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Geophysical properties of cap rocks in Qiongdongnan Basin, South China Sea
Marine and Petroleum Geology 17 (2000) 547±555
Geophysical properties of cap rocks in Qiongdongnan Basin, South China Sea Shisheng Hao*, Zhilong Huang, Guangdi Liu, Yuling Zheng University of Petroleum, Beijing, 100083, People's Republic of China Received 20 October 1998; received in revised form 5 January 2000; accepted 18 January 2000
Abstract Based on measured petrophysical properties and statistical methods, relationship between petrophysical properties and well log and seismic parameters are established empirically to evaluate the quality of cap rocks in Qiongdongnan Basin. Good cap rocks show high density and low sonic travel time. Four zones can be divided vertically according to displacement pressures in Qiongdongnan Basin, best seal quality is found in the depth range of 2000±4000 m. The Ya 13-1 structure has excellent cap rock, which contributes to its abundant gas accumulation. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Cap rock is a crucial and sometimes overlooked factor in the evaluation of a potential hydrocarbon accumulation. Cap rock, together with source rock, reservoir rock and overburden rock, consists four essential elements of petroleum system (Magoon & Dow, 1994). Although cap rock directly aects the accumulation and preservation of both oil and gas, it is more important for the gas pool. Gas accumulations is considered to persist through extended periods of geologic time only as dynamic systems reaching an equilibrium between dissipation through the cap rock and continuous replenishment from source rock (Leythaeuser, Schaefer, & Yukler, 1982; Hao, Huang & Yang, 1994; Nelson & Simmons, 1995). When assessing cap rock quality, both microscopic and macroscopic properties of cap rock should be taken into consideration, such as displacement pressure (capillary pressure), lithology, thickness, ductility, lateral continuity (Grunau, 1981, 1987; Watts, 1987; Downey, 1984). Eective cap rocks for hydrocarbon accumulations are typically * Corresponding author. C/o Zhiyong He, ARCO Exploration and Production Tech., 2300 W. Plano Parkway, Plano, TX 75075, USA. E-mail address: [email protected] (S. Hao).
thick, laterally continuous and ductile with high capillary entry pressure (Downey, 1984). The quality assessment of cap rock can be conducted from geophysical data, such as well log and seismic data. The porosity of cap rocks can be calculated from well logs, such as sonic, density and neutron logs (Wyllie, Gregory & Gardner, 1956, 1958; Jian, Chork, Taggart, Mckay & Bartlett, 1994). Permeability may be inferred from the calculated porosity. Statistical techniques are often used to ®nd empirical relationships between porosity and log derived sonic travel time or density (e.g. Burnett, Parry & Willmott, 1970). Experimental studies suggest that relatively simple relations between seismic velocity and such important rock parameters as porosity and clay content do exist (Wyllie et al., 1956, 1958; Tosaya & Nur, 1982; Kowallis, Jones & Wang, 1984; Han, Nur & Morgan, 1986; Vernik & Nur, 1992a,b).
2. Geological setting Qiongdongnan Basin lies in the western part of the South China Sea between 108852 ' E±110.478E and 16847 ' N±19800' N. Covering about 45,000 km2, it trends east to northeast. The formation and evolution
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S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
Table 1 Generalized stratigraphy and evolution of Qiongdongnan Basin Age Quaternary Pliocene Miocene Miocene Oligocene Oligocene Eocene Paleocene Cretaceous
Formation
Thickness (km)
Depositional environment
Basin development
Ying-Huang Meisan Sanya Lingshui Yacheng
0.5±2.8 0.25±2.7 0.2±1.4 0±1.2 0.1±3.8 0.2±2.8 0±0.2
Littoral Littoral±Bathyal Littoral±Neritic Littoral±Neritic Coastal±Restricted marine Littoral±Restricted marine
Transtension Transtension Post-rift subsidence Post-rift subsidence Rifting Rifting Rifting Rifting Rifting
Basement
of Qiongdongnan Basin is closely related to the opening of the South China Sea (Chen, Chen & Zhang, 1993). It shows a characteristic passive margin development from rifting to regional subsidence (Hao, Sun, Li & Zhang, 1995). During Pliocene and Quaternary, the basin is featured with another episode of tensile rifting. The generalized stratigraphy of Qiongdongnan Basin is shown in Table 1. A dozen structural elements can be divided within the basin, among which Ya 13-1 low uplift and Ya 21-1 low uplift are the most favorable for oil and gas accumulation (Fig. 1). 3. Well-log response of cap rocks There are mainly three cap rocks in Qiongdongnan Basin: Meisan formation, Ying-Huang formation and
the second member of Lingshui formation (Fig. 2). The lithology of Meisan formation is mainly limestone, mudstone and muddy sandstone. Deltaic, coastal plain and shallow-water plateau facies developed in the northern border area, while littoral-shallow marine and bathyl to pelagic facies developed in other areas. The data from ®ve wells in Ya 13-1 structure show that Meisan formation is 292.5±370.5 m thick with good lateral seal continuity. It is a good regional seal cap rock without the destruction by the activities of faults. Its density is much higher than that of other the cap rocks. The maximum density is greater than 2.65 g/cm3, average density is greater than 2.60 g/cm3. The sonic travel time is generally less than 68 ms/ft (4.48 km/s) in normally compacted zone, while in the undercompacted zone, its value is often less than 75 ms/ft
Fig. 1. Structural elements map of Qiongdongnan Basin.
