Oil & Gas Geology-2003, Volume 24 Issue 3

GENETIC TYPES, DISTRIBUTION AND EXPLORATION SIGNIFICANCE OF MULTISTAGE SLOPE BREAKS IN RIFT LACUSTRINE BASIN

Wang Yingmin, Jin Wudi, Liu Shuhui, Qiu Guiqiang, Li Qun, Liu Hao, Xin Renchen, Yang Fei

2003, 24(3): 199-203,214. doi:10.11743/ogg20030301

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Slope break is a geomorphologic concept, which refers to a zone where there is an abrupt change in topographic slope.Slope break can develop not only in sedimentary basins,but also in denudation areas. It would be of stratigraphic sequences in marine sedimentary basins shows that the type I sequences in a basin with shelf slope break margin or growth fault (faulted slope break)margin,unconformity and incised valley can be formed above the slope break,while basin-floor fan,slopee fan and other aktian deposits,as well as lowstand wedges can be deposited below the slope break during the lowstand.This sedimentary system constitute a typical LST sediments.The concept of structural slope break has been advanced in recent years in the rift lacustrine basins. Structural slope break refers to the sedimentary slope break caused by the long activity of syndepositional structures. The structural slope break, on the one hand, has a strong impact on the accommodation and deposition in the basin, and on the other hand, controls the spatial distribution of the sedimentary facies.There are various kinds of faulted slope breaks,such as salient-gentle slope margin, gentle slope-subsag margin,salient-steep slope margin, steep slope-subsag margin, etc.And it has been further clarified that incised valleys, lowstand river-delta and lake-floor fans can be developed in the LST in the lake basin, which would constitute the potential reservoirs in deeper part in the basin.The distribution of these sediments have clearly been controlled by the structural slope breaks in the basin margin.The study of the Paleogene in Dongying sag shows that there are multistage slope breaks ,such as faulted, flexural, depositional and erosional slope breaks in the half graben-like rift lacustrine basin. The large syndepositional faults that act as the boundary of tectonic subunits, such as salient, slope (fault bench) and subsag, would often become faulted slope breaks.As a result,there will be various types of faulted slope breaks successively developed from the steep slope to gentle slope belts,including salient-steep slope, steep slope-subsag,salient-gentle slope, gentle slope-subsag margins.Nearby these slope breaks,some smaller syndepositional faults can become auxiliary faulted slope breaks and constitute various complex plane combination patterns. Above and below the faulted slope break on the edge of salient may develop erosional slope break. Flexural slope break are mainly associated with syndepositional anticline and nose,which are developed best on both limbs of central anticline-belt and on delta front.The most typical depositional slope break is that developed along the long-axis on Dongying delta front.Faulted slope break controls the development of lowstand fan and compound lithologic-structural traps, depositional slope break controls the distribution of lowland fan,highstand slumping turbidity fan and lithologic traps,while flexural and erosional slope breaks control not only the development of sandbody,but more apparently control the development of stratigraphic overlap, truncation wedge-out and stratigraphic traps.Finally, different kinds of slope breaks developed in the rift lake basin distribute extensively and appears multistage slope breaks.The slope break controls significantly the development of stratigraphic depositionnal and non-structural traps. Combination of oil and gas reservoirs usually occur near the slope break.A thorough study of the genetic type and distribution of slope breaks and their control on sedimentary facies and reservoirs in the maturely explored area as Dongying sag would certainly establish a vast domain in the study of rift basins in eastern China.

HYDROCARBON ACCUMULATION THRESHOLD AND ITS APPLICATION IN JIYANG DEPRESSION

Pang Xiongqi, Li Pilong, Jin Zhijun, Zhang Shanwen, Zuo Shengjie, Chen Dongxia

2003, 24(3): 204-209. doi:10.11743/ogg20030302

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Hydrocarbon accumulation threshold quantitatively expounds the regularity of hydrocarbon distribution. Hydrocarbon accumulation threshold refers to the least amount of oil and gas dissipated during the process of pool formation in the accumulation system. Dissipated hydrocarbon includes those left within the source rock, detained in the reservoir bed, discharged before cap-rock being formed, adsorbed by adjacent rock, and dissolved in water, as well as oil and gas diffusion. The mechanisms of hydrocarbons dissipation within the pool-forming system mainly include being adsorbed by rocks, blocked by capillary and dissolved by subsurface water and oil. The manners for hydrocarbons dissipated out of the pool-forming system mainly involves oil and gas diffused upward, oil and gas dissolved in formation water-flow and destroyed by tectonic movement after the formation of pools. Statistics, physical modeling experiment, multiphase percolation theory and geological phenomena analyses prove that hydrocarbon accumulation threshold, like hydrocarbon generation threshold and hydrocarbon expelling threshold,exist objectively.The essential research of hydrocarbon accumulation threshold is modeling and calculating the amount of hydrocarbon disspation. On the basis of studying the regional geological condition, dividing different accumulation systems, and geological analyses of source, cap and reservoir rocks, the research is,firstly,determining the temperature and pressure conditions of pool formation and the history of oil/gas reservoir evolution,as well as measuring the amount of hydrocarbon dissipation through adsorption,dissolution and diffusion for the sake of finding out the main factors controlling the dissipation and confirming the quantitative relationship;secondly, modeling the total amount of hydrocarbon generation in the source rocks and the total amount of hydrocarbon dissipation; and lastly, discriminating the accumulation threshold and recovering the critical geological condition.In the geological history, hydrocarbon begins to accumulate when the amount of hydrocarbon generated in source rock reaches or exceeds the least amount of oil and gas dissipated. Accumulation period, amount of hydrocarbon migration and accumulation and the largest reservoir size are all constrained by hydrocarbon accumulation threshold. The research of hydrocarbon accumulation threshold of 28 accumulation systems in Jiyang depression shows that the least amount of oil and gas dissipation is from 0.036 billion tons to 1.84 billion tons, which increases with the increase intensity of hydrocarbon expulsion, range of migration and accumulation and dip angle of objective formation and decreases with the increase of sand-stratum ratio,tectonic movements and denuded thickness. Finally, on the basis of calculation and comparison the amount of hydrocarbon generation,the resources, largest oil/gas field and favorable targets can then be predicted.

PHYSICAL SIMULATION FOR PETROLEUM MIGRATION IN THERMODYNAMIC SYSTEM

Qiu Nansheng, Fang Jiahu

2003, 24(3): 210-214. doi:10.11743/ogg20030303

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The mechanism and genesis of ore deposit can be basically concluded as the dynamics of mineralization. Temperature will play an important role in hydrocarbon migration under subsurface environment, i.e. migration in reservoir rocks deeply buried in sedimentary basins with high temperature, high pressure and low permeability. Temperature can not only change the physical property of petroleum (e.g. specific gravity and viscosity) during migration,but can also act as the major dynamic force of fluid migration,especially in deep geological condition.An experimental equipment has been installed to simulate the petroleum migration with temperature difference to be the thermodynamic force under normal temperature and pressure environment. The installation is composed of two thermoregulators and a glass tube filled with sands of different sizes. The main objective is to explore the mechanism of heat as the direct diving force during the petroleum migration, by means of the physical simulation of petroleum migration under different thermodynamic systems. The result will provide some helpful information to the study of the mechanism of hydrocarbon accumulation, especially hydrocarbon charging and accumulating in deeper part of the basin. The modeling results include:(1)Oil can migrate under the action of temperature difference, and it migrates from the site with high temperature to the site with lower temperature, indicating that temperature can act as the dynamic force for fluid flow. Under the experimental condition, the temperature difference and time control, the migration velocity and distance. Given in the same time interval,the higher the temperature difference is, the higher velocity and larger distance the migration would be. However, the velocity of oil migration is much slower in water-saturated sands.(2)As to a certain temperature difference, the distance of oil migration is constant no matter how long the experiment would be continued, which indicates that the temperature difference is the most important force to oil migration under the experiment condition.(3) Since the pressure within the tube decreases when the confined glass tube is opened, the velocity and amount of oil migration in the open system would be larger than that in confined system, and they also depend on the temperature difference. (4)The migration distances of oil are almost the same in sands with different grain sizes. The result indicates that the poroperm characteristics of reservoir do not affect the migration distance, but the temperature difference does. (5)No matter how long the experiment would be continued under a certain temperature difference, more oil supple would not increase the distance of oil migration. This also shows that the migration distance has no relation to the amount of oil supply, it just depends on the temperature difference.

