Table of Content

28 February 2011, Volume 32 Issue 1

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Sequence stratigraphic features of the Cambrian carbonate rocks in the Tarim Basin

2011, 32(1):  1-10.  doi:10.11743/ogg20110101

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Sequence stratigraphic theories of carbonate are employed in a sequence stratigraphic study on the Lower Paleozoic carbonates in Tarim Basin through outcrop observation, drilling data analysis as well as integra ted seismic interpretations. Together with the study, the sequence boundaries are identified and stratigraphic framework is established. As a result, eight sequences identified in the Cambrian carbonates are described. Except for sequence 1(SQ 1) as a ascending base level semicycle sequence, the other sequences are all falling base level semicycle sequences. Sequences 5, 6, 7 and 8 are considered key hydrocarbon exploration targets. In Lunnan area, exploration activities shall be focused on sequences 6, 7 and 8 for complex structurallithologic traps. A model for the Cambrian carbonate sequences in the Tarim Basin is developed and provides helpful aid to searching for places with favorable conditions for the formation of stratigraphic lithologic traps.

Geobiological conditions for the formation of the Lower Cambrian source rocks in Yangba area of Nanjiang County in the Sichuan Basin

2011, 32(1):  11-16.  doi:10.11743/ogg20110102

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Evaluating conditions of source rock development by geobiological approaches is an important supplement for the traditional evaluation method of source rocks based on analyses of remnant organic matter. This method is especially suitable for source rock evaluation in areas with highlymatured marine carbonate rocks. Our study is based on observation of outcrops and focused on a systematic analysis of the depositional environment, fossil organism, trace element data and total organic carbon in the Lower Cambrian of the Yangba area in Nanjiang County, Sichuan Basin. The forming conditions for the Lower Cambrian source rocks are recognized through a study on the habitat types, oceanic paleoproductivity, paleooxygenation facies and burial efficiency by using geobiolgical principles. The results show that the lower part of the Qiongzhusi Formation has the most favorable conditions for development of highquality hydrocarbon source rocks as it has higher original organic content, anaerobic sedimentary environment and higher burial efficiency of organic carbon. In contrast, the lower part of the Shilongdong Formation possesses basic geobiological conditions for development of ordinary source rocks.

Geochemical behaviors of the Wumishan Formation in the Jibei Depression of Yanshan region

2011, 32(1):  17-28.  doi:10.11743/ogg20110103

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The Wumishan Formation in Jibei Depression is of carbonate tideflat sedimentary, and can be divided into three subfacies (supralittoral zone, subtidal zone, intertidal zone) and four microfacies (upper and lower intertidal zones as well as highenergy and lowenergy subtidal zones). An analysis of trace element and oxide enrichment pattern is carried out by using factor analysis method. The result shows that the carbonates in the area are affected by terrigenous muddy water sedimentation and that there is a close relationship between its geochemical behaviors and sedimentary environment. Contents of both trace elements and oxide significantly increase from supralittoral zone to intertidal zone and to subtidal zone. As the water depth increases, the content of Sr and B, which are sensitive to carbonate sedimentary environment, and Sr/Ba, Sr/Ca and Mn/Fe ratios, rise from the supralittoral zone to subtidal zone. These features indicate that the geochemical characteristics of the area can be used as important marks for facies analysis and that the accuracy of field division of stratum and sedimentary facies is relatively high.

Regressive continental shelf as an important location for the development of source rocks—an example from the Upper Permian Dalong Farmation in the northern margin of the UpperMiddle Yangtze region

2011, 32(1):  29-37.  doi:10.11743/ogg20110104

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This paper examines carefully the sedimentary features, biological associations and characteristics of organic carbon of the Upper Permian Dalong Formation in the northern margin of the MiddleUpper Yangtze region. We also discuss the relationship between regressive sequences and highquality source rocks. According to the study, we believe that the depositional setting of the Upper Permian Dalong Formation belong to regressive continental shelf facies which changes from the outer shelf in the early period to the inner shelf in the middletolate period. The change of facies belt brought about shallowwater organism, especially plenty of bacillusalga, which continuously provided organic materials for the development of highquality source rocks in the Dalong Formation in the MiddleUpper Yangtze region. The organic carbon content in the upper part is higher than that in the lower part of the Dalong Formation. In addition, ascending current repeatedly brought nutritious salt from deep water to sallow water, which made the biological groups thrive and promoted the productivity of organic matter. Therefore, highquality source rocks developed in large scale in the Dalong Formation. From this study we come to the conclusion that the regressive continental shelf is also an important location for the development of source rocks.

