Shiyou shiyan dizhi (Jan 2024)
Geological characteristics and sweet spot selection of Permian organic-rich sedimentary tuff series in northern Fuling, Chongqing
Abstract
The sedimentary tuff in the second member of Permian Wujiaping Formation in the northern Fuling area of Chongqing is a new type of organic-rich source rock, which is a new field of unconventional gas reservoir exploration. Clarifying the distribution of key sweet spot parameters of the sedimentary tuff series in the second member of Wujiaping Formation is a primary issue in exploration and evaluation. Based on drilling, logging, experimental analysis and seismic data, this paper evaluated the target sweet spot starting from the sedimentary tuff series' organic geochemistry, lithological characteristics, thickness distribution, formation pressure, fracture development and other geological conditions for reservoir formation and enrichment, and focusing on the two key factors of lithology quality and compressible fracture network. The results show that: (1) The sedimentary tuff series in the second member of Wujiaping Formation in the northern Fuling area is in the deep-water shelf facies, with high organic matter abundance. The TOC content is mainly 5%-6%, the Ro value ranges 1.90%-2.44%, the organic matter types are mainly Ⅰ and Ⅱ, and the thickness of high-quality sedimentary tuff section (TOC≥4%) is 15-20 m, with a wide distribution area. (2) The reservoir space of the high-quality sedimentary tuff section is diversified, the porosity is high, the brittle mineral content is high, the formation pressure coefficient is high to ultra high pressure level, and small-micro fracture zone is developed at the hinge zone, reflecting a good gas exploration potential of "self-generation and self-storage". (3) The sedimentary tuff gas reservoir in the second member of Wujiaping Formation in the northern Fuling area has the laws of high quality control of enrichment, overpressure control of reservoir and fracture network control of sweet spot. The sweet spot evaluation criteria for sedimentary tuff series in the second member of Wujiaping Formation are preliminarily established in combination with the main control factors, and three target sweet spots are selected, with an area of 186.4 km2. The L2-L1 well area in the northwest of the exploration area has good oil and gas shows, which can be a favorable target for recent exploration.
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