Depositional characteristics and petroleum geological significance of wetland

Wetland is a new type of sedimentary facies proposed for the first time in this paper. It is a type of sedimentary environment, transitional between land facies and water (sea or lake) facies. In this paper, wetland is redefined as “a type of sedimentary environments whose ground surface is flat and wet throughout the year, which may be covered with very shallow water (less than 2 m deep), and is covered with lush plant growth”. Wetland is reclassified into two types, swamp wetland (swamp in brief) and wet plain wetland (wet plain in brief). Swamp deposits are coal, while wet plain deposits are dark colored mudstones or silty mudstones rich in plant fossils and carbon debris. The deposits of wet plain are different from those of lake and floodplain in the abundance of plant fossils, color, resistivity well logging curve, and other sedimentary characteristics. In boreholes, resistivity logging can be used to distinguish between wet plain mudstones and lake mudstones. Understanding the sedimentary characteristics of wet plain wetland can help to identify wet plain deposits which were formerly classified as floodplain or lake deposits. This will help to reconstruct the palaeogeography and to understand the history of basin and climate evolution accurately, and is especially important in evaluation of the hydrocarbon generating potential of a basin. With the Jurassic Sangonghe Formation of the Yanqi Basin as an example, the identification characteristics of wetland are described.

Research on Thin Layer Structure Identification and Sedimentary Facies of Middle and Deep Layers Based on Reflection Coefficient Inversion—By Taking Dongying Formation of CFD Oilfield in Bohai Offshore as an Example

The sand layer B of Dongying Formation of CFD oilfield in Bohai offshore belongs to the middle deep layer of buried hill overlap deposit. Its reservoir distribution has the characteristics of large burial depth, thin thickness and rapidly lateral change. Because of low resolution of seismic data and overlying sand layer. It is difficult to identify and interpret the structure of sand layer accurately. The uncertainty of structure and reservoir restricts the fine development of B sand layer. In order to identify the top surface of reservoir effectively. The seismic data are processed by using the reflection coefficient inversion method. The results show that the inversion resolution of reflection coefficient is significantly higher than that of original data. The top surface of sand layer B and its overlying sand layer can be well identified and traced. Carrying out structural interpretation of B sand layer based on reflection coefficient inversion data and the microstructure and the formation tip extinction point are implemented. Based on the constraint of new interpretation level, the sedimentary facies plane distribution of B sand layer is described and make prediction of dominant reservoir development area in detail combining with sedimentary paleogeomorphology, along layer attribute section and limited drilling data. The research shows that the study area is mainly from the northwest material sources, the slope belt in the northwest is close to the lake shoreline with a gentle slope and shallow water depositional environment, which is located on the main transport and deposition channels. The shallow water gentle slope landform is suitable for forming large-area sand bar deposition, mainly composed of underwater distributary channel and debouch bars facies, which is the dominant reservoir development area. The research conclusion guides the deployment and implementation of the development well location effectively.

Distribution prediction of the glutenite in Member 3 of Triassic Xujiahe Formation in Yuanba area,northern Sichuan Basin

Although the Triassic Xujiahe Formation in Yuanba area of northern Sichuan has a large amount of resources,the proportion of proved reserves to the amount of resources is low.Production of most of the pilot production wells is high at the initial stage,but it declines rapidly,mainly due to tight rock properties and strong heterogeneity of Xujiahe Formation.Thus,it is important to investigate the distribution of high-quality reservoirs.The Member 3 of Xujiahe Formation develops sandstone interbedded with mudstone,and its lateral thickness of different sandstone groups varies greatly,so it is a challenge to identify appropriate time windows for seismic attribute analysis,thus,so the prediction effect of the glutenite in the Member 3 of Xujiahe Formation.In this study,through analysis of core data and logging facies,single-well sedimentary microfacies subdivision is carried out.Single-well lithological interpretation is also studied with application of lithological identification technique.Then,lithological associations and its seismic responses at the top and bottom interfaces of each sandstone group in the Member 3 of Xujiahe Formation are well investigated,The lithological association with the glutenite above and the mudstone below the stratigraphic interface respectively,corresponds to a trough in seismic profiles;in this case,when extracting the seismic attributes of sandstone groups above the stratigraphic interface,only the difference of between drilling horizons and seismic horizons needs to be considered.The lithological association with the mudstone above and glutenite below the stratigraphic interface respectively,corresponds to a peak in seismic profiles;in this case,when extracting the seismic attributes of sandstone groups above the stratigraphic interface,influence of which should be eliminated;thus,a method of time window determination for seismic attribute extraction is established.Among amplitude,frequency,and phase attributes,the total peak energy has the good correlation with thickness of the glutenite at well sites and it also can reflect the distribution of distributary channels well.The results show that belt-like glutenite in each sandstone group in the Member 3 of Xujiahe Formation extends in a NW-SE direction and thins out toward the southeast,and the distribution of glutenite has certain inheritance in the longitudinal direction,the glutenite in the third sandstone group in the Member 3 of Xujiahe Formation(TX33)and the first sandstone group in the Member 3 of Xujiahe Formation(TX31)are both developed.