Distribution And Formation Mechanism Of Pressure System In Huatugou Oilfield Of Qaidam Basin

Huatugou oilfield locates on shallow layer high spot in Shizigou-Youshashan anticline structure in the west region of the Qaidam Basin, and belongs to structural-lithologic combination oil pools. The reservoir locates in Xiayoushashan and Shangganchaigou group of neogene, and has the strong heterogeneity. The average formation pressure of which is 8.27Mpa with the original formation pressure coefficient 0.61 to 0.80, which reveals the feature of abnormal low pressure reservoir. The Huatugou oilfield shows the considerable petroleum geology reserves because of the shallow burying and the good crude oil quality. However, the shortage of natural energy of oil reservoir brings the very major difficulty to oilfield exploration and exploitation due to the complicated geological conditions. Therefore, to study the structural evolution, the distribution features of pressure systems, the thermal evolution history, the characteristic of paleofluid evolution, recovery of formation temperature and formation pressure, recovery of denudation thickness, the depositional configuration of pressure compartment, the feature of hydrocarbon accumulation, the relationship between the formation water and hydrocarbon accumulation and the controlling factors forming abnormal pressure will be very important to realize the causation of abnormal pressure and promote the effective exploration and development of Huatugou oilfoeld.This dissertation focuses on the main controlling factors of the causation of abnormal low pressure in Huatugou oilfield. Through the analysis on structural evolution, the distribution features of pressure systems, the relation between sedimentation and pressure compartment, hydrocarbon generating, migrating and accumulating, the characteristics of reservoir fluid and the controlling factors of the formation of abnormal pressure system, the influences of tectonic uplift, temperature decrease, the proliferation of light hydrocarbon and the depth of the water table to the formation of abnormal pressure system in Huatugou oilfield are illustrated, and further the main controlling factor is clarified.Based on the studies above, the following conclusions can be drawn: the formation pressure of neogene in Huatugou area is abnormal low pressure, and the low-pressure layers of the Huatugou oil field are mainly located in the distal bar, the thick and dense prodelta mud, shore and shallow lake mud and algal mound constitute the top and bottom boundaries of low-pressure compartments, which have independent, closed systems, respectively, and feature vertical overlay and horizontal juxtapositions. The formation water type in the Huatugou oil field is CaCl₂ type, which shows that the reservoir has good sealing ability and is beneficial for oil and gas concentration and preservation. The obvious diversity of fluorescent color of organic inclusions shows that there is two times hydrocarbon injection. The low pressure reservoirs have the features of multi-stages hydrocarbon generation and short-distances migration and accumulation. The main controlling factors of the formation of abnormal low pressure system are the rock pore volume rebounded caused by tectonic uplift in the late Himalayan period and the pore fluid shrunk due to the decrease of formation temperature, and simultaneously, the effect of the proliferation of light hydrocarbon and the depth of the water table to formation pressure are little.