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Seismic prediction of sandstone diagenetic facies: Applied to Cretaceous Qingshankou Formation in Qijia Depression, Songliao Basin, East China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 3, June 2013 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(3): 287–295.
RESEARCH PAPER
Seismic prediction of sandstone diagenetic facies: Applied to Cretaceous Qingshankou Formation in Qijia Depression, Songliao Basin, East China ZENG Hongliu1,*, ZHU Xiaomin2, ZHU Rukai3, ZHANG Qingshi4 1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Texas 78712, USA; 2. China University of Petroleum (Beijing), Beijing 102249, China; 3. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China; 4. Research Institute of PetroChina Daqing Oilfield Company, Daqing 163000, China
Abstract: Seismic diagenetic facies is an important control on reservoir quality. This study investigated the feasibility of predicting sandstone diagenetic facies using conventional 3D seismic data by seismic sedimentology and calibrated by laboratory core-analysis data in the Qijia area of the Songliao Basin. Three core issues are related to seismic characterization of diagenetic facies, including how to correlate stratigraphic and diagenetic units from seismic data, how to evaluate the relationship between core-based diagenetic facies and seismic attributes, and how to find an effective way of mapping seismic diagenetic facies. Well- and seismic-based, high-resolution sequence analysis and seismic stratal slicing provide reservoir-scale (20 m) seismic representations of diagenetic units. Core-based analyses of sandstone diagenetic processes and diagenetic sequences reveal the kind of diagenesis that most influences reservoir quality. An investigation of reservoir parameters and acoustic rock properties further reveals the link between diagenetic facies and impedance, leading to a recognition of calcite cementation as the process that can be detected by seismic data. A seismic-based lithology cube (e.g., 90°-phased seismic volume) provides the amplitude (impedance) signal for detection of diagenetic facies. Stratal slices made from the seismic-based lithology cube are then used to interpret depositional facies and systems. Eventually a seismic diagenetic-facies map is generated through analysis of the relationships between depositional facies, impedance, and diagenetic facies. A case study of clay- and calcite-cemented sandstone in the Qijia area shows that although still in its infancy, seismic detection of sandstone diagenetic facies using conventional seismic data is definitely feasible. Key words: seismic diagenetic facies, seismic detection of diagenetic facies, seismic sedimentology, diagenesis, sandstone
Introduction Reservoir diagenetic facies have an important control on reservoir quality. For years both domestic and foreign research on diagenetic facies has been confined primarily to core and microscopic thin-section analyses, seismic data seldom being adopted for description or prediction of diagenetic facies. Theoretically, seismic data can predict diagenetic facies as long as acoustic characteristics of the rock mass in the diagenetic unit are significantly affected by a specific diagenesis. For instance, in clastic sedimentary basins, because sandstone reservoirs are commonly modified by carbonate (calcite, dolomite, etc.) cementation, sandstone velocity and density could vary as the content of carbonate cement varies in pores. If the velocity and density of carbonate cement are very different from those of other cements and the carbonate cement makes up a large proportion, the seismic
signal would be significant. For example, calcite (predominantly matrix) cement, which has a P-wave velocity (vp) of 6.53 km/s, a bulk density (ρb) of 2.71 g/cm3, and an acoustic impedance (AI) of 1.77×106 g/(cm2·s), is very different in impedance (up to 44%) from that of argillaceous cements, which consist of clay diagenetic minerals, including matrix and micropores (vp = 4.00 km/s, ρb= 2.50 g/cm3, AI = 1×106 g/(cm2·s). Seismic detection should therefore be relatively easy. The main challenge for diagenetic-facies description and prediction using seismic data lies in establishing the link between core diagenetic facies and detectable seismic attributes, as well as dividing and identifying lithofacies units using seismic data. Dutta et al. [1] first attempted to link the content of calcareous cement in a sandstone to acoustic impedance of the sandstone and discussed the effects of cement-mineral composition,
Received date: 17 Aug. 2012; Revised date: 29 Mar. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by PetroChina International Cooperation Project (2009B-0102-01). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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cement content, shale content, grain size, stratigraphic position, and sequence-stratigraphic surfaces on range of acoustic impedance. However, they failed to link results of well and model analysis to those of actual seismic data interpretation or to put their method into practice [1]. On the basis of numerous core- and laboratory-analysis data of lacustrine sandstones in the Cretaceous Qingshankou Formation in the Qijia Depression, Songliao Basin, as well as high-quality 3D seismic data, we tried to determine diagenetic facies and diagenetic sequence. We used core analysis, linking the main diagenetic facies to acoustic parameters by means of wireline logs, followed by subdividing seismic sedimentological and diagenetic units, and eventually predicting diagenetic facies using seismic data. Because of the limitation of seismic resolution, conventional seismic-stratigraphic mapping techniques cannot meet the precision required for seismic diagenetic-facies analysis. At present, seismic sedimentology is the only interpretation tool available for extracting seismic-stratigraphic images at relatively high resolution (single-event imaging to detect 10-m thin beds) from conventionally stacked and migrated seismic data. Seismic sedimentology comprises seismic geomorphology and seismic lithology. Using the seismic-lithologic technique, a 3D seismic data cube can be converted to a log-lithology cube, in which the lithologic log (e.g., GR and SP) correlates to the well-site seismic trace with little error, so that the best fit between wireline-log and seismic data can be ensured at the reservoir scale. Using the seismic-geomorphologic technique, seismic data can be further converted to depositional facies, with lithofacies being identified on stratal slices. Therefore, establishment of a seismic-lithology cube and preparation of stratal slices are basic conditions for seismic imaging of sandstone diagenetic facies. For details please refer to papers by Zeng and others [2−5].
1 High-resolution sequence-stratigraphic framework and diagenetic units Songliao Basin is a large lacustrine depression that devel-
Fig. 1
oped during deposition of the Cretaceous Quantou to Nenjiang Formations [6, 7]. The Qijia Depression (Figure 1) is next to the west slope of the Songliao Basin in the north and west and is adjacent to the Daqing Anticline in the east. The Qingshankou Formation, as the target interval of this study, is 400–500 m thick. The sequence-stratigraphic framework was established in a previous study of seismic sedimentology [8]. As per the principles of sequence stratigraphy, the well-seismic stratigraphic framework was interpreted mainly on the basis of maximum flooding surfaces. Four continuous seismic reflections were picked in the 3D seismic cube (Figure 2), which correspond mostly to maximum flooding surfaces and which have chronostratigraphic meaning. Using wireline-log-based sequence-stratigraphic analysis and seismic facies characteristics, we correlated three third-order sequences and six fourth-order sequences in the Qingshankou Formation. Fifth-order sequences were recognized in accordance with GR (natural gamma ray) and SP (spontaneous potential) logs and the seismic event trend. However, chronostratigraphic significance of fifth-order sequence boundaries cannot be guaranteed because of the limitation of seismic resolution. The interpreted fifth-order sequence boundaries are of symbolic meaning only. A more effective seismic expression of a fifth-order sequence is the stratal slice. The dominant frequency of 3D seismic data in the Qingshankou Formation is around 50 Hz, which corresponds to a tuning thickness (limit of seismic resolution) of 10 ms (or 20 m for sandstones of 4 000-m/s velocity), setting the minimum mapping unit for seismic-stratigraphic study in this area. Because the average thickness of a fifth-order sequence ranges from 40 to 50 m, thickness of a minimum mapping unit is roughly equivalent to the thickness of a systems tract (LST/TST, or HST). As a result, seismic events are well correlated to systems tracts of fifth-order sequences (G11–G42). On average, each fifth-order sequence includes two seismic events. Lower red and upper black events correspond roughly to LST/TST and HST, respectively (Figure 2).
