Geophysical methods for the study of sedimentary cycles

We present the wavelet depth-frequency analysis and variable-scale frequency cycle analysis methods to study sedimentary cycles. The spectrum analysis, variable-scale frequency cycle analysis, and wavelet depth-frequency analysis methods are mainly discussed to distinguish sedimentary cycles of different levels. The spectrum analysis method established the relationship between the spectrum characteristics and the thickness and number of sedimentary cycles. Both the variable-scale frequency cycle analysis and the wavelet depth-frequency analysis are based on the wavelet transform. The variable-scale frequency cycle analysis is used to obtain the relationship between the periodic changes of frequency in different scales and sedimentary cycles, and the wavelet depth-frequency analysis is used to obtain the relationship between migration changes of frequency energy clusters and sedimentary cycles. We designed a soft-ware system to process actual logging data from the Changqing Oilfield to analyze the sedimentary cycles, which verified the effectiveness of the three methods, and good results were obtained.

Study of sedimentary sequence cycles by well-seismic calibration

In order to solve the problems of the fine division of sedimentary sequence cycles and their change in two-dimensional space as well as lateral extension contrast, we developed a method of wavelet depth-frequency analysis. The single signal and composite signal of different Milankovitch cycles are obtained by numerical simulation. The simulated composite signal can be separated into single signals of a single frequency cycle. We also develop a well-seismic calibration insertion technology which helps to realize the calibration from the spectrum characteristics of a single well to the seismic profile. And then we determine the change and distribution characteristics of spectrum cycles in the two-dimensional space. It points out the direction in determining the variations of the regional sedimentary sequence cycles, underground strata structure and the contact relationship.

Lateral downslope transport and tentative sedimentary organic carbon box model in the southern Yap Trench,western Pacific Ocean

Sediment collapse and subsequent lateral downslope migration play important roles in shaping the habitats and regulating sedimentary organic carbon(SOC)cycling in hadal trenches.In this study,three sediment cores were collected using a human-occupied vehicle across the axis of the southern Yap Trench(SYT).The total organic carbon(TOC)and total nitrogen(TN)contents,δ13C,radiocarbon ages,specific surface areas,and grain size compositions of sediments from three cores were measured.We explored the influence of the lateral downslope transport on the dispersal of the sediments and established a tentative box model for the SOC balance.In the SYT,the surface TOC content decreased with water depth and was decoupled by the funneling effect of the V-shaped hadal trench.However,the sedimentation(0.0025 cm/a)and SOC accumulation rates(∼0.038 g/(m^(2)·a)(in terms of OC))were approximately 50%higher in the deeper hadal region than in the abyssal region(0.0016 cm/a and∼0.026 g/(m^(2)·a)(in terms of OC),respectively),indicating the occurrence of lateral downslope transport.The fluctuating variations in the prokaryotic abundances and the SOC accumulation rate suggest the periodic input of surficial sediments from the shallow region.The similar average TOC(0.31%–0.38%),TN(0.06%–0.07%)contents,and SOC compositions(terrestrial OC(11%–18%),marine phytoplanktonic OC(45%–53%),and microbial OC(32%–44%))of the three sites indicate that the lateral downslope transport has a significant mixing effect on the SOC composition.The output fluxes of the laterally transported SOC(0.44–0.56 g/(m^(2)·a)(in terms of OC))contributed approximately(47%–73%)of the total SOC input,and this proportion increased with water depth.The results of this study demonstrate the importance of lateral downslope transport in the spatial distribution and development of biomes.

Oil／Gas Accumulation Characteristics and Exploration Methods of the Deltaic Lithologic Reservoirs in Northern Shaanxi Area

There are large deltaic systems in the Triassic Yanchang Formation in the northern Shaanxi area of the Ordos Basin, and developed two sets of good source-reservoir-caprock assemblages and many sets of oil-bearing beds. Exploration experience demonstrates that the formation and distribution of the reservoir were controlled by the generative depression of the Yanchang Formation, and deltaic reservoir sand body is the material basis for large-scale oilfields. In addition, secondary laumontite in a low permeable area was dissolved and then a high permeable area was formed. The updip lithologic variety of reservoir sand bodies is favorable to the formation of subtle lithologic traps, and the deltaic reservoirs are characterized by large multi-beds of oil-generation and abundant hydrocarbon resources. In this paper, the petroleum geologic settings of the studied area are analyzed, and the accumulation characteristics and exploration methods of lithologic reservoirs are summarized. It is of theoretical significance for the study of the exploration theories of lithologic reservoirs, and also expedites the exploration steps of deltaic reservoirs in the northern Shaanxi area.

Magnetostratigraphic Study of Meishan Permian-Triassic Section, Changxing, Zhejiang Province, China

The authors present new paleomagnetic results obtained at the Meishan Permian Triassic (P/T) boundary section, Changxing County, Zhejiang Province. 330 samples in Changxing Group of Permian, 9 samples at P/T boundary, and 25 samples in Qinglong Group of Triassic are studied. Thermal and alternative filed demagnetization isolated a stable magnetization from most samples. This stable magnetization was thought to be acquired at or close to the time of formation of the rocks. The constructed polarity column shows that the bottom of the sampling section is forming a short reversed polarity. Above this short reversed polarity, there is a normal polarity, which lasts about 1.0 Ma. Below this short reversed polarity, are present 6 normal polarities and 7 reversed polarities between the upper Lower Changxing and Upper Changxing Group. Near the P/T boundary, the magnetization polarity transfers from reversal to normal, and the normal polarity may characterize the rest of the section. Susceptibility measurement of 550 samples shows a sharp change at the P/T boundary and a high correspondence with the minor frequency cycle of sedimentary. The spectrum of susceptibility indicates that there are 4 sedimentary cycles during 252-248 Ma in the section.

Characteristics and formation of sinian(Ediacaran)carbonate karstic reservoirs in Dengying Formation in Sichuan Basin,China

The Gaoshiti-Moxi Gas Field has been discovered recently in the central part of Sichuan Basin,which might be the biggest gas field in China.In this gas field,a large amount of gas is from Neoproterozoic Sinian(Ediacaran)karstic carbonate rocks.The gas was originated from cracking of oil,and the oil source rocks are the Sinian and Cambrian strata.In this study,the characteristics of these carbonate reservoir rocks have been studied based on subsurface data and outcrop observation.The carbonate rocks are at the Sinian Dengying Formation,and the reservoir properties are controlled by sedimentary facies and diagenesis.The Dengying Formation is composed of algal mounds and grainstone build-ups,which were developed in a restricted platform setting from glacial period to interglacial period.Due to tectonic movement and eustatic cycle in Sichuan Basin during Neoproterozoic Era,five sedimentary cycles of transgression and regression have been developed in the Dengying Formation.According to the sedimentary facies and carbon-oxygen isotope variation,five sedimentary cycles have been recognized.Due to the penecontemporaneous dolomitization,together with hypergene karstification and late burial dissolution,the reservoir properties of the carbonates rocks have been improved greatly.