ANALYSIS OF PETROLEUM ACCUMULATION PHASE IN DONGYING SAG

Jiang Youlu, Liu Hua, Zhang Yue, Tan Lijuan, Wang Ning

2003, 24(3): 215-218,259. doi:10.11743/ogg20030304

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Based on composite data of trap formation and evolution, hydrocarbon-generated and hydrocarbon-expulsed process, reservoir saturated pressure and fluid inclusion of diagenetic minerals,the accumulation phase of oil and gas reservoirs in Dongying sag have been divided into two phases,i.e. the late Oliocene phase and the late Miocene to Pliocene phase.Large oil and gas pools were accumulated in late Miocene to Pliocene,especially in the middle-late period deposited in Pilocene Minghuazhen Fm.The main petroleum accumulation phases varies in the oil and gas reservoirs in different stratigraphic horizons and different structural units.In general, it is relatively early in the lithologic pool enclosed in source rocks. There is a tendency for the main accumulation phase of oil and gas reservoirs to become late ordered stratigraphically from the bottom to the top and from the center area to the margin of Dongying sag.

TRAP FILL OF LITHOLOGIC POOLS AND ITS MAIN CONTROLLING FACTORS IN DONGYING SAG

Zeng Jianhui, Zhang Shanwen, Qiu Nansheng, Jiang Zhenxue

2003, 24(3): 219-222. doi:10.11743/ogg20030305

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Dongying sag is the largest petroliferous region in Jiyang depression,Bohai Bay basin, which is the largest reserves distribution region in the lithologic pools, Jiyang depression. The main reservoir rock of lithologic pools in Dongying sag is sandstones in the middle and lower parts of the 3rd member of Shahejie Fm(Es₃).The main source rock is mudstones deposited in the Es₃ and the lower part of Es₄. The exploration results of lithologic pools in Dongying sag show that the lithologic traps has a great variation in trap fill in Dongying sag. In order to study the trap fill as well as its distribution and controlling factors in lithologic pools,the methods of statistic analysis and principal factor analysis have been used. In general,the trap fill in the lithologic pools of Dongying sag ranges from 6.8% to 94.1%,with an average value of 46.5%. Among these,50% lithologic traps has less than 40% trap fill; 46.7% lithologic pools is more than 50%, only a few trap fill is more than 80%, with the maximum value of 94.1%.In terms of the types of sandbody,the trap fill in lithologic pools of sandbody isolated by source rock is relatively high,with the average 60%,while the trap fill of the lithologic pools laterally connected with faults is relatively low, with average about 40%. In stratigraphic horizon,the lithologic pools in the lower and middle parts of Es₃ have trap fill as high as 52%, while the lithologic pools in the Es₂ is only 23% trap fill.In the lithologic traps of Dongying sag,the most important factor of controlling the trap fill is hydrocarbon migration conditions, including the distances between lithologic traps and source area, the quantity of faults and the residual pressure within the lithologic traps. The second one is the hydrocarbon charge conditions, i.e. the petrophysical property (average porosity and permeability) and geometry (thickness and area) of sandbody.The oil source condition has a relatively less influence on the trap fill, because of more abundant oil source in Dongying sag.

MODEL TEST AND MECHANISM INTERPRETAION OF THE PETROLEUM ACCUMULATION OF LENTICULAR SANDSTONE IN JIYANG DEPRESSION

Jiang Zhenxue, Chen Dongxia, Miao Sheng, Zeng Jianhui, Qiu Nansheng

2003, 24(3): 223-227. doi:10.11743/ogg20030306

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The oil and gas was accumulated under the surface condition of high-temperature and high-pressure.This process is not only complex but also difficult to observe. Petrophysical properties are important parameters for evaluating reservior quality and for controlling petroleum accumulation in the lithologic sandbody.Under the condition of geology, this influence is mainly represented in reservoir space and nature of sandbody. As we have seen,the sandstones with better petrophysical properties have high storage capacity, larger storage space and better permeability,and oil and gas easily enter into the reservoir spaces of sandstones.Moreover,sandstone cann't be filled with oil and gas from external source rock until the porosity and permeability in sandstones reach their critical values. With respect to the control of interior conditions of lithologic sandstone on hydrocarbon-bearing capacity, Chen Zhangming suggested by experimental analysis that the coarse-grained sandstone is better in hydrocarbon-bearing capacity and the fine-grained sandstone possibly has no hydrocarbon-accumulation. Zeng Jianhui found, by the petroleum accumulation test of lithologic sandbody, that the sandstone cann't be filled with hydrocarbon until the sandstone grain diameter is up to certain critical value. Stainforth indicates that the capillary transmission under the action of hydrocarbon concentration gradient is key mechanism that cause the expulsion of hydrocarbon from source rock, and this mechanism is also suitable for the petroleum accumulation of lenticular sandstone.In order to make more intensive study of petroleum accumulation mechanism and model in lenticular sandstone, the nuclear magnetic resonance (NMR) technique was introduced in this model test. This technique is used to describe quantitatively the magnitude and space distribution of rock porosity and permeability as well as the transmission and percolation mechanism of rock pore fluid. By using this technique,the distribution of oil (in lenticular sandbody )and water(in enclosing rock)and their replacing process can be more directly observed and the process and mechanism of petroleum accumulation can be deeply analyzed. Lentiular pools refer to petroleum accumulation in lenticular reservior or other irregular reservoirs confined by impermeable and low-permeable strata. Test model of nuclear magnetic resonance was built according to characteristics of the geological model. The experiment was carried out in group. Under the conditions of T=30℃,P=20 MPa,and 40% of oil saturation in argillaceous rock, the artifical cement cores (5 centimeters in diameter and 4 centimeter in height) with permeability of 988×10^-3,585×10^-3,203×10^-3,56×10^-3,13×10^-3 μm² were separately measured. From the results of experiment, under the pressure difference of penetration and diffusion caused by hydrocarbon concentration difference, there is a tendency for oil in enclosing rock to migrate towards core (sandstone). When oil migrated to the sand-shale boundary, the capillary force difference exist in contact belt between large and small pores as the migration channel of sand lens is pore. Under the action of the capillary force difference and hydrocarbon concentration difference, oil and gas first entered into the sand lens with larger pores,where the intergranular water is replaced to enter into the shale with smaller pores. With the progress of the above process, oil and gas constantly entered into sand lens and pore water is continuously replaced until sand lens are saturated by oil and gas. There is no hydrocarbon accumulation in the sandstone bodies with smaller pore throat radius and poor petrophysical properties.