Sedimentary environment and geochemical characteristics of hydrocarbon source rocks in the Pingliang Formation,southern margin of the Ordos Basin

2011, 32(1):  38-46.  doi:10.11743/ogg20110105

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Hydrocarbon source rocks were developed in the Pingliang Formation of the Middle Ordovician in southern Ordos Basin. Based on test of over 40 outcrop samples from five sections in southern Ordos Basin, we studied the paleosalinity, hydrodynamic conditions and redox conditions during the Pingliang Age of the Middle Ordovician through trace element analysis. Two major paleosedimentary environments are identified: deepwater slope with brackish, weak hydrodynamics and reducing conditions; and carbonate platform with brine, strong hydrodynamics and oxidizingprone and weak reducing conditions. The former was favorable for development of quality hydrocarbon source rocks. By conducting a comprehensive evaluation of the TOC, organic types and maturity, we come to the conclusion that the mudstone at the lower part of the Formation is the major source rocks, which are mediocre to excellent gas source rocks featuring in large thickness, high TOC, favorable kerogen types (sapropeltype predominant) and high maturity. It is therefore suggested that the northern Weibei high be favorable for exploring the lower Paleozoic in southern Ordos Basin.

Water bridges mechanism of organosmectite interaction in argillaceous hydrocarbon source rocks: evidences from in situ DRIFT spectroscopic study

2011, 32(1):  47-56.  doi:10.11743/ogg20110106

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To examine the role of water in the interaction of organic matter with smectite in argillaceous source rocks with situ DRIFTS, we make a study of clays with grainsize less than 2μm extracted from hydrocarbon source rocks with high clay content in the Neocene, Jiyang Depression, eastern China. The DRIFT spectra show that the evolution of the adsorption water is accompanied by significant changes in the location and shape of the carboxyl above 250°C and both of them almost disappear at 400℃. An attempt for semiquantitative measurement of vibration peaks is also carried out. Result shows that the loss of adsorption water and carboxyl are in proportion above 250°C, suggesting most carboxylic acids interact with interlayer hydrated cations of smectite through hydrogen bond, and water molecules are bridges between carboxylic acids and smectite, a most important mechanism for organosmectite interaction. Due to the effect of water bridge, dehydrate temperature of water and carboxylic acids are higher than that of pure smectite and carboxylic acids. As water bridge breakdown at higher temperature, water molecules and carboxylic acids were expelled from interlayer of smectite simultaneously. The water bridge mechanism of organosmectite interaction and dehydration lagging of water bridge and carboxylic acids have a great influence on hydrocarbon generation, expulsion and primary migration. 

Reservoir characteristics of different sedimentary facies in the Changxing and Feixianguan Formations, northeast of the Sichuan Basin

2011, 32(1):  56-63.  doi:10.11743/ogg20110107

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Previous exploration activities indicate that the Changxing Formation of the Late Permian and the Feixianguan Formation of the Early Triassic in northeast of Sichuan Basin have great potential of gas discoveries. The reservoirs are mostly developed in platform margin reef & bank facies, restricted platform facies, open platform facies and platform margin rampcontinental shelf facies. In order to deepen the understanding of the reservoir characteristics，we studied the petrophysical characteristics of rock samples from different sedimentary facieses in the two formations from three blocks namely as Xuanhan, Tongnanba and Yuanba in Da County of Sichuan Province based on available log interpretations and sample test results. It turns out that the sedimentary facies controlled the rock types, which in turn result in varying reservoirs petrophysical characteristics due to differences in internal textures and primary porosity. It also points out that reservoirs are better developed in facies deposited in an environment with higher energy. Reservoirs in the platform margin reef & bank facies have the best petrophysical characteristics and are of porous types (type Ⅱ). The platform facies contains complex reservoirs of fractureporosity type (type Ⅲ) with second best petrophysical characteristics. The reservoirs in the platform margin rampcontinental shelf facies have poor petrophysical characteristics and are of fracture type. We therefore suggest that the platform margin reef & bank facies be the focus of exploration activities.