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
(4.06 km/s). Meisan Formation is the best regional cap rock in Qiongdongnan Basin. Ying-Huang formation is mainly a thick, laterally continuous mudstone of shallow-sea and bathypelagic facies. The lower part of Ying-Huang formation shows undercompaction. Due to its ductility and lack of fault penetration, Ying-Huang formation is another regional cap rock. From the log measurements, in the undercompacted zone, the average density is more than 2.47 g/cm3, the maximum is mostly 2.51 g/cm3. In the normally compacted zone, the average density is more than 2.53 g/cm3; the maximum is mostly 2.60 g/cm3. The average sonic travel time is about 92 ms/ft in normally compacted zone and about 94 ms/ft in undercompacted zone. The second member of Lingshui formation is a local cap rock. It is developed in fault sags and is eroded in those structural high areas (e.g., in Ya 13-1-1 well). Its sedimentary facies is mainly restricted shallow marine, littoral and shallow-water plateau facies. The average
549
density is more than 2.55 g/cm3, the average sonic travel time is generally less than 74 ms/ft (4.12 km/s). Good sealing quality of Meisan formation and the second member of Lingshui formation can be de®ned as: average density is more than 2.55 g/cm3, average sonic travel time is less than 75 ms/ft (4.06 km/s). As for Ying-Huang formation, only density can be used to de®ne good seals because of undercompaction. Good seal of Ying-Huang formation is identi®ed with average density of 2.48 g/cm3 (Fig. 3). Generally speaking, formations with good seal ability in Qiongdongnan Basin show high density and low sonic travel time, though some dierence exists among the formations. 4. Relationship between well log parameters and petrophysical properties The petrophysical parameters under consideration
Fig. 2. Cross-section of cap rock in Y13-1 structure.
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Table 2 Petrophysical properties of mudstone in Qiongdongnan Basin Zone
Depth (m)
Porosity (%)
Permeability (10ÿ6mm2)
Density (g/cm3)
Displacement pressure (MPa)
Seal quality
1 2 3 4
< 2000 2000±3000 3000±4000 > 4000
> 15 8±15 5±8 <5
>5 5.0±1.2 1.2±0.6 <3
< 2.35 2.35±2.50 2.50±2.65 > 2.65
<1 7.0±13.0 8.0±10.0 <8
Poor±fair Good Good±fair Fair±poor
include porosity, permeability and displacement pressure. The porosity is normally predicted from well logs, such as sonic, density and neutron logs. The sonic travel-time and density for rock matrix, ¯uids and clay minerals as well as clay content in the formation can be empirically estimated. Because of the heterogeneous nature of the rocks, it is dicult to obtain absolute values of grain density and clay content. Statistical methods, (linear and multiple regressions) are applied here to ®nd a relationship between measured core porosity and one or more log quantities such as sonic, density, neutron porosity and g-ray. Similarly, permeability is calculated by empirical methods (Timur, 1968; Beard & Weyl, 1973; Bateman, 1985; Swanson, 1981; Wendt, Sakurai & Nelson, 1986; Ungerer, Burrus, Doligez, Chenet & Bessis, 1990). The quality of a seal, at a given time, is de®ned by the minimum pressure
required to displace connate water from pores or fractures in the seal, thereby allowing leakage (Downey, 1984). This minimum pressure is called displacement pressure. It is the capillary forces of a seal that act to con®ne hydrocarbon within an accumulation (Melrose & Brandner, 1974; Berg, 1975; Vavra, Kaldi & Sneider, 1992). In Qiongdongnan Basin, four zones can be divided vertically according to petrophysical properties measured from 44 rock samples, such as porosity, permeability, density and displacement pressures, especially. The best seal quality is found in the depth interval 2000±4000 m (Table 2). The following empirical relationships are developed between petrophysical parameters and log parameters and/or seismic velocity. They can then be used to predict petrophysical properties in uncored wells or intervals.