DISCUSSION AND PRELIMINARY APPLICATION OF TERNARY GENETIC MECHANISM OF LITHOLOGIC RESERVOIR

Chen Dongxia, Pang Xiongqi, Weng Qingping, Jiang Zhenxue, Zhang Jun

2003, 24(3): 228-232. doi:10.11743/ogg20030307

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Based on the systematical research and statistical analysis on lithologic oil reserviors in Jiyang depression, it is found that the formation of lithologic oil reservior is mainly controlled by hydrocarbon resource, reservoir petrophysical property and sandbody genetic type.In the geological history, the effective hydrocarbon source rocks were subjected to increasing temperature and pressure during burial period.When the isolated sandstone body contacts with or is surrounded by effective source rock and only when the hydrocarbon source rock reaches hydrocarbon generation and expulsion threshold during burial to greater depths, migration and accumulation of petroleum can take place in significant quantities.When those sandstone does not contact with effective hydrocarbon source rock, migration and accumulation of petroleum can also take place if faults link up the lower, upper or deep source rocks. Although the lithological oil reservoir can be developed in various sedimentary environment (channel,delta,cestal bar and turbidite fan), the sandbodies developed in different sedimentary environment have different hydrocarbon-potential.The areal distribution and hydrocarbon-potential of lithologic sandbody are controlled by deposition. The lithologic sandbody is best developed in turbidite fan and has better hydrocarbon-potential, the delta and subsea-fan sandstones have secondary position in hydrocarbon-potential. Even in turbidite sandbody, the deep-water turbidite is different from delta front fluxoturbidite in hydrocarbon-potential. The hydrocarbon filling factor in deep-water turbidite is higher than that in delta front fluxoturbidite.The porosity and permeability in sandstone are also major factors that affect and control the hydrocarbon filling factor in sandstone. Sandstone cann't be filled with oil and gas from hydrocarbon source rock until the porosity and permeability in sandstone reach their critical values.From analysis of major factors that control the formation lithologic oil reservoirs, it is found that the hydrocarbon source rock cann't generate enough hydrocarbon for oil and gas accumulation in trap until the source rocks are buried to depth threshold of hydrocarbon generation.The sedimentary environment of trap formation affects hydrocarbon accumulation. Under the control of deposition, oil and gas can only be accumulated in some types of traps.Moreover,the internal condition of sandstone,i.e.petrophysical properties of sandstone, also control hydrocarbon accumulation in traps. Therefore, hydrocarbon accumulation in lithologic traps can be explained by three-genetic factors. In the ternary composite accumulation model,three end members are represented by sandstone type (i.e. sedimentary environment) buried depth and hydrocarbon-expelling intensity(i.e.hydrocarbon generation and expulsion), sandstone petrophysical property (i.e. internal condition of sandstone)respectively.The hydrocarbon generation and expulsion condition and sandstone internal-condition range from bad to good, sedimentary environment range from unfavorable to favorable. Trap hasn't hydrocarbon accumulation until all three-genetic factors satisfy certain conditions.On the basis of fine analysis on hydrocarbon source condition, sedimentary system, sedimentary facies and reservoir petrophysical properties for the sequence I in lower Es₃ in Chezhen depression, the ternary genetic mechanism was used to make preliminary and comprehensive prediction on the lithologic pools in sequence I in lower Es₃ in Chezhen depression. According to prediction results, we suggest that the important turbidite fan bodies in Chexi and Guojuzi sags are important exploration targets, where the Che 271 and Da 355 wellblocks are favorable regions for searching lithologic trap and lithologic pools.

PETROLEUM ACCUMULATION MECHANISM AND ITS MAIN CONTROLLING FACTORS OF SAND LENS IN NIUZHUANG SUBSAG

Zhang Jun, Pang Xiongqi, Chen Dongxia, Jiang Zhenxue

2003, 24(3): 233-237. doi:10.11743/ogg20030308

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Petroleum exploration and research show that the small-medium lenticular sand bodies in the middle or the lower 3rd member of Shahejie Fm in the Niuzhuang subsag have different petroliferous property. Three lenticular turbidite bodies (Niu 20C3, Wang 70B and Niu 35B1)were selected as case studies,including the sand body horizon,buried depth,tectonic position,sedimentary system and facies, petrologic character, grain size, sorting and roundness, porosity and permeability, pore type,pore throat radius,diagenesis,petroliferous property, sand body characteristics,trap filling,reservoir pressure and so on. Through detailed geological analyses, it is found that the Niu 20C3 sand body has the trap filling of 74.56%,with the best conditions of hydrocarbon supply and petrophysical property;with Wang70B has better conditions of hydrocarbon supply and petrophysical property;with the medium petroliferous property (trap filling of 53.25%);Niu 35B has the lowest petroliferous property (trap filling of 29.02%),with the worst conditions of hydrocarbon supply and petrophysical property.The research results of three cases studies indicate that the main factors which control oil and gas accumulation in sand lens reservoir are sedimentary environment or genetic type of sand body, hydrocarbon supply of surrounding rock,and petroliferous property of sand body. In the turbidite sand reservoirs,the petroliferous property proportional to hydrocarbon supply condition of surrounding rock as well as porosity and permeability of sand body.The genetic type of sand body is controlled by sedimentary environment and lithofacies palaeogeograph. Sedimentary environment not only controls the reservoirs macroscopical distribution and development, but also have an important influence on lithology and microscopical feature.Turbidite sand lens reservoirs confined in source rock are favorable for petroleum accumulation, because the oil and gas sourced from mud rock can move into them from all different directions.Oil and gas can be accumulated only when the hydrocarbon-generating amount,which is transformed from organic matter contacting with sand body,execeds residual hydrocarbon amount (including the absorbed hydrocarbon,soluble hydrocarbon in porous water,soluble gas in oil and blocking hydrocarbon by capillary pressure)during the burial evolution process of source rock.Petrophysical characteristics of sand body, including porosity, permeability, grain size, sorting and heterogeneity, play especially important role in petroleum accumulation of sand lens reservoir. Experiments demonstrate that only if the grain size of lithologic reservoir exceeds some critical size, oil and gas can be accumulated in the sand body.During the geological history, the evolution of oil and gas accumulation in lithologic reservoir could be divided into three phases. The first one is in shallow depth where such a lithologic oil and gas pool does not exist because the hydrocarbon amounts,sourced from the mud rock confinding sand body,does not exceed the dispersed and residual hydrocarbon amounts.The second one is in the medium depth where the lithologic oil and gas pools could be formed because the hydrocarbon-generating amount is enough to supply the oil and gas accumulation in lithologic pool and organic acid formed by organic matter transformation might improve the reservoir petrophysical property.The last one is in a great depth where oil and gas can not be accumulated owing to the poor petrophysical property of reservoir.Therefore,as the burial depth is increased,the lithologic pool formed by stages,petroleum is only accumulated in the second phase with medium depth.

