7 3 D Seismic Sedimentology of Nearshore Subaqueous Fans – A Case Study from Dongying Depression , Eastern China

. Journal of Chengdu University of Technology(Science & Technology Edition), Vol.34, No.2, (April 2007), pp.174-179, ISSN 1671-9727 Ling, Y.; Sun, D.S. & Gao, J. (2005). Interpretation of depositional bodies in the parasequence sets from 3-D seismic data (in Chinese). Geophysical Prospecting for Petroleum, Vol.44, No.6, (November 2005), pp.568-577, ISSN 1000-1441 Liu, B.G. &Liu, L.H. (2008). Application of applied seismic sedimentology in sedimentary facies analysis (in Chinese). Geophysical Prospecting for Petroleum, Vol.47, No.3, (May 2008), pp.266-271, ISSN 1000-1441 Lu, Z.Y. (2008). Influence of the Paleogene structural styles on deposition and reservoir in Chezhen Sag, Bohai Bay Basin (in Chinese with English abstract). Journal of Palaeogeography, Vol.10, No.3, (June 2008), pp.277-285, ISSN 1671-1505 Luo, Q.S.; Zhao, M. & Zhang X.J. (2006). Application of Zero Mean-Based Curve Reconstruction to Seismic Inversion (in Chinese with English abstract). Xinjiang petroleum geology, Vol.27, No.4, (August 2006), pp.478-480, ISSN 1011-3873 Nordfjord, S.; Goff, J; Jr, J. & Sommerfield, C. (2005). Seismic geomorphology of buried channel systems on the New Jersey outer shelf: assessing past environmental conditions. Marine Geology, Vol.214, No.4, (February 2005), pp.339-364., ISSN 00253227 Posamentier, H. & Kolla, V. (2003). Seismic geomorphology and stratigraphy of depositional elements in deep-water settings. Journal of Sedimentary Reserch, Vol.73, No.3, (May 2003), pp. 367–388, ISSN 1527-1404 Prather, B. (2003). Controls on reservoir distribution, architecture and stratigraphic trapping in slope settings. Marine and Petroleum Geology, Vol..20, No.6-8, (June-September 2003), pp.529-545, ISSN 0264-8172 Richard, M. & Bowman, M. (1998). Submarine fans and related depositional II: variability in reservoir architecture and wireline log character. Marine and Petroleum Geology, Vol.15, No.8, (December 1998), pp.821-839, ISSN 0264-8172 Schwab, A.; Tremblay, S. & Hurst, A. (2007). Seismic expression of turbiditycurrent and bottom-current processes on the Northern Mauritanian continental slope, In: Seismic Geomorphology: Applications to Hydrocarbon Exploration and Production, Geological Society, R.J. Davies.; H.W. Posamentier.; L.J. Wood. & J.A. Cartwright, (Ed.), pp.237-252, Special Publications, ISBN 978-1-86239-223-6, London Shen, X.C. & Yang, J.F. (2006). The Application of the Reconstructed Characteristic Curve of Reservoir in the Inversion (in Chinese with English abstract). West China Petroleum Geosciences, Vol..2, No.4, (December 2006), pp.436-439, ISSN 5021-5850 Song, N. (2004). The study of stratigraphic sequence with gravel rock in northern steep of the Dongying sag stratigraphic research study (in Chinese with English abstract). Ph.D. Thesis, nanjing university, Nanjing Song, R.C.; Zhang, S.N. & Dong, S.Y. et al. (2007). Characteristics and Controlling Factors Analyze of Nearshore Subaqueous Fans in Langgu Depression (in Chinese with English abstract). Journal of Earth Sciences and Environment, Vol.2, No.29, (June 2007), pp.145-158, ISSN 1672-6561 Sullivan, E.; Marfurt, K.; Blumentritt, C. & Ammerman, M. (2007). Seismic geomorphology of Paleozoic collapse features in the Fort Worth Basin (USA), In: Applications to Hydrocarbon Exploration and Production, Geological Society, R.J. Davies.; H.W.

126 2007; Liu, B.G. & Liu, L.H., 2008;Nordfjord et al., 2005;Posamentier & Killa, 2003;Prather, 2003;Sullivan et al., 2007;Schwab et al., 2007;Wang et al , 2004;Wu et al, 2005;Zeng et al., 2003Zeng et al., , 2004Zeng et al., , 2007;;Zhang et al., 2007), but rarely used to study nearshore subaqueous fans.In this paper, we took the nearshore subaqueous fan in the Dongying Depression as a case, and used the pseudo-acoustic 3D seismic inversion method on characteristic logs to reconstruct 3D seismic sedimentological structures of the nearshore subaqueous fans including the distribution of the effective sand-conglomerate reservoirs and the temporospatial evolution of individual nearshore subaqueous fan system.Over the years, six exploratory wells were drilled into the lower Es4 Formation in the northern Dongying Depression and four of them encountered commercial oil and gas.The logging data from all six wells yield good coverage with 0.125 m or 0.25 m sampling spacing.An industry-standard 3D seismic data of 600 km 2 acquired in 2005 was processed using high-fidelity prestack time migration technique with 25m × 25m track spacing, 1ms sampling interval, 25HZ dominant frequency and 10-60Hz effective frequency bandwidth in the target formation.

Characteristics of the nearshore subaqueous fan in the northern Dongying Depression
The Dongying Depression is a typical sub-structural unit in the Bohai Bay Basin, Eastern China, surrounded by a series of uplifts, including the Luxi Massif in the south, Chenjiazhuang Uplift in the north, Qingtuozi Uplift in the east, Binxian Uplift and Qingcheng Uplift in the west (Fig. 1).As a rift basin, the Dongying Depression is characterized by a structural style of half-graben with a northern faulting and southward overlaping.The Depression can be divided in to three structural belts: a northern steep belt (NSB), a middle sag belt (MSB) and a southern slope belt (SSB) (Fig. 1c).During the lower Es4 Formation in the early Paleogene, under the controlling of the northern Chenjiazhuang boundary extensional fault, many nearshore subaqueous fans developed in the footwall of the Chenjiazhuang major fault along the northern steep belt and extending into the deep and semi-deep lacustrine facies of the middle sag belt (Fig. 1d) with sources mainly from the northern Chenjiazhuang Uplift (Gao et al., 2008;Xie et al., 2004;Yan et al., 2005) (Fig. 1).The burial depth of these subaqueous fans now reaches more than 3500m.Facies analyses shows that the Dongying nearshore subaqueous fan consists of three subfacies including a root sub-fan, a mid sub -fan and a marginal sub -fan (Gao et al., 2008;Song, 2004;Yan et al., 2005).The root sub-fan is composed by one or more major channel Fig. 2. The single-well facies analysis of well f8 shows that the 4th member of the lower Es4 Formation is a nearshore subaqueous fans with three sub-facies (modified from Gao et al., 2008;Song, 2004;Yan et al., 2005).

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Advances in Data, Methods, Models and Their Applications in Geoscience 128 sediments and the main lithology includes gray matrix-supported conglomerates, sandy conglomerates and black shales.The mid sub-fan, which is the main part of the nearshore subaqueous fan and forms the effective reservoir in the study area, is characterized by braided channels with braided channel microfacies, intra-channel microfacies and leafy sandbody microfacies.The main lithology of the mid sub-fan includes pebbly sandstones, conglomerate and block sandstones, with thickness varying between 1 and 55 m.The marginal sub-fan consists of siltstones, muddy siltstones and mudstone interbedding rocks (Fig. 2).According to well stratigraphic cyclicities and 3D seismic reflection features, the lower Es4 Formation can be divided further into 5 members, standing for 5 individual nearshore subaqueous fans with several sub-facies (Fig. 2).In general, 3D seismic reflection profiles of the nearshore subaqueous fan are characterized by wedge-shaped, mound-shaped or lenticular-shaped systems, and sub-fans can be further identified.On the synthetic seismograms record calibration, the root sub-fan is characterized by weak reflection, non-reflection or chaotic reflection, the mid sub-fan is characterized by weak to moderate intensity amplitudes, sub-parallel, weak continuous reflection, and the marginal sub-fan is characterized by continuous medium frequency, moderate to low intensity amplitudes.The deep lacustrine facies in the sag belt is characterized by either weak reflection or non-reflection.However, to identify the sub-facies of the nearshore subaqueous fan using the 3D seismic section is difficult (Fig. 3).

Methodology
The pseudo-acoustic 3D seismic inversion method different from the conventional 3D seismic impedance inversion method not only in working through logs reconstruction, inversion, interpolation and extrapolation, but also adding or replacing characteristic curves to the density logs or, more commonly, velocity logs in order to achieve the ability to identify the reservoir from the surrounding rock in the case of small impedance difference (Shen & Yang, 2006;Zhang et al., 2005).The potential reservoir may show no direct relationship with the seismic reflection but can be distinguished from different lithological changes.The velocity and the time-depth relationship after logs reconstruction may change so deviations between seismic reflection horizon and synthetic seismogram calibration's horizon should be established to reflect these changes (Luo et al., 2006).The pseudo-acoustic seismic inversion results based on logs reconstruction may be not accurately reveals the corresponding lithological changes of the target layers.To solve this problem, the zero Mean-Based logs reconstruction techniques, which keeps the original time-depth relationship unchanged, can be applied.The principle is to set the characteristic curves or logs involved in seismic inversion to a mean of 0, that is ΣAi = 0 (Ai is characteristic curve sample values of target layers).Adding or subtracting the normalized curve and acoustic characteristics curve, then properly magnifying the normalized characteristics curve in order to highlight lithology information.This process can be expressed as: pseudo-acoustic curve = acoustic logs ± characteristic curves × K, while K stands for the curve amplification factor.As the characteristic curves keep the information of the target layer, and the velocity curves of the upper and lower target layers are kept unchanged, so the original time-depth relationship will remaine unchange (Luo et al., 2006).The implementation process of this method includes the following: 遖selection of the characteristic curves; 遘standardization of the characteristic curves; 遞normalization and reconstruction of the pseudo-acoustic curve; 遨seismic wavelet extraction and the establishment of the initial model; 遯 pseudo-acoustic seismic inversion.

The pseudo-acoustic 3D seismic inversion based on logs reconstruction 1. Selection of the characteristic curves
To select the characteristic curves of the target layers, quantitative and semi-quantitative correlations through statistical analysis are established between different lithologyies (such as the conglomerate in the fan-root, sand-conglomerate in the mid-fan, mudstone, gypsumsalt rock in the marginal-fan), effective reservoir (such as gas sand-conglomerate in the midfan), logs (such as acoustic time (ac), natural gamma (gr), neutron porosity (cnl), spontaneous potential (sp), and logging parameters that correspond to different lithology types in different fans.The results show that single logs parameter cannot identify the different lithologies in different fans, but combinations of any two of logging parameters (ac, gr or sp) can effectively indentify them to some extent.Further analysis also show that any two logs parameter's combinations between ac, gr, and sp could distinguish the effective and ineffective sand -conglomerate reservoir with a thicknesses greater than 6 m (Fig. 4).Therefore, we can use any two logs combination between ac, gr, and sp as the characteristic curves.Fig. 4. Statistical analysis between the different lithology, logging parameters of gr and sp and effective reservoir with a thicknesses of > 6 m in wells f1,f2,f3,f8.

The standardization of the characteristic curve
In order to eliminate the systematic error caused by different measuring apparatuses and time, the characteristic curves need to be standardized by depth correction, environment correction, mudstone baseline correction, outliers removal, wave filtering and so on.

Normalization and the creation of the pseudo-acoustic curve
In order to avoid the systematic error caused by differences in dimension and value range, the characteristic curves need to be normalized before creating the pseudo-acoustic curve.Firstly, the natural gamma (gr) and spontaneous potential (sp) logs will be normalized by regulating the numerical range to the [0 ,1] ,and conducting the [0 ,100] amplification process before summing them for a GS (gr+sp) curve.Then, the asonic logging curve (ac) is processed for the treatment filter values that exceed 100 in order to remain the lowfrequency information and eliminate high-frequency information of ac.Finally, the pseudoacoustic curve GS is obtained by adding the characteristic curve (GS) to the filtered ac.It is clear that the pseudo-acoustic curve GS contain not only the high frequency information of both gr and sp, but also the low frequency information of ac, thus the ability to identify lithologies and strata is greatly improved.

