Petroleum Exploration and Development

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Hydrocarbon geology and reservoir formation characteristics of Niger Delta Basin

DENG Rongjing; DENG Yunhua; YU Shui and HOU Dujie

, 2008, 35(6): 1652-0.

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Niger Delta Basin is abundant in oil and gas resources. Based on over 50 years of petroleum exploration and development and major geological data for the basin, this paper studies its petroleum geology characteristics and summarizes its reservoir formation and hydrocarbon enrichment rules. Niger Delta Basin is a passive continental margin basin developed in early Cretaceous in association with the breakup of Gondwana and the opening of South and Equatorial Atlantic and evolved two tectonic stages, namely rifting and drifting. Long-term marine regression since Eocene resulted in the Niger Delta, with three lithostratic units developed from bottom to top, i.e. Akata Formation, Agbada Formation and Benin Formation. During deltaic progradation, under the continental gravity, three structural zones were developed from north to south, which are extensional structure of Niger Delta, diapiric structure and obduction nappe structure. The large diapiric-obduction structural trap in deep water and the deep large faulted trap, faulted-nose structural trap and large lithologic trap at flank of structure in land-shallow water extensional structure are major future targets to explore large oil and gas fields in Niger Delta Basin.

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Principal characteristics and forming conditions for mediumlow abundance large scale oil/gas fields in China

ZHAO Wenzhi; WANG Zecheng; WANG Hongjun and WANG Zhaoyun

, 2008, 35(6): 1653-0.

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In recent years, some of largescale oil or gas fields with mediumlow abundance have been discovered in onshore China. They have such characteristics in common as low porosity and permeability, small oil or gas column, low reserves abundance, and large scale. Studies conducted by the authors show that the formations of wide reservoirs with mediumlow abundance have the following favorable conditions: the largescale sandbodies caused by drag flow in the hinterland of large lacustrine basins, alternating broadly with source rocks like "sandwichtype" architecture of reservoirsource rock association, provide fundamental basis of oil/gas accumulation in large scale; the small oil or gas column and normal to low pressure system reduce the restrict requirement on caprock quality which leads to largescale reservoir formation; the strong reservoir heterogeneity with poor connectivity minimizes gas escape energy and ensures wide oil/gas accumulations even in the area with poorer geological conditions; the late uplifting and unloading environment makes it possible for source rock desorb and discharge hydrocarbon to form reservoirs with young generation. Studies indicate that there are nice prospects in discovering largescale lithological reservoirs in the synclinal areas of large terrestrial depression basins of China. This concept has guided the oil/gas discoveries in synclinal areas of Ordos, Songliao and Sichuan Basins, promoting the great increase of oil and gas reserves and showing a good potential for future exploration.

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Sequence and systems tract of Paleogene Kumugeliemu Group in Western Kuqa Depression, Tarim Basin

LIU Jingyan; WANG Qinghua; LIN Changsong; LIN Changsong; LEI Yanping; HU Guangcheng and HU Bo

, 2008, 35(6): 1654-0.

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Sequence stratigraphy is used for analytical comparison and palaeogeographic reconstruction of lithologic sections in Paleogene Kumugeliemu Group in western Kuqa depression under the framework of sequence and systems tract, to establish the reservoir-caprock assemblage and allocation system and to predict the distribution of favorable reservoir sandstones. Two sets of coarse fragment sands are developed in the 3rd order sequences Esq1 lowstand systems tract and Esq2 lowstand to transgressive systems tract. The widely distributed salt and gypsum muddy section of Esq1 and Esq2 has good sealing and forms good reservoir-caprock assemblage with the two reservoirs in the bottom sand section of Kumugeliemu Group. According to the reservoir sandbody distribution, the forwarding fluvial system at mountain front to the north, fan delta at east flank of Wensu uplift to the west and the downcut valley filling and fluvial delta system at Yingmai low paleo-uplift to the south form favorable reservoir sandbodies. Kelasu-Dabei zone on north margin of the depression, Qiulitage-Quele and its southward parts, and northern and eastern Wensu uplift are the favorable plays.