SEGMENTARY CHARACTERISTICS OF MAXI FAULT AND THEIR RELATIONSHIPS WITH OIL AND GAS IN JIZHONG DEPRESSION

Sun Dongsheng, Liu Chiyang, Yang Minghui, Du Jinhu, Zhang Yiming, Zhang Ruifeng

2003, 24(3): 238-244. doi:10.11743/ogg20030309

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Maxi fault is the eastern boundary fault of Maxi trough, and is also an important oil-controlling fault. Based on the interpretation of a series of seismic profiles and the analysis of deformation characterisitcs along the fault, as well as the influence on the deposition, the fault can be divided into three parts or segments,i.e. the strike-slip part in the north, the major uplifting and subsiding part in the middle, and the splays in the south. The middle segment is located between Houliugusi and Nanmazhuang with an extension of about 11 km, the strike of the fault turns from nearly NS in the south to NNW in the north.It can be seen from the section that the fault plane is flat and straight,and steeply dipping to the west with an angle of about 60?. The fault movement has been strongest in the Paleogene and formed the Maxi trough in the footwall. The southern segment of the fault is located to the south of Liugusi and to the north of Hejian with an extension of about 12 km. A series of subsidiary faults that are dispersing to the south and the middle segment of the major fault formed a typical brush structural system. The throw and the dip angle of those faults decrease with depth, and finally flock together to a nearly horizontal detachment surface in the basment rock. It is kind of end effect commonly seen in the strike-slip and tenso-shear faults. The strike-slip fault segment in the north is located to the north of Nanmazhuang, and extends through the eastern side of Mozhou buried hill to Gaojiazhuang area in Baxian sag, with an extension of about 30 km. The fault became a strike-slip fault to the north of Ren 105 well. The plane is steeply dipping to the west and nearly erecting, and reflect as a straight line in seismic profile,and is characterized by narrow crushed zone in the basement and dispersing upward in the overlying rocks as flower-like structure, which is apparently the result of the strike-slip fault in later period. Maxi fault is actually a thrust fault zone developed in front of a large detached structure in the pre-Paleogene Fuping metamorphic complex.In Paleogene,a strong extensional rift formed under the effect of regional dextral shear and extensional stresses with Maxi trough to be in the thrown wall. Maxi fault underwent sinistral and dextral strike-slip movements at the end of both Paleogene and Neogene,which formed many associated structures, such as an echelon faults, diapiric structures, brush structures, flower-like structures,compressional anticlines, and pull-apart depression. The steeply dipping of Maxi fault and quickly subsiding of the footwall result in the Maxi trough to be rich in hydrocarbon source rocks, as well as to have the richest resources in Raoyang sag. Maxi fault also controlled the distribution of petroleum. Firstly, many traps have been formed along the fault through the multiple activities; secondly, faults become ideal pathways for oil and gas migration;thirdly,sandstones develop along the fault. Therefore,Maxi fault has not only controlled the source rocks but also controlled the distribution of oil and gas.

STUDY OF CENOZOIC TECTONIC EVOLUTION AND HYDROCARBON ACCUMULATION IN SOUTHWEST GENTLE SLOPE BELT, HUIMIN SAG

Han Tianyou, Qi Jiafu, Lin Huixi

2003, 24(3): 245-248. doi:10.11743/ogg20030310

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The southwest gentle slope belt in Huimin sag is located in the structural slope to the south of Xiakou fault. In the north,it is adjacent to Linnan depression, an important oil kitchen,and is bounded by Xiakou fault zone; it is adjacent to Luxi uplift in the south, Qudi horst and Jiyang graben in the east, and Yucheng depression in the west. Its area is about 500 km². The Paleogene in the gentle slope belt is relatively thin. On the whole, it appears to be a wide and gentle monocline,uplifted in the south and dipping to the north.There are only a few NNE conjugated normal faults with small displacement in it. At the same time, these faults cut through the strata into the basement and pinch out upward in the Paleogene or end up beneath the unconformity.The Cenozoic tectonic evolution in that area can be divided into 4 stages:(1)During the deposition of Kongdian Fm to Es₄,Huimin sag was a simple half-graben fault depression and the southwest gentle slope belt was located on the structural slope of the half-graben. The tilting movement resulted in the whole area to be dipping to the north,and so deposited a wedge-shaped sedimentary sequence composed of Kongdian Fm -Es₄. During this stage, the displacement of Xiakou fault was small,and it could be regarded as accommodating the counter-inclined fault developed on the hanging wall of Linshang fault. (2)During the deposition of Es₃-Es₂,the activity of Xiakou fault was intensive,which became the southern boundary fault of the graben-type Linnan depression. The southwest gentle slope belt located on the footwall of Xiakou fault subsided slowly,and the tilting had apparently been restrained. There were conjugated normal faults with small displacement in the belt. The thickness of the Es₃ and Es₂ were relatively thin and uniform. (3)During the deposition of Es₁ to Dongying Fm,the faults in the southwest gentle slop belt had stronger activity. During the depositon of the Dongying Fm,the tilting made the whole belt to be dipping to the north .(4)During the deposition of the Neogene to Quaternary, Xiakou fault and the faults within the southwest gentle slope belt were basically inactive. As a result, a regional unconformity occurred between Guantao Fm and the underlying formations. In the southwest gentle slope belt near the basin margin,source rock can only be found locally in some well-developed Es₄. The sealing characteristics of Xiakou fault have an important effect on the hydrocarbon migration and accumulation in the southwest gentle slope belt. Xiakou fault is characterized by listric and high-angle plane fault. The smearing coefficient analysis of Xiakou fault indicate that the fault's sealing property becomes poorer with increasing depth. The middle Es₃ and younger formations have relatively good sealing characteristics, while the lower Es₃, Es₄ and older formation have relatively poor sealing characteristics. The Es₄ source rock in the southwest gentle slope belt reached the hydrocarbon-generating threshold (2 600 m) after Dongying Fm was deposited. The hydrocarbon generated from Es₄ source rock on the hanging wall of Xiakou fault can migrate laterally through the fault to the southwest gentle slope belt. Because of the lack of source rock in Es₃ and sequences above it, and the sealing of Xiakou fault, the hydrocarbon generated in Linnan depression have been hampered to migrate to the gentle slope belt. The characteristics of the structural evolution and the hydrocarbon accumulation model set up from the migration condition show that fault-block and stratigraphic traps might be formed in the Es₄ and older formations in the southwest gentle slop belt,but oil and gas reservoirs would hardly be formed in the Es₃ and younger formations.