Reconstruction of thermal evolutionary history of the Upper Paleozoic in the southern North China

2011, 32(1):  64-74.  doi:10.11743/ogg20110108

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The southern North China is considered one of the areas with great hydrocarbon potentials. The Upper Paleozoic coalbearing strata are major exploration targets in the area. An analysis of more than 600 sets of vitrinite reflectance (Ro) data and other geothermometer data such as Tmax and spore color index indicates that the thermal evolution of the Upper Paleozoic is uneven, showing a general trend of higher in the north and west but lower in the south and east. By combining a reconstruction of the eroded strata with an analysis of the paleostructure evolution, fission track dating and magmatic rocks dating, we concluded that the Upper Paleozoic strata experienced complex thermal evolution history and hydrocarbon generation history, and identified three hydrocarbon generation types including the early, the earlylate and the continuous types. The organic matter in the Upper Paleozoic strata mainly experienced hypozonal metamorphism in early stage. Magmatic thermal metamorphism and dynamic metamorphism were the major causes of local abnormal high thermal evolution. The regional tectonicthermal events in the MidLate Mesozoic were the crucial factors controlling the regional high thermal evolution of the Paleozoic in TaikangJiyuan area. The Upper Paleozoic with favorable conditions for secondary hydrocarbon generation, accumulation and preservation are the realistic exploration targets. They mainly occur in deep of MesoCenozoic depression, such as the middle part of the Niqiuji sag and the middle part of the TanzhuangShenqiu sags.

Diagenesis of tight sandstone in the Silurian Xiaoheba Formation of the Shizhu Synclinoyium，western Hubeieastern Chongqing Area

2011, 32(1):  75-82.  doi:10.11743/ogg20110109

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Based on observations of a large number of cores and thin sections, and the experimental analysis of vitrinite reflectance(Ro),Xray diffraction of clay minerals,scanning election microscope(SEM), this paper discusses in detail diagenetic phenomenon, sequence and evolution of the Silurian Xiaoheba Formation in the Shizhu Synclinoyium,western Hubei easternChongqing Area,It is defined that the Xiaoheba Formation currently is in the epigenetictelogenetic stage. We establish the diagenetic evolution sequence in the area and discuss the influence of diagenesis on reservoir porosity and the densification process of reservoirs. According to this study, sandstone reservoirs in the Xiaoheba Formation experienced a densification process during sedimentation, synsedimentation and eogenetic stage and a positive alteration process during the mesogenetic and telogenetic stages. When eogenetic phase ended, reservoirs in the study area had become very tight, which badly affected the development of secondary pores. Therefore, secondary pores failed to be formed in the best period of their development.

Log evaluation of fractured igneous reservoirs: a case study of the Carboniferous igneous reservoirs in the Junggar Basin

2011, 32(1):  83-90.  doi:10.11743/ogg20110110

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Taking the Carboniferous igneous reservoirs of Junggar Basin as an example, the authors examine the origin and characteristics of fractures in the igneous rocks by using reservoir geology, core data and conventional log data. Lithology of the igneous rocks are identified by MN cross plot. Taking the impact of fractures on porosity and permeability into consideration, we put forward methods for calculating elastic parameters of fractures. In combination with imaging log data, formation testing data and core description, we establish a composite evaluation method for fractured igneous reservoirs in this area. The study shows that the fractures feature in low Yang modulus and high Poisson ratio. The method can meet the needs of studying fractured igneous reservoirs in this area.

Characteristics and potential of gas charging in the PermoCarboniferous of Sulige region

2011, 32(1):  91-97.  doi:10.11743/ogg20110111

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The PermoCarboniferous source rocks are widely distributed in the Sulige region. With a flat regional structural relief and poor reservoir poroperm characteristics as background, the secondary migration of gas in the region is mainly driven by gas expansion. Gas charging potential is not only depended on the amount of gas expelled from the source rocks but also closely related to the thickness and porosity of the reservoirs to be charged. The author therefore proposes in the paper a quantitative method for gas charging potential evaluation by using reservoir charging index (RCI). Experimental results of two phase (gaswater) flow were also used to determine the evaluation standards of RCI at high, mediocre and low gas charging potentials respectively. For the PermoCarboniferous in Sulige Region, effective gas charging mainly occurs between the Carboniferous Benxi Formation and the 8th member of the Permian Shihezi Formation. Their RCI values are in the range from 0.4 to 0.6, indicating a medium to high gas charging potential as a whole.

Genetic types and evolutionary model of mixed clasticcarbonate deposits in the lower part of the Sha-1 Formation, the Huanghua Depression

2011, 32(1):  98-107.  doi:10.11743/ogg20110112

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This paper makes a study of the mixed clasticcarbonate deposits widely distributed in the lower part of the 1st member of the Shahejie Formation, Huanghua Depression. Since there is little research on this kind of mixed deposits, it is hard to identify sedimentary facies, reservoir distribution, poolforming elements and their distribution patterns. Therefore, by examining sedimentary microfacies and based on the sequence stratigraphic framework of the study area, we made a comprehensive study of petrologic features, genetic types and evolutionary patterns of mixed clasticcarbonate deposits and the controlling effect of sequence stratigraphic framework on their distribution. According to this study, we recognize four types of mixed deposits: parent material mixing, mixing due to gradual facies change, insitu mixing and gravity flow mixing. The mixed deposits are highly developed in lowstand system tract, but poorly developed in highstand and transgression system tracts. The lower part of the 1^st member of the Shahejie Formation, Huanghua Depression, is a typical example of modern mixed clasticcarbonate deposits. Results from this study can provide thoughts and foundations for further exploration and development in the study area.

Hydrocarbon plays and poolforming patterns in the western part of the Pearl River Mouth Basin

2011, 32(1):  108-117.  doi:10.11743/ogg20110113

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There were many hydrocarbon plays with long geological time span in the western part of Pearl River Mouth Basin. The major plays were different from one structural zone to another. Starting from exploring reasons for the differences of hydrocarbon play distribution, we analyze characteristics, distribution patterns and main controlling factors of the upper, middle and lower plays in the Hanjiang Formation and Wenchang Formation in the western part of this basin. On the basis of above findings, we establish three poolforming patterns: 1) “longdistance lateral migration and accumulation in anticlines” for the Neogene Zhujiang Formation in the uplift; 2) “vertical migration and accumulation in secondary structure and lithologic reservoirs” for the Neogene Zhujiang Formation; 3) “vertical migration and accumulation in composite reservoirs” for the faulted slope belt in the sag. Three favorable plays are recognized: 1) lowamplitude anticline reservoir and relevant secondary lowresistivity reservoirs in the Qionghai uplift; 2) fault block reservoirs in the 2^nd member of the Zhujiang Formation of the faulted slope belt in the WenchangB sag; 3) subtle stratigraphiclithologic reservoirs in the northern slope of the WenchangB sag. The predictions are proved by drilling with the discovery of three commercial oilfields at the X structure in the downthrow wall of the south fault belt, the Y structure in the Qionghai uplift and the Z structure in the Qionghai uplift.

Oil migration and accumulation under the influence of fluid dynamics in the Yanchang Formation, eastern Gansu Province, the Ordos Basin

2011, 32(1):  118-125.  doi:10.11743/ogg20110114

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Based on the study of compaction, overpressure and oil potential at the deepest burial depth in Early Cretaceous were restored for the Yanchang Formation, eastern Gansu Province, Ordos Basin. We carefully examine the relationship between vertical distribution of overpressure and testing production. The results show that the oil flow is relatively higher where it is close to the oil source and has lower overpressure, indicating that a close relationship between overpressure and oil distribution. Distribution of oil potential in the main period of accumulation shows that oil tends to migrate and accumulate at the lower oil potential area which coincides with current oil distribution. From this study, we believe that, under the condition of low permeability of reservoirs, fluid dynamics in the main period of accumulation is the main controlling factor for oil migration and accumulation.

