Cycles and stages division of volcanic effusive phase of the third member of Kongdian Formation in Weibei Sag, Bohaiwan Basin

Key words:  volcanic  effusive phase  eruptive cycle  eruptive stage  Weibei Sag  Bohai Bay Basin

Abstract:

The volcanic strata were widely developed in the 3rd member of Kongdian Formation of Weibei Sag, where the effective reservoirs were mainly developed in effusive phases. Eruption cycles and stages of the effusive phases were categorized comprehensively using cores, logging and seismic data. The results show that volcanic rocks can be divided into two eruptive cycles and seven eruptive stages. Each eruptive cycle can be vertically divided into three subfacies:the upper subface, the central subfaces and the lower subface, respectively. Air holes and almond constructions were developed in the upper subface, which is the best reservoir type of volcanic rocks. According to the scale of lava flows, effusive phases are divided into two types, progradational and retrogradational cycles. Favorable reservoirs are mainly distributed in the upper part of the progradational layers, and the lower part of the retrogradational layers. Progradational layers have larger scales, developed during the early eruption (from 4th to 7th) stages; while retrogradational layers have smaller scales, developed during later eruption (from 1st to 3rd) stages. It is suggested that early volcanic activities were intensive, which were gradually weakened through time.