PALEOGENE LOWSTAND SYSTEM TRACT DEPOSITS AND NONSTRUCTURAL TRAPS IN DONGYING SAG

Jin Wudi, Wang Yingmin, Liu Shuhui, Chen Chonghe

2003, 24(3): 249-252. doi:10.11743/ogg20030311

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Lowstand systems tract (LST) is a concept used in sequence stratigraphy.It is referred to as sediments deposited above the type I sequence boundary,and below the first flooding surface.It is usually composed of basin-floor fan,slope fan and lowstand prograding wedge.When the low-water datum declines down below the slope-break,then the area above the slope-break will become exposed or denudation,and there will be unconformity and incised valley;while that below the slop-break will become a depositional area deposited with aktian deposit, such as basin-floor fan,slope fan,and lowstand wedges, All these will constitute a typical LST deposited in a basin with shelf slope-break margin or growth fault margin.Since there are various types of slope-breaks controlling the development of LST,there will also be different compositions of litholosomic bodies in the LST. On the basis of a vast amount of 3D seismic interpretation and analyses of well and log data, the LST developed in the Paleogene Shahejie Fm in Dongying sag has been studied and identified.It is suggested that the LST developed in the slop-break with steep slope faults in northern Dongying sag is characterized by alluvial fan,small lowstand delta,nearshore submerged fan,and the LST developed in the gentle slope belt in southern Dongying sag is mainly composed of lowstand delta and coastal shallow lacustrine sediments.Being situated in a specific position in the stratigraphic sequences and characterized by the development of its lithosomic body,the LST would be in an extremely favorable position for the LST is usually developed on the sequence interface or uncomformity interface.These interfaces are mostly good migration pathways, oil and gas can migrate along these interfaces,and would enter and accumulated in the sand bodies in the LST;secondly,the deposition of lowstand wedges,lowstand fans and basin-floor fans in the LST are mostly related to tectonic movement,therefore the dimension of single sand body would often be bigger than the other sand bodies within the sequence,and have relatively good poroperm characteristics,so they can be good reservoirs.In addition, the sand bodies in the LST are located on the transgressive and highstand system tracts (TST and HST) of the former sequence and under the TST and HST of its own sequence,i.e. they would be in interrelated between the mudstone deposited in the two sets of TST and HST,and thus form a good vertical sealing.All these would be favorable for the development of nonstructural traps in the LST.During this research, it is found that various types of nonstructural traps have been developed in the LSTs in Dongying sag,such as the developing stratigraphic traps in northern and southern slope-breaks,and the lithologic traps in the eastern depositional slope-break. Through detailed description and preliminary assessment of these traps, it is believed that the hydrocarbon prospects are optimistic, and it is very important to study the LST through research of slope-break and unconformity in Dongying sag.A large contribution would be made in the exploration of nonstructural traps in Dongying sag.

FAULT ACTIVITIES AND THEIR CONTROLS ON THE DEVELOPMENT OF LITHOLOGIC SEQUENCES IN ZHANHUA SAG

Zhang Fanqin, Wang Weifeng, Dai Junsheng

2003, 24(3): 253-259. doi:10.11743/ogg20030312

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Zhanhua sag is located in northeastern Jiyang depression. The development of Zhanhua sag was controlled by the NE and NEE extensional and tenso-shear faults,which constituted tectonic framework characterized by alternation of sag and arch. Complex half graben is the characteristics of Zhanhua sag was controlled mainly by the first- and second-order faults with the strike of NE and NEE.The third- and the fourth-order faults are associated faults or antithetic adjustment faults. In Zhanhua sag,many boundary faults lack of footwalls, so, falls of the faults have been used to indicate the intensity of fault activities. Fall of fault refers to the amplitude of relatively fluctuation of the two walls. The larger the fall is, the more intensive the fault activity would be. In the early Es₃ period, Gubei fault had the largest fall; in the middle Es₃ period, Chengdong fault had the largest fall; during the late Es₃ to the early Es₂ period, Gubei fault had the largest fall; during the late Es₂ to Es₁ period, Yidong fault had the largest fall; in the early Ed period, Chengdong fault had the largest fall; and in the late Ed period, Gubei fault had the largest fall. In Zhanhua sag, the most tectonic intensive activity occurred during the deposition of upper Es₂ to Es₁; the next is that developed during the deposition of upper Es₃, and then are those developed during the middle Es₃, late Es₃ to early Es₂ and early Ed periods; and the weakest tectonic activities was in late Ed. Obviously,the tectonic activities were characterized by episodes, i.e.tectonic activity was intensive in early Es₃ period,became weak in middle Es₃,late Es₃ to early Es₂; in late Es₂ to Es₁ period,tectonic activities became intensive again,and weak again in early and late Ed period.In Zhanhua sag, the Paleogene Es₃ to Ed sediments can be divided into two second-order sequences and six third-order sequences.Total fall of faults is used to indicate the intensity of fault activity here. The largest fall amplitude of fault corresponds to the beginning of the second-order sequence, i.e. the total fall of faults had been the largest at the beginning of each second-order sequence,and then gradually became smaller, which indicates that faults have controlled the development of second-order sequences.In Jiyang depression,there are six third-order tectonic cycles from Es₃ to Ed. They are characterized by strong extensional movement in the early period of each cycle,extensional movement in the middle and late periods of each cycle, and compressional movements in the final period of each cycle. Their activity periods correspond to the development periods of the six third-order sequences in Zhanhua sag. The responses of high-resolution sequence stratigraphy logging rhythms of Fu-111 and other eight wells to tectonic activities show that tectonic activities have controlled the overlapping mode of parasequences, i.e. they are characterized by intensive extension and retrograding parasequence set in the early period of each third-order cycle,and by intensive compression and progradational parasequence set in the final period of each cycle.Fault activities controlled the thicknesses and distributions of third-order sequences,as well as the migration of depocenter .As a starting point,the depocenter had been in the area near to Wuhaozhuang-Changdi fault zone during the development period of sequence I,and then migrated successively in clockwise direction to the southern part of Bonan sub-sag and the eastern side of Gubei fault during the development period of sequence II,to the northern side of the western sector of Gubei fault during the development period of sequence III,to the the southern part of Bonan sub-sag during the development period of sequence IV,to the area near the Chengdong fault during the development period of sequence V,and to the western part of Wuhaozhuang sub-sag during the development period of sequence VI .

SIMULATION TEST OF VERTICAL MICRO-SEEPAGE OF HYDROCARBON COMPOSITION

Xia Xianghua, Yang Ruiyan, Bao Zhengyu

2003, 24(3): 260-263. doi:10.11743/ogg20030313

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Vertical micro-seepage of hydrocarbon can be not directly observed but measured by the instruments,which is usually referred to as a circle-liked halo distributed on the top of oil and gas reservoirs.The study shows that the equilibrium time of vertical micro-seepage system has a linear relation with strata thickness,and the equilibrium time of this system is exponentially decreased with porosity increase.When annual adsorption rate of soil changes from average 1% to near 0, the equilibrium time of this system is exponentially increased.As the consumption rate of hydrocarbon derived from microorganism is increased, the equilibrium time of this system is exponentially decreased.Through the comprehensive analysis of the influence of soil adsorption rate of and microorganism-derived hydrocarbon consumption rate on the system equilibrium time, respectively,it is shown that the influence of the former would be less than the latter's.For the overlying and underlying formations,the influences of either soil adsorption rate and rock porosity on the equilibrium time of micro-seepage system are nearly equivalent.When the microorganism-derived hydrocarbon consumption rate both in overlying and underlying formations is about 2% per year,their influence on equilibrium time are nearly equivalent,i.e. the influence of microorganism's consumption rate in overlying and underlying formations on the equilibrium time of micro-seepage system are nearly equivalent.When the microorganism-derived hydrocarbon consumption rate both in overlying and underlying formations is less than 2% per year,the microorganism's effect in underlying formation would be more than that in overlying formation;while the microorganism-derived hydrocarbon consumption rate both in overlying and underlying formations is more than 2% per year, microorganism's effect in underlying formation would be less than that in overlying formation.

