Rapid Recognition Approach for Sequence Stratigraphy and Its Application in Xiuwu Basin

Abstract

In the conventional oil and gas exploration, sequence stratigraphy has been widely used to identify the favorable section of the longitudinal direction. The sedimentary response of shale to sea-level change is weak for which are deposited in deep water. Therefore, the sequence identification method in the theory of classical sequence stratigraphy is not applicable to shale. Some methods of sequence identification in shale have been put forward in which core is needed. Based on some easily obtained data, like lithology and formation element logging curve, a new method of second- and third-order sequences in shale is proposed and then be used in predicting the favorable section of Lower Cambrian shale, in Xiuwu basin. First, identifying the lithology changes according to the logging data. The lithology and sedimentary environment changed suddenly in the boundary between Ediacaran to Cambrian and Lower Cambrian to Middle Cambrian which has been identified as second-order sequence boundaries. Second, obtaining the redox index (U/Th ratio), salinity index [Ca/(Ca+Fe) ratio] and provenance distance index (Ti/Al ratio) according to the calculation of formation elements logging data. High U/Th ratio is supposed to be related to reducing environment (transgression) while low U/Th ratio related to oxidizing environment (regression). High Ca/(Ca+Fe) ratio indicates the increased salinity (regression) while low Ca/(Ca+Fe) ratio represents decreased salinity (transgression). Besides, high Ti/Al ratio indicates proximity to the provenance (regression) while low Ti/Al ratio indirectly suggests a distance from the provenance (transgression). Wangyinpu Formation of Lower Cambiran present a high U/Th ratio, low Ca/(Ca+Fe) and Ti/Al ratio. On the contrary, Guanyintang Formation of Lower Cambrian over Wangyinpu presents a lower U/Th ratio, a higher Ca/(Ca+Fe) and Ti/Al ratio. Thus, the boundary of Wangyinpu and Guanyintang Formation is the Maximum Flooding Surface (MFS). On the level of second-order sequence, Wangyinpu Formation is the Transgressive Systems Tract (TST), and Guanyintang is the Regressive Systems Tract (RST). On the level of third-order sequence, the Lower Cambrian was divided into 5 third-order sequence (SQ1-5) and each SQ was divided into a TST and a RST. The mineral content, effective porosity, TOC content and gas content of 5 third-order sequence shown that TST2 of SQ2 and TST3 of SQ3 are the most favorable section.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018