Location of (a) Songliao Basin and (b) 3D seismic survey in the Qijia area [8]
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Fig. 2
Well-seismic section A-A′ showing sequence correlation in the Qingshankou Formation in the Qijia area, Songliao Basin [8]
In outcrops, Taylor et al. [9] observed that distribution of carbonate cements in sandstone is related to depositional environment and formation architecture. In the Songliao Basin, lacustrine sandstones are related to all types of Cretaceous sandstone diagenetic facies. Because lacustrine depositional environments controlled the distribution of lacustrine sand bodies, some factors affecting sandstone diagenesis, such as sandy grain composition, size and sorting, clay content, pore-water salinity, etc., are all related to depositional environments. Therefore, the connection between diagenetic facies and depositional facies units should be a close one. In this article, we assume that because the main diagenetic unit equals the depositional facies unit, seismic recognition of diagenetic-facies units will be consistent with that of seismic depositional facies. On that basis, in our study of diagenetic facies using seismic data, we infer that 20 m is the minimum mapping unit for both stratigraphic and diagenetic-facies correlations. The actual thickness of diagenetic facies, of course, is related to the detection limit. A reasonable estimation is that we should be able to detect 3- to 4-m-thick diagenetic-facies units under certain conditions.
2 Sandstone diagenetic facies According to analysis of numerous core, thin sections, and scanning electron microscope data, reservoirs in the Qingshankou Formation in the Qijia Depression are composed mainly of calcite-cemented, fine feldspar sandstone (Figures 3 and 4). A small amount of ostracod limestone is observed. The feldspar, quartz, and rock fragments in the sandstone are 36%
to 46%, 36% to 53%, and 12% to 24%, respectively. Rock fragments are predominately volcanic debris. Sand grains are approximately 0.02 to 0.16 mm in diameter, medium to good sorting, and mostly subrounded, revealing petrologic characteristics of high compositional maturity and medium textural maturity. A fair number of ostracod fragments and carbonate cements are present (content ranges from 0 to 55%, average 11.2%) in grains. Because of cementation and compaction, sandstones in the Qingshankou Formation are tight, with grain particles in spot-line contact (Figures 3a and 4a). Secondary enlargement of quartz grains was observed. Pores are a combination of original intergranular and intergranular–intragranular pores. Carbonate cements are ferriferous calcite and dolomite (Figure 3a). Clay minerals are mainly mixed illite-smectite, with illite and chlorite cements (Figure 4b). All evidence indicates that the formation is in the early stages of the middle diagenetic phase. Sandstone reservoirs went through a series of diagenetic processes, including compaction, calcite cementation, and dissolution of feldspar grains and calcite cement. A diagenetic sequence is inferred to span from early compaction, to calcite cementation, to feldspar dissolution and generation of metasomatic calcite of quartz particles, followed by occasional pressure solution, secondary enlargement of quartz grains, and chlorite growth. As the most important facet of diagenesis affecting reservoir quality, calcite content directly controls porosity and permeability. Sandstones with a high calcite-cement content (>10%) have low porosity because most intergranular pores
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Fig. 3 Photo of calcite-cemented, fine-grained lithic feldspar sandstone, 2145.8 m, Well Gu-933
are blocked (Figure 3), and hardly any intergranular pores can be seen under the scanning electron microscope (Figure 3b). Sandstones with low calcite-cement content (<10%) (Figure 4) are of higher porosity because intergranular pores are better preserved; therefore, partially preserved intergranular pores with few fillings of needlelike illite and foliated chlorite can be observed under the scanning electron microscope (Figure 4b). Sandstones with high calcite cement belong to calcareous sandstone from the perspective of diagenetic facies, and “calcareous sandstone” can therefore be called calcite-cemented sandstone facies; similarly, sandstones with low calcite content can be called clay-cemented sandstone facies. Qualitative seismic identification of these two types of diagenetic facies will be discussed next.
3 Basis for seismic identification of calcite- and clay-cemented sandstone facies Analysis of lab and microscopic data from log-cored sections in the studied formation reveals relationships between
Fig. 4 Photo of clay-cemented, fine-grained lithic feldspar sandstone, 1658.5 m, Well Jin-56
sandstone diagenetic facies and acoustic properties necessary for seismic mapping of diagenetic units. For instance, a linear relationship exists between calcareous-cement (calcite) content measured from thin sections and porosity measured from core plugs (effective porosity) in Well Jin-44 (Figure 5): porosity drops from 16% to less than 4% as the calcite content in pore cements increases from 0 to 40%. Therefore, the proportion of calcite in pore cements has a direct impact on sandstone reservoir quality; i.e., higher calcite content corresponds to poorer reservoir quality. Furthermore, calcite content in pore cement is also related to clay content (Figures 5 and 6). Clay-cement content (illite, mixed illite-smectite, and chlorite, etc.) ranges from 2% to 20%, corresponding to a calcite content of from 45 to 0%, respectively. Generally speaking, moderate increase of clay content in sandstone cements would lead to a decrease of calcite content (Figure 6) or improvement of reservoir quality (porosity increase, Figure 5). In other words, calcite cementation would be weak for some facies units (e.g., channel-filled sand bodies), where a small amount of clay exists among sand particles; depositional facies units with low clay content, such as
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Fig. 5 Relationship between calcareous-cement (calcite) content measured in thin sections and porosity measured in core
Fig. 6 Relationship between calcite content and clay content in pore cements measured in thin sections
alongshore sand bars and delta-front sand sheets, would lead to strong calcite cementation at later stages, which is unfavorable for pore preservation after diagenesis of the sandstone. Further analysis indicates a linear, positive correlation between calcite content in sandstone and sandstone impedance in the Qingshankou Formation (Figure 7). Calcite content is linearly related to bulk density, and a similar relationship exists between calcite content and impedance, although with error increasing because sonic logs are used to replace lab measurements of sandstone velocity. Given this relationship, 10% of calcite content change would lead to a change of 0.02 to 0.03 in the reflection coefficient, which is sufficient to generate a detectable seismic signal. By adding the effects of lithology, we can evaluate the feasibility of predicting diagenetic facies using acoustic impedance. Figure 8 shows the relationship between impedance, effective porosity, and clay content of different lithologies calculated using wireline logs. Among the four types of lithologies, clay content is useful in distinguishing between mudstone and other rocks. The difference between reservoir-quality, clay-cemented sandstone
Fig. 7 Relationship between calcite content, bulk density, and impedance of sandstones
Fig. 8 Distinguishing between four major types of lithology and diagenetic facies using impedance, effective porosity, and clay content
and calcite-cemented sandstone and limestone is effective porosity; only those sandstones with higher effective porosity (>5%) and lower impedance (<1,1000 m/s g/cc) can be counted as reservoirs. A calcite content of 50% (13,000 m/s g/cc) is used to differentiate between calcite-cemented sandstones and limestones. Distinguishing between semideep lake mudstone and calcite-cemented sandstone is easy if impedance is used; clay-cemented sandstone generally has lower impedance than calcite-cemented sandstone; however, shallow-lake mudstone and clay-cemented sandstone are characterized by similar impedance ranges, which can cause confusion in interpretation. Other evidence (e.g., different depositional characteristics interpreted on stratal slices) should therefore be considered.
