Development prospects and geological characteristics of nearly tight and tight sandstone gas reservoirs in western secondary sag and western slope zone of Xihu depression, East China Sea

Abstract: Based on tectonic and sedimentary background as well as a large number of drilling and logging data, the authors studied geological characteristics and development prospects of nearly tight and tight sandstone oil and gas reservoirs in western secondary sag and western slope zone of Xihu depression, East China Sea by using casting thin section, laser scanning confocal microscope, scanning electron microscope, cathodoluminescence, stable carbon isotope composition and light hydrocarbon components. The results indicate that nearly tight and tight sandstone gas reservoirs are distributed under the conventional gas reservoir, but their development horizon are different, belonging respectively to middle and lower Pinghu Formation and lower Huagang Formation. Nearly tight and tight sandstone gas is coal-type gas and came from coal measures source rock in Pinghu Formation. The reservoir is above source rock in western secondary sag, and multiple reservoir and source rock are superimposed upon each other in western slope zone. Western secondary sag reservoir belongs to tight sandstone and western slope zone reservoir is nearly tight sandstone. Strong compaction, siliceous cement, authigenic illite cementation and carbonate cementation are important factors for densification of the reservoir. The pore types of two zones mainly include intergranular dissolved pore, native intergranular pore and feldspar intragranular dissolved pore. There are relatively high porosity and high permeability reservoirs in different depths of the two zones, resulting from acid dissolution. However, western slope zone reservoir has larger corrosion scale and does not develop microfracture. There is a certain size of microfracture which is “microfracture” sweet spot in deep layer of western secondary sag. The former has weak overpressure and the latter has overpressure-strong overpressure. The source rocks which have high abundance of organic matter in Pinghu Formation are distributed widely in Xihu depression. The supply of continuous gas constitutes the material basis of large-scale development of nearly tight and tight sandstone gas reservoirs. Large-sized nearly tight and tight sandstone provides effective reservoirs. So, there are rich tight sandstone oil and gas resources in East China Sea.