Seismic Response and Wave Group Characteristics of Reef Zeng zhongyu Zheng xuyao Hong qiyu Zeng zhongyu Zheng xuyao Hong qiyu Institute of Geophysics, China Earthquake Administration, Beijing 100081, China, Zengzhongyu2001@163.com Institute of Geophysics, China Earthquake Administration, Beijing 100081, China, xuyaozheng@yahoo.com.cn Institute of Geophysics, China Earthquake Administration, Beijing 100081, China, iamhhnn@126.com Abstract In this article, we built a reef complex geological model, which is based on the geological characteristics and electric characteristics on the upper Jurassic Karloff-Oxford Formation of Asser Block at the Right Bank of the Amu River, and studied seismic response of carbonate formation by the wave equation forward modeling. Seismic response characteristics of reef complex in the area are the mound-shaped and lenticular reflections characteristics. Seismic wave in the edges of the reef complex often appear onlap and diffraction seismic reflection phenomenon, and there is upper bend phenomenon on the top of the reflection wave. The reflection wave on the top of the reef complex is trough weak reflection, with weak amplitude and discontinuous wave-front. The internal reflection wave in reef complex are unorganized or weakly, and there is a big difference in continuity between it and that of the surrounding rock on the amplitude, frequency and phase. Moreover, compared with formations of two wings, we found out that the thickness suggested by the reflection waves in the internal reef complex is bigger. The characteristics mentioned above are consistent with seismic section characteristics of reef complex over well. So that, we built a characteristics to recognize pattern of seismic of reef complex by using forward modeling results of a reef complex. Keywords: Karloff-Oxford Group; carbonate formation of reef; forward modeling response 1. Introduction Asser block lies in the right bank of the Amu River of Turkmenistan Basin and north of Sd3D block, which is covered by desert and gobi mostly. The 3d seismic covering the area is 500 km 2, and there are already 26 drilled wells. The composition of sedimentary formation of the Amu River Basin is the Upper Jurassic, Cretaceous, Paleogene and Neogene-Quaternary stratum. The bearing gas target formation is a carbonate rock of Karloff-Oxford group, and its thickness is 330m-410m. The conditions of the block on oil growing, storing, building, migration and accumulation are good. The coal-bearing mudstone and thin coal seam, which is in the Middle-Lower Jurassic under carbonate rock are a good source rock. The fault development of the internal carbonate rock forming the vertical migration pathway, covering the carbonate rock with gypsum-salt strata of thick-layer in the meantime, and then constituting a good enclosed gas reservoir combination with growing oil in lower interval, reserving oil in middle interval, and covering oil in the upper interval. International Journal of Digital Content Technology and its Applications(JDCTA) Volume 7, Number 10, June 2013 doi : 10.4156/jdcta.vol7.issue10.14 139
The reef formation of Karloff-Oxford in this area includes the Upper Reef complex, Reef complex Body and Lower Reef complex (Figure 1); it is gray carbonate rock layer containing two thin dense limestone dissections. The reef formation covered with evaporite of Kimmeridgian of Upper Jurassic, which contains 3 gypsum beds and 2 salt beds, in all 5 layer sections: Upper Gypsum, Upper Salt, Middle Gypsum, Lower Salt and Lower Gypsum. The evaporate is very thick, which causes serious energy absorption of high frequency component of seismic wave, so the resolution of seismic data processed by the conventional methods is generally low. Some experts have studied on the characteristics of reef complex and exploration of the block of the right bank of the Amu River. For example Wang Lin etc. [1] discussed the characteristics of the reef complex of this block and the recognition method. According to the distribution characteristics of bioherm limestone, comprehensive logging and geological data, a geological model, a structural model and a seismic inversion model were built, then geological and reservoir characteristics were analyzed. Qi Baoquan etc. [2] studied logging facies of reef complex and built a logging interpretation model. Using various methods, Lin Sun etc. [3] recognized and predicted the reef formation of the Upper Jurassic in Turkmenistan Basin Bargleid block of Karloff-Oxford Group, and summarized some effective methods and means to identify reef complex. While these results focus on the geological and logging explanation of reef complex, and not very clear and comprehensive on the characteristics of seismic response of reef complex [4]. Based on the basic characteristics of reef formation as of the block of the right bank of the Amu River, with drilled geology data, well log data and explanation of seismic data, a reasonable geological model of reef complex was built in this research. Applying this model to wave equation forward modeling, we got the seismic response of the geological model of reef complex. Moreover, we analyzed characteristics of seismic response of the geological model of wells with and without reefs. By comparing the result of forward modeling with seismic profile of cross-well seismic section, we got seismic response and methods of seismic characteristics recognition of reef complex. 2. Forward modeling of the reef complex stratum Figure 1. comprehensive stratigraphic column of the Upper Jurassic in the right bank of the Seismic forward modeling is an important means to investigate kinematic and dynamic behaviors of propagating seismic waves. From seismic forward simulation result could be 140
obtained seismic response and wave group characteristics of reef complex. There are two basic problems involved here: one is to build a geological model, and the other is numerical method we choose. First, we need to create a geological body model with rock of underground target formation. According to the essential characteristics of the stratum of the reef complex and their surrounding rocks, we set up a reasonable geological model of reef complex; analyzed characteristics of lithology, electrical property and physical property of anhydrite, salt rock and limestone. Combining the results with the explanation of seismic data, welling and logging data of the study district, a seismic-geological model of this area was set up. The building geological model should include overlying surrounding rock of reef complex (upper gypsum (BA), upper salt (BC), middle gypsum (CA), lower salt (HC) and lower gypsum (HA)) and carbonate formation with reef complex (thin strata subdividing into: lime mudstone (GAP), upper strata of reef complex (Hp), (XVa1), compacted limestone strata (Z), lower strata of reef complex (XVa2), compact layered mudstone (XVI)). Combining the thickness of the mentioned layers and velocity, density parameter of four wells(yg5, YG11, YG1, YG6), we set up a geological model, which contains continuous line of the 4 wells (Figure 2). The purpose of designing the geological model of cross-well section is to make use of geological and test data of these wells. By observing and comparing the characteristics of seismic response of the geological model with and without reef complex, the characteristics of the model with the reef complex were found out. Among the geological model, YG5 well come across reef complex, and the pore of the matrix of the text section is a little bigger than average, and the matrix is mainly massive reservoir. The pore of the lower segment of the matrix of YG1 well is not so high while the upper segment of the matrix is a reef complex reservoir with high porosity, appears massive characteristics. YG11 well did not come across a reef complex, and the porosity of the matrix in the test section is small; YG6 well did not come across a reef complex while the porosity of the matrix is a bit smaller than average. Figure 2. cross-well geological model through YG5, YG11, YG1 and YG6 141
Wave-equation modeling method is an ideal way to obtain realistic seismic data from a model. Here, we applied wave-equation finite difference modeling to recognize the seismic waves shape in dolomite gas reservoir. u x x 2 u t 2 2 u u u 2 f 2 According to the spectrum characteristics of the real seismic data, we choosed Ricker wavelet with the main frequency of 30 HZ as the incident pulse, and we got the seismic response of the wave equation forward modeling. Figure 3 shows the forward modeling result of cross-well section of YG5, YG11, YG1, YG6, YG11. Combining the test result of the well and comparing it with the seismic response (Figure 3) of the geological model (Figure 2), we can inferred that: i i Figure 3. characteristics of forward modeling response of cross-well geological model through YG5, YG11, YG1 and YG6 (1) The seismic responses in well YG5 and YG1, which contains a reef complex, are mound shape and show lenticular reflection. The amplitude of reflection wave of GAP top of carbonate rocks in the reef complex is weak. The wave shape of the wave is discontinuous, and it is a weak reflection of trough wave. The reflection coefficient is negative because of wave travels from lower gypsum layer (HA), with high velocity, to lime mudstone layer (GAP), with low velocity, so that the reflection coefficient is negative. The formation surrounding the reef complex shows onlap and diffraction seismic reflection phenomenon, and there is upper bend phenomenon. Comparing with the two wings of formations of reef complex, we found out that the thickness of the reflection wave is big significantly, and there is a big difference between reef complex and the surrounding rock on the amplitude, frequency and phase continuity in reflection wave section. (2) Amplitude energy of well YG11 and YG6 without a reef complex is higher, the wave shape on YG11 and YG6 is continuous, there is no similar seismic response characteristics of reef complex on well YG5 or YG1. 3. Seismic characteristics recognition of reef complex 3.1 Calibration with the synthetic seismogram 142
Before calibration, in order to improve the resolution of seismic data, based on an analysis of the effective high frequency components of CRP gathers, we used the common reflection bin stacking technique to preserve amplitude and eliminate noise. Then analyze reliability of the effective low frequency and high frequency signals on the gathers after noise elimination. To make the amplitude spectrum close to that of the reflection coefficient, we widen the effective frequency band of seismic signals The frequency band of high resolution data is 0 Hz wider than that of the conventional data. Enlarge the dominant frequency by 15 Hz, and significantly improve the signal-to-noise ratio. Thus, we can effectively identified thin reservoirs with thickness of about 10m, recognized reef complex with seismic characteristics and provided a good basis for fine structural description and lateral reservoir prediction. According to the results of forward modeling of reef formation model, we can found out the characteristics of seismic response of the formation. To get seismic response of reflection and its location accurately, we also need an exact calibration. We can got the characteristics of the wave group corresponding to the reef complex by calibrating the synthetic seismogram by VSP and acoustic logging data. Extracting wavelet from near-well seismic traces and adjusting parameters of the wavelet continuously, we can got the best calibration and the best wavelet for the interactive iteration of wavelet inversion and horizontal calibration. Make sure the synthetic seismogram, which is the result of the convolution of the best wavelet and the parameters of reflection, and the practical seismic section reaches maximum correlation. While there is a small error between seismic horizon and geologic horizon, we took the seismic as the standard to align the main target bed and ensure accurate time-depth calibration of formation with reef complex. Make the most of logging data, analyzed the lithology and electrical property of target bed. Realized multi-well and multi-layer location according to the conclusion of the synthetic seismogram of reef formation. On the basic of accurate calibration of formation group, we did secondary fine calibration of the six small layer section of Karloff-Oxford group (GAP, Hp, XVa1, Z, XVa2, XVI) by taking wave group characteristics as reference section, enlarging comparison time and then finely modifying the internal characteristics of the small layer. From the comparison of the result of Figure 4. SDK4 well synthetic seismogram calibration result calibration and the synthetic seismogram on the similarity of wave shape, relative energy, the amplitude of wave shape, the corresponding of frequency and phase, and the corresponding of geological horizon and seismic trace (Figure 4), we found out that the calibration result is good. 3.2 Seismic characteristics recognition of reef complex According to the result of synthetic seismogram and fine calibration, combining the characteristics of seismic section of well with the reef complex in the study area, we drew the conclusion that: 143
(1) The reflection characteristics of seismic section with the reef complex in the study area are mound-shaped and show lenticular reflection. The characteristics of edge wave group of reef complex show onlap and diffraction seismic reflection phenomenon. The reflection wave appears upper bend phenomenon. (2) The reflection wave around the formation of the reef complex top shows fragmentation phenomena. It is reflection of trough wave. Its amplitude is weak and its wave shape is discontinuous. (3) The reflection wave of internal reef complex is unorganized or weak, and there is a big difference between surrounding rock and that on the amplitude, and continuity of frequency and phase. By the reflection characteristics of the seismic section of the reef complex and examination of seismic section, we found out a similar characteristics at the reflection characteristics of the seismic section of reef complex in the seismic section through well YG1 (Figure 5)and YG10 (Figure 6). The upper test section of well YG1 comes across reef complex reservoir formation with high porosity. While well YG10 come cross reef complex, the porosity of the matrix of the test section become slightly higher. We have verified the reflection characteristics of the seismic section of reef complex, and then we can identified reef complex from seismic section in non-well areas. 4. Conclusion According to the result of the wave equation forwarding modeling, the characteristics of seismic response of the reef complex are mound-shaped and lenticular reflection. The reflection wave appears upper bend phenomenon. The reflection wave around the formation of the reef complex top shows fragmentation phenomenon, the amplitude of the reef complex becomes weaker, and wave shape of the reef complex is discontinuous. The wave group characteristics of carbonate formation is the weaker reflection on wave trough. These phenomenons match the characteristics of the seismic section of well with reef complex.. Figure 5. the reef complex reflection characteristics in the section through well YG1 144
According to the result of synthetic seismogram and fine calibration, combining the characteristics of the seismic section of well with a reef complex in the project, we can built a characteristics recognition pattern of seismic of a reef complex. Through forward modeling to formation with reef complex, we can reduce the multiplicity and limitation of the prediction and seismic recognition of reef complex, and improve accuracy and precision of recognition. Figure 6. the reef complex reflection characteristics in the seismic section through well YG10 5. Reference: [1] Wang Ling, Zhang yan, Wu Lei, Ma Xiaoyu, Xu Minghua. Characteristics and identification of bioherms in the Amu Darya Right Bank Block,Turkmenistan, Natural Gas Industry, 2010, 30(5): 30-33. [2] Qi Baoquan, Ran Zhibing, Wang Xueqin, Su Xiaoyong. Identification of limestone reservoirs and prediction of their fluid properties in the Amu Darya Right Bank Block,Turkmenistan, Natural 145
Gas Industry. 2010, 30(5):21-25 [3] Sun Lin, Chen Xiaoqing, Zhang Yanzhong. Several mean to predict or recognize subsalt reef complex. Oil Geophysical Prospecting, 2009, 44(supplement issue 1):79-83. [4] Xiong Xiaojun, He Zhenhua, Huang Deji. Numerical simulation on seismic response characteristics of reef complex. Acta Petrolei Sinica, 2009, 30(1):75-79. 146