High-frequency Cycles Of The 3rd And 4th Member Of Shahejie Formation In Dongying Depression And Its Geological Significance

As the most typical hydrocarbon accumulation zone in East China, Dongying Depression has kept new discoveries in recent decades and shows great potential for oil and gas resources. At the same time, with the deepening of exploration degree and increasing of exploration difficulty, subtle reserviors gradually change as the main exploration targets, and the 3rd and 4th member of Shahejie Formation is the most important layer that subtle reserviors developed in. On the other hand, a series of theories and technical methods for subtle reserviors exploration have been an unprecedented developed and success greatly in the exploration practice. But more refined strata classification subject to many restrictions like that strong tectonic activity, complex lithologic formation mechanism and distribution and incomplete stratigraphic cycle records of continental rift basin. High frequency cycles study provided a new way to this issue, the complete seismic and core and log and test data in Dongying Depression provided strong data protection and a higher degree of previous study also had laid a solid foundation.For controlling factors are complicated and be lack of continued profile, the study of high frequency cycle of continental basins is obviously lagging to marine basins. The 4th to 3rd member of Shahejie formation in Dongying depression experienced distinguished change of climate. And it is sure to contain the information of Milankovitch cycle. In this paper, The 4rth to the 3rd member of Shahejie formation where hydrocarbon accumulation mostly developed is chosen as the formation of interest. And GR data which is closely related with climate changes was chosen as the study object. After the environmental correction and standardization process, using sequence stratigraphy and sedimentology and Milankovitch cyclostratigraphy theory and method to make a system study on high frequence cycle, obtained the following results and conclusions:1. With the data including well core, seismic data and well logging data,4 sequence boundary of 3rd order has been recognized in the Es4s to Es3z member. Based on these boundaries, the stratigraphic sequence framework has been set up and the features of depositional systems of sequences have been released. Moreover, this paper also analyzed the effects of climate towards the sedimentary characteristics.2. This study calculated the earth orbit parameters during 30-50Ma in Dongying Depression with the La (2004) solutions. Based on the parameters, the method of spectrum analysis and wavelet analysis has been used to figure out the cycles for the eccentricity, gradient and precession. The results showed that:in 30-40Ma the eccentricity cycles mainly lie in 405ka, 124ka,95ka and the gradient cycles are mainly of 52ka,40ka and 39ka while during 40-50Ma, the eccentricity cycles are majorly of 405ka,125ka,96ka and the gradient cycles are 51ka,40ka, 38ka. Yet, the precession cycles are both of 23ka,22ka and 19ka. Furthermore, the curve of variation in theory has been computed and the ratios for the cycles during 30-40Ma and 40-50Ma have been calculated.3.After the correction and nomalization of the Es4s-Es3z GR data, spectral and wavelet analysis showed that Es4s was dominated by obliquity 40ka, however Es3x and Es3z were controlled by eccentricity 405ka and 125ka significantly.4. This study tuned the high frequency cycles in the Es4s to Es3z formation and selected the key curves of Milankovitch cycles in formation. It turned out that the cycle for Es4s member is 40ka and there are 34.5 obliquity cycles in total; the cycle for Es3x member is 405ka and there are 4.5 long eccentricity cycles in all; for Es3z member, there are about 40 short eccentricity cycles and the main cycle is 125ka.Then, the high frequency astronomical time scale for each formation has been established, leading to the more accurate geological time for each boundary and depth.5.Under the control of stratigraphic framework, according to the seismic and core and well logging data, the Dongying delta was divided into 9 periods named Z1-Z9, based on the sliding windows of spectral analysis the sedimentary rates of each period in the Es3z increased first and then decreased upwards, Z4 reached the maxmum 0.127m/ka at the most prosperous period of delta prograding. Finally the duration of each period and more accurate geological age of the interfaces were calculated on the basis of the sedimentary rates.This paper analyzed the average depositional rate of the Es4s and Es3x formation by sliding spectrum analysis, using the window of 100m and the step length,50m. The result showed the average depositional rate was O.llm/ka,0.19m/ka for Es4s and Es3x member separately. Moreover, the paper calculated the lasting time for each window.6. Take the strata 100m above and below the bottom boundary for instance, with the depositional rate data referred, this paper converted depth domain to time domain and gained the time series. The sequence was later analyzed with the tools of spectrum analysis and wavelet analysis and lead to the results that in the 50m strata above the base of Sha3 formation there were cycles of 12.06ka,11.29ka,7.94ka and 6.6ka, of which the cycle 11.29ka is the most dependable. In correspondence, the cycles of 11.7ka,8.35ka,6.5ka and 5.85ka were released from the 50 strata at the bottom of the lower Sha3 formation, with credibility of more than 99%.7. This paper studied the GR data from 10 wells in EW and NS sections that ran through the entire basin and recognized the high frequency cycles with spectrum analysis and wavelet analysis. By selecting 7 wells and setting their short cycles of eccentricity as reference for classifying the 5th sequence, this paper divided the high frequency cycles of Es3z member and made comparisons of them and then established the unified high-precision stratigraphic framework for the entire study area.