New sequence division method of shallow platform with natural gamma spectrometry data: implication for reservoir distribution—a case study from Majiagou Formation of Bozhong 21–22 structure, Bohai Bay Basin

Abstract

Dividing carbonate sequence with natural gamma data is the popular method, while it is not suitable for shallow platform because high gamma of mud flat represent shallow-water environment. Taking Majiagou Formation of Ordovician in Bozhong 21–22 structure of Bohai Sea as an example, a new sequence division method for shallow platform was presented based on natural gamma spectrum data, and the relationship between the sequences and reservoirs was discussed as well in this paper. The high natural gammas in Majiagou Formation of the study area are response for the weathering, uranium element enrichment in reservoirs and argillaceous sediment. The comparison result between well logging parameters and geological data shows that the ratio of Th/K is an effective parameter for sequence division, which can distinguish the origin of different high gammas effectively, even the mudstone from oxidized environment and reductive environment. Based on the ratio of Th/K, five third-order sequences were divided in Majiagou Formation of the study area; among them, all the sequences are developed in well BZ21-B-1, while SQ5 is missing in well BZ22-A-2 for denudation. The corresponding relationship between sequences and reservoirs indicates that the reservoirs of SQ4–SQ5 are dominated by structural fractures, which are mainly controlled by tectonic movement and weathering; while those of SQ1–SQ3 are dominated by dissolved pores, which may be related to the internal dissolution.