4 Relative-impedance meaning of 90°-phase seismic amplitude Conversion of a 3D seismic data cube to a lithology cube is recommended before high-resolution seismic diagenetic analysis is performed. The physical attributes in this lithologic volume can be converted to high-resolution depositional facies maps of lithologic meaning and diagenetic-facies maps on stratal slices. Under current technical conditions, the most
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economic and effective way of lithologically calibrating conventional seismic data is 90° phasing [10−11]. A 90°-phase wavelet shifts the maximum point of the main lobe of the seismic response to the midpoint of a thin bed. As a result, seismic response will correspond to the midpoint rather than the top or base of the thin bed, and the seismic event will correlate to a reservoir unit defined geologically, such as a sandstone bed. In other words, seismic polarity is able to correspond to lithology within a wavelength. Although not very precise for thin beds of less than one-quarter wavelength, the top and base of the bed can be determined at zero crossing points. While these changes are being applied to actual data, a one-to-one correlation between seismic events and thin lithologic units could be established, making interpretation of depositional lithology—e.g., distinguishing sandstone from mudstone—much easier [4]. These advantages for interpretation, however, do not exist in zero-phased and other-phased data. Furthermore, seismic inversion, seismic-attribute analysis, and time-frequency analysis can also convert a seismic volume into a lithologic volume. In cases of multiple types of lithology and diagenetic variations, lithologic calibration for seismic attributes would be complex. An analysis of spontaneous potential and sonic logs in Well Jin-44 (Figure 9) reveals a complex lithology–velocity relationship in the Qingshankou Formation. For example, differences in velocity between semideep lake mudstones in the Qing-1 member and shallow-lake mudstones in the Qing-2 and -3 are significant; sandstones include low-velocity, clay-cemented sandstones and high-velocity, calcite-cemented sandstones; the velocity of limestone is close to but higher than that of calcite-cemented sandstones. Besides, lithology–velocity (impedance) relationships also vary with depth and are affected significantly by the stratigraphic framework. In this situation, lithologic and diagenetic identification using 90°-phase seismic amplitude would be nonunique, but at least a close relationship can be observed between the 90°-phase seismic data cube and relative impedance. For instance, in the Well Jin-44 seismic section (Figure 10), although SP and GR curves do not indicate a good correlation between distribution of mudstones and sandstones and seismic events, the correlation between the log-velocity curve averaged by seismic event and seismic polarity is good; i.e., a high-impedance event (positive polarity in red) indicates a high-log-velocity block, whereas a low-impedance event (negative polarity in black) indicates a low-log-velocity block. This relationship lays the foundation for distinguishing calcite- and clay-cemented sandstone facies by relative impedance. In order to decrease the nonuniqueness of seismic prediction of diagenetic facies, stratal slices rather than seismic sections should be adopted as the interpretation platform [12]. An agreement between average velocity and seismic polarity (Figure 10) indicates that seismic polarity and average amplitude mainly represent the cyclicity of depositional sequences (lake transgression and regression or migration of systems
Fig. 9 Log interpretation of major lithologies and sandstone diagenetic facies in Well Jin-44
tracts), rather than internal lithologic and lithofacies variation of depositional systems. Eliminating this type of system error by using stratal slices is convenient, i.e. by neglecting absolute changes of amplitude in the vertical section and observing relative amplitude variations in plane (slice) view. Besides, on stratal slices seismic geomorphology revealed by amplitude patterns can be taken full advantage of, so that lithology and
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Fig. 11 Black-and-white amplitude stratal slice representing lowstand systems tract of a lower G31 high-frequency sequence
Fig. 10
90°-phase seismic section through Well Jin-44[8]
diagenetic facies based on differences in facies distribution, might be interpreted, even when those facies are characterized by similar amplitudes (impedances).
5 Seismic diagenetic facies in the Qingshankou Formation Seismic identification of diagenetic facies should be based on seismic depositional-facies interpretation because (1) if a major diagenetic-facies unit is roughly consistent with a depositional-facies unit, then a connection would exist between distributions of diagenetic and depositional facies and depositional-facies prediction would therefore benefit diagenetic-facies prediction, and (2) distinguishing between some diagenetic facies and other diagenetic facies or lithofacies with similar impedances would be difficult, and diageneticfacies identification using knowledge of depositional facies would reduce ambiguity considerably. Figure 11 shows a seismic stratal slice in the lower part of G31, a high-frequency sequence in the Qingshankou Formation. In black and white, many channel-like, low-amplitude (negative) anomalies can be easily recognized. However, identification of seismic-geomorphologic units in other shapes is difficult. Adoption of a spectral color scheme (Figure 12) improves identification of various types of nonchannel seismic-geomorphologic units, including narrow, irregular zones of relatively low amplitude along the channel margin; leaflike (lobe), relatively low amplitude bodies in front of channels;
Fig. 12 Spectrum amplitude stratal slice representing lowstand systems tracts of a lower G31 high-frequency sequence
relatively low- and high-amplitude zones that are roughly perpendicular to channels; and sheet and irregular, relatively low- and high-amplitude zones distributed in front of channelized areas. The lithology of each seismic-geomorphologic unit can be interpreted on the basis of the relationship between relative amplitude and relative impedance (Figure 10). Fluvial channels are most likely to be filled by sandstones; channel margins, leaflike bodies, and zonal bodies are probably sandstones, siltstones, or mudstones; and sheetlike bodies are likely to be mudstones (low amplitude and low impedance) or ostracod limestone (high amplitude and high impedance). Collectively this seismic-sedimentologic mapping unit in the study area can be interpreted as a lacustrine shallow-water delta (Figure 13). Large amounts of sandy and muddy sediments from the northeast (Daqing Anticline) were carried into the shallow-water lacustrine basin by distributary channels. Sandy sediments were deposited mainly in the channels. Some mouth-bar sediments accumulated to form leaflike sand bodies, and the rest were modified by wave and alongshore
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Fig. 13 Lowstand, shallow-water delta depositional facies map of lower G31 interpreted from Figure 12
currents to form delta-front sand sheets. The deltaic system underwent multiple phases of progradation, migrating basinward with multiple shorelines and forming delta-front sand sheets predominantly in band shapes orthogonal to distributary channels. Seismic identification of diagenetic facies is based mainly on the relationship between calcite-cement content and impedance in sandstone (Figure 7). Clay-cemented sandstone facies (calcite content <10%) with low impedance (<1.15×106 g/(cm2⋅s)) is shown as low amplitude on a stratal slice; conversely, calcite-cemented sandstone facies (calcite content >10%) with high impedance (>1.15×106 g/(cm2⋅s)) is shown as high amplitude on a stratal slice. Furthermore, seismic identification of diagenetic facies would also be inferred by the existence of other lithologies. For example, high-impedance limestone is similar to calcite-cemented sandstone in high-amplitude expression. However, both high-impedance limestone and calcite-cemented sandstone belong to nonreservoir facies and thus are not necessary to distinguish between in practice. Shallow-lake mudstone and clay-cemented sandstone facies are characterized by similar impedance ranges and cannot be distinguished by amplitude. An interpretation can be made only by clear depositional facies identiTable 1
Fig. 14 Seismic diagenetic-facies map of lower G31 lowstand systems tracts
Seismic-sedimentologic and seismic diagenetic-facies characteristics for lower G31 lowstand systems tracts Relative
Relative
amplitude
impedance
Channel
Low
Channel margin
Geomorphology
fication on the stratal slice. From these analyses, a seismic depositional-facies map (Figure 13) could be converted to a diagenetic-facies map (Figure 14): clay-cemented sandstone facies were developed in distributary channels. Clay-cemented sandstone facies (could be mixed with mudstone in the outer periphery) were formed on distributary-channel margins, delta-front lobes, and low-amplitude/low- impedance delta-front sand sheets, whereas high-amplitude/high-impedance delta-front sand sheets correspond to calcite-cemented sandstone facies. Finally, sheet facies are recognized on the basis of the location of facies mapped so far and then subdivided into shallow-lake mudstone zones and ostracod-bank limestone zones on the basis of the magnitude of relative amplitude and impedance. Wireline logs from wells can be utilized for calibrating diagenetic- facies maps. For example, calcite-cemented sandstone facies in delta-front sand sheets correspond to high-log-velocity zones, whereas log velocity decreases considerably in clay-cemented sandstone facies (Figure 14). Table 1 summarizes seismic-geomorphologic features, amplitude and impedance characteristics, lithologic and depositional facies interpretation results, and corresponding seismic diagenetic facies for various facies in the lower G31 high-frequency sequence.