Fig. 3. Diagram of density and sonic travel time dierential value between compacted zone and undercompacted zone: (A) density; (B) sonic travel time.
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
Fig. 4. Relation between displacement pressure of mudstone and sonic travel time.
1. Relation between displacement pressure of mudstone Pm d and sonic travel time, Dt (Fig. 4), ÿ Dt ÿ23:78 log Pm d 123:22 r 0:78 2. Relation between displacement pressure of mudstone Pm d and density, D (Fig. 5), ÿ D 0:404 log Pm d 1:75 r 0:75 3. Relation between displacement pressure of sandstone, Psd , and sonic travel time, Dt (Fig. 6): Dt 86:555 Psd ÿ 0:117 r 0:89
551
Fig. 6. Relation between displacement pressure of sandstone and sonic travel time.
ÿ ÿ D 2:104 exp 0:0456 Psd r 0:89 based on these empirical correlation equations, displacement pressures can be predicted in uncored wells. We recommend using the combination of density and sonic travel time to predict the displacement pressure. However, the method based on density log may be more reliable in under-compacted zones. 5. Prediction of displacement pressure from seismic velocity
4. Relation between displacement pressure of sandstone, Psd , and log density, D (Fig. 7):
Although bore hole and laboratory measurements are more accurate and can provide a physical understanding of cap rock properties, they are only of lim-
Fig. 5. Relation between displacement pressure of mudstone and density.
Fig. 7. Relation between displacement pressure of sandstone and density.
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S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
Fig. 8. Comparison of displacement pressures of Ying-Huang formation predicted using well log and seismic data.
ited value as they represent ``point'' measurements (Downey, 1984). The ability for a cap rock to trap hydrocarbons is determined by the seal capacity at the weakest point of the seal, not the average values. As random samples of point measurements have little relevance in determining the weakest point of the seal, seismic velocity data should be used to determine the three-dimensional distribution of displacement pressure to assess seal capacity. Based on the relationship between sonic travel time and seismic velocity, an empirical equation of displacement pressure and seismic velocity in Qiongdongnan Basin is derived with the controls of the lab data and well logs near seven seismic lines: ÿ Pd exp 5:175 ÿ 1:28 104 =Vn , where Pd is displacement pressure in MPa and Vn the seismic velocity in m/s. The results of displacement pressure predicted by sonic data and seismic data are compared in Figs. 8 and 9. According to the two ®gures, the dierences between calculated values of displacement pressure from the two methods are small. The displacement pressures increase with depth and their variation also agree with the interpreted sedimentary facies and diagenetic processes. Therefore, it is reasonable to evaluate cap rock quality with displacement pressure predicted by seismic velocity. The seismic velocities along 15 seismic lines were converted to displacement pressures based on the established equation. These values are then contoured, taking into consideration the measured points and sedimentary facies (Figs. 10±12). For the second member of Lingshui formation, the displacement pressure map shows several high value areas (Fig. 10), one near
Fig. 9. Comparison of displacement pressure of Meisan formation predicted using well log and seismic data.
the center of Ya13-1, Ya21-1 and Ya14-1 structures, and three others lie in the East Central Depression, West Central Depression and southeastern uplift of Yinggehai Basin. The displacement pressures of Meisan formation are high in the area of Ya13-1 and Ya21-1 structures and become lower outward (Fig. 11). In West Central Depression and southeastern uplift of Yinggehai Basin, there are two subordinate areas of high displacement pressures. There are two high value areas of displacement pressure in Ying-Huang formation (Fig. 12), one is located between Ya13-1 and Ya21-1 structures, and the other is located at the southeastern uplift in Yinggehai Basin. And the values decrease outward, which is consistent with sedimentary facies. Overall, Ya13-1 and Ya21-1 structures have higher displacement pressures in all the cap rocks indicating excellent seal quality, which we believe is a key factor contributing to the abundant gas accumulation in Ya13-1 structure, the largest oshore gas ®eld in China.
