Hong Liu

High-precision time-frequency (TF) analysis (TFA) can accurately detect anomalies in oil and gas reservoirs. However, conventional TFA methods cannot effectively identify fractured-vuggy carbonate reservoirs, which are deep and strongly heterogeneous. In this paper, a TFA method based on analytical mode decomposition (AMD) and the Hilbert-Huang transform (HHT) is proposed. Specifically, seismic signals are decomposed by empirical mode decomposition (EMD) and then by AMD. Finally, the TF distribution of the seismic signals is obtained after Hilbert spectrum analysis (HSA). Synthetic seismic data demonstrate that the AMD-HHT method can effectively eliminate mode mixing and provide higher time and frequency resolutions and energy focus than conventional TFA methods. In addition, model and field seismic data also verify that the AMD-HHT method can effectively detect hydrocarbon-related energy anomalies hidden in broadband seismic data.

Based on data from cores, cuttings, thin sections, and logging and seismic activities, the lithofacies, geophysical response, and spatial distribution characteristics of the Permian igneous rocks in the northern part of central Sichuan were analyzed and summarized. Furthermore, the eruption and filling processes of the igneous rocks were restored, and the hydrocarbon geologic significance of the volcanic eruption events was investigated. The results reveal that the igneous rocks in this area are volcanic overflow facies and sedimentary facies, with volcanic lava and pyroclastic rocks developed. Only one eruption cycle occurred vertically. The igneous rocks exhibit low natural gamma ray, low acoustic, high compensation neutron logging, high density, and high resistivity logging values. The seismic responses are characterized by multiaxial, flexural, and upward arching. The plane distribution shape is an inverted U, with a nearly east–west linear distribution. The igneous rocks in the area were emplaced via a fissure eruption along the basement faults in the early Longtan. The eruption-filling process can be roughly divided into four stages: the deposition of the Maokou Formation, the geomorphological embryonic stage, the geomorphological differentiation stage, and the volcanic eruption and filling stage. The underdevelopment of primary pores and the lack of dissolution in the later stage led to poor igneous reservoir performance, which limits the igneous reservoir hydrocarbon exploration prospect. However, the early faulting activity resulted in the differentiation of the Maokou Formation sedimentary and karst landforms. The thermodynamic conditions created by the later eruption of the igneous rocks, sufficient Mg2+ sources, and favorable geomorphological units laid a good foundation for the development of the dolomite reservoirs in the Maokou Formation. Among them, the karst slopes and highland areas of the Maokou Formation along the two sides of the strip of igneous rocks are dominant areas for the development of the dolomite reservoirs in the Maokou Formation.

Corrigendum: Image recognition–based identification of multifractal features of faults

The requirements for the accurate characterization of shoal sediments have increased in view of the fact that strata are eroded due to uplift and it is difficult to calculate the real thickness of granular shoal. To solve this problem, with the shoal-to-strata ratio, the type and distribution range of carbonate rock shoal facies are described, and the characterization of sedimentary facies is subsequently presented. Taking the P₂m²a in Eastern Sichuan as an example, the following conclusions were obtained. (1) The primary rock types of the shoal facies of the P₂m²a are bioclastic limestone and micrite limestone. The logging facies of platform margin shoals, intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals are large funnel-type, small funnel-type, box-type, and finger-like, respectively. The platform margin shoals and intra-platform high-energy shoals are imbricate seismic facies, and the intra-platform low-energy shoals and sloping low-energy shoals are S-type progradation seismic facies and oblique progradation. (2) The shoal-to-strata ratio of platform margin shoals is the highest, followed by intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals. (3) Shoal facies deposits have a certain controlling effect on gas production, and platform margin shoals have greater exploration potential. (4) This example shows that in the areas where carbonate rock strata are eroded, compared with the method of characterizing the sedimentary facies of carbonate rock using the shoal thickness, the shoal-to-strata ratios are more accurate. (5) The planar distribution diagram of the shoal-to-strata ratio can be used as a supporting map for compiling sedimentary facies maps. This method is worth applying in related research fields.

The classification method of karst formations is widely used in engineering and environmental geology but is seldom used in petroleum geology. In this study, the classification method of karst formations is applied to the sealing study of shale gas roof and floor carbonate rocks, and the influence on shale gas accumulation and drilling is discussed. The Paleozoic black shale in southern China is primarily formed by marine and transitional faces, and the intergrowth between shale and carbonate rocks is a basic geological feature of the Paleozoic strata in southern China. Carbonate karst is an unavoidable problem in shale gas exploration in southern China. Around the black shale target layer, in the Upper Paleozoic trait region, the study starts from the development strength of karst strata, through geological profile survey, spring flow statistics, test, and other methods and means; the shale and the carbonate rock contacted with it are taken as a whole to explore the impact of karst strata on shale gas drilling. The Upper Paleozoic karst strata in the study area were divided into two kinds, four types, and six subtypes. It was determined that the limestone continuous karst strata of the Sidazhai Formation and the second member of the Nandan Formation are the sensitive layers of shale gas drilling, whereas the first number of Nandan and Wuzhishan formations are shale reservoir-forming packers. In addition, a method for evaluating the karst sensitivity of shale gas exploration is summarized. The karst avoidance, karst-sensitive, and karst-insensitive areas for shale gas exploration were divided. Combined with the surface and underground conditions and the basic geological conditions of shale gas not being significantly different, shale gas drilling should avoid the fold core and fault zones.

Geologists have made several advances in applying multifractal theory in geology; however, some questions such as a large statistical workload and low efficiency remain unanswered. Thus, this study proposes an image recognition–based method for calculating fault multifractality. First, grayscale processing and binarization of the fault distribution map were performed. The image was then gridded, and the grids were numbered. Subsequently, computer image recognition technology was used to count the number of faults in each grid as a replacement for manual counting. Finally, the fractal dimensions of the faults were calculated using a multifractal box-counting algorithm. This method was successfully applied to fracture studies of the Maokou Formation in southeast Sichuan. Compared to the conventional approach, the proposed method demonstrated considerably improved work efficiency and accuracy. The results showed that the faults in the study area exhibited good statistical self-similarity in the scale range, indicating fractal characteristics. The fractal dimensions of faults with different orientations and the planar distribution of the fractal dimension contours indicate tectonic stages and stress magnitude in the study area. The results indicate that the tectonic setting of southeast Sichuan was formed primarily during the Indosinian, Yanshanian, and Himalayan periods. From the Indosinian to the early Yanshanian periods, NE-trending faults with relatively large fractal dimensions developed under NW–SE compressional tectonic stress. From the Late Yanshanian to Early Himalayan, EW-trending faults were formed by relatively weak N–S compressional stress and had the lowest fractal dimensions. The NW-trending faults formed by intense NE–SW compressional tectonic stress in the Late Himalayan region had the highest fractal dimensions. To promote oil and gas migration and ensure that faults do not destroy the caprock, oil and gas reservoirs must be in a relatively mild tectonic environment. Thus, the fractal dimensions of faults in favorable areas should be neither too high nor too low. The relationship between the fractal dimensions of faults and well test results in southeast Sichuan indicates that the region along the wells “ls1–xia14–guan3” (with fractal dimensions of 1.49–1.57) in the study area is a relatively favorable region for oil and gas preservation.