Controls of the Sandbody Scale and Fault Throw on the Lithology and Composite Reservoir Formation in the Baoyunting Slope, East China Sea

Abstract

Under the conditions of many faults, sandbodies, and hydrocarbon sources on the slopes of faulted basins where structural traps are scarce, only a few sandbodies are capable of forming hydrocarbon pools, while most sandbodies act as aquifers. This situation presents a challenge for predicting favorable hydrocarbon accumulation areas and understanding controlling factors. The Pinghu Formation reservoirs in the Baoyunting nose structure of the Xihu Sag in the East China Sea exemplify this characteristic. Among the 19 small-scale oil and gas reservoirs discovered in this area, 10 are faulted sandbody composite traps and 9 are lithological traps, while the majority of the remaining sand layers, especially the thick layers, act as aquifers, resulting in significantly lower accumulation probabilities compared to the adjacent northern and southern areas. We analyzed the relationship between the sandstone thickness and the amplitude through the 1-D forward modeling of wells and dissected the 3-D seismic event to obtain the planar distribution of a single sandbody. Further comprehensive research on fault sealing and kinetic reservoir formation processes suggests that the gas pool formation in this area is closely related to fault sealing and lateral oil and gas transport. A small fault-to-caprock ratio is beneficial for the sealing of mudstone caprocks, while a large fault-to-sand thickness ratio is beneficial for the lateral sealing of faults and the formation of fault–sand composite pools. The tidal microfacies sandbody has a small scale, poor lateral transport ability, and a low probability of gas reservoir formation. The barrier and delta front sandbodies have a large scale, good lateral transport, and a high probability of reservoir formation. Based on the above methods, favorable pool formation traps were identified in the area, and high-yield gas wells were drilled.