The Distribution Characteristics and Favorable Exploration Zones of Karst Reservoirs in the Beijing-Tianjin-Hebei Plain

摘要

The main part of the Beijing-Tianjin-Hebei Plain is located in the Jizhong Depression and the Cangxian Uplift of the Bohai Bay Basin, and is the main area for the development and utilization of the medium-to-deep geothermal energy in China. However, very few study has been done on plane distributions and forming mechanisms of the karst geothermal reservoirs in this area. In this paper, based on drilling data and seismic interpretations, a comprehensive analysis of the basin tectonic evolution and karst reservoir forming mechanism of the basin was carried out. The results show that the karst geothermal reservoirs that developed in the Wumishan Group of the Jixian System and the Ordovician System are the two main geothermal reservoirs in the Beijing-Tianjin-Hebei Plain area. Controlled by different denudation degrees from the Indosinian to Yanshanian periods, the plane distribution of the geothermal reservoir age is characterized by segmentation from south to north inside the Jizhong Depression, and zoning from east to west in the whole area. In the Jizhong Depression, main geothermal reservoirs are the Wumishan group and the Middle Ordovician in the middle and south parts respectively, and there areresidual Cambrian - lower Ordovician locally existing in the north part. The lithology of the Wumishan group is mainly composed of tidal-flat facies polycyclic deposited stromatolites with siliceous band dolomite, while the Ordovician system is mainly composed of leopard limestones, grainstones and micrites which were deposited during four third-level sea level eustacies. There had been four phases of karstification that determined the formation of karst reservoirs, the Caledonian, the Indosinian, the Yanshanian and the Himalayan periods. Two main types (epigenetic and buried) with six hypotypes of Karstifications are recognized. As a result, the reservoir space can be divided into three types: pores, caverns and fractures, which can be further divided into 9 subcategories. The karst geothermal reservoirs are characterized with multi-layer reservoirs, large effective thickness, good porosity and permeability conditions, and large geothermal water volume. Both the central basal bulge of the Jizhong Depression and the Cangxian Uplift are favorable karst zones and have relatively shallow-buried karst reservoirs. Therefore, they are the most favorable exploration zones for karst reservoirs in the Beijing-Tianjin-Hebei Plain area. The existed geothermal resource exploration results have confirmed that the geothermal resource conditions of the central basal bulge of the Jizhong Depression and the Cangxian Uplift are superior, with 55 to 110°C wellhead water temperature and 80 to 120m~3/h water volume. They are the potential areas for forming medium to large geothermal fields.