Lithology
Depositional facies
Seismic diagenetic facies
Low
Sandstone
Distributary channel
Clay-cemented sandstone facies
Low
Low
Sandstone/siltstone/mudstone
Distributary-channel margin
Clay-cemented sandstone facies
Leaflike
Low
Low
Sandstone/siltstone/mudstone
Delta-front lobe
Clay-cemented sandstone facies
Zonal
Low
Low
Sandstone/siltstone/mudstone
Delta-front sand sheet
Clay-cemented sandstone facies
Zonal
High
High
Sandstone/siltstone
Delta-front sand sheet
Calcite-cemented sandstone facies
Sheet
Low
Low
Mudstone
Shallow lake
Sheet
High
High
Limestone
Ostracod bank
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Seismic diagenetic-facies analysis shows that distributarychannel, clay-cemented sandstones are the most favorable hydrocarbon reservoirs in the Qingshankou Formation in the Qijia Depression. Channel sands failed to receive sufficient washing at the time of deposition, leaving a certain amount of clay among grain particles, which later would play an important role in hindering calcite cementation and preserving primary intergranular-pore space. Some channel sand bodies, of course, went through calcite cementation anyway and probably lost the amplitude characteristics of channel forms (cf. interpreted channels in depositional facies map in Figure 13 and identified channels in the stratal slice in Figure 12). Clay-cemented sandstones and siltstones of distributary-channel margins, delta-front lobes, and low-amplitude/ low-impedance delta-front sand sheets also possess a certain exploration potential. The main target for exploration of lithologic stratigraphic traps in the Qijia Depression in the future will be clay-cemented sandstones of distributary-channel facies. Seismic identification of sandstone diagenetic facies is just the beginning. Numerous problems are yet to be resolved. For example, to recognize different diagenetic facies with similar acoustic impedances, seismic attributes that are more sensitive to diagenetic facies should apply to different formations; a full understanding of relationships between stratigraphic units and diagenetic units and between depositional facies and diagenetic facies is still lacking; and determination of seismic and diagenetic facies is still qualitative and somewhat arbitrary. Research should be expanded beyond this single case study.
by 90°-phasing processing, which was then used as a seismic signal for identifying diagenetic facies. A lacustrine, shallow-water delta system was identified on stratal slices created by the seismic-lithologic cube. Relationships between depositional facies, impedance, and diagenetic facies were analyzed on a depositional map, leading to a high-resolution diagenetic facies map, which will one day provide references for finding lithologic stratigraphic traps in the Qijia Depression.
Acknowledgments We want to thank Sun Yu, Wang Rui, Zhou Chuanmin, and Bai Bin, etc. for participating in this study. Publication authorized by the Director, Bureau of Economic Geology.
References
6 Conclusions Study of seismic diagenetic facies involves three key steps: (1) dividing stratigraphic and diagenetic units using seismic data, (2) establishing a link between core-based diagenetic facies and seismic attributes, and (3) searching for an effective seismic diagenetic-facies mapping method. Reservoir-scale (20 m, detection limit 3 to 4 m) depositional sequence and diagenetic-facies unit framework were established using welland seismic-based, high-resolution sequence correlation and seismic stratal slicing of the Qingshankou Formation in the Qijia Depression, Songliao Basin. Core-based sandstone diagenetic-facies analysis revealed a series of diagenetic phenomena, such as compaction, calcite cementation, dissolution of feldspar particles and calcite cement, and demonstrated that calcite cementation is the most important diagenetic activity affecting reservoir quality. Clay-cemented sandstones with low calcite content formed the primary reservoirs. Reservoir and acoustic parameters, such as porosity, clay content, calcite content, and acoustic impedance, were analyzed to explain the relationship between calcite content, porosity, and impedance, confirming that clay- and calcite-cemented sandstone facies are appropriate diagenetic facies for seismic detection. A seismic data cube was converted to a seismic-lithologic cube
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[1]
Dutta T, Mukerji T, Mavko G. How does carbonate cementation in sandstones affect seismic response? Tulsa: SEG Las Vegas 2008 Annual Meeting, 2008.
[2]
Zeng H, Backus M M, Barrow K T, et al. Stratal slicing: Part I: Realistic 3-D seismic model. Geophysics, 1998, 63(2): 502–513.
[3]
Zeng H, Henry S C, Riola J P. Stratal slicing: Part II: Real 3-D seismic data. Geophysics, 1998, 63(2): 514–522.
[4]
Zeng H, Hentz T F. High-frequency sequence stratigraphy from seismic sedimentology: Applied to Miocene, Vermilion Block 50, Tiger Shoal area, offshore Louisiana. AAPG Bulletin, 2004, 88(2): 153–174.
[5]
Zeng H. Seismic sedimentology in China: A review. Acta
[6]
Yang W L. Daqing Oilfield, People’s Republic of China, a
Sedimentologica Sinica, 2011, 29(3): 61–70. giant field with non-marine origin. AAPG Bulletin, 1985, 69: 1101–1111. [7]
Feng Z Q, Jia C Z, Xie X N, et al. Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao Basin, northeast China. Basin Research, 2010, 22: 79–95.
[8]
Zeng H, Zhu X, Zhu R, et al. Guidelines for seismic sedimentologic study in non-marine postrift basins. Petroleum Exploration and Development, 2012, 39(3): 275–284.
[9]
Taylor K G, Gawthorpe R L, Curtis C D, et al. Carbonate cementation in a sequence-stratigraphic framework: Upper Cretaceous sandstones, Book Cliffs, Utah-Colorado. Journal of Sedimentary Research, 2000, 70(2): 360–372.
[10] Zeng H, Backus M M. Interpretive advantages of 90°-phase wavelets: Part 1: Modeling. Geophysics, 2005, 70(3): 7–15. [11] Zeng H, Backus M M. Interpretive advantages of 90°-phase wavelets: Part 2: Seismic applications: Geophysics, 2005, 70(3): 17–24. [12] Zhang X, Lin C, Zhang T, et al. Seismic sedimentologic research in shallow sea areas, Dagang. Petroleum Exploration and Development, 2011, 38(1): 40–46.