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Hydrocarbon generation and reservoir formation: A case from Chang 7 source rock in Majiatan area, west margin of Ordos Basin

XI Shengli; LI Wenhou and LI Rong-xi

, 2008, 35(6): 1655-0.

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By recovering the initial maturity of Chang 7 source rocks in Majiatan area in different structural thermal evolution periods, the hydrocarbon generation stages and reservoir formation features are studied. Chang 7 source rocks experienced 3 times of hydrocarbon generation. The first hydrocarbon generation took place at Late Jurassic with the maturity (Ro) of Chang 7 source rocks being 0.71%, entering the large-scale hydrocarbon-generation stage. But the oil produced during the stage escaped completely because of tectonic uplift in later Late Jurassic. The second hydrocarbon generation took place at Early Cretaceous with the Chang 7 source rocks maturity being 0.89%, entering the hydrocarbon-generation peak stage. This is the most important process for hydrocarbon generation and reservoir formation. The nappe structural traps related to this process are the main targets for hydrocarbon exploration in the study area. The Chang 7 source rocks were buried deeper because of Himalayan movement after late Paleogene. There are good geologic conditions for third-time hydrocarbon generation and reservoir formation in the Majiatan fault fold zone. The combination hydrocarbon reservoirs related to later fault structures are the valuable targets for exploration.

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Distribution and genesis of large-scale Upper Palaeozoic lithologic gas reservoirs on Yi-Shaan Slope

FU Jinhua; WEI Xinshan and REN Junfeng

, 2008, 35(6): 1656-0.

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Lithologic reservoirs are widely distributed at Upper Paleozoic of Yi-Shaan Slope in Ordos Basin. With over 40 years of exploration, six large gas fields were discovered, including Sulige, Wushenqi, Yulin, Daniudi, Zizhou and Mizhi, with total proved gas in place up to 13 126.54×108 m3. Based on the study on reservoir formation, the key factors of large-scale lithologic reservoirs include: widely-distributed coal measure source rocks and widely-covered hydrocarbon generation, laying a physical foundation for wide gas accumulation; wide development of fluvial-delta sand deposits in the north of the basin, providing good reservoir spaces for wide gas accumulation; reservoir formation after densification of Upper Paleozic reservoirs and stable structures in the basin ensure the later conservation of gas reservoirs. The effective allocation and interaction of these factors in space and time lead to the large-scale distribution of Upper Palaeozic lithologic gas reservoirs.

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Basin-mountain tectonic pattern and hydrocarbon exploration domain in north margin of Qaidam Basin

WANG Guihong; MA Dade; ZHANG Qiquan and LI Jun

, 2008, 35(6): 1657-0.

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Seismic profile crossing southern boundary of Saishiteng Mountain shows the shear tectonic characteristic of fault zone at southern boundary of Saishiteng Mountain. In the west of Saishiteng, the fault zone thrusts towards top and north; in the center, the fault zone shows high angle, in the east (line 1001 profile), it represents as low-angle and basin-toward thrusted fault, which may be converted structure at the end of strike-slip faulted structure, in pattern of associated structure. The north boundary didn't overthrust to basin; Wells Dongtai 1 and Gaqiu 1 at foothill of Saishiteng didn't penetrate into new formations overlapped by the mountains, but into associated products of high-angle faulted structure, which indicates that the mountain was less thrusted to the basin. So the structure deformation of Northern Qaidam Basin had been mainly strike slip since Cenozoic and the thrust nappe is associated with the strike-slip structure. The strike-slip faulted zone in north boundary showed reversal deformation, so it is not a favorable play to form original reservoirs; the masses between strike-slip trips, that may be stable relatively, are favorable for forming reservoirs.

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Pyrolysis-gas chromatography of the oil shale in Lucaogou Formation, Dahuangshan, Southern Junggar Basin

LI Jingjing; TANG Dazhen; XU Hao; WANG Dongying; TAO Shu; ZHOU Chuanyi and GAO Guanfeng

, 2008, 35(6): 1658-0.

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Dahuangshan oil shale mine is located at the main position of oil shale deposit belt in Northern Bogda Mountain. The majority of the pyrolysis products of Dahuangshan oil shale are n-alkanes and n-alkenes, which approximately account for 90% of total products. Aliphatic groups break from kerogen nuclear when heated and turn into alkyl radicals. Alkyl radicals, which are very active, quickly turn into n-alkanes or n-alkenes depending on capturing or losing hydrogen radicals. Due to the effect of space steric hindrance, the formation of n-alkenes need more energy than n-alkanes, and the formation ratio of short chain n-alkenes (C14－) is bigger than long chain n-alkenes (C15＋). The relative contents of C1—C4 n-alkanes and n-alkenes, C5—C14 n-alkanes and n-alkenes, and C15＋ n-alkanes and n-alkenes are effective parameters to distinguish kerogen type, as proved. Also the composition ratio of all n-alkanes and all n-alkenes is also an accurate parameter, for the type Ⅰ kerogen is below 2.17 while the type Ⅱ is above it. The pyrolysis products of Dahuangshan oil shale are mainly high wax paraffinic oil, which is similar to that of Green River shale in America.

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Integrated identification method for low-resistivity hydrocarbon layers in Nanpu Sag

ZHOU Fengming; SI Zhaowei; MA Yuejiao; XU Feng; PENG Yinhui and HE Zhongwen

, 2008, 35(6): 1659-0.

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Plenty of low-resistivity oil/gas layers exist in Nanpu sag in Bohai Bay Basin. The intense variation of lithology, physical property and water property and the deep invasion of brine mud are two key factors for sharp decrease of resistivity and reducing differences of electric characteristics of oil and water layers. Based on the logging data of mid-shallow reservoir in Nanpu sag and using the effective logging information including natural gamma and SP logging and MDT technology, this paper proposes the integrated identification method for low-resistivity reservoirs which focuses on lithology identification and uses the multi-parameter crossplot and non-logging information such as MDT and gas logging.The correspondence rate for oil and water layer interpretation by Rwa(Rt)-ΔGR，Rwa(Rt)/Rwa(SP)-ΔGR, \[Rwa(Rt)/Rwa(SP)\]/(1－ΔGR)-C3 crossplot is over 96%. The oil/gas layer identification is made in wells ××3, ××4 and ××5 in Nanpu sag, providing the same conclusions as the formation and production test and proving the accuracy of integrated identification and assessment for low-resistivity oil/gas layers.

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Double provenance depositional model and exploration prospect in deepwater area of Qiongdongnan Basin

YAO Genshun; YUAN Shengqiang; WU Shiguo and ZHONG Chuang

, 2008, 35(6): 1660-0.

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Based on the newly collected seismic data and the provenance supply mode, the depositional sequence, provenance features and oil-gas exploration prospects were analyzed. The study area has the typical double-layer geologic structure of rift-depression, and has double provenance supply in the north and south. The provenance supply in rifting period was mainly from the southern Xisha Uplift and the southwestern Indo-China Peninsula, and was also from the northern South China Block; while the provenance supply in depression period was mainly from Indo-China Peninsula. In the rifting period, the southern sediment formed a large-scale progradation body in Lingshui Formation, and encountered the subsea apron formed by the northern provenance in the central depression; in the depression period, the large-scale turbidite channel developed in Sanya Formation, the large-scale reef developed in the southeast of the study area in Meishan Formation in the depositional stage, and the large-scale incised valleys developed in Huangliu Formation in the late depositional stage. In the Late Quaternary Period, the northern provenance and southern provenance joined in the deepwater area of Qiongdongnan Basin. The seismic data and basin modeling indicate that, the reservoirs of subsea apron and sand-rich turbidite channel above the hydrocarbon-generating sag can be the good exploration targets in the deepwater area.

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Types, accumulation models and exploration concepts of marine carbonate reservoirs

ZHAO Zongju

, 2008, 35(6): 1661-0.

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There are 6 types of marine carbonate reservoirs, i.e. reef shoal, ancient buried hill (ancient weathering crust-type karst), internal dolomite, slope fan, chalk and fractured limestone. The exploration for reef shoal reservoirs is mainly to ascertain the space-time distribution of reef shoal at different paleogeographic locations under the sequence & cycle stratigraphy framework; the exploration for limestone ancient buried hill reservoirs rests in identifying the fractured karst caves systems and nature of fillings (oil, gas, water and cave sediments etc.); the exploration for dolomite (internal dolomite, dolomite ancient buried hill) reservoirs is mainly to seek structural (anticlinal) traps with effective caprock/reservoir assemblage; the priority in exploration for slope fan reservoirs is to confirm the space-time distribution of slope fans relating to type-I sequence boundary under sequence stratigraphy framework; the exploration for chalk and fractured limestone reservoirs focuses on the structural (anticlinal or faulted anticlinal) traps with effective fractures.

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A method to optimize seismic layout based on remote sensing information

YE Yong; LEI Yingchun; ZHANG Youyan; LUO Luncai and HU Yan

, 2008, 35(6): 1662-0.

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For seismic exploration, it is the basis for better data acquisition to select better line layout in line with morphosequent and geologic conditions, provided that the objective of underground exploration is met. To ensure that the seismic line layout in areas with high-steep mountains and low signal-noise ratio is helpful for the seismic execution, launching and receiving, a line assessment and semi-automatic optimization method was defined on the basis of remote sensing information through exploitation, study and practices in complex-surface seismic area. The method is used to evaluate the applicability and launching and receiving conditions at line positions with such information as digital elevation model (DEM), slope, relative humidity, surface roughness and lithology, so as to select the better layout from optional seismic lines generated from the original seismic layout. The application in the Qiulitage exploration area in the west of Tarim Basin proved that the method can improve the seismic data acquisition conditions and applicability of lines, promoting the data quality and cutting down the exploration cost.

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A new analysis method for C5-C13 fractions of organic matter in source rocks and its primary application to oil-source correlation

XU Guanjun; WANG Peirong; LIU Jianxian; XIAO Tingrong and REN Dongling

, 2008, 35(6): 1663-0.

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By the online analysis system of liquid nitrogen freezing-impact crushing-closed thermal desorption-helium purging-cold trap capturing-gas chromatogram (Patent Application No. 2007101194606), the online analysis is made for the source rock samples of about 2 cm grain size. Due to the large grain size of the samples, the light fraction loss of the samples before and after crushing is greatly reduced. As a result, the original status of light fractions of organic matter in the source rock is maintained well. In the acquired chromatogram, about 203 peaks from C5-C13 have been got, pair peaks of 2-methylhexane and 2,3-dimethylpentane could be separated to lower than half of the peak width, and 3-ethylpentane could also be seen as a shoulder peak. The repeatability of the measurement is good. With the source rock samples from Junggar Basin as an example, this analysis method is applied in oil-source correlation. This method may provide a new research tool for identifying the oil sources and calculating the remaining hydrocarbon volume in oil-gas exploration.

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Four fundamental principles for design and follow-up of steam flooding in heavy oil reservoirs

ZHANG Yitang; LI Xiuluan and ZHANG Xia

, 2008, 35(6): 1664-0.

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Numerical simulation is used to understand the impacts of steam injection rate, bottom-hole steam quality, production/injection ratio and reservoir pressure on the performance of steam flooding in heavy oil reservoirs, providing the concrete ranges of quantified indices. For conventional heavy oil reservoirs, to realize better performance by steam flooding, the following conditions must be met: (1)steam injection rate should not be less than 1.6 m3/(d·hm2·m), thus keeping the continuous extension of steam chamber and maintaining the bottom hole steam quality; (2)bottom hole steam quality should not be less than 40%, which reflects the requirement of enthalpy for unit injected fluid by steam flooding; (3)reservoir pressure should be less than 5.0 MPa, had better in 1.0-3.0 MPa, thus maintaining a lower reservoir pressure to improve the swept volume of steam; (4)instant production/injection ratio should not be less than 1.2 during the stable stage of steam flooding, realizing practical steam flooding. The design, follow-up and evaluation of steam flooding in pilot test and extension test in Qi40 Block of Liaohe Oilfield show that the quantified indices have become four fundamental principles for such design, follow-up and evaluation.

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Deep fluid diverting mechanism of soft diverting agent

MA Hongwei; LIU Yuzhang; LI Yikun; XIONG Chunming; LI Zhiyan; LI Caixiong and TIAN Jianru

, 2008, 35(6): 1665-0.

