity is an important parameter to evaluate reservoir physical properties and calculate petroleum reserves. The porosity of tight reservoir is less than 10%, and that of gas shale is usually less than 5%, which is a great challenge of porosity determination apparatuses. Currently the Boyle's Law Double-Cell (Matrix Cup) Method is widely used. The porosity determination for gas shale and tight sandstone needs ultra high accuracy, and it is necessary to carry theory analysis and optimize determination parameters. The objective pressure distinction function is established and numerical simulation is carried out. To obtain higher accuracy, it is necessary to set higher reference cell primary pressure, higher sample cell primary pressure, lower standard bulk volume and lower sample bulk volume. The relative error of experimental porosity of the standard sample of 5.0% can decline as much as 14%, and the absolute error can decline by 0.7%. This study developed a theory calculation model, which will be useful for technical support for tight reservoir porosity determination.
Prediction of single sand body and its application in Shanxi Formation of Permian, Daniudi Gas Field
Sun Suqin
2012, 34(3): 340-344. doi: 10.11781/sysydz201203340
Abstract(4051) PDF-CN(1589)
Abstract:
The Permian Shanxi Formation in the Daniudi Gas Field was deposited in delta plain environment. It was composed of interbedded sandstone and mudstone, and also generated several sets of coal and coal line. Due to the low-frequency strong interference of coal and coal line, reservoir prediction was very difficult. The model-based well-seismic inversion technique was introduced in this paper, by means of which the wave impedance of time and depth domains, lithology, physical property and gas-bearing parameters might be obtained. The prediction of single sand body was successfully carried out, and the trajectory design of horizontal wells was made. The predicted vertical well depth and thickness matched well with actual drilling in two wells. In the predicted horizontal segment in well DF2, it was drilled for over 733 m along gas reservoir, achieving good results.
2012, 34(3): 345-345. doi: 10.11781/sysydz201203345
Abstract: