Volcaniclastic fan facies and reservoir characteristics: a case study of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay Basin, East China

Abstract

This paper investigates volcaniclastic fan facies and reservoir characteristics of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay basin, Eastern China. Core and log data from the Guantao 3 and 4 Formations were analyzed in order to characterize volcaniclastic fan facies. Comprehensive analyses show that volcaniclastic fan sediments have been characterized as rapid proximal deposition of well-rounded and poorly sorted large-grain sediments. During the developmental processes of the volcanic fans, shallow to moderately deep lacustrine mudstones are deposited on the volcanoes. They vertically form superimposed sedimentary bodies that were characterized by laterally alternating fingers of volcaniclastic fan sediments and interchannel mudstone. Differences in pyroclastic supply and the volcanic terrain during the deposition account for the variations in volcanic rocks distribution. Distribution of volcaniclastic fans also exhibited characteristic features. Volcaniclastic fans around the volcanic highlands show variable distribution of volcanic rocks. The deep valley sizes of the debris flows and streams indicate variable sediment supply. The extent and thickness of the lahar deposits from the volcaniclastic debris flows indicate variable size of sedimentation processes. Most volcaniclastic fans are composed of debris-transported deposits having small clasts. The bed thickness increases with increasing distance from the volcanic source. The volcaniclastic rocks show high content of intermediate–basic volcanic rocks debris. The original and dissolved intergranular and dissolved intragranular porosities in the volcaniclastic rocks provide the best reservoir properties. Identification of lithofacies and their assemblages, and volcaniclastic sedimentary facies buried within volcanos and adjacent regions provide a better understanding that can be applied to similar rift basins. A method to identify depositional geometry and accumulation patterns of volcaniclastic fans was employed. This method assisted in constructing depositional models, reconstructing the depositional history of volcaniclastic fans and the related history of volcanic activity based on geological and geophysical data within volcanic areas in rift basins.