Genetic pattern of lowresistivity gas reservoirs in the Xujiahe Formation, southeastern Sichuan Province

2011, 32(1):  124-132.  doi:10.11743/ogg20110115

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Gas reservoirs in the Upper Triassic Xujiahe Formation, southwestern Sichuan Province are characterized by low resistivity, complex genetic types and complex gaswater relationships, which make them hard to identify. Using data from core analysis, logging, structures, testing and logging responses, we thoroughly examine genetic pattern of low resistivity gas reservoirs in the study area from macro and microviews. According to this study, main controlling factors responsible for the development of these low resistivity gas reservoirs are: 1) muddy sandstone reservoirs with low porosity and permeability developing in the context of lowamplitude structures; 2) complex pore configurations, high water saturation, and high formation water salinity; 3) lowenergy sedimentary environment resulting in fine particles in reservoirs, high mud content, some clay minerals with conductivity and enrichment of conductive minerals in certain areas. Reservoir quality has a negative correlation with resistivity.

Performance evaluation on production tests in the extralow permeability reservoirs of complex faultblocks and development strategies—examples from oil reservoirs in the Yingcheng Formation and the first member of the Shahezi Formation in Shiwu oilfield of the southern Songliao Basin

2011, 32(1):  133-141.  doi:10.11743/ogg20110116

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The Yingcheng Formation and the Shahezi Formation in the Shiwu oilfield harbor oil/gas reservoirs of low to ultralow permeability in complex faultblocks. Single thin sand bodies there distribute unevenly and are compartmentalized by faults in certain places. Fluids of various types inside the reservoirs are controlled both by structure and lithology. Most wells drilled in the field have to be fractured before production with only a few can produce by natural energy. Performance of production test in single wells shows a low formation volume factor, a bigger startup pressure gradient (meaning a low production), and a faster production declining rate for wells in natural depletion. The results also suggest that wells producing both oil and gas have a better performance than those produce oil only, and gas wells are the most productive ones. To tackle these problems, we proposed a comprehensive development strategy which includes the following aspects. The first is performing a separatelayer fine reservoir description based on separatelayer fracturing, testing and evaluating. The second is increasing single well production capacity by commingled production after separatelayer fracturing and supplementing formation energy with wholelayer water injection followed by separatelayer water injection. The third is carrying out water injection pilot with irregular well patterns in the selected favorable fault blocks at first and then implementing the successful experience to other areas. The fourth is performing largescale fracturing in gas/oilbearing layers and CO2 huff-n’puff in single oil well with low gasoil ratio.

Logging identification methods of clastic and carbonate rocks and their differences

2011, 32(1):  142-149.  doi:10.11743/ogg20110117

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:Log technique have been employed in the recognition of many meaningful geological events. However, the subtle changes of logs are seldom recognized and used, thus limiting the accuracy and reliability of interpretation. The issue could be tackled be establishing genetic relationships between the variety of important geological events and logs, and that between geological evolution and the vertical and horizontal changes of the logs based on a comprehensive study on geological responses and the combined features of associated logs. The method is used to identify geological events of clastic and carbonate rocks as well as their formation mechanism. Based on an analysis of the variety of geological events, an approach for identifying geological events of clastic rocks is proposed. Through identification and tracing of isogenesis geologic boundaries, an approach for identifying contemporaneous carbonate rocks of different facies. Approaches for identifying geological events of these two kinds of rocks are different as they vary in lithology and genesis. Their major differences include tracing basis and log responses of geologic boundary, and corresponding relationships between log data and geologic boundaries. Application of the abovementioned methods in Dagang Oilfield, Puguang Gasfield and many other fields in Australia has successfully solved the problem of identifying geologic boundaries.

Application of pore fluid pressure prediction by velocity spectrum to Tahe oilfield Zhu Ding，Li Zongjie and Fu Tongtong

2011, 32(1):  150-156.  doi:10.11743/ogg20110118

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The reasons for anomalous pore pressure are analyzed in this paper. The methods of equivalent depth to predict pore pressure from well logging data and Fillippone’ formula based on seismic velocity are introduced and compared with the results from MDT pressure data. The results of prediction of pore pressure from well sonic data in the Tahe oilfield TPT area shows that this area pressure coefficient change from 1.0 to 1.2, can be classified as normal pressure system. The maximum error between MDT and predicted pore pressure by Fillippone’ formula reaches 5% as predictied pore pressure by equivalent depth method reaches 15%, so Fillippone’ formula has high precision and reliability than the equivalent depth method. This study show that prediction of pore pressure using seismic velocity can predict the layer pressure which embodies the pressure change trend of a heavy layer and design the mud density before drill. It can also can improve the drill productivity and approve the support for safe drill.