Abstract:

In recent years, basement gas reservoirs in Qaidam Basin have demonstrated significant potential for exploration and development. The Kun 2 block in Kunteyi gasfield, as an ultra-deep basement gas reservoir, poses significant challenges for reservoir prediction and favorable area selection due to its complex internal geology, strong heterogeneity, and dual-porosity characteristics of fractures and dissolution pores. By integrating geological, logging, seismic, and production data, this study developed an innovative integrated method combining “well-seismic integration, static-dynamic fusion, and multi-attribute synergy” to systematically characterize reservoir characteristics and predict natural gas accumulation zones, aiming to reveal the spatial distribution of ultra-deep basement reservoirs and provide guidance for the optimization of well placement. The results showed that: (1) The basement reservoirs in the Kun 2 block primarily consisted of granitic gneiss, with storage spaces characterized by a dual-porosity system of fractures and dissolution pores. The fractures exhibited a network-like development and were distributed in NE-SW trending bands in the plane view. Lateral heterogeneity was significant. The fracture densities ranged from 3 to 10 m^-1, the average fracture porosity was about 0.015%, and the matrix porosity ranged from 1.8% to 6.8%. Overall, this demonstrated tight and low-permeability characteristics. (2) The development of dissolution pores was fault-controlled, primarily distributed along Kun 1, North Kun 2, and North Kun 101 faults. These faults formed fracture-pore coupled reservoirs. The interval 100-300 m below the top of the basement was a concentrated development zone, with the maximum reservoir thickness reaching up to 200 m. (3) The innovative integration of maximum likelihood attributes and structure tensor-acoustic impedance inversion technologies achieved high-precision characterization of the spatial distribution of fractures and dissolution pores. The prediction of maximum likelihood attributes revealed that high-angle fractures were mainly located on the upthrown side of faults, exhibiting an 85% consistency rate with imaging logging results. Additionally, the structure tensor-acoustic impedance inversion revealed that zones of dissolution pore development aligned closely with fault orientations, thereby validating the controlling mechanism of fault activity on dissolution process. (4) Based on reservoir classification and evaluation criteria, along with seismic prediction and dynamic production data, this study proposed a reservoir development model of “fault-controlled fractures, fracture-controlled pores” for the first time, indicating structurally high positions on upthrown side as the core zones of natural gas accumulation. Five favorable areas for natural gas accumulation were selected, including four on the upthrown side and one on the downthrown side. Among them, the NE-SW strip zones on both sides of the North Kun 2 Fault were identified as the optimal target areas.

Key words: Qaidam Basin, basement gas reservoir, fracture prediction, dissolution pores, favorable area optimization