RESEARCH PAPER
Seismic prediction of sandstone diagenetic facies: Applied to Cretaceous Qingshankou Formation in Qijia Depression, Songliao Basin, East China ZENG Hongliu1,*, ZHU Xiaomin2, ZHU Rukai3, ZHANG Qingshi4 1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Texas 78712, USA; 2. China University of Petroleum (Beijing), Beijing 102249, China; 3. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China; 4. Research Institute of PetroChina Daqing Oilfield Company, Daqing 163000, China
Abstract: Seismic diagenetic facies is an important control on reservoir quality. This study investigated the feasibility of predicting sandstone diagenetic facies using conventional 3D seismic data by seismic sedimentology and calibrated by laboratory core-analysis data in the Qijia area of the Songliao Basin. Three core issues are related to seismic characterization of diagenetic facies, including how to correlate stratigraphic and diagenetic units from seismic data, how to evaluate the relationship between core-based diagenetic facies and seismic attributes, and how to find an effective way of mapping seismic diagenetic facies. Well- and seismic-based, high-resolution sequence analysis and seismic stratal slicing provide reservoir-scale (20 m) seismic representations of diagenetic units. Core-based analyses of sandstone diagenetic processes and diagenetic sequences reveal the kind of diagenesis that most influences reservoir quality. An investigation of reservoir parameters and acoustic rock properties further reveals the link between diagenetic facies and impedance, leading to a recognition of calcite cementation as the process that can be detected by seismic data. A seismic-based lithology cube (e.g., 90°-phased seismic volume) provides the amplitude (impedance) signal for detection of diagenetic facies. Stratal slices made from the seismic-based lithology cube are then used to interpret depositional facies and systems. Eventually a seismic diagenetic-facies map is generated through analysis of the relationships between depositional facies, impedance, and diagenetic facies. A case study of clay- and calcite-cemented sandstone in the Qijia area shows that although still in its infancy, seismic detection of sandstone diagenetic facies using conventional seismic data is definitely feasible. Key words: seismic diagenetic facies, seismic detection of diagenetic facies, seismic sedimentology, diagenesis, sandstone
Introduction Reservoir diagenetic facies have an important control on reservoir quality. For years both domestic and foreign research on diagenetic facies has been confined primarily to core and microscopic thin-section analyses, seismic data seldom being adopted for description or prediction of diagenetic facies. Theoretically, seismic data can predict diagenetic facies as long as acoustic characteristics of the rock mass in the diagenetic unit are significantly affected by a specific diagenesis. For instance, in clastic sedimentary basins, because sandstone reservoirs are commonly modified by carbonate (calcite, dolomite, etc.) cementation, sandstone velocity and density could vary as the content of carbonate cement varies in pores. If the velocity and density of carbonate cement are very different from those of other cements and the carbonate cement makes up a large proportion, the seismic
signal would be significant. For example, calcite (predominantly matrix) cement, which has a P-wave velocity (vp) of 6.53 km/s, a bulk density (ρb) of 2.71 g/cm3, and an acoustic impedance (AI) of 1.77×106 g/(cm2·s), is very different in impedance (up to 44%) from that of argillaceous cements, which consist of clay diagenetic minerals, including matrix and micropores (vp = 4.00 km/s, ρb= 2.50 g/cm3, AI = 1×106 g/(cm2·s). Seismic detection should therefore be relatively easy. The main challenge for diagenetic-facies description and prediction using seismic data lies in establishing the link between core diagenetic facies and detectable seismic attributes, as well as dividing and identifying lithofacies units using seismic data. Dutta et al. [1] first attempted to link the content of calcareous cement in a sandstone to acoustic impedance of the sandstone and discussed the effects of cement-mineral composition,
Received date: 17 Aug. 2012; Revised date: 29 Mar. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by PetroChina International Cooperation Project (2009B-0102-01). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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cement content, shale content, grain size, stratigraphic position, and sequence-stratigraphic surfaces on range of acoustic impedance. However, they failed to link results of well and model analysis to those of actual seismic data interpretation or to put their method into practice [1]. On the basis of numerous core- and laboratory-analysis data of lacustrine sandstones in the Cretaceous Qingshankou Formation in the Qijia Depression, Songliao Basin, as well as high-quality 3D seismic data, we tried to determine diagenetic facies and diagenetic sequence. We used core analysis, linking the main diagenetic facies to acoustic parameters by means of wireline logs, followed by subdividing seismic sedimentological and diagenetic units, and eventually predicting diagenetic facies using seismic data. Because of the limitation of seismic resolution, conventional seismic-stratigraphic mapping techniques cannot meet the precision required for seismic diagenetic-facies analysis. At present, seismic sedimentology is the only interpretation tool available for extracting seismic-stratigraphic images at relatively high resolution (single-event imaging to detect 10-m thin beds) from conventionally stacked and migrated seismic data. Seismic sedimentology comprises seismic geomorphology and seismic lithology. Using the seismic-lithologic technique, a 3D seismic data cube can be converted to a log-lithology cube, in which the lithologic log (e.g., GR and SP) correlates to the well-site seismic trace with little error, so that the best fit between wireline-log and seismic data can be ensured at the reservoir scale. Using the seismic-geomorphologic technique, seismic data can be further converted to depositional facies, with lithofacies being identified on stratal slices. Therefore, establishment of a seismic-lithology cube and preparation of stratal slices are basic conditions for seismic imaging of sandstone diagenetic facies. For details please refer to papers by Zeng and others [2−5].
1 High-resolution sequence-stratigraphic framework and diagenetic units Songliao Basin is a large lacustrine depression that devel-
Fig. 1
oped during deposition of the Cretaceous Quantou to Nenjiang Formations [6, 7]. The Qijia Depression (Figure 1) is next to the west slope of the Songliao Basin in the north and west and is adjacent to the Daqing Anticline in the east. The Qingshankou Formation, as the target interval of this study, is 400–500 m thick. The sequence-stratigraphic framework was established in a previous study of seismic sedimentology [8]. As per the principles of sequence stratigraphy, the well-seismic stratigraphic framework was interpreted mainly on the basis of maximum flooding surfaces. Four continuous seismic reflections were picked in the 3D seismic cube (Figure 2), which correspond mostly to maximum flooding surfaces and which have chronostratigraphic meaning. Using wireline-log-based sequence-stratigraphic analysis and seismic facies characteristics, we correlated three third-order sequences and six fourth-order sequences in the Qingshankou Formation. Fifth-order sequences were recognized in accordance with GR (natural gamma ray) and SP (spontaneous potential) logs and the seismic event trend. However, chronostratigraphic significance of fifth-order sequence boundaries cannot be guaranteed because of the limitation of seismic resolution. The interpreted fifth-order sequence boundaries are of symbolic meaning only. A more effective seismic expression of a fifth-order sequence is the stratal slice. The dominant frequency of 3D seismic data in the Qingshankou Formation is around 50 Hz, which corresponds to a tuning thickness (limit of seismic resolution) of 10 ms (or 20 m for sandstones of 4 000-m/s velocity), setting the minimum mapping unit for seismic-stratigraphic study in this area. Because the average thickness of a fifth-order sequence ranges from 40 to 50 m, thickness of a minimum mapping unit is roughly equivalent to the thickness of a systems tract (LST/TST, or HST). As a result, seismic events are well correlated to systems tracts of fifth-order sequences (G11–G42). On average, each fifth-order sequence includes two seismic events. Lower red and upper black events correspond roughly to LST/TST and HST, respectively (Figure 2).