6. Conclusion In summary, the following conclusions can be drawn on the basis of above analysis: 1. Formations with good seal quality in Qiongdongnan Basin show high density and low sonic travel time, though some scatters exist in these formations. 2. Four zones are divided vertically according to displacement pressures in Qiongdongnan Basin. Best seal quality is found between the depths of 2000± 4000 m. 3. According to statistical correlation, displacement
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
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Fig. 10. Contour map of predicted displacement pressure of the second member of Lingshui formation in Qiongdongnan Basin.
pressure increases exponentially with density and decreases exponentially with sonic travel time. With the same density or sonic travel time, mudstone has higher displacement pressure than sandstone. 4. Because the consistency between the displacement
pressures predicted by the two methods, it is reasonable to use seismic velocity to calculate displacement pressures, which makes it possible to evaluate the capacity of cap rock in the study area in two or three dimensions.
Fig. 11. Contour map of predicted displacement pressure of Meisan formation in Qiongdongnan Basin.
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Fig. 12. Contour map of predicted displacement pressure of Ying-Huang formation in Qiongdongnan Basin.
5. The Ya13-1 structure has excellent cap rock, which contributes to its abundant gas accumulation.
References Bateman, R. M. (1985). In Open-hole log analysis and formation evaluation (p. 647). Boston: Reidel. Beard, D. C., & Weryl, P. K (1973). In¯uence of texture on porosity and permeability of unconsolidated sands. AAPG Bulletin, 57, 349±369. Berg, R. H. (1975). Capillary pressure in stratigraphic traps. AAPG Bulletin, 59, 939±956. Burnett, J. W., Parry, J. C., & Willmott, S. P. (1970). Evaluation of the Windalia sand, an empirical approach. APEA Journ, 10, 91± 96. Chen, P. H., Chen, Z. Y., & Zhang, Q. M. (1993). Sequence stratigraphy and continental margin development of northwestern shelf of the South China Sea. AAPG Bulletin, 77, 842±862. Vavra, Charles L., Kaldi, John G., & Sneider, Robert M. (1992). Geological applications of capillary pressure: a review. AAPG Bulletin, 76(6), 840±850. Downey, M. W. (1984). Evaluating seals for hydrocarbon accumulations. AAPG Bulletin, 68, 1752±1763. Grunau, H. R. (1981). Worldwide review of seals for major accumulations of natural gas (abs.). AAPG Bulletin, 65, 933. Grunau, H. R. (1987). A worldwide look at the cap rock problem. Journal of Petroleum Geology, 10, 245±266. Han, D. H., Nur, A., & Morgan, D. (1986). Eects of porosity and clay content on wave velocities in sandstones. Geophysics, 51, 2093±2107. Hao, Fang, Sun, Yongchuang, Li, Sitian, & Zhang, Qiming (1995). Overpressure retardation of organic-matter maturation and petroleum generation: a case study from the Yinggehai and
Qiongdongnan Basins, South China Sea. AAPG Bulletin, 79, 551± 562. Hao, Shisheng, Huang, Zhilong, & Yang, Jiaqi (1994). Dynamic equilibrium for natural gas migration and accumulation and its application. Beijing: Petroleum Industry Press. Jian, F. X., Chork, C. Y., Taggart, I. J., McKay, D. M., & Bartlett, R. M. (1994). A genetic approach to the prediction of petrophysical properties. Journal of Petroleum Geology, 17, 71±88. Kowallis, B., Jones, L. E., & Wang, H. F. (1984). Velocity-porosityclay content; systematics of poorly consolidated sandstones. Journal of Geophysical Research, 89, 10355±10364. Leythaeuser, D., Schaefer, R. G., & Yukler, A. (1982). Role of diusion in primary migration of hydrocarbons. AAPG Bulletin, 66, 408±429. Magoon, L. B., & Dow, W. G. (1994). The petroleum system. In L. B. Magoon, & W. G. Dow (Eds.), The petroleum system Ð from source to trap. AAPG Memoir60. Melrose, J. C., & Brandner, C. F. (1974). Role of capillary forces in determining microscopic displacement eciency for oil recovery by water¯ooding. Journal of Canadian Petroleum Technology, 13, 54±62. Nelson, J. S., & Simmons, E. C. (1995). Diusion