Kongyou Wu

Characteristics and formation mechanisms of large volcanic rock oil reservoirs: A case study of the Carboniferous rocks in the Kebai fault zone of Junggar Basin, China

Volcanic hydrocarbon reservoirs are rare and may be overlooked. The Carboniferous volcanic rocks of the Kebai fault zone in the western Junggar Basin contain hydrocarbon (HC) reservoirs in volcanic rock with proven oil reserves of 9.76 × 108 bbl that have a complex filling history. We have investigated the lithology and properties of these volcanic rock HC reservoirs as well as diagenesis and control of faults and fractures in oil reservoirs. The lithology of these Carboniferous volcanic rocks is primarily andesite and tuff. Also present were volcanic breccia and metamorphic rock in addition to rhyolite, felsite, diabase, and granite in the volcanic lava. On the basis of microscopic examination, five types of pores and fractures were observed: (1) fracture–dissolved phenocrystal pore, (2) fracture–intergranular pore, (3) fracture–gas pore, (4) fracture–dissolved intragranular pore, and (5) fracture–dissolved matrix pore. The fractures in these rocks are a significant factor in connecting the pores. Diagenetic processes that control reservoir quality include compaction, filling of pores and fractures, cementation, metasomatism, and grain dissolution. The volcanic reservoirs show a variety of lithologies, and oil has been discovered in all types of Carboniferous rocks. The controlling factors for oil distribution in these Carboniferous volcanic rocks are faulting, fracture development, and degree of weathering when they were subaerially exposed in the Permian. The area in which these faults and fractures developed is the primary area of oil enrichment with high yields. The objectives of this study were to (1) describe the characteristics of different types of volcanic rocks and reservoirs found in this basin and (2) characterize the diagenetic history of these rocks and document how diagenesis controls porosity and permeability.

Unraveling the influence of throw and stratigraphy in controlling subseismic fault architecture of fold-thrust belts: An example from the Qaidam Basin, northeast Tibetan Plateau

Understanding the detailed fault architecture of reverse faulting is critical for understanding the processes involved in fold-thrust belts as well as predicting the degree of fault compartmentalization, the relationship between folds and faults, the distribution of strain, and subseismic faulting deformation. The Lenghu5 fold-thrust belt provides an exceptionally well-exposed outcrop example of a reverse fault-related fold. Detailed stratigraphic logging coupled with high-resolution cross sections provides a unique insight into the three-dimensional geometry of a thrust fault at both basin and outcrop scale. In this study, we observe that 85%–90% of the estimated throw is accommodated on the main fault zone, which has sufficient throw to be imaged on a seismic profile, whereas 15%–20% of the throw is accommodated on smaller-scale folds and faults that are beyond seismic resolution. The plan-view mapping of the structure reveals that there is significant variation in how strain is accommodated along the structure, which is associated with the throw variations in the main fault. In addition, by coupling the structural observations within a stratigraphic context, we can demonstrate that although the main fault controls the overall strain in the system, the local stratigraphy plays a critical role in how the strain is accommodated and whether it is partitioned into single faults, multiple-fault splays, or folding. By demonstrating the remarkable geometric similarity between the outcrop observations with a comparable structure in the subsurface (Niger Delta), the study provides an insight into the potential subseismic fault-zone geometry present in poorly imaged fold-thrust systems.

Structural characteristics and implication on tectonic evolution of the Daerbute strike-slip fault in West Junggar area, NW China

The Daerbute fault zone, located in the northwestern margin of the Junggar basin, in the Central Asian Orogenic Belt, is a regional strike-slip fault with a length of ~ 400 km. The NE-SW trending Daerbute fault zone presents a distinct linear trend in plain view, cutting through both the Zair Mountain and the Hala’alate Mountain. Because of the intense contraction and shearing, the rocks within the fault zone experienced high degree of cataclasis, schistosity, and mylonization, resulting in rocks that are easily eroded to form a valley with a width of 300–500 m and a depth of 50–100 m after weathering and erosion. The well-exposed outcrops along the Daerbute fault zone present sub-horizontal striations and sub-vertical fault steps, indicating sub-horizontal shearing along the observed fault planes. Flower structures and horizontal drag folds are also observed in both the well-exposed outcrops and high-resolution satellite images. The distribution of accommodating strike-slip splay faults, e.g., the 973-pluton fault and the Great Jurassic Trough fault, are in accordance with the Riedel model of simple shear. The seismic and time-frequency electromagnetic (TFEM) sections also demonstrate the typical strike-slip characteristics of the Daerbute fault zone. Based on detailed field observations of well-exposed outcrops and seismic sections, the Daerbute fault can be subdivided into two segments: the western segment presents multiple fault cores and damage zones, whereas the eastern segment only presents a single fault core, in which the rocks experienced a higher degree of rock cataclasis, schistosity, and mylonization. In the central overlapping portion between the two segments, the sediments within the fault zone are primarily reddish sandstones, conglomerates, and some mudstones, of which the palynological tests suggest middle Permian as the timing of deposition. The deformation timing of the Daerbute fault was estimated by integrating the depocenters’ basinward migration and initiation of the splay faults (e.g., the Great Jurassic Trough fault and the 973-pluton fault). These results indicate that there were probably two periods of faulting deformation for the Daerbute fault. By integrating our study with previous studies, we speculate that the Daerbute fault experienced a two-phase strike-slip faulting deformation, commencing with the initial dextral strike-slip faulting in mid-late Permian, and then being inversed to sinistral strike-slip faulting since the Triassic. The results of this study can provide useful insights for the regional tectonics and local hydrocarbon exploration.