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Soft diverting agent features high chemical stability. Uniaxial extension and compression tests indicate that the agent could be deformed randomly and hard to be broken for its high tenacity. 1D pore throat model tests indicate that there is an exponential relationship between threshold-pressure of the agent and throat-particle diameter ratio. If the formation pressure differential is higher than the threshold-pressure, the agent can move; otherwise, the agent will remain in the reservoir. Network model test indicates that the particles of the agent migrate alternatively in four forms: elastic deformation, plastic deformation, grain generation and grain conglutinating, and are distributed at microthroats in the formations like skirmishers, thus forming a temporary plugging peristalsis strip through overlapping to realize the whole-process profile control over the formation and finally settle at the deep section of the reservoir, so the diverting of deep fluid is completed. The field test of Guan74 block in Wangguantun Oilfield, with expected results obtained, illustrated that the soft diverting agent can overcome the problems of inefficient circulating of injected water in large pore channels between oil and water wells and of the low sweep efficiency in late development of high water cut oilfields.

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Sectional combination optimization model of horizontal well perforating parameters

WANG Zhiming; WEI Jianguang and WANG Xiaoqiu

, 2008, 35(6): 1666-0.

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Because of the pressure loss from the heel to the toe of the horizontal well, and the uniformity of perforating parameters along the whole horizontal wellbore, the earlier water breakthrough and higher water cut will be caused. So the sectional combination of perforating parameter to control the inflow profile along the wellbore is very important to control the water cut in the horizontal well. Based on the inflow control theory of horizontal well, considering the wellbore pipe flow, the wellbore inflow, formation filtration and other factors comprehensively, the sectional combination optimization model of perforating parameters of horizontal well was established, and the results of sectional optimization to control inflow profile with different perforating parameters combination were compared and analyzed. The result shows that the perforating density is the most important parameter in the controlling of inflow profile, besides that, the charge type, the perforating phase angle and the perforating angle of orientation are also important. With more parameters, the result of the optimization of inflow profile gets better.

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Prediction of gas hydrate formation region in the wellbore of deepwater drilling

WANG Zhiyuan; SUN Baojiang; CHENG Haiqing and GAO Yonghai

, 2008, 35(6): 1667-0.

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It is essential to consider the thermodynamics, temperature and pressure conditions of gas hydrate formation, in order to predict the gas hydrate formation region in the wellbore of deepwater drilling. Multiphase flow governing equations (including continuity equation and momentum conservation equation), temperature field equations in annulus and drill pipe, and hydrate formation thermodynamics equation are established, based on the characteristics of deepwater drilling. The definite conditions, discrete methods and iterative steps are given to solve the equations under different working conditions. Through examples of calculation, the impacts of relevant parameters on gas hydrate formation region during drilling, off drilling and well killing process are discussed. The result shows that the hydrate formation region is getting small when circulation volume becomes great, inhibitor concentration becomes high, downtime becomes short, or choke line size becomes large. And the result also indicates that it's more efficient to consider more factors to suppress hydrate formation. The parameters can be optimized based on the method.

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A correction method of deep laterolog responses under under-balanced logging condition

LI Changxi; TANG Liangmin; ZHOU Cancan; WANG Changxue; ZHANG Shudong and YANG Renlin

, 2008, 35(6): 1668-0.

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Under-balanced drilling and under-balanced logging are very important techniques for directly identifying and protecting hydrocarbon reservoirs and enhancing productivity. The wellhead assembly under under-balanced logging is greatly different from that under conventional logging. Therefore, the length of cable electrode for dual laterolog should be reduced to adapt to the wellhead assembly, which causes the deep laterolog responses to drop, and can not truly reflect the reservoir property. The rule of electrode coefficient varying with the cable electrode length under under-balanced condition has been computed using finite element method, that is, the longer the cable electrode, the smaller the electrode coefficient. The deep laterolog resistivity correction method under under-balanced condition is introduced. Field data examples are used to test the new method, and shows that this method is reliable. This method can be used to any length of cable electrode and can be applied to field pretreatment directly. The corrected deep laterolog resistivity can be used in logging interpretation and quantitative hydrocarbon evaluation.