of methane and ethane through the reservoir cap rock: implications for the timing and duration of catagenesis. AAPG Bulletin, 79, 1064±1074. Swanson, B. F. (1981). A simple correlation between permeabilities and mercury capillary pressure. Journal of Petroleum Technology, 33, 2498±2504. Timur, A. (1968). An investigation of permeability, porosity and residual water saturation relationship for sandstone reservoirs. The Log Analyst, 9, 8±17. Tosaya, C., & Nur, A. (1982). Eects of diagenesis and clays on compressional velocities in rocks. Geophysical Research Letter, 9, 5±8. Ungerer, P., Burrus, J., Doligez, B., Chenet, P. Y., & Bessis, F. (1990). Basin evaluation by integrated two-dimensional modeling of heat transfer, ¯uid ¯ow, hydrocarbon generation, and migration. AAPG Bulletin, 74, 73±96.
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555 Vernik, L., & Nur, A. (1992a). Petrophysical analysis of the Cajon Pass scienti®c well: implications for ¯uid and seismic studies in the continental crust. Journal of Geophysical Research, 97, 5121± 5134. Vernik, L., & Nur, A. (1992b). Petrophysical classi®cation of siliciclastics for lithology and porosity prediction from seismic velocities. AAPG Bulletin, 76, 1295±1309. Watts, N. L. (1987). Theoretical aspects of cap rock and fault seals for single- and two-phase hydrocarbon columns. Marine and Petroleum Geology, 4(4), 273±307.
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Wendt, W. A., Sakurai, S., & Nelson, P. H. (1986). Permeability prediction from well logs using multiple regression. In L. W. Lake, & H. B Carroll, Reservoir characterization (pp. 181±222). New York: Academic Press. Wyllie, M. R. J., Gregory, A. R., & Gardner, L. W. (1956). Elastic wave velocities in heterogeneous and porous media. Geophysics, 21, 41±70. Wyllie, M. R. J., Gregory, A. R., & Gardner, L. W. (1958). An experimental investigation of factors aecting elastic wave velocity in porous media. Geophysics, 23, 459±493.
Geophysical properties of cap rocks in Qiongdongnan Basin, South China Sea Shisheng Hao*, Zhilong Huang, Guangdi Liu, Yuling Zheng University of Petroleum, Beijing, 100083, People's Republic of China Received 20 October 1998; received in revised form 5 January 2000; accepted 18 January 2000
Abstract Based on measured petrophysical properties and statistical methods, relationship between petrophysical properties and well log and seismic parameters are established empirically to evaluate the quality of cap rocks in Qiongdongnan Basin. Good cap rocks show high density and low sonic travel time. Four zones can be divided vertically according to displacement pressures in Qiongdongnan Basin, best seal quality is found in the depth range of 2000±4000 m. The Ya 13-1 structure has excellent cap rock, which contributes to its abundant gas accumulation. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Cap rock is a crucial and sometimes overlooked factor in the evaluation of a potential hydrocarbon accumulation. Cap rock, together with source rock, reservoir rock and overburden rock, consists four essential elements of petroleum system (Magoon & Dow, 1994). Although cap rock directly aects the accumulation and preservation of both oil and gas, it is more important for the gas pool. Gas accumulations is considered to persist through extended periods of geologic time only as dynamic systems reaching an equilibrium between dissipation through the cap rock and continuous replenishment from source rock (Leythaeuser, Schaefer, & Yukler, 1982; Hao, Huang & Yang, 1994; Nelson & Simmons, 1995). When assessing cap rock quality, both microscopic and macroscopic properties of cap rock should be taken into consideration, such as displacement pressure (capillary pressure), lithology, thickness, ductility, lateral continuity (Grunau, 1981, 1987; Watts, 1987; Downey, 1984). Eective cap rocks for hydrocarbon accumulations are typically * Corresponding author. C/o Zhiyong He, ARCO Exploration and Production Tech., 2300 W. Plano Parkway, Plano, TX 75075, USA. E-mail address: [email protected] (S. Hao).
