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Grain shoals are one of principal reservoir facies of carbonate rocks in the oil and gas exploration domains. With the discovery of oil and gas in the grain shoal reservoirs of Longwangmiao Formation in central Sichuan paleo-uplift, the characteristics and distribution of grain shoal reservoirs are still unclear in the northern slope of the central Sichuan paleo-uplift. Through integrated analysis of wireline logs and high-quality 3D seismic data, combined with seismic forward modeling and waveform classification, reservoir seismic facies, reservoir classification, and reservoir distribution of different grades are investigated. The results show that: (1) Under the thin thickness (i.e., the thickness is around 80 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the seismic waveform is manifested as a single peak, and the top interface of the Longwangmiao Formation corresponds to 1/8 λ above the single peak. (ii) When a single set of reservoirs is merely developed in the second member of the Longwangmiao Formation (SMLF), the waveform still displays a single peak, and the top peak moves downward. In addition, when multiple sets of reservoirs are developed in the SMLF, the seismic reflections of multiple reservoirs are consistent with those of a single reservoir with the same cumulative thickness. (iii) When the reservoirs are developed in both the first and second members of the Longwangmiao Formation, the single peak exhibits moving downward, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (2) Under the thick thickness (i.e., the thickness is approximately 110 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the waveform of the Longwangmiao Formation exhibits a double peak, and the top interface of the Longwangmiao Formation corresponds to the extreme value of the upper peak. (ii) When a single set of reservoirs or multiple sets of reservoirs are only developed in the SMLF, the Longwangmiao Formation exhibits a single peak, and the top interface of the Longwangmiao Formation corresponds to the trough. (iii) With the reservoir development in both the first and second members of the Longwangmiao Formation, the waveform of the Longwangmiao Formation shows an asymmetric low-frequency peak, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (3) Using the waveform classification method, the seismic waveforms corresponding to the Longwangmiao Formation could be summarized into four types, including two types of reservoir waveforms. Two reservoir waveforms are distributed as two strips spread from north to south in the central study area. This paper recommends that the waveform classification method is more refined than amplitude attribute extraction for carbonate reservoir prediction.
Grain shoals are one of principal reservoir facies of carbonate rocks in the oil and gas exploration domains. With the discovery of oil and gas in the grain shoal reservoirs of Longwangmiao Formation in central Sichuan paleo-uplift, the characteristics and distribution of grain shoal reservoirs are still unclear in the northern slope of the central Sichuan paleo-uplift. Through integrated analysis of wireline logs and high-quality 3D seismic data, combined with seismic forward modeling and waveform classification, reservoir seismic facies, reservoir classification, and reservoir distribution of different grades are investigated. The results show that: (1) Under the thin thickness (i.e., the thickness is around 80 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the seismic waveform is manifested as a single peak, and the top interface of the Longwangmiao Formation corresponds to 1/8 λ above the single peak. (ii) When a single set of reservoirs is merely developed in the second member of the Longwangmiao Formation (SMLF), the waveform still displays a single peak, and the top peak moves downward. In addition, when multiple sets of reservoirs are developed in the SMLF, the seismic reflections of multiple reservoirs are consistent with those of a single reservoir with the same cumulative thickness. (iii) When the reservoirs are developed in both the first and second members of the Longwangmiao Formation, the single peak exhibits moving downward, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (2) Under the thick thickness (i.e., the thickness is approximately 110 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the waveform of the Longwangmiao Formation exhibits a double peak, and the top interface of the Longwangmiao Formation corresponds to the extreme value of the upper peak. (ii) When a single set of reservoirs or multiple sets of reservoirs are only developed in the SMLF, the Longwangmiao Formation exhibits a single peak, and the top interface of the Longwangmiao Formation corresponds to the trough. (iii) With the reservoir development in both the first and second members of the Longwangmiao Formation, the waveform of the Longwangmiao Formation shows an asymmetric low-frequency peak, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (3) Using the waveform classification method, the seismic waveforms corresponding to the Longwangmiao Formation could be summarized into four types, including two types of reservoir waveforms. Two reservoir waveforms are distributed as two strips spread from north to south in the central study area. This paper recommends that the waveform classification method is more refined than amplitude attribute extraction for carbonate reservoir prediction.
The Eastern Sichuan area is a typical fold-and-impulse zone. It is bounded by the Qiyue Mountain fault, with septal folds on the east and on the west. Based on previous geological mapping and surface geological structure research as well as the fault-related folding theory, in this study, tectonophysical simulation experiments were conducted to investigate the tectonic geometry and kinematics of the Eastern Sichuan fold-impulse zone. The experiments were conducted by selecting different experimental materials and changing the physical properties of the cover, the friction between the cover and the basement, the number of slip layers, the burial depth, and other factors. Finally, the similarity of the results to the morphological characteristics of the real geological structure was assessed. The results show that the deformation style is influenced to some extent by the brittle shear strength and the ductile shear strength of the basement, and the different interlayer cohesions between the competent and incompetent layers, the high rheology, and the burial depth of the slickensides played crucial roles in the formation of the final evolution of the Eastern Sichuan fold fault zone. The step-like system, composed of a lower crustal crystalline basement detachment surface, regional boundary fractures, and the overlying Cambrian and Triassic synclines, is a necessary condition for the formation of the Eastern Sichuan fold and fault zone and controls the overall evolution of the zone. Under the action of multiple phases of tectonic activity with different main stress orientations, the geological phenomenon of multiple superimposed phases of folding in different directions observed at present in the Eastern Sichuan area was formed. The well-developed karst features are not conducive to the construction of a project tunnel in the area where tightly closed back-slope, fissures, and normal faults have developed in the Eastern Sichuan fold and fault zone.
The past decade has witnessed a breakthrough in the gas exploration of deep marine carbonates of the central Sichuan Basin. Deep faults research has also attracted increasing attention, as faulting plays an important role in reservoir control. Previous studies have suggested a developed series of high-angle strike-slip fault systems in the central Sichuan Basin, but correlated exploration activities are limited, as distribution rules and dynamic mechanisms remain unclear. In this study, the spectral decomposition coherence method was used to describe the geometric and kinematic characteristics of these strike-slip faults. Using a comprehensive analysis technique to assess the strike-slip fault tectonic activity history, the formation and evolution processes of strike-slip faults and their control on hydrocarbon distribution were examined. The results showed that the deep strike-slip fault system, mostly distributed in the Dengying Formation, can be divided into four stages, three levels, and three groups of orientation, which controlled the structural framework and shape of the central Sichuan area, as well as the zoning from north to south, and blocking from west to east. The faults showed features of layered deformation and staged evolution in the vertical direction. The segmentation of strike-slip faults strongly controls the quality of fractured vuggy reservoirs. Reservoirs of the hard-linked zone of the strike-slip fault are the most developed, followed by those of soft-linked segments, with translational sections of the strike-slip fault being relatively undeveloped. Strike-slip faults are important hydrocarbon migration paths, and their multistage activities have different controlling effects on hydrocarbon accumulation.
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