APPLICATION OF GEOCHEMICAL METHODS IN THE RECOGNITION OF HIGH-RESOLUTION SEQUENCE BOUNDARY

Zhao Junqing, Ji Youliang, Zhang Shanwen

2003, 24(3): 264-268,273. doi:10.11743/ogg20030314

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Clay minerals and trace elements are widely distributed in the different types of sediments,which are so sensitive to environment change.The characteristics of clay minerals (sedimentary differentiation,mineral assemblages, and mineral composition and contents) has provided the information of environment change during their formation process.After studying the typical lithogeochemical parameters of high-resolution sequence units in the 3rd and 4th of Shahejie Fm in the fall-wall of Shengbei fault,Dongying depression,it has been found that mineral composition (including clay minerals and cements),chemical constituents (trace elements) would be periodically changed with the pace of change cycle of lake surface controlled by paleoclimate.The contents of illite + mixing-stratified illite/semectite and kaolinite +chlorite as well as paleosalinity show distinct change features above and below high-resolution sequences boundary.In the water transgressive system, the content of kaolinite is increased but the content of semectite and value of paleosalinity are gradually reduced from bottom to top.However these characteristics in water regressive system would be opposite. On the sequence boundary under different transgressive and regressive water systems,the contents of dolomitic cement is relatively high,while the contents of calcitic and mixing cements is relatively low.

PALEOSEDIMENTARY ENVIRONMENT IN TERTIARY AND ITS EFFECT ON PETROLEUM DISTRIBUTION IN NIUZHUANG SUBSAG, DONGYING SAG

Li Sumei, Li Xue, Zhang Qinghong, Ju Lirong, Ma Xiaochang

2003, 24(3): 269-273. doi:10.11743/ogg20030315

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31 rock samples and 20 oil samples have been collected from Niuzhuang subsag and its southern slope for detail GC,andGC-MS analyses.The rocks have been sampled from Es₃ and Es₄ members, which are respecting the two main source rocks being deposited in freshwater lake and brackish to saline lake environments, respectively. The oils have been sampled from Bamianhe, Guangli and Shaziling oilfields in the southern slope of Niuzhuang subsag.Our oil-oil and oil-rock correlations show that there are two genetic types of oils in Niuzhuang subsag and its periphery.They are separately corresponding to the two major source rocks in middle to lower Es₃ and upper Es₄ members.Rock extracts of Es₄ member and the related oil are characterized by Pr/Ph<1, high gammacerane, sterane and dibenzothiophene contents, relatively low diasterane content,and high C₃₅/C₃₄ homohopane ratios, suggesting that they have been generated in a reduction and brackish to saline lacustrine paleoenvironment. However, rock extracts of middle to lower Es₃ and the related oil are characterized by Pr/Ph>1,and relatively high concentration of hopane and dibenzofuran contents,representing freshwater lacustrine genesis. It is found that methylsteranes including dinosteranes are of importance to indicating sedimentary environment and even specific species of organisms. The alge-rich Es₄ shales deposited in brackish water in the southern slope of Niuzhuang subsag are rich in C_28-30 4-methyl steranes and dinosteranes, and but very small amount of dinosterane in the Es₄ source rock buried at a depth over 2700 m. Similar phenomenon has been observed in Es₃ mudstones deposited in fresh water,i.e.C₂₈, C₂₉, C₃₀ 4α methyl-ααα-20 Rsterane of Es₃ mudstone distribute in the shape of “V” in the m/z 231 mass chromatogram, while those in Es₄ source rock have a linear distribution, suggesting that they have apparently different paleoenviornment and related geochemical environment. Analyses of formation water and trace element also show that the sedimentary environments in Es₃ and Es₄ are quite different. The Sr/Ba ratio,boron content,paleo-salinity and saltiness in Es₄ are relatively higher than those in Es₃.In a word,the development of the Tertiary lake in this area has been closely related to the tectonic movement and paleo-climates. Sedimentary environment and biomarker index show that the oil field in the deeper part of Niuzhuang subsag have mainly been sourced by the Es₃ source rocks.They are actually of lenticular oil reservoirs characterized by short-distance migration and indigenous oil generation and accumulation. The oils generated from Es₄ have mainly been accumulated in the tectonically high position,such as southern slope of Niuzhuang subsag and the adjacent area.They are characterized by relatively long-distance migration. The sedimentary environment indexes indicate that most of the oils discovered in the southern slope have been sourced from Niuzhuang subsag,only Guangli oilfield and the oil pools in northern Bamianhe oilfield have been sourced from Guangli subsag and adjacent area. It can,therefore,be concluded that papleoenvironment is one of the most important factors having controlled oil and gas distribution in the area.

EFFECT OF OIL CHARGING ON RESERVOIR'S DIAGENETIC MINERAL EVOLUTION

Li Yanxia, Liu Hongjun, Yuan Dongshan, Zhang Zhihuan, Zhu Xiaomin, Zhong Dakang, Cai Jingong, Xie Zhonghuai

2003, 24(3): 274-280,295. doi:10.11743/ogg20030316

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All secondary enlargement of quartz and albitization of potash feldspar in water-saturated zone (water layer) are stronger than those in oil-saturated zone. Homogenization temperatures of fluid inclusions in quartz overgrowth (60～140℃) shows that both the temperatures of fluid inclusions in oil-saturated and water-saturated zones are very similar,which proves that the enlargement process of quartz in oil-saturated zone has been continuous after the charging of oil,but the process has been restrained in contrast to that in water-saturated zone. There are mainly five kinds of diagenetic changes for feldspar minerals,including dissolution,carbonatization,kaolinization,illitization and albitization.The pH of pore water and concentration of carbon dioxide (fractional pressure) play an important role in feldspar dissolution.Three different results would occur after feldspar dissolution:(1) all the dissolved substances would migrate along with the solution to other places,which is advantageous to the formation of secondary pores.Migration capacity of dissolved substances depends on the transport capacity of pore water or percolating capability of pore water.Both the poroperm characteristics and oil saturation of the reservoir can affect the percolating capability of pore water.Besides,it is also affected by the property of the pore water.The organic acid in the pore water is favorable to the formation of metal organic complex,thus accelerating the migration of dissolved substances.At the same time,the increase of organic acid concentration would cause the precipitation of SiO₂ and the formation of some clay minerals. (2) K⁺、Na⁺ and Ca²⁺ cations in the pore water lead to the formation and transformation of clay minerals, such as illite; while SiO₂ and Al₂O₃ precipitate nearby as kaolinite and SiO₂, cause the albitization of potash feldspar, or even become authigenic albite or albite overgrowth. (3) The remaining SiO₂ then become the pore filling material, silicon framework or “silicon ball” accumulation. It is the alteration of pore water property caused by oil charging that creates this phenomenon. Since there is a certain amount of organic acid in the oil, and when oil migrate into the reservoir,some of the water soluble substances, such as organic acid, dissolve in formation water, which reduce the pH of formation water and lead to the increase of acidity of pore water, associated with oil with a pH of 3～5, thus boost the dissolution of feldspar. The SiO₂ generated from the dissolution of feldspar, cannot easily migrate in the acidic water, it thus precipitates as the so-called “silicon ball”. But if the oil saturation is continuously increasing so that the reservoir to be rich in oil or immersed in oil, the flowing of pore water would then be restrained, and the diagenesis, including dissolution and precipitation processes, would be impeded. Similarly, the development of calcite and dolomite is better in water-saturated and low oil-saturated zones than in high oil-saturated zone.Theoretically,charging of hydrocarbons alters the property of fluid in the pores, and the diagenetic environment alters accordingly and leads to the ceasing of illite growth. Therefore, the amount of illite is always different in sands containing varying amount of oil. In oil-saturated or oil-rich zone, illite is relatively low, while in sands with small amount of oil, such as the oil patch sands,or with only trace of oil and water layer, illite content is relatively high. The relative content of chlorite in sands varies similarly as that of illite. All mentioned above show that oil charging can change the microenvironment for growing of minerals in reservoir pores, and thus would restrain,to some extent, the diagenesis of reservoir rock.