thick, laterally continuous and ductile with high capillary entry pressure (Downey, 1984). The quality assessment of cap rock can be conducted from geophysical data, such as well log and seismic data. The porosity of cap rocks can be calculated from well logs, such as sonic, density and neutron logs (Wyllie, Gregory & Gardner, 1956, 1958; Jian, Chork, Taggart, Mckay & Bartlett, 1994). Permeability may be inferred from the calculated porosity. Statistical techniques are often used to ®nd empirical relationships between porosity and log derived sonic travel time or density (e.g. Burnett, Parry & Willmott, 1970). Experimental studies suggest that relatively simple relations between seismic velocity and such important rock parameters as porosity and clay content do exist (Wyllie et al., 1956, 1958; Tosaya & Nur, 1982; Kowallis, Jones & Wang, 1984; Han, Nur & Morgan, 1986; Vernik & Nur, 1992a,b).
2. Geological setting Qiongdongnan Basin lies in the western part of the South China Sea between 108852 ' E±110.478E and 16847 ' N±19800' N. Covering about 45,000 km2, it trends east to northeast. The formation and evolution
0264-8172/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 8 1 7 2 ( 0 0 ) 0 0 0 0 5 - 2
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S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
Table 1 Generalized stratigraphy and evolution of Qiongdongnan Basin Age Quaternary Pliocene Miocene Miocene Oligocene Oligocene Eocene Paleocene Cretaceous
Formation
Thickness (km)
Depositional environment
Basin development
Ying-Huang Meisan Sanya Lingshui Yacheng
0.5±2.8 0.25±2.7 0.2±1.4 0±1.2 0.1±3.8 0.2±2.8 0±0.2
Littoral Littoral±Bathyal Littoral±Neritic Littoral±Neritic Coastal±Restricted marine Littoral±Restricted marine
Transtension Transtension Post-rift subsidence Post-rift subsidence Rifting Rifting Rifting Rifting Rifting
Basement
of Qiongdongnan Basin is closely related to the opening of the South China Sea (Chen, Chen & Zhang, 1993). It shows a characteristic passive margin development from rifting to regional subsidence (Hao, Sun, Li & Zhang, 1995). During Pliocene and Quaternary, the basin is featured with another episode of tensile rifting. The generalized stratigraphy of Qiongdongnan Basin is shown in Table 1. A dozen structural elements can be divided within the basin, among which Ya 13-1 low uplift and Ya 21-1 low uplift are the most favorable for oil and gas accumulation (Fig. 1). 3. Well-log response of cap rocks There are mainly three cap rocks in Qiongdongnan Basin: Meisan formation, Ying-Huang formation and
the second member of Lingshui formation (Fig. 2). The lithology of Meisan formation is mainly limestone, mudstone and muddy sandstone. Deltaic, coastal plain and shallow-water plateau facies developed in the northern border area, while littoral-shallow marine and bathyl to pelagic facies developed in other areas. The data from ®ve wells in Ya 13-1 structure show that Meisan formation is 292.5±370.5 m thick with good lateral seal continuity. It is a good regional seal cap rock without the destruction by the activities of faults. Its density is much higher than that of other the cap rocks. The maximum density is greater than 2.65 g/cm3, average density is greater than 2.60 g/cm3. The sonic travel time is generally less than 68 ms/ft (4.48 km/s) in normally compacted zone, while in the undercompacted zone, its value is often less than 75 ms/ft
Fig. 1. Structural elements map of Qiongdongnan Basin.
