Integration of charging time, migration pathways and sealing analysis to understand hydrocarbon accumulation in complex fault blocks, the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin

Abstract

A large number of oil and gas fields have been successfully discovered in complex fault blocks worldwide. However, the role played by the fault-rock seal in the hanging wall traps is not fully understood due to the paucity of datasets. The trap examples of the Pinghu Slope Belt are used to analyze the migration and accumulation of hydrocarbons in the complex fault blocks, as well as the fault-sealing mechanism of the hanging wall traps. The burial history in typical wells, combined with the homogenization temperature of fluid inclusions, is used to determine the hydrocarbon charging time. Based on detailed 3D seismic interpretation, the paleo-structural features at critical moments and their control on hydrocarbon migration were discussed. Hydrocarbon accumulation characteristics were illustrated based on triangle juxtaposition diagrams. The result shows that there are two hydrocarbon charging episodes in the study area, respectively, 10.6–8.1 Ma and 2.8 Ma-present, with the latter being predominant. This phenomenon that the homogenization temperatures of inclusions are generally higher than the measured formation temperatures at the same depth is attributed to late fluid accumulation and not reaching thermal equilibrium with the reservoir fluids. The analysis of hydrocarbon generation history, trap fixation history and fluid inclusions indicate that the critical moment of the generation-migration-accumulation of most hydrocarbons ranges from 2.8 Ma to the present. Due to the difficulty in effectively recognizing the reflector (the top of the Santan Fm) and the nearly horizontal sediments above T20, the reflector T20 (5 Ma) near the top of the Santan Fm was chosen as an approximate proxy for this critical time (2.8 Ma). The paleo-structures of the Pinghu Slope Belt at 5 Ma are a gentle southeast-dipping slope. Based on the numerical simulation of three different fault scenarios (no faults, closed faults and open faults), wells A-H are located on the preferential migration pathways. Fault seal analysis shows that multiple juxtaposition types on the fault surface and moderate-potential to effective SGR values at the sandstone reservoirs in the middle and lower Pinghu Formation. Lateral and vertical reservoir compartmentalizations are presented in clean sand-sand juxtapositions with low-potential sealing (SGR<20%) in well A. The SGR-buoyancy pressure relationship from hanging wall traps and the global calibration plots from footwall traps have a consistent trend. The fault sealing mechanism of hanging wall traps is similar to that of the footwall traps. Therefore， those global calibration plots can be simultaneously applied to the fault seal evaluation of both hanging wall traps and footwall traps.