Control of negative inversion structures on high-quality Carboniferous buried hill reservoirs in the Weixinan Sag, Beibu Gulf Basin, China

Based on drilling, mud logging, core, seismic and imaging logging data, this paper studies the identification and evolution process of negative inversion structures in the Carboniferous buried hills in the No. 1 and No. 2 fault zones of Weixinan Sag, Beibu Gulf Basin, China, and reveals the controls of these structures on high-quality reservoirs. The No. 2 fault zone develops significant negative inversion structures in the Carboniferous buried hills, as a result of multi-stage transformations of compressive–tensile stress fields in the period from the Late Hercynian to the Himalayan. The Hercynian carbonates laid the material basis for the formation of high-quality reservoirs. The negative inversion structures mainly control the development of high-quality reservoirs in buried hills through: (1) creating large-scale fractures to increase reservoir space and improve oil-gas flow pathways; (2) regulating stratigraphic differential denudation to highlight dominant lithology for later reservoir transformation; (3) shaping the paleogeomorphological highlands to provide favorable conditions for superficial karstification. The negative inversion structures form a high-quality, composite reservoir space with the synergistic existence of superficial dissolution fractures/cavities and burial-enhanced karst systems through the coupling of fracture network creation, formation denudation screening and multi-stage karst transformation. The research results have guided the breakthrough of the first exploratory well with a daily oil production over 1 000 m 3 in carbonate buried-hill reservoir in the Beibu Gulf Basin, and provide referential geological basis for finding more reserves and achieving higher production in the Carboniferous buried hills in the Weixinan Sag.