Accumulation Characteristics and Preservation Conditions of Niutitang Formation of Lower Cambrian Series Shale Gas in West Hubei: A Case Study of Well XD1

To find out the accumulation characteristics of the Cambrian Niutitang Formation shale gas in West Hubei,reservoir fracture characteristics,fluid inclusion testing of fracture-filling calcite veins,and detrital zircon U-Pb dating of Cambrian sedimentary rocks were conducted based on the data of Well XD1. This well contains developed,high-angle,mostly calcite-filled fractures. Inclusions inside the fractures include liquid inclusions typically consisting of H₂O and CH₄,and gas inclusions that contain CO₂,H₂S,CH₄,N₂ and H₂. The fluid inclusions display a high level of divergence in their homogenization temperature,spanning from 168 to 293℃,and are roughly distinguished by three temperature intervals:168-175℃ low-temperature intervals,185-227℃ medium-temperature intervals,and 244-293℃ high-temperature intervals,suggesting a multistage history of reservoir formation. Most of the late secondary inclusions are found in the medium-temperature intervals and contain high levels of CH₄,which indicates an origin of late secondary accumulation. According to the valid age data of 178 groups of detrital zircon obtained,the ages range from 2 638 to 330 Ma and mostly fall within five time intervals of 2 638-2 010 Ma,1 997-1 516 Ma,1 450-904 Ma,890-722 Ma and 699-330 Ma. It shows that XD1 has undergone multistage tectonic transformation. Well XD1 shale gas has the material basis of hydrocarbon generation. This suggests that extensive fracture and rupture zones resulting from multistage tectonic reformation in the area have led to destruction of shale gas preservation conditions,resulting in the loss of shale gas. Therefore,the stable structure regions are predicted as the favorable shale exploration areas in West Hubei.