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
(4.06 km/s). Meisan Formation is the best regional cap rock in Qiongdongnan Basin. Ying-Huang formation is mainly a thick, laterally continuous mudstone of shallow-sea and bathypelagic facies. The lower part of Ying-Huang formation shows undercompaction. Due to its ductility and lack of fault penetration, Ying-Huang formation is another regional cap rock. From the log measurements, in the undercompacted zone, the average density is more than 2.47 g/cm3, the maximum is mostly 2.51 g/cm3. In the normally compacted zone, the average density is more than 2.53 g/cm3; the maximum is mostly 2.60 g/cm3. The average sonic travel time is about 92 ms/ft in normally compacted zone and about 94 ms/ft in undercompacted zone. The second member of Lingshui formation is a local cap rock. It is developed in fault sags and is eroded in those structural high areas (e.g., in Ya 13-1-1 well). Its sedimentary facies is mainly restricted shallow marine, littoral and shallow-water plateau facies. The average
549
density is more than 2.55 g/cm3, the average sonic travel time is generally less than 74 ms/ft (4.12 km/s). Good sealing quality of Meisan formation and the second member of Lingshui formation can be de®ned as: average density is more than 2.55 g/cm3, average sonic travel time is less than 75 ms/ft (4.06 km/s). As for Ying-Huang formation, only density can be used to de®ne good seals because of undercompaction. Good seal of Ying-Huang formation is identi®ed with average density of 2.48 g/cm3 (Fig. 3). Generally speaking, formations with good seal ability in Qiongdongnan Basin show high density and low sonic travel time, though some dierence exists among the formations. 4. Relationship between well log parameters and petrophysical properties The petrophysical parameters under consideration
Fig. 2. Cross-section of cap rock in Y13-1 structure.
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Table 2 Petrophysical properties of mudstone in Qiongdongnan Basin Zone
Depth (m)
Porosity (%)
Permeability (10ÿ6mm2)
Density (g/cm3)
Displacement pressure (MPa)
Seal quality
1 2 3 4
< 2000 2000±3000 3000±4000 > 4000
> 15 8±15 5±8 <5
>5 5.0±1.2 1.2±0.6 <3
< 2.35 2.35±2.50 2.50±2.65 > 2.65
<1 7.0±13.0 8.0±10.0 <8
Poor±fair Good Good±fair Fair±poor
include porosity, permeability and displacement pressure. The porosity is normally predicted from well logs, such as sonic, density and neutron logs. The sonic travel-time and density for rock matrix, ¯uids and clay minerals as well as clay content in the formation can be empirically estimated. Because of the heterogeneous nature of the rocks, it is dicult to obtain absolute values of grain density and clay content. Statistical methods, (linear and multiple regressions) are applied here to ®nd a relationship between measured core porosity and one or more log quantities such as sonic, density, neutron porosity and g-ray. Similarly, permeability is calculated by empirical methods (Timur, 1968; Beard & Weyl, 1973; Bateman, 1985; Swanson, 1981; Wendt, Sakurai & Nelson, 1986; Ungerer, Burrus, Doligez, Chenet & Bessis, 1990). The quality of a seal, at a given time, is de®ned by the minimum pressure
required to displace connate water from pores or fractures in the seal, thereby allowing leakage (Downey, 1984). This minimum pressure is called displacement pressure. It is the capillary forces of a seal that act to con®ne hydrocarbon within an accumulation (Melrose & Brandner, 1974; Berg, 1975; Vavra, Kaldi & Sneider, 1992). In Qiongdongnan Basin, four zones can be divided vertically according to petrophysical properties measured from 44 rock samples, such as porosity, permeability, density and displacement pressures, especially. The best seal quality is found in the depth interval 2000±4000 m (Table 2). The following empirical relationships are developed between petrophysical parameters and log parameters and/or seismic velocity. They can then be used to predict petrophysical properties in uncored wells or intervals.
Fig. 3. Diagram of density and sonic travel time dierential value between compacted zone and undercompacted zone: (A) density; (B) sonic travel time.
S. Hao et al. / Marine and Petroleum Geology 17 (2000) 547±555
Fig. 4. Relation between displacement pressure of mudstone and sonic travel time.
1. Relation between displacement pressure of mudstone Pm d and sonic travel time, Dt (Fig. 4), ÿ Dt ÿ23:78 log Pm d 123:22 r 0:78 2. Relation between displacement pressure of mudstone Pm d and density, D (Fig. 5), ÿ D 0:404 log Pm d 1:75 r 0:75 3. Relation between displacement pressure of sandstone, Psd , and sonic travel time, Dt (Fig. 6): Dt 86:555 Psd ÿ 0:117 r 0:89
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Fig. 6. Relation between displacement pressure of sandstone and sonic travel time.