PORE EVOLUTION AND GENESIS OF SECONDARY PORES IN PALEOGENE CLASTIC RESERVOIRS IN DONGYING SAG

Zhang Qin, Zhong Dakang, Zhu Xiaomin, Song Gang, Zhang Shanwen, Cai Jingong

2003, 24(3): 281-285. doi:10.11743/ogg20030317

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This paper analyses the type and evolution of primary pores, the genesis of secondary pores and the factors of controlling their distribution in the Paleogene clastic reservoirs in Dongying sag. Study of various data,including thin section analysis,SEM,cathodoluminescense,poroperm characteristics,carbonate content and other data,shows that the pores in Paleogene clastic reservoirs in Dongying sag experienced the evolutionary process from primary to secondary pores,i.e.those occur at a depth less than 1 650 m are mainly primary pores;those occur between 1 650m and 1 900 m are the combination or coexistence of solution and primary pores,due to the relatively weak solution action; while at a depth of over 1 900 m, solution and cementation would play the main role,which result in to be mainly of secondary pores. In different areas and buried depths, secondary pores appear to have been developed differently: secondary pores mainly develop at a depth of 1 650～2 450 m on the steep slope zone in the north, especially best develop at 2 100 m in central uplift zone,secondary pores have a relatively good development at a depth of over 1 600 m,with the best development in the interval of 1 850～2 500 m on the gentle slop in the south,secondary pores are generally developed at a depth of 1 900～2 600 m.As a whole,the development of secondary pores tends to be deeper and deeper from north to south,but the extent is apparently become weaker and weaker.All these have mainly been affected by the different reservoir facies, paleo-geothermal gradient and organic maturation in different areas,as well as by the distances of the areas from the oil-generating sag and fault zone. The formation of secondary pores has mainly been related with the solution of carbonate and feldspar, and the dehydration of clay minerals; in other words,it is the result of inorganic diagenesis and the soluble constituents to be dissolved by the carbon dioxide and organic acid during the organic maturation. According to thin section observation, most secondary pores have been formed from the dissolution of intergranular carbonate cement, which has been proved by the negative correlation between porosity & permeability and carbonate content. The dissolved carbonate materials consist of calcites formed in early diagenesis and ferrocalcite & ferrodolomite formed in late diagenesis. Part of secondary pores has been created by the dissolution of feldspars (including plagioclase and potash feldspar) and lithoclasts of intermediate-basic volcanic rocks. In addition, the main depth that montmorillonite to be dehydrated its interlayer water and to be transformed into mixed-layer illite& montmorillonite and chlorite is consistent with the depth interval of secondary pores. The low salinity of interlayer water dehydrated from clay minerals contributes to the further solution of carbonate and feldspar. Secondary pores of clastic reservoirs in the study area have the following distribution characteristics. (1) Secondary pores have been controlled vertically by the time of source rock maturation,i.e.have been controlled by the burial depth(or geotemperature) of sandstones. (2)Secondary pores have been controlled horizontally by the location of sources acidic water. Secondary pores are less developed in the areas far from the oil-generating center. The oil-generating center has been located in Lijin depression in the northern part of Dongying sag, so secondary pores have better development in Binnan, Lijin and Shengtuo areas on the steep slope zone in the north than those on gentle slope zone in the south. (3)Secondary pores are closely related to the sedimentary facies zone, i.e. sandstones deposited in subaqueous environment (strong hydrodynamic condition) have better development of secondary pores than those deposited in terrestrial environment (weak hydrodynamic condition).(4) Secondary pores are more developed in the sandstones near the fault zone. Strong movement of fault would not only produce fissures in the sandstones, but would also become good pathways of acidic water.The sandstones near the pathways have long been affected by acidic water, so there have been more opportunities to be dissolved into secondary pores. For example, the steep slop zone in the north has better development of secondary pores than the gentle slope zone in the south has, because there are more faults on the steep slope zone.

VERTICAL DISTRIBUTION OF SECONDARY PORES IN PALEOGENE SANDSTONES IN ZHANHUA DEPRESSION

Zhong Dakang, Zhu Xiaomin, Cai Jingong

2003, 24(3): 286-290. doi:10.11743/ogg20030318

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This paper mainly discusses the vertical distribution of secondary pores in Paleogene sandstones in Zhanhua depression,based on the thin section, SEM,carbonate contents, vitrinite reflectance, clay minerals, and porosity and permeability data. Thin section and SEM observation shows that the pores in sandstones are mainly of primary pores preserved after compaction at a depth of less than 1 500 m, at a depth interval of 1 500～1 900 m, pores are mainly of primary intergranular pores left after cementation made by carbonate, authigenic clay minerals and quartz overgrowth; there are a few secondary pores at a depth of 1 900 m,and then become mixed pore zone,characterized by the coexistence of primary and secondary pores in a depth range of 1 900～2 200 m; a large amount of secondary pores occur at 2 200～3 500 m interval; and over 3 500 m, it is characterized by residual pores preserved after compaction and cementation. The vertical variation pattern of 15 000 porosity and permeability data from more than 200 wells can also directly confirm such evolutionary processes. From the surface to 1 900 m, the poroperm characteristics of sandstones gradually become poorer and poorer; at the 2 200～3 500 m interval, there is a zone with higher porosity and permeability; and over 3 500 m, the poroperm characteristics of sandstones are getting poor again. It is found from thin section and SEM data that secondary pores have mainly been formed by feldspar dissolution, with minor by carbonate cement dissolution. The vertical distribution of authigenic kaolinite and carbonate cements can also give such explaination. At the 2 200～3 500 m interval where secondary pores are well developed, feldspar dissolution has resulted in the increase of authigenic kaolinite and large amount of quartz overgrowth, while the amount of carbonate cements is still relatively high. The phenomena of feldspar being dissolved and no or only trace of carbonate cement being dissolved are determined by free energy of chemical reaction between organic acid and feldspar/carbonate cement. Dissolution of feldspar in organic acids is characterized by relatively low chemical energy,as for calcium feldspar, ΔG=-154.49 kJ/mol;and for K-Feldspar, ΔG=-17.92 kJ/mol,while for carbonate cement, ΔG= 46.89 kJ/mol.Secondary pores are largely distributed in a depth range of 2 200～3 500 m. Such distribution is closely related to the maturation of source rock and diagenitic evolution of sandstones. In Zhanhua depression, the source rock reached the hydrocarbon-generating window at a burial depth of 2 000 m,with the peak of hydrocarbon-generation to be at 2 000～3 000 m. At this interval, the organic acids generated from source rock maturation caused the dissolution of feldspar, precipitation of kaolinite,and quartz overgrowth,as well as the formation of secondary pores. On the other hand, at 1 900～2 800 m interval, the interlayer water has rapidly been dehydrated from the clay minerals in mudstones,which would result in a further corrasion in sandstones. Therefore, a large amount of organic acid generated during source rock maturation,and water dehydrated from clay minerals would cause the dissolution of feldspar, as a result,a large amount of secondary pores have been formed in Paleogene sandstones in Zhanhua depression. These pores are mainly distributed at a depth range of 2 200～3 500 m.