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Borehole stability prediction before drilling for formations under the second section in an exploration well

JIN Yan; CHEN Mian and WU Chao

, 2008, 35(6): 1669-0.

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Based on the comprehensive analysis of some complicated situations met in drilling, such as circulation loss, borehole shrinkage and collapse, the collapse pressure and fracture pressure of the drilled well interval were determined. And the nonlinear relationships between the seismic interval velocity, the collapse pressure and the fracture pressure were established by neural network theory. A new borehole stability prediction method was put forward, that is, predicting the collapse pressure and fracture pressure of the formation under the second section of the first exploration well in prospecting area using seismic data. Mudstone collapse happened when drilling the second section of well YS-D in a field, northwest China. Before drilling the third section, the equivalent drilling fluid density of the collapse pressure and that of the fracture pressure of the third section were predicted by this method, and the density of drilling fluid was adjusted and the drilling made good progress. The method has been successfully applied to 52 exploration wells since April, 2001. The predicted pressure change is close to the actual result within 300 m under the bit in the third section, meeting the demands of predicting borehole stability before drilling.

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An improved clipping evaluation method at preliminary trap prospecting stage

LIANG Kun; GUO Qiulin and LI Xiaodi

, 2008, 35(6): 1670-0.

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On the basis of analyzing the strong and weak points of two ways in the economic evaluation at the trap preliminary prospecting stage before drilling, that is, the integrated exploration and development evaluation and the traditional evaluation by phase, this article establishes a phase evaluation model for reserves prediction and reserves classification, which takes reserves value as cash inflow and exploration cost as the cash outflow. Considering geologic risk and fund efficiency, this model takes the expected net present value and the expected net present value ratio as the criteria for entrapment optimizing. It achieves the single trap evaluation and the trap optimizing under the conditions of non-fund limit and fund limit. The practical application indicates that this model can realize rapid evaluation with reserves value instead of the oil price in the integrated evaluation, and the results are well consistent with the integrated evaluation. This model needs fewer parameters and is easy to operate.

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Methodology for determining productivity of new oilfields

YANG Siyu; HU Yongle; JIANG Manqi and OUYANG Ting

, 2008, 35(6): 1671-0.

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In recent years, there emerged some evaluation methods for determining the productivity of new oilfields that calculate mathematically without taking account of the actual physical properties of the reservoirs. In that case, the result of productivity evaluation may be incorrect, leading to unscientific basis for investments and unreachable economic targets. This paper presents a method for determining the productivity of new oilfields, based on the basic concepts of reservoir engineering. As for different production stages and performance, the valuing principles and calculation methods for some key parameters, such as producing pressure drop and productivity index, are provided. Taking well Lu-9 in Luliang oilfield as an example, the productivity of two major reservoirs were calculated and compared with the actual average single-well production after it was put into operation completely. Based on the results, in Hutubi Formation, the actual average single-well production is 14.26 m3/d, and the calculated production is 16.68 m3/d; in Toutunhe Formation, the actual is 8.30 m3/d, and the calculated is 8.67 m3/d. Therefore, the calculation is reliable, showing that the productivity evaluation method is reasonable and correct.

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A method for petrophysical classification of low porosity and low permeability reservoirs

ZHANG Longhai; LIU Zhonghua; ZHOU Cancan and LI Changxi

, 2008, 35(6): 1672-0.

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In order to establish a rock electrical parameters model under the condition that pore structures and petrophysical characteristics are identical, it is necessary to classify reservoirs according to petrophysical characteristics. The disparity of two macroscopic parameters, flow zone index and reservoir quality index, is contrasted by analyzing actual data, and an effective method of classifying reservoirs is proposed based on petrophysical parameters and microscopic pore structures. The mercury injection experimental data of sixty rock samples of typical low porosity and low permeability reservoirs in the Daqingzijing (Songliao basin) and Jiyuan (Ordos basin) areas are classified based on these two indexes. The result shows that the reservoir classification by reservoir quality index is more reasonable. The theoretical analysis also demonstrates that reservoir quality index is the monotone function of pore structure while flow zone index is not. Compared with flow zone index, reservoir quality index can reflect the variation of reservoir pore structures and petrophysical properties more exactly.

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, Volume 35 Issue 6