ÿ ÿ D 2:104 exp 0:0456 Psd r 0:89 based on these empirical correlation equations, displacement pressures can be predicted in uncored wells. We recommend using the combination of density and sonic travel time to predict the displacement pressure. However, the method based on density log may be more reliable in under-compacted zones. 5. Prediction of displacement pressure from seismic velocity
4. Relation between displacement pressure of sandstone, Psd , and log density, D (Fig. 7):
Although bore hole and laboratory measurements are more accurate and can provide a physical understanding of cap rock properties, they are only of lim-
Fig. 5. Relation between displacement pressure of mudstone and density.
Fig. 7. Relation between displacement pressure of sandstone and density.
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Fig. 8. Comparison of displacement pressures of Ying-Huang formation predicted using well log and seismic data.
ited value as they represent ``point'' measurements (Downey, 1984). The ability for a cap rock to trap hydrocarbons is determined by the seal capacity at the weakest point of the seal, not the average values. As random samples of point measurements have little relevance in determining the weakest point of the seal, seismic velocity data should be used to determine the three-dimensional distribution of displacement pressure to assess seal capacity. Based on the relationship between sonic travel time and seismic velocity, an empirical equation of displacement pressure and seismic velocity in Qiongdongnan Basin is derived with the controls of the lab data and well logs near seven seismic lines: ÿ Pd exp 5:175 ÿ 1:28 104 =Vn , where Pd is displacement pressure in MPa and Vn the seismic velocity in m/s. The results of displacement pressure predicted by sonic data and seismic data are compared in Figs. 8 and 9. According to the two ®gures, the dierences between calculated values of displacement pressure from the two methods are small. The displacement pressures increase with depth and their variation also agree with the interpreted sedimentary facies and diagenetic processes. Therefore, it is reasonable to evaluate cap rock quality with displacement pressure predicted by seismic velocity. The seismic velocities along 15 seismic lines were converted to displacement pressures based on the established equation. These values are then contoured, taking into consideration the measured points and sedimentary facies (Figs. 10±12). For the second member of Lingshui formation, the displacement pressure map shows several high value areas (Fig. 10), one near
Fig. 9. Comparison of displacement pressure of Meisan formation predicted using well log and seismic data.
the center of Ya13-1, Ya21-1 and Ya14-1 structures, and three others lie in the East Central Depression, West Central Depression and southeastern uplift of Yinggehai Basin. The displacement pressures of Meisan formation are high in the area of Ya13-1 and Ya21-1 structures and become lower outward (Fig. 11). In West Central Depression and southeastern uplift of Yinggehai Basin, there are two subordinate areas of high displacement pressures. There are two high value areas of displacement pressure in Ying-Huang formation (Fig. 12), one is located between Ya13-1 and Ya21-1 structures, and the other is located at the southeastern uplift in Yinggehai Basin. And the values decrease outward, which is consistent with sedimentary facies. Overall, Ya13-1 and Ya21-1 structures have higher displacement pressures in all the cap rocks indicating excellent seal quality, which we believe is a key factor contributing to the abundant gas accumulation in Ya13-1 structure, the largest oshore gas ®eld in China.
6. Conclusion In summary, the following conclusions can be drawn on the basis of above analysis: 1. Formations with good seal quality in Qiongdongnan Basin show high density and low sonic travel time, though some scatters exist in these formations. 2. Four zones are divided vertically according to displacement pressures in Qiongdongnan Basin. Best seal quality is found between the depths of 2000± 4000 m. 3. According to statistical correlation, displacement
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Fig. 10. Contour map of predicted displacement pressure of the second member of Lingshui formation in Qiongdongnan Basin.
pressure increases exponentially with density and decreases exponentially with sonic travel time. With the same density or sonic travel time, mudstone has higher displacement pressure than sandstone. 4. Because the consistency between the displacement
pressures predicted by the two methods, it is reasonable to use seismic velocity to calculate displacement pressures, which makes it possible to evaluate the capacity of cap rock in the study area in two or three dimensions.
Fig. 11. Contour map of predicted displacement pressure of Meisan formation in Qiongdongnan Basin.
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Fig. 12. Contour map of predicted displacement pressure of Ying-Huang formation in Qiongdongnan Basin.
5. The Ya13-1 structure has excellent cap rock, which contributes to its abundant gas accumulation.
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