Location of (a) Songliao Basin and (b) 3D seismic survey in the Qijia area [8]
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Fig. 2
Well-seismic section A-A′ showing sequence correlation in the Qingshankou Formation in the Qijia area, Songliao Basin [8]
In outcrops, Taylor et al. [9] observed that distribution of carbonate cements in sandstone is related to depositional environment and formation architecture. In the Songliao Basin, lacustrine sandstones are related to all types of Cretaceous sandstone diagenetic facies. Because lacustrine depositional environments controlled the distribution of lacustrine sand bodies, some factors affecting sandstone diagenesis, such as sandy grain composition, size and sorting, clay content, pore-water salinity, etc., are all related to depositional environments. Therefore, the connection between diagenetic facies and depositional facies units should be a close one. In this article, we assume that because the main diagenetic unit equals the depositional facies unit, seismic recognition of diagenetic-facies units will be consistent with that of seismic depositional facies. On that basis, in our study of diagenetic facies using seismic data, we infer that 20 m is the minimum mapping unit for both stratigraphic and diagenetic-facies correlations. The actual thickness of diagenetic facies, of course, is related to the detection limit. A reasonable estimation is that we should be able to detect 3- to 4-m-thick diagenetic-facies units under certain conditions.
2 Sandstone diagenetic facies According to analysis of numerous core, thin sections, and scanning electron microscope data, reservoirs in the Qingshankou Formation in the Qijia Depression are composed mainly of calcite-cemented, fine feldspar sandstone (Figures 3 and 4). A small amount of ostracod limestone is observed. The feldspar, quartz, and rock fragments in the sandstone are 36%
to 46%, 36% to 53%, and 12% to 24%, respectively. Rock fragments are predominately volcanic debris. Sand grains are approximately 0.02 to 0.16 mm in diameter, medium to good sorting, and mostly subrounded, revealing petrologic characteristics of high compositional maturity and medium textural maturity. A fair number of ostracod fragments and carbonate cements are present (content ranges from 0 to 55%, average 11.2%) in grains. Because of cementation and compaction, sandstones in the Qingshankou Formation are tight, with grain particles in spot-line contact (Figures 3a and 4a). Secondary enlargement of quartz grains was observed. Pores are a combination of original intergranular and intergranular–intragranular pores. Carbonate cements are ferriferous calcite and dolomite (Figure 3a). Clay minerals are mainly mixed illite-smectite, with illite and chlorite cements (Figure 4b). All evidence indicates that the formation is in the early stages of the middle diagenetic phase. Sandstone reservoirs went through a series of diagenetic processes, including compaction, calcite cementation, and dissolution of feldspar grains and calcite cement. A diagenetic sequence is inferred to span from early compaction, to calcite cementation, to feldspar dissolution and generation of metasomatic calcite of quartz particles, followed by occasional pressure solution, secondary enlargement of quartz grains, and chlorite growth. As the most important facet of diagenesis affecting reservoir quality, calcite content directly controls porosity and permeability. Sandstones with a high calcite-cement content (>10%) have low porosity because most intergranular pores
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Fig. 3 Photo of calcite-cemented, fine-grained lithic feldspar sandstone, 2145.8 m, Well Gu-933
are blocked (Figure 3), and hardly any intergranular pores can be seen under the scanning electron microscope (Figure 3b). Sandstones with low calcite-cement content (<10%) (Figure 4) are of higher porosity because intergranular pores are better preserved; therefore, partially preserved intergranular pores with few fillings of needlelike illite and foliated chlorite can be observed under the scanning electron microscope (Figure 4b). Sandstones with high calcite cement belong to calcareous sandstone from the perspective of diagenetic facies, and “calcareous sandstone” can therefore be called calcite-cemented sandstone facies; similarly, sandstones with low calcite content can be called clay-cemented sandstone facies. Qualitative seismic identification of these two types of diagenetic facies will be discussed next.
3 Basis for seismic identification of calcite- and clay-cemented sandstone facies Analysis of lab and microscopic data from log-cored sections in the studied formation reveals relationships between
Fig. 4 Photo of clay-cemented, fine-grained lithic feldspar sandstone, 1658.5 m, Well Jin-56
sandstone diagenetic facies and acoustic properties necessary for seismic mapping of diagenetic units. For instance, a linear relationship exists between calcareous-cement (calcite) content measured from thin sections and porosity measured from core plugs (effective porosity) in Well Jin-44 (Figure 5): porosity drops from 16% to less than 4% as the calcite content in pore cements increases from 0 to 40%. Therefore, the proportion of calcite in pore cements has a direct impact on sandstone reservoir quality; i.e., higher calcite content corresponds to poorer reservoir quality. Furthermore, calcite content in pore cement is also related to clay content (Figures 5 and 6). Clay-cement content (illite, mixed illite-smectite, and chlorite, etc.) ranges from 2% to 20%, corresponding to a calcite content of from 45 to 0%, respectively. Generally speaking, moderate increase of clay content in sandstone cements would lead to a decrease of calcite content (Figure 6) or improvement of reservoir quality (porosity increase, Figure 5). In other words, calcite cementation would be weak for some facies units (e.g., channel-filled sand bodies), where a small amount of clay exists among sand particles; depositional facies units with low clay content, such as
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Fig. 5 Relationship between calcareous-cement (calcite) content measured in thin sections and porosity measured in core
Fig. 6 Relationship between calcite content and clay content in pore cements measured in thin sections
alongshore sand bars and delta-front sand sheets, would lead to strong calcite cementation at later stages, which is unfavorable for pore preservation after diagenesis of the sandstone. Further analysis indicates a linear, positive correlation between calcite content in sandstone and sandstone impedance in the Qingshankou Formation (Figure 7). Calcite content is linearly related to bulk density, and a similar relationship exists between calcite content and impedance, although with error increasing because sonic logs are used to replace lab measurements of sandstone velocity. Given this relationship, 10% of calcite content change would lead to a change of 0.02 to 0.03 in the reflection coefficient, which is sufficient to generate a detectable seismic signal. By adding the effects of lithology, we can evaluate the feasibility of predicting diagenetic facies using acoustic impedance. Figure 8 shows the relationship between impedance, effective porosity, and clay content of different lithologies calculated using wireline logs. Among the four types of lithologies, clay content is useful in distinguishing between mudstone and other rocks. The difference between reservoir-quality, clay-cemented sandstone
Fig. 7 Relationship between calcite content, bulk density, and impedance of sandstones
Fig. 8 Distinguishing between four major types of lithology and diagenetic facies using impedance, effective porosity, and clay content
and calcite-cemented sandstone and limestone is effective porosity; only those sandstones with higher effective porosity (>5%) and lower impedance (<1,1000 m/s g/cc) can be counted as reservoirs. A calcite content of 50% (13,000 m/s g/cc) is used to differentiate between calcite-cemented sandstones and limestones. Distinguishing between semideep lake mudstone and calcite-cemented sandstone is easy if impedance is used; clay-cemented sandstone generally has lower impedance than calcite-cemented sandstone; however, shallow-lake mudstone and clay-cemented sandstone are characterized by similar impedance ranges, which can cause confusion in interpretation. Other evidence (e.g., different depositional characteristics interpreted on stratal slices) should therefore be considered.
