Shiyou shiyan dizhi (Mar 2024)
Development model and exploration practice of fault-lithologic composite traps in Oligocene Weizhou Formation, Weixinan Sag, Beibuwan Basin
Abstract
In order to address the challenges related to exploration target search and sustainable development of oilfields in the Weixinan Sag of Beibuwan Basin, this study focused on fault-lithologic composite traps within the structural trap development section of the third member of Oligocene Weizhou Formation in the No.2 fracture zone of Weixinan Sag. By examining the relationship between the evolution of depression-controlling faults and the sedimentary characteristics of the third member of Weizhou Formation, it was observed that the No.2 fault zone exhibited weak activity during the sedimentary period of the third member, facilitating the continuous transportation of underwater distributary channels of the braided river delta along the long axis direction of the sag towards the depression. Conversely, the regional tectonic activities were intense during the sedimentary periods of the second and first members of Weizhou Formation, leading to the formation of a series of en-echelon faults. The interaction between relatively isolated underwater distributary channels of the braided river delta front and en-echelon faults resulted in angled staggered formations and intersecting channels, ultimately creating a distinctive channel-type structural-lithologic trap group characterized by NE-SW trending fault sealing and NW-SE trending sand body pinching. The research findings indicate the development of a three-factor coupling fault-lithologic trap model in the third member of the Weizhou Formation. This model includes the depression faulted uplift background of the No.2 fracture zone, low-frequency oscillating channel, and en-echelon tension-torsion fault. The intersection area of the main stream line of underwater distributary channel in a braided river delta and the No.2 fracture zone is identified as a favorable zone for structural-lithologic traps. Additionally, composite structural-lithologic traps are predominantly found in the near-source intersection area, while isolated structural-lithologic traps are more common in the far-source intersection area, leading to the formation of an accumulation model involving axial channel sand control, fault, and sand body composite control. The fault-lithologic trap model and accumulation model have demonstrated effective application in practice. Several favorable fault-lithologic composite trap groups with distinct fault sealing and sand body pinching orientations have been identified, particularly in the Wei B-E structure. Notably, well WB-E 1 has successfully drilled nearly 100 meters of oil layer, with subsequent wells yielding positive results. The exploration efforts in the third member of the Weizhou Formation within the No.2 fault zone of the Weixinan Sag have led to the discovery of geological reserves exceeding 10 million cubic meters, underscoring the significance of fault-lithologic composite traps in this region.
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