Prediction of grain shoal reservoirs via seismic forward modeling and waveform classification: Application to the northern slope of the central Sichuan uplift, Sichuan Basin, SW China
Authors: Lei Chao , Xu Shaohua , Li Yuhan , Li Ming , Wu Chen , Wu Shihu , Shang Wenliang , Luo Kaitong , Li Xiaogang , Wu Kangjun , Cao Zheng , Xiong Yixue 
Published: December 2023

Abstract
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.