Based on the research results of sediments at home and abroad, the relationship between lithology and composition of sediments and natural gas hydrate mainly has the following characteristics:
1) Marine natural gas hydrate is mainly produced in soft and loose sediments with coarse particles, such as sandy ooze. This kind of sediment is generally coarse in particle size and large in porosity. For example, the grain size of hydrate sediments found in DSDP570 station of Sino-American Trough is much larger than that of upper and lower non-hydrate sediments, and the grain size content of sand and silt is obviously increased.
2) As far as its sedimentary age is concerned, most soft and loose sediments containing natural gas hydrate are strata since Miocene. For example, the natural gas hydrate obtained from ODP of hole 997 in Heiji is distributed in Pliocene strata; However, some natural gas hydrates seeping through structural fractures or salt fractures can be distributed in Holocene strata, such as the natural gas hydrate samples obtained by ocean drilling at sousaphone and Heiji 996 Station in the East Pacific Ocean.
3) Some research results show that the sediments in the gas hydrate stability zone are rich in diatom fossils. It is speculated that the existence of a large number of diatoms increases the porosity and permeability of sediments because they have more pore structures. In addition, these diatom-rich sediments are formed in the local environment with suitable paleoclimate and high biological productivity, and are one of the sources of organic carbon.
4) The sediments containing natural gas hydrate are rich in organic carbon (TOC), and the organic carbon content is generally above 0.5%.
5) Natural gas hydrate producing areas are often accompanied by authigenic carbonate minerals or other authigenic minerals. The results of ODP sampling analysis in hole 997 of Heiji show that the lithology and mineral composition at the top boundary of natural gas hydrate suddenly change at 180m below the seabed, and the contents of calcite and plagioclase increase, but the timely contents decrease.
6) The sedimentation rate of gas hydrate-bearing strata is generally faster, exceeding 30m/Ma. The Cenozoic gas hydrate deposition rate in the Central American Trough in the East Pacific Ocean is as high as1055 m/ma; /Horse. Of the four hydrate accumulation areas in the continental margin of the United States in the western Pacific Ocean, three are related to rapid deposition areas, and the deposition rate of sediments from late Oligocene to Holocene in Blackridge reached 160 ~ 190 m/ma. The reason is that most marine natural gas hydrates are biogenic methane gas (Kvenvolden et al., 1980), which accumulates a large amount of organic debris in the rapidly deposited semi-deep-sea sedimentary area, and is preserved because it is quickly buried in the seabed without oxidation, and is converted into a large amount of methane by bacteria in sediments. Therefore, in the rapid sedimentary area, combined with gas source analysis, it is usually possible to predict that there are abundant natural gas hydrates.
Second, the sedimentary characteristics of ocean drilling core data
During the period of 1999, the international ocean drilling program voyage 184 conducted drilling sampling in Nansha and Dongsha waters of the South China Sea. Because there are only these boreholes in the South China Sea slope at present, the obtained sediments and related research results are valuable and practical materials for understanding the characteristics of sediments below the surface of the South China Sea slope, such as sedimentary age, lithological characteristics and sedimentary environment.
Black Ridge Ocean Drilling 164 voyage is a special drilling study of marine natural gas hydrate. This voyage not only obtained natural gas hydrate samples, but also analyzed the related sedimentary characteristics. When analyzing the sediments drilled in the South China Sea on voyage 184, this report will compare with the data of voyage 164, so as to know in more detail which sedimentary conditions are favorable for the accumulation and distribution of natural gas hydrates in the South China Sea.
Based on several drilling data of international ocean drilling in the South China Sea and Black Ridge, this project has carried out the following research: collecting and synthesizing biostratigraphic and paleomagnetic data, determining the stratigraphic age framework of these drilling cores since Oligocene, and determining some main stratigraphic boundaries and sedimentary thickness; On this basis, the change of sedimentation rate in this area since Oligocene is analyzed, and the correlation diagram of sedimentation rate of these borehole cores is drawn, and the sedimentary sections with high sedimentation rate are delineated and marked. Collect and synthesize the descriptions of these drilling cores, analyze, sort out and calibrate the intervals with coarse lithology, collect and synthesize some existing paleontological and geochemical analysis results in these wells, and sort out and calibrate the intervals with high diatom abundance and high paleoproductivity on this basis; In addition, some related geochemical data, such as methane gas in sediments and chloride ion content in pore water, are collected, analyzed and compared in order to better understand and evaluate the possible gas hydrate areas.
Three. Sedimentary characteristics of seismic data
Studies at home and abroad show that natural gas hydrates are mainly formed on slopes and foot of slopes (Dou Shi et al., 1992), because the slope area is conducive to rapid deposition, forming a sandy or porous sedimentary system, especially different fans formed under various gravity forces provide good storage space for the formation of natural gas hydrates; At the same time, due to the limitation of temperature and pressure conditions, it is speculated that the stable zone of natural gas hydrate is mainly distributed in shallow sea sediments above BSR, and the BSR in this area is basically above Miocene, so it is necessary to divide the stratigraphic sequence (base level cycle) since late Miocene (10.4Ma).
