Developmental characteristics and exploration prospects of the Neogene Miocene volcanic rock reservoirs in the Songnan-Baodao Sag and its periphery,Qiongdongnan Basin

In the Qiongdongnan Basin, volcanic gas reservoirs are revealed for the first time in the S29-A structure, suggesting promising prospects for hydrocarbon exploration. This study aims to ascertain the developmental characteristics of the Neogene Miocene volcanic rock reservoirs in the Qiongdongnan Basin. Using the latest data from drilling, seismic survey, logging, thin section observation, and nuclear magnetic resonance (NMR) testing, we analyze the lithology, lithofacies, reservoir development characteristics, primary factors controlling reservoir development, and natural gas accumulation model of volcanic rocks in the S29-A block. The research results indicate that intermediate-basic pyroclastic rocks occur in the Miocene strata in the S29-A block, with three microfacies identified in the base surge subfacies of the explosive facies: crater to near-crater microfacies, proximal microfacies, and distal microfacies. A total of nine types of reservoir spaces in seven categories are determined. The reservoirs, proving porous-fractured, exhibit favorable physical properties and a high proportion of large-diameter pores, establishing them as high-quality volcanic rock reservoirs overall. The reservoir development is jointly governed by lithology, lithofacies, dissolution, and tectonism. The volcanic breccia reservoirs of the proximal microfacies exhibit the most favorable reservoir physical properties, with dissolution contributing more than 50% to their porosity enhancement. The Songnan-Baodao Sag and its periphery are characterized by extensive volcanic edifices in the Miocene strata, which have delineated three major volcanic rock zones, suggesting great potential for hydrocarbon exploration. Notably, volcanic edifices on the north side of the Wanhu volcanic rock zone exhibit favorable hydrocarbon accumulation conditions and a large scale, implying promising prospects for hydrocarbon exploration. Therefore, these volcanic edifices are expected to be a natural gas accumulation area with total resource potential exceeding 100 billion cm³.