APPLICATION OF SEISMIC ATTRIBUTE OPTIMIZATION IN RESERVOIR PREDICTION

Yu Jianguo, Jiang Xiuqing

2003, 24(3): 291-295. doi:10.11743/ogg20030319

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A seismic attributes includes physical attribute and geometry attribute, which quantifies specifically the characters of geometry, kinematics, dynamics or statistics in seismic data. Although geometry attributes can be easily accepted and straightly identified by the sense organ of men,the physical attributes derived from abstract process and mathematics are better than geometry attribute in seismic reservoir prediction.Therefore,seismic attribute is mainly referred to physical attribute calculated by mathematical algorithm. Reservoir prediction by seismic attributes is widely used in geophysics. Since 1980s,pattern recognition technique is paid great attention to,and the reservoir prediction techniques,such as a fuzzy pattern recognition, statistic pattern recognition, neural network pattern recognition and function approach,have been successively developed.The predicted objects include hydrocarbon, reservoir thickness, lithology and reservoir porosity.In resevoir prediction,the selection of seismic attribute is accomplished by experiences of interpreters,whose effect is subject to better geological conditions, simple predicted objects,and higher S/N in original seismic data. However, under the other conditions, the effect of prediction is worse. In fact,there exist complex relations between predicted objects and their seismic attributes. The seismic attributes sensitive to predicted objects are not totally the same in different areas and reservoirs. They are also somewhat different even for same reservoir and same area. The optimization technique of seismic attributes is an effective means for solving the above questions. The optimum methods of seismic attributes mainly include the dimension-reduced projected profile and cluster analysis etc. Its purpose is to optimize the minimum seismic attributes or seismic attributes group, which are the most sensitive (or most effective, most representative) to studied problem,in order to increase reservoirs prediction precision and to improve the effect of processing and interpretation related to seismic attributes. The obvious effect of reservoir prediction in CB31 area has been achieved by using the optimum method of seismic attributes. This area is located at the graben belt that is between the drape structures of CB30 buried hill and the drap structures of Changdi buried hill. Its main reservoirs are the sand bars,formed by meander river sediment,in Neogene Guantao Fm,with poor continuity and small distribution. Because the distribution of depositional sand bodies are unstable in lateral and vertical direction and because the drilled thickness of sand bodies is usually less than 10m,the sand bodies have no corresponding reflection on seismic sections, resulting in the bigger difficulty in reservoir prediction and description.Therefore, the recognition of its pool-forming factors and hydrocarbon accumulation rules has been disputed for many years. With the help of the theory and methods of seismic attribute optimization, the seismic attributes of predicted reservoir are optimized. Meanwhile, one of the optimized seismic attributes, the wave peak number, is verified with seismic forward modeling, and demonstrating the reliability of the method. Based on the above works, the stratigraphy, reflected wave velocity, structure and electric property have been studied;the pool-forming factors and hydrocarbon accumulation have been analyzed;the areal distribution of sand bodies have been basically identified;more than 30 sand bodies have been described.As a result,the oil in place reserve in the CB31 wellblock is estimated to be 40 million tons.

POTENTIAL OF REMAINING HYDROCARBON RESOURCES AND MAIN EXPLORATION DOMAINS IN EASTERN MATURE EXPLORATION AREAS IN EASTERN CHINA

Wang Junling Zheng Herong

2003, 24(3): 296-300. doi:10.11743/ogg20030320

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The eleven oil-rich sags in mature exploration areas in eastern China,including Dongying, Zhanhua, Chezhen, Huimin, Biyang, Nanyang, Qianjiang, Gaoyou, Jinhu, and Baise basin,are rich in oil and gas resources. According to the third resource evaluation, the total oil and gas resources in these 11 sags are 9.66168 billion tons and 363.98 billion m³,respectively. The distribution of hydrocarbon resources is very unbalanced in different regions and horizons,because of differences in oil and gas accumulation conditions. Laterally,the oil resources mainly distribute in Dongying, Zhanhua, Dongpu, Huimin and Chezhen sags. Vertically the oil resources mainly distribute in the Paleogene.The total oil resources in these sags are 8.0466 billion tons,accounting for 83.28% of the total oil resources in the 11 sags. The oil resources mainly distribute in moderate and deep reservoirs(1 500～3 500 m), with oil resources of 7.40816 billion tons.Laterally,the gas resources mainly distribute in Dongpu sag, with the gas resources to be 235.5 billion m³.Vertically,the gas resources mainly distribute in the Paleogene, with gas resources of 212.98 billion m³.The gas resources mainly distribute in the moderate and deep reservoirs.Although the exploration areas in eastern China have been in mature and high-mature exploration stages after over 40-years exploration and development, the hydrocarbon resources that have been proved are still less than 50%. According to the experience in the world that 70% of oil resources might finally be proved, there will still be a prodigious amount of remaining hydrocarbon resources in these mature exploration areas. By the end of 2002, the cumulative proved oil reserves in these 11 sags is 4.88659 billion tons,i.e.an average of 50.57% of the oil resources have been proved and the remaining oil resources is 4.77509 billion tons. The cumulative proven gas reserves in these 11 sags is 99.393 billion m³, i.e.an average of 27.31% of the gas resources have been proved,and the remaining gas resources is 264.587 billion m³.The distribution of remaining hydrocarbon resources is very unbalanced because various sags, horizons and reservoir types have different degrees of hydrocarbon accumulation, prospecting and ascertainment. The exploration results,got recently,show that the remaining oil resources distribute mainly in Paleogene, with remaining oil resources of 4.33176 billion tons, accounting for 90.7% of the total remaining oil resources in these 11 sags. Laterally,the remaining oil resources distribute mainly in Dongying, Zhanhua, Chezhen, Huimin and Dongpu sags, with remaining oil resources of 3.94505 billion tons,accounting for 83.2% of the total remaining oil resources in these 11 sags.The remaining oil resources distribute mainly in subtle reservoirs, such as lithologic, complex fault-block reservoir and buried hill reservoirs. Vertically,the remaining gas resources (excluding the deep gas in Jiyang depression) mainly distribute in the Paleogene,with remaining gas resources of 262.9 billion m³,accounting for 99.3% of the total remaining gas resources in the 11 sags. Laterally,the remaining gas resources distribute mainly in Dongpu sag, with remaining gas resources of 176.77 billion m³. Analysis of hydrocarbon resource potential and exploration achievements show that the key exploration domains in the mature exploration areas in eastern China will be the Paleogene, subtle reserviors (including lithologic, complex fault-block and buried hill reservoirs) and new stratigraphic horizons in the depths.

COMPOSITE METHOD OF ROCK PERMEABILITY IN RESERVOIR

Sun Xixin

2003, 24(3): 301-303. doi:10.11743/ogg20030321

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The composite method of rock permeability have always been applied in the calculation of oil reserves,complication of oilfield development plan,numerical simulation of reservoirs and other reservoir engineering research, which mainly include the permeability compositon of vertically different stratigraphic horizons and the permeability composition of different permeable zones along filtrational directions.Based on Darcy's law and combined with established filtration pattern,two composite formulas of permeability have been derived from mathematic calculation.The composite formula of permeability in vertically different stratigraphic horizons corresponds to that in historical literature,but the composite formula in different permeable zone is more different from the forefathers' formula.Based on the examination of actual data, calculated error of new formula is about 0.28%-5.35%,which is basically within permissible error range.

SIMULTANEOUS EXPLORATION AND DEVELOPMENT OF COMPLEX FAULT-BLOCK OIL RESERVOIRS IN SUBEI OILFIELD

Zhang Renling

2003, 24(3): 304-308. doi:10.11743/ogg20030322

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