Sedimentary background and sedimentary facies control the sedimentary characteristics of sedimentary bodies. Therefore, in order to analyze the sedimentary conditions of natural gas hydrate, it is necessary to study the sedimentary background and sedimentary facies of its possible development areas, so as to understand the sedimentary process of sedimentary bodies, analyze sedimentary bodies conducive to the accumulation of natural gas hydrate, and then find favorable reservoirs for natural gas hydrate.
Due to the lack of other data, the study of sedimentary facies in this area depends entirely on the understanding of seismic facies. The so-called seismic facies is the seismic characteristics formed by sedimentary environment. Some people also understand it as a three-dimensional seismic reflection unit that can be mapped, which is composed of reflection wave groups with different seismic parameters of adjacent seismic phase units. However, due to the multiplicity of seismic facies, it must be limited by other methods when it is converted into sedimentary facies. Therefore, this study introduces the concept of seismic reflection architecture, which combines seismic facies with sedimentation. Its greatest feature is to organically combine the internal structure, external geometry and sedimentation represented by seismic reflection, thus making the judgment of sedimentary facies more practical and scientific.
1985 A.D.Miall put forward the concept of seismic reflection configuration, and further emphasized the theory of genesis of sedimentation, which is mainly based on the inevitability between sedimentary environment and sedimentary facies.
1. Related concepts and definitions
The so-called configuration is a lithologic body expressed by geometric shape, facies association and its scale, which can represent a specific sedimentation or a set of sedimentation processes in the sedimentary system. Seismic reflection structure is a combination of seismic reflection properties in sedimentary body and geometric characteristics formed by its specific deposition or process. Its form consists of four basic elements: internal seismic reflection structure, reflection structure, external geometry of seismic unit and sedimentation that forms sedimentary bodies.
2. Classification of seismic reflection structures
According to the concept of seismic reflection structure, a seismic reflection structure should include four basic elements, so the structure classification is also based on these four basic elements. If ordinary sedimentation is the main line (because it is the main factor controlling seismic reflection characteristics), five seismic configurations are identified in the study area. The definitions and characteristics of various reflection configurations are as follows (Table 2- 1).
Table 2- 1 Classification and characteristics of seismic reflection structures
(1) vertical product configuration
The reflection configuration is mainly vertical accretion. Vertical accretion means that during the whole deposition process, the topographic features of the deposition surface only extend directly upward without any lateral movement, including bottom load and suspension load during mechanical handling. The reflection structure of vertical sedimentary structure can be divided into two types according to the grain size of sediments: ① When the grain size is coarse, the reflection structure is chaotic or weak, and the reflection structure is parallel or nearly parallel, and its shape is wedge-shaped and lenticular, which is mainly formed by vertical accretion of the incised valley with strong hydrodynamic force; ② When the particle size is fine, the reflection structure can be weak reflection or no reflection, the reflection structure is concave, parallel or nearly parallel, and the seismic unit is mat-shaped. This reflective structure is mainly formed by the vertical accumulation of fine particles in still water. The river structure generally reflects the characteristics that the waterway is not easy to migrate and bifurcate, but the sedimentation rate is fast and changes greatly.
(2) Forward product configuration
The reflection structure is mainly formed by progradation or downstream accretion. Downstream accumulation (downstream accumulation) refers to the continuous accumulation of debris under certain circumstances. Usually accretion means that when the terrain is suddenly open and the slope is steep, the sediment carried by the river is deposited downstream, that is, the sediment begins to unload and gradually advance or accumulate at the part with open terrain and increasing slope. The formed reflection structure is weak reflection, strong reflection and chaotic reflection, with good continuity; Reflective structures mainly include various progradation reflection structures: S-shaped progradation structure, top supertype progradation structure, lower supertype progradation structure, inclined progradation structure, imbricate progradation structure, disordered progradation structure, composite progradation structure, bidirectional progradation structure and bidirectional (mound-like) reflection structure; The shape of seismic unit is wedge-shaped, lenticular or banded. Progressive structures are ubiquitous in the delta environment, and it is sedimentation that forms various delta sedimentary systems.
(3) Selective product configuration
The reflective configuration is dominated by selective products. The so-called selective deposition is due to the wave action in the catchment basin, which makes the sandy particles above the wave base leach back and forth, forming beach and bar deposition. The reflection structure of this structure is weak reflection with good continuity; Reflective structures mainly include parallel or nearly parallel and wavy reflective structures; The shape of seismic unit is plate or mat. This configuration is mainly a thin sand body formed in coastal environment.
(4) Filling configuration
Mainly filling. Filling mainly refers to the filling and siltation in the waterway. This process is the accumulation form in which a large amount of sediment carried by the water flow is unloaded and filled in the waterway when the energy of the water flow is less than the weight of the particles themselves. The filling structure is mainly erosion filling reflection structure, which is weak reflection, strong reflection or chaotic reflection with good continuity; Most seismic units are lenticular.
(5) Turbidite structure
The reflection structure is dominated by turbidity. Turbidity refers to the process that the particles in the mixture of sediment and water are supported by the upward component of fluid turbulence, so that the sediment is suspended and forms an obvious density difference with the overlying water body. Under the action of gravity caused by density difference, sediment flows along the (underwater) slope and accumulates forward. The reflection structure formed is weak reflection or chaotic reflection; Reflection structures mainly include oblique progradation structure, chaotic progradation structure and bidirectional (mound-like) reflection structure, and seismic facies units are wedge-shaped and lenticular.