4 Relative-impedance meaning of 90°-phase seismic amplitude Conversion of a 3D seismic data cube to a lithology cube is recommended before high-resolution seismic diagenetic analysis is performed. The physical attributes in this lithologic volume can be converted to high-resolution depositional facies maps of lithologic meaning and diagenetic-facies maps on stratal slices. Under current technical conditions, the most
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economic and effective way of lithologically calibrating conventional seismic data is 90° phasing [10−11]. A 90°-phase wavelet shifts the maximum point of the main lobe of the seismic response to the midpoint of a thin bed. As a result, seismic response will correspond to the midpoint rather than the top or base of the thin bed, and the seismic event will correlate to a reservoir unit defined geologically, such as a sandstone bed. In other words, seismic polarity is able to correspond to lithology within a wavelength. Although not very precise for thin beds of less than one-quarter wavelength, the top and base of the bed can be determined at zero crossing points. While these changes are being applied to actual data, a one-to-one correlation between seismic events and thin lithologic units could be established, making interpretation of depositional lithology—e.g., distinguishing sandstone from mudstone—much easier [4]. These advantages for interpretation, however, do not exist in zero-phased and other-phased data. Furthermore, seismic inversion, seismic-attribute analysis, and time-frequency analysis can also convert a seismic volume into a lithologic volume. In cases of multiple types of lithology and diagenetic variations, lithologic calibration for seismic attributes would be complex. An analysis of spontaneous potential and sonic logs in Well Jin-44 (Figure 9) reveals a complex lithology–velocity relationship in the Qingshankou Formation. For example, differences in velocity between semideep lake mudstones in the Qing-1 member and shallow-lake mudstones in the Qing-2 and -3 are significant; sandstones include low-velocity, clay-cemented sandstones and high-velocity, calcite-cemented sandstones; the velocity of limestone is close to but higher than that of calcite-cemented sandstones. Besides, lithology–velocity (impedance) relationships also vary with depth and are affected significantly by the stratigraphic framework. In this situation, lithologic and diagenetic identification using 90°-phase seismic amplitude would be nonunique, but at least a close relationship can be observed between the 90°-phase seismic data cube and relative impedance. For instance, in the Well Jin-44 seismic section (Figure 10), although SP and GR curves do not indicate a good correlation between distribution of mudstones and sandstones and seismic events, the correlation between the log-velocity curve averaged by seismic event and seismic polarity is good; i.e., a high-impedance event (positive polarity in red) indicates a high-log-velocity block, whereas a low-impedance event (negative polarity in black) indicates a low-log-velocity block. This relationship lays the foundation for distinguishing calcite- and clay-cemented sandstone facies by relative impedance. In order to decrease the nonuniqueness of seismic prediction of diagenetic facies, stratal slices rather than seismic sections should be adopted as the interpretation platform [12]. An agreement between average velocity and seismic polarity (Figure 10) indicates that seismic polarity and average amplitude mainly represent the cyclicity of depositional sequences (lake transgression and regression or migration of systems
Fig. 9 Log interpretation of major lithologies and sandstone diagenetic facies in Well Jin-44
tracts), rather than internal lithologic and lithofacies variation of depositional systems. Eliminating this type of system error by using stratal slices is convenient, i.e. by neglecting absolute changes of amplitude in the vertical section and observing relative amplitude variations in plane (slice) view. Besides, on stratal slices seismic geomorphology revealed by amplitude patterns can be taken full advantage of, so that lithology and
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Fig. 11 Black-and-white amplitude stratal slice representing lowstand systems tract of a lower G31 high-frequency sequence
Fig. 10
90°-phase seismic section through Well Jin-44[8]
diagenetic facies based on differences in facies distribution, might be interpreted, even when those facies are characterized by similar amplitudes (impedances).
5 Seismic diagenetic facies in the Qingshankou Formation Seismic identification of diagenetic facies should be based on seismic depositional-facies interpretation because (1) if a major diagenetic-facies unit is roughly consistent with a depositional-facies unit, then a connection would exist between distributions of diagenetic and depositional facies and depositional-facies prediction would therefore benefit diagenetic-facies prediction, and (2) distinguishing between some diagenetic facies and other diagenetic facies or lithofacies with similar impedances would be difficult, and diageneticfacies identification using knowledge of depositional facies would reduce ambiguity considerably. Figure 11 shows a seismic stratal slice in the lower part of G31, a high-frequency sequence in the Qingshankou Formation. In black and white, many channel-like, low-amplitude (negative) anomalies can be easily recognized. However, identification of seismic-geomorphologic units in other shapes is difficult. Adoption of a spectral color scheme (Figure 12) improves identification of various types of nonchannel seismic-geomorphologic units, including narrow, irregular zones of relatively low amplitude along the channel margin; leaflike (lobe), relatively low amplitude bodies in front of channels;
Fig. 12 Spectrum amplitude stratal slice representing lowstand systems tracts of a lower G31 high-frequency sequence
relatively low- and high-amplitude zones that are roughly perpendicular to channels; and sheet and irregular, relatively low- and high-amplitude zones distributed in front of channelized areas. The lithology of each seismic-geomorphologic unit can be interpreted on the basis of the relationship between relative amplitude and relative impedance (Figure 10). Fluvial channels are most likely to be filled by sandstones; channel margins, leaflike bodies, and zonal bodies are probably sandstones, siltstones, or mudstones; and sheetlike bodies are likely to be mudstones (low amplitude and low impedance) or ostracod limestone (high amplitude and high impedance). Collectively this seismic-sedimentologic mapping unit in the study area can be interpreted as a lacustrine shallow-water delta (Figure 13). Large amounts of sandy and muddy sediments from the northeast (Daqing Anticline) were carried into the shallow-water lacustrine basin by distributary channels. Sandy sediments were deposited mainly in the channels. Some mouth-bar sediments accumulated to form leaflike sand bodies, and the rest were modified by wave and alongshore
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Fig. 13 Lowstand, shallow-water delta depositional facies map of lower G31 interpreted from Figure 12
currents to form delta-front sand sheets. The deltaic system underwent multiple phases of progradation, migrating basinward with multiple shorelines and forming delta-front sand sheets predominantly in band shapes orthogonal to distributary channels. Seismic identification of diagenetic facies is based mainly on the relationship between calcite-cement content and impedance in sandstone (Figure 7). Clay-cemented sandstone facies (calcite content <10%) with low impedance (<1.15×106 g/(cm2⋅s)) is shown as low amplitude on a stratal slice; conversely, calcite-cemented sandstone facies (calcite content >10%) with high impedance (>1.15×106 g/(cm2⋅s)) is shown as high amplitude on a stratal slice. Furthermore, seismic identification of diagenetic facies would also be inferred by the existence of other lithologies. For example, high-impedance limestone is similar to calcite-cemented sandstone in high-amplitude expression. However, both high-impedance limestone and calcite-cemented sandstone belong to nonreservoir facies and thus are not necessary to distinguish between in practice. Shallow-lake mudstone and clay-cemented sandstone facies are characterized by similar impedance ranges and cannot be distinguished by amplitude. An interpretation can be made only by clear depositional facies identiTable 1
Fig. 14 Seismic diagenetic-facies map of lower G31 lowstand systems tracts
Seismic-sedimentologic and seismic diagenetic-facies characteristics for lower G31 lowstand systems tracts Relative
Relative
amplitude
impedance
Channel
Low
Channel margin
Geomorphology
fication on the stratal slice. From these analyses, a seismic depositional-facies map (Figure 13) could be converted to a diagenetic-facies map (Figure 14): clay-cemented sandstone facies were developed in distributary channels. Clay-cemented sandstone facies (could be mixed with mudstone in the outer periphery) were formed on distributary-channel margins, delta-front lobes, and low-amplitude/low- impedance delta-front sand sheets, whereas high-amplitude/high-impedance delta-front sand sheets correspond to calcite-cemented sandstone facies. Finally, sheet facies are recognized on the basis of the location of facies mapped so far and then subdivided into shallow-lake mudstone zones and ostracod-bank limestone zones on the basis of the magnitude of relative amplitude and impedance. Wireline logs from wells can be utilized for calibrating diagenetic- facies maps. For example, calcite-cemented sandstone facies in delta-front sand sheets correspond to high-log-velocity zones, whereas log velocity decreases considerably in clay-cemented sandstone facies (Figure 14). Table 1 summarizes seismic-geomorphologic features, amplitude and impedance characteristics, lithologic and depositional facies interpretation results, and corresponding seismic diagenetic facies for various facies in the lower G31 high-frequency sequence.
