The Upper Cambrian Xixiangchi Formation in the Southern Sichuan Basin, China, has favorable hydrocarbon accumulation conditions. The accumulation factors and enrichment conditions of this formation were key considerations in this study. By analyzing core, thin section, seismic, and geochemical data, the research shows that there are many sets of granular and crystalline dolomite reservoirs in the Xixiangchi Formation vertically, with thin thickness of single-reservoir. During the transformation of karst and tectonism, dissolution pores and fractures developed to form an ideal reservoir space. The reservoir of the Xixiangchi Formation is connected to the Lower Cambrian source rock through a fault system. The high-energy shoal facies of the Xixiangchi Formation are located on the oil and gas migration path, providing an appropriate reservoir space for forming the source reservoir configuration relationship between the lower generation and upper reservoir. The key factors affecting hydrocarbon accumulation in the Xixiangchi Formation are sufficient oil and gas supply, development of inherited paleo-uplift, effective transportation system, and favorable reservoir-forming combination. The inherited paleo-uplift controls the distribution of gas reservoirs. Owing to the short migration distance of oil and gas, hydrocarbon is found near source hydrocarbon accumulation, and the paleo-uplift slope area should be targeted for exploration in future studies.

Low-frequency shadows beneath gas reservoirs can be regarded as a time delay relative to the reflection from the reservoir zone, but they cannot be reasonably explained by the high-frequency attenuation or velocity dispersion observed in normal P-waves. According to the new asymptotic theory for porous media, seismic P-waves undergo multiple conversions between fast and slow modes during seismic waves passing through layered permeable reservoirs at low frequencies, and changes in the velocity and amplitude (i.e., energy) of slow P-waves can lead to low-frequency shadows. In this study, a forward analysis was performed on the dispersion and attenuation of fast and slow P-waves within the seismic frequency band based on the asymptotic theory for porous media; the results revealed that fast P-waves do not undergo frequency dispersion and attenuation within the seismic frequency band and that slow P-waves are the primary contributor of dispersion and attenuation. In addition, methods used to calculate the frequencies at which low-frequency shadows occur were analyzed and are discussed. Finally, S-transform time-frequency analysis method was used to calculate and analyze the low-frequency shadows of three-dimensional seismic data acquired from work area M in Sichuan. The low-frequency shadow anomalies determined by this method were found to be highly consistent with those identified based on the data acquired from wells in the target reservoir. These results indicate the good application performance of this method.

Despite the great success in the global exploration and development of reef reservoirs, research on bioclastic shoals under reefs is still in its infancy. Bioclastic shoal reservoirs are very thin, with multiple vertical levels and fast lateral changes, which makes accurate prediction of the reservoir’s location much tougher. To further implement the reservoir distribution, under the guidance of sequence stratigraphy, the prediction of thin shoals under the control of an isochronous stratigraphic framework was established. Using the combination of spectrum shaping and F-X domain noise suppression techniques and utilizing the signal-to-noise ratio spectrum set as the reference, logging curve as supervision, and well seismic calibration and isochronal amplitude slicing as quality control, the seismic frequency band was extended, and the seismic data resolution and signal-to-noise ratio were improved. After frequency extension, the global optimal seismic automatic interpretation technique was used to construct an isochronal stratigraphic framework model. Through waveform facies-controlled inversion and waveform facies-controlled simulation techniques, the elastic properties of the shoal reservoir were obtained, from which the planar distribution of the reservoir was accurately predicted. The above methods were applied to the prediction of the bioclastic shoal reservoir in the lower submember of the Changxing formation in the Yuanba gas field (China). The plane distribution of bioclastic shoal in the first and second levels was identified, which provides a guideline for the prediction of thin shoal reservoirs.

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.

In recent years, it has become more and more common to drill deep karst caves as a part of deep shale gas resource exploration and engineering construction in South China. However, the amount of research on the genesis and development mechanism of deep karst caves is relatively low. Based on drilling core karst morphology analysis and two-dimensional (2D) seismic and wide-field geophysical exploration methods, it is revealed that the deep karst in the Huanjiang area is mainly composed of net cracks, holes expanding along cracks and dolomite honeycomb pores, and that large karst caves are also developed, which are related to NW-trending faults. The deep karst is developed in the hanging wall of the fault, with a width of 500 m and a height of 1500 m, and a linear distribution along NW faults in the region. Based on Th-U dating, inclusion testing and rare earth elements from cave fillings, it is revealed that the development of deep karst space is related to two deep karst genetic types: The first is the hypogene hydrothermal karst, which developed in the Yanshanian period and is related to regional magmatism. The second is the groundwater deep circulation karst, mainly developed after the Himalayan period, which is related to the deep circulation of meteoric water. The genesis of deep karst space is the result of multi-stage karst superposition and is mainly controlled by faults. It is difficult to determine the specific time points of these two types of deep karst transformation, but according to regional tectonic evolution, we speculate that the Yunnan-Guizhou Plateau has been uplifted since the Himalayan period (>3.54 Ma), and the Carboniferous carbonate rocks and early faults in the Huanjiang area have been exposed, leading to the change and evolution of deep karst. Through comprehensive analysis, a fault-controlled hypogene hydrothermal karst pattern and a meteoric water deep karst pattern are established. The genetic pattern of deep karst provides theoretical support for predicting this kind of karst in southern China and for avoiding drilling deep karst caves as a part of resource exploitation.