Utilizing 2D seismic forward modeling to constrain the seismic response and plane distribution of grain shoal reservoir in the northern slope of Central Sichuan Paleo-uplift, Sichuan Basin

Abstract

Carbonate grain shoal reservoirs are well developed in the first member of the Longwangmiao Formation on the northern slope of the Central Sichuan Paleo-uplift. Influenced by the stratal thickness, reservoir heterogeneity, and development patterns, it is difficult to predict reservoirs by using the seismic data present. In this contribution, multiple sets of seismic models were established, and then the seismic forward modeling method was used to constrain the seismic response of the reservoirs. The results show: (1) When reservoirs are not developed in the Longwangmiao Formation, the top interface of the Longwangmiao Formation corresponds to 1/8 λ above the central lobe of the wave peak while the bottom interface corresponds to the central lobe of the wave trough; (2) If a single set of reservoirs is developed in the first member of the Longwangmiao Formation, the waveform of the top interface will be constant, but the bottom trough will shift upward and the frequency will decrease. Additionally, the scale of trough upward movement and frequency reduction is negatively correlated with reservoir velocity and positively correlated with reservoir thickness. (3) If multiple sets of reservoirs are developed in the first member of the Longwangmiao Formation, the waveforms of the top and bottom interface are consistent with that of a single reservoir which has the same total thickness. Similarly, the corresponding relationship between the wave trough variation and reservoir velocity or thickness remains constant. (4) When reservoirs are both developed in the first and second members of the Longwangmiao Formation, the top and bottom waveforms will vary regularly with the difference in reservoir velocity between them. If the reservoir velocity in the first member is less than that of the second member, the top peak is constant while the bottom trough shifts upward, and the frequency decreases. On the contrary, if the reservoir velocity in the second member is lower than that of the first member, the bottom trough is constant while the top peak moves downward, and the frequency increases. Finally, using the waveform clustering method, the reservoir is divided into three grades, namely high-, medium-, and poor-quality. Among them, high-quality reservoirs are mainly distributed in the southeast and northeast of the study area which is the key area for oil and gas exploration in the next step.