Lithology
Depositional facies
Seismic diagenetic facies
Low
Sandstone
Distributary channel
Clay-cemented sandstone facies
Low
Low
Sandstone/siltstone/mudstone
Distributary-channel margin
Clay-cemented sandstone facies
Leaflike
Low
Low
Sandstone/siltstone/mudstone
Delta-front lobe
Clay-cemented sandstone facies
Zonal
Low
Low
Sandstone/siltstone/mudstone
Delta-front sand sheet
Clay-cemented sandstone facies
Zonal
High
High
Sandstone/siltstone
Delta-front sand sheet
Calcite-cemented sandstone facies
Sheet
Low
Low
Mudstone
Shallow lake
Sheet
High
High
Limestone
Ostracod bank
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Seismic diagenetic-facies analysis shows that distributarychannel, clay-cemented sandstones are the most favorable hydrocarbon reservoirs in the Qingshankou Formation in the Qijia Depression. Channel sands failed to receive sufficient washing at the time of deposition, leaving a certain amount of clay among grain particles, which later would play an important role in hindering calcite cementation and preserving primary intergranular-pore space. Some channel sand bodies, of course, went through calcite cementation anyway and probably lost the amplitude characteristics of channel forms (cf. interpreted channels in depositional facies map in Figure 13 and identified channels in the stratal slice in Figure 12). Clay-cemented sandstones and siltstones of distributary-channel margins, delta-front lobes, and low-amplitude/ low-impedance delta-front sand sheets also possess a certain exploration potential. The main target for exploration of lithologic stratigraphic traps in the Qijia Depression in the future will be clay-cemented sandstones of distributary-channel facies. Seismic identification of sandstone diagenetic facies is just the beginning. Numerous problems are yet to be resolved. For example, to recognize different diagenetic facies with similar acoustic impedances, seismic attributes that are more sensitive to diagenetic facies should apply to different formations; a full understanding of relationships between stratigraphic units and diagenetic units and between depositional facies and diagenetic facies is still lacking; and determination of seismic and diagenetic facies is still qualitative and somewhat arbitrary. Research should be expanded beyond this single case study.
by 90°-phasing processing, which was then used as a seismic signal for identifying diagenetic facies. A lacustrine, shallow-water delta system was identified on stratal slices created by the seismic-lithologic cube. Relationships between depositional facies, impedance, and diagenetic facies were analyzed on a depositional map, leading to a high-resolution diagenetic facies map, which will one day provide references for finding lithologic stratigraphic traps in the Qijia Depression.
Acknowledgments We want to thank Sun Yu, Wang Rui, Zhou Chuanmin, and Bai Bin, etc. for participating in this study. Publication authorized by the Director, Bureau of Economic Geology.
References
6 Conclusions Study of seismic diagenetic facies involves three key steps: (1) dividing stratigraphic and diagenetic units using seismic data, (2) establishing a link between core-based diagenetic facies and seismic attributes, and (3) searching for an effective seismic diagenetic-facies mapping method. Reservoir-scale (20 m, detection limit 3 to 4 m) depositional sequence and diagenetic-facies unit framework were established using welland seismic-based, high-resolution sequence correlation and seismic stratal slicing of the Qingshankou Formation in the Qijia Depression, Songliao Basin. Core-based sandstone diagenetic-facies analysis revealed a series of diagenetic phenomena, such as compaction, calcite cementation, dissolution of feldspar particles and calcite cement, and demonstrated that calcite cementation is the most important diagenetic activity affecting reservoir quality. Clay-cemented sandstones with low calcite content formed the primary reservoirs. Reservoir and acoustic parameters, such as porosity, clay content, calcite content, and acoustic impedance, were analyzed to explain the relationship between calcite content, porosity, and impedance, confirming that clay- and calcite-cemented sandstone facies are appropriate diagenetic facies for seismic detection. A seismic data cube was converted to a seismic-lithologic cube
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[1]
Dutta T, Mukerji T, Mavko G. How does carbonate cementation in sandstones affect seismic response? Tulsa: SEG Las Vegas 2008 Annual Meeting, 2008.
[2]
Zeng H, Backus M M, Barrow K T, et al. Stratal slicing: Part I: Realistic 3-D seismic model. Geophysics, 1998, 63(2): 502–513.
[3]
Zeng H, Henry S C, Riola J P. Stratal slicing: Part II: Real 3-D seismic data. Geophysics, 1998, 63(2): 514–522.
[4]
Zeng H, Hentz T F. High-frequency sequence stratigraphy from seismic sedimentology: Applied to Miocene, Vermilion Block 50, Tiger Shoal area, offshore Louisiana. AAPG Bulletin, 2004, 88(2): 153–174.
[5]
Zeng H. Seismic sedimentology in China: A review. Acta
[6]
Yang W L. Daqing Oilfield, People’s Republic of China, a
Sedimentologica Sinica, 2011, 29(3): 61–70. giant field with non-marine origin. AAPG Bulletin, 1985, 69: 1101–1111. [7]
Feng Z Q, Jia C Z, Xie X N, et al. Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao Basin, northeast China. Basin Research, 2010, 22: 79–95.
[8]
Zeng H, Zhu X, Zhu R, et al. Guidelines for seismic sedimentologic study in non-marine postrift basins. Petroleum Exploration and Development, 2012, 39(3): 275–284.
[9]
Taylor K G, Gawthorpe R L, Curtis C D, et al. Carbonate cementation in a sequence-stratigraphic framework: Upper Cretaceous sandstones, Book Cliffs, Utah-Colorado. Journal of Sedimentary Research, 2000, 70(2): 360–372.
[10] Zeng H, Backus M M. Interpretive advantages of 90°-phase wavelets: Part 1: Modeling. Geophysics, 2005, 70(3): 7–15. [11] Zeng H, Backus M M. Interpretive advantages of 90°-phase wavelets: Part 2: Seismic applications: Geophysics, 2005, 70(3): 17–24. [12] Zhang X, Lin C, Zhang T, et al. Seismic sedimentologic research in shallow sea areas, Dagang. Petroleum Exploration and Development, 2011, 38(1): 40–46.