Sedimentary cycles of evaporites and their application in sequence division in the upper member of the Xiaganchaigou For
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ORIGINAL ARTICLE
Sedimentary cycles of evaporites and their application in sequence division in the upper member of the Xiaganchaigou Formation in Yingxi Area, Southwestern Qaidam Basin, China Bin Xu1 · Jianming Li2,3 · Zhiping Wu1 · Yongshu Zhang4 · Jun Cui4 Accepted: 26 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The upper member of the Xiaganchaigou Formation in the Yingxi Area of the Southwestern Qaidam Basin is a set of evaporite-bearing lacustrine series. In this study, petrographic analyses of rock thin sections, cores, drillings, logging as well as geochemistry were used to study the characteristics of the sedimentary cycles of the series. Concurrently, this study adopts a transgressive–regressive (T–R) cycle sequence theory, based on lithology, logging curves, the stacking pattern of the subsequences, and unconformity and seismic reflection terminal relations. This approach uses evaporites as the primary marker layer for identifying the initial lake flood surface and dividing the sequence of the evaporite-bearing series in the upper member of the Xiaganchaigou Formation. Results show a zonal distribution of evaporites transitioning laterally from the center to the margin of the basin to mixed carbonate rocks and then to clastic rocks. Vertically, there are two third-order sedimentary cycles and six fourth-order sedimentary cycles. Five types of evaporite lithofacies combinations were identified. Evaporites were formed by evaporative crystallization at a particular depth beneath lake water during a lacustrine regression during an arid paleo-climate and rapid subsidence of the basin basement. It has been established based on the asymmetric sedimentary cycle development model that during the lacustrine transgression stage, the mixed carbonate rocks were transformed into mudstone upward, and during the lacustrine regression stage, the mixed carbonate rocks transformed into gypsum rocks, glauberite, and halite, upward, while a delta and reef flat limestones developed at the edge of the basin. There is a one-to-one correspondence between the T–R cycle sequence division and the sedimentary cycle division. Keywords Southwestern Qaidam basin · Yingxi area · Xiaganchaigou Formation · Evaporites · Sedimentary cycles · Sequence division
Introduction
* Jianming Li [email protected] Bin Xu [email protected] 1
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
2
School of Geosciences, Yangtze University, Wuhan 430100, China
3
Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Wuhan 430100, China
4
Exploration and Development, Research Institute of Qinghai Oilfield Company, PetroChina, Dunhuang 736202, China
The formation of hydrocarbon reservoirs in many petroliferous basins in the world is associated with evaporites (Zhao et al. 2007). Based on previous statistics, about 33% of the 334 large oil and gas fields in the world are covered by halite and gypsum (Ulisses and Carry 1996). In the petroliferous ba
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