Recovery mechanisms and formation influencing factors of miscible CO2 huff-n-puff processes in shale oil reservoirs: A systematic review

Dynamic mechanisms of tight gas accumulation and numerical simulation methods: Narrowing the gap between theory and field application

Although significant progress has been made in tight gas exploration and development, there is still a limited understanding of the gas accumulation mechanism in tight formation. The description of heterogeneity in the tight reservoir is still an obstacle during tight gas exploration. In this work, we will develop a 3D gas accumulation facility that can reflect the high temperature and pressure under the geological condition to simulate the gas accumulation process in the tight formation of the Jurassic Formation, Sichuan Basin. Both the physical experiment simulation and numerical simulation methods will be combined to reveal the mechanism of gas accumulation. The results show that (1) the permeability ratio can characterize the heterogeneity of tight reservoirs. Based on this parameter, petroleum geologists can understand the interlayer heterogeneity during fluvial deposition and predict the sweet spots of tight gas; (2) the permeability ratio relationship between hydrocarbon accumulation of the tight sandstone reservoir mainly manifests as long as there is the existence of the differential permeability and there will be differences in natural gas migration dynamics, so that the natural gas accumulates in pores and depends on the extent of source rock; and (3) although the difference between capillary force curves is at the core of the influence of the permeability ratio on the gas-bearing capacity of the “sweet spot” sand body, the permeability ratio is a parameter of practical engineering significance. This work adds new methods to tight gas formation heterogeneity characterization and sheds light on the mechanisms of hydrocarbon accumulation in tight formation.

The identification of the oil-source correlation plays a significant role in petroleum exploration and development. In this study, we identify the oil-source correlation by a hierarchical cluster analysis method combined with traditional methods. The results shed light on the oil-source correlation in Minfeng area and revealed the oil migration and accumulation process. The crude oil in different structural belts and different horizons has different geochemical characteristics. According to the four types of crude oil and their planner distribution, it was considered that the crude oil mainly migrates along with favorable sand bodies and unconformity surfaces in the lateral direction and then charged and accumulated in the glutenite of Sha3 and Sha4 members since the area from sag to Yan Jia Oil and the gas field was lacking of oil source faults. Further analysis shows that the traps of fault blocks in Yong’anzhen are formed in the same phase, while the crude oil generated in the early stage is charged and accumulated in the fault block of the near source. Along with increasing of the buried depth of source rocks, the overlying source rocks gradually entered into the hydrocarbon generation phase, when crude oil started to charge in the fault blocks farther away.

The Ordos Basin, one of China’s most significant onshore oil and gas basins, hosts the Triassic Yanchang Formation, renowned for its multi-layered oil production and three-dimensional distribution of diverse hydrocarbon resources. The formation’s abundant crude oil reserves, coupled with the stratified distribution of shale oil, tight oil, and conventional oil, have garnered substantial attention from researchers globally. Understanding the distribution patterns and controlling factors of these reservoirs is crucial for optimizing exploration and development strategies. This study provides a comprehensive analysis of the sedimentary facies within the Yanchang Formation, elucidating the interplay between sedimentary sequences and environmental-climatic changes. It challenges the traditional view that lake-level fluctuations predominantly control reservoir sand body distribution, proposing instead a new sedimentary model driven by the combined effects of tectonic activity and alternating dry-wet climatic conditions. Our findings reveal that the Yanchang Formation’s climate exhibited a tripartite division, with abrupt climatic fluctuations likely influenced by major tectonic events such as those related to the Quinling orogeny such as the Qinling orogeny. The middle Triassic (Chang 10∼Chang 8) and late Triassic (Chang 3∼Chang 1) stages were characterized by arid conditions. During these periods, the eastern basin, with its gentle topography and limited catchment areas, was dominated by flood-driven sedimentation events. In contrast, the steeper western region developed extensive braided river deltas. The middle stage (Chang 7∼Chang 4 + 5), influenced by the Carnian Pluvial Episode (CPE), experienced a humid climate, fostering fluvial, deltaic, lacustrine and gravity flow deposits and the development of high-quality source rocks. Additionally, this study identifies and systematically analyzes sedimentary facies unique to arid environments during low lake-level stages: the fluvial fan system. We define its developmental conditions, distribution patterns, and sedimentary sequence variations in response to climatic changes. This discovery challenges the conventional view that the Yanchang Formation is dominated by shallow-water delta deposits, offering new insights into the sedimentary theory of continental basins and providing valuable guidance for future oil and gas exploration.

Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.