Seismic wavelet extraction and initial model creation
Establishing a reasonable initial geological model is the key for getting a good pseudoacoustic seismic inversion.In fact it is a process of deciphering interpolation and extrapolation of well data under the constraints of the geological concept; the quality of the seismic inversion results are largely dependant on the initial model, which is decided by previous geological knowledge.In order to acquire a good model of impedance inversion, we not only replace the sonic logging curve (ac) by the GS logging curve and by extract Ricker wavelet from the target layer, but also combine the available well information based on the synthetic seismograms calibration and test runs repeatedly. 5. Pseudo-acoustic 3D seismic inversion On the Strata5.2inversion software platform, the GS, the GS pseudo-acoustic 3D seismic inversion data are obtained by calculation after importing the GS.The results show that 3D seismic inversion data based on gr+sp logs reconstruction is better than the conventional 3D seismic inversion using ac+den loggings to distinguish the internal structure of the nearshore subagueous fans (Figs. 5, 6) Fig. 6. 3D seismic reflection characteristics of the internal structure in the nearshore subaqueous fans based on GS 3D seismic inversion data along line B'B (see Line L location in Fig. 1) 4. 3D seismic sedimentology analysis of nearshore subaqueous fans 4.1 Evolution characteristics of seismic palaeogeomorphology of nearshore subaqueous fans By using the GS pseudo-acoustic 3D seismic inversion data coupled with calibration of the synthetic seismograms, the internal sub-facies in each member of the lower Es4 Formation can be identified and the temporospatial evolution of the nearshore subaqueous fans can be extrapolated (Fig. 6).The analysis finds that each member of the lower Es4 generally consists www.intechopen.com3D Seismic Sedimentology of Nearshore Subaqueous Fans -A Case Study from Dongying Depression, Eastern China 133 of 2-5 sub-facies (Fig. 6).The time for high frequent sub-facies development is during deposition of the 4th member of the lower Es4 Formation, which includes at least 5 subfacies.Fig. 7 shows the instantaneous frequency level slices of sub-layers' bottom boundary of the lower Es4 Formation as characterized by a low frequency in the main channel in the fan-root, a middle-low frequency in the mid-fan and a high frequency in the marginal-fan.These results clearly reveal the paleogeographic characteristics and different temporospatial evolution stages of sub-facies in the nearshore subaqueous fan system.

The distribution characteristics of effective reservoir in nearshore subaqueous fan
The synthetic seismogram calibration results show significantly higher dimension values of 12000-15500 in the GS pseudo-acoustic 3D seismic inversion for the effective sandconglomerate reservoir but lower dimension values<12,000 for the ineffective reservoir in the lower Es 4 Formation (Fig. 8).Accordingly, quantifying the thickness and the distribution of the effective sand-conglomerate reservoir in the lower Es 4 Formation can be relatively easy (Fig. 8).Compared with the conventional 3D seismic inversion, the pseudo-acoustic 3D seismic inversion based on characteristic logs reconstruction greatly improves the ability to identify internal seismic sub-facies.Several internal sub-facies in each member of the nearshore subaqueous fan in the lower Es4 Formation have been identified.4. The pseudo-acoustic 3D seismic inversion technique based on logs reconstruction reveals the 3D seismic sedimentological characteristics of nearshore subaqueous fans including the internal sub-facies structure and various temporospatial evolution stages in different sub-facies.The distribution of the effective sand-conglomerate reservoirs can be better quantified by using this method than the conventional 3D seismic impedance inversion.

Fig. 1 .
Fig. 1.Location and distribution of sedimentary facies of the lower Es4 Formation in the northern Dongying Depression, Bohai Bay Basin, Eastern China.3D seismic area is marked by Red box.

Fig. 3 .
Fig. 3. 3D seismic reflection characteristics of the nearshore subaqueous fans along line B'B (see Line location in Fig.1) Fig. 5. Comparison between results using different 3D seismic inversion parameters.a. Conventional 3D seismic inversion data using ac+den loggings; b.Pseudo-acoustic (GS) 3D seismic inversion data based on gr + sp Logs reconstruction

Fig. 7 .
Fig. 7.The instantaneous frequency level slices of sub-layers' bottom boundary of the lower Es4 Formation reflect the paleogeographic characteristics and space-time evolution of different sub-layers

Fig. 8 .
Fig. 8.The effective reservoirs range of values in the GS 3D seismic inversion data for the blue zone (see location in Fig.5b)