A Study On Formation Conditions Of Deep Gas And Anatomy Of A Typical Oil And Gas Reservoirs In The Western Qaidam

It is difficult to discover new shallow gas reservoir according to the exploration research and production practice of recent ten years in Western Qaidam Basin, which results in a stagnant passive situation in natural gas exploration for a long time. However, as the deep gas exploration is effected by the risks of trap implement, reservoir characteristics and trap efficiency, together with high investment, no determination is made unless it is sure to drilling exploration. So it is necessary to do further research on distribution of deep gas reservoir and choice of favorable exploration target in western Qaidam Basin.Based on logging, seismic data interpretation and basin simulation technology, by means of study in tectonic and sedimentary evolution, we do some research on deep petroleum system of Western Qaidam Basin in this paper, describing its reservoir cap distribution and quality, implementing the trap type and scope, analysing the carrier network and migrational direction of deep gas, elucidating its reservoir forming condition, as well as accumulation process and rules, pointing out the favorable zone for deep gas exploration.The quality and hydrocarbon supplying ability of all the main hydrocarbon source rocks are confirmed through analysis on geochemical characteristics of the source rocks in this area, it is concluded that the Tertiary of Western Qaidam develops E₁₊₂,E₃¹,E₃² and N₂¹ these four sets of hydrocarbon source rocks, which mainly distribute in the source depression of Mangya, and it is basically dominated by sapropelic mixed type from E to N₂¹. According to comprehensive evaluation of Western Qaidam reservoirs, three primary types of reservoirs such as pore type clastic rocks, erode pore type algal limestone and fracture type marl are identified. While deep reservoirs are mainly clastic rocks and pore type reservoirs. Fracture type marl reservoirs has great variation in porosity-permeability, and with high heterogeneity, which frequently concentrated distributes on the tectonic axis or high position, and it is widely developed in Higher Ganchai channel (N₁) and Youshashan Formation(N₂) from the structure of Nanyishan, Oil Spring and Katmerica. Two sets of deep and shallow regional seals were existed in the Tertiary of this area, which mainly are lime mudstone, gypsum-salt rock and marl. Deep regional seal is widely developed, almost proliferates the western area, with large thickness, primarily consists of lacustrine lime mudstone, and marl and gypsum-salt layer interbeded with them, among which gypsum-salt bed is the most perfect cap rock for deep petroleum reservoir. Two kinds of carrier system are included in Western Qaidam area: sand body and fault compound carrier system as well as fault carrier system. Faults act as the main carrier system for deep gas, and therefore hydrocarbons migrate mainly in vertical direction with short migration distance. On the basis of the comprehensive analysis of the geological characteristics and reservoir forming factors, it can be inferred that Western Qaidam possesses good reservoirs for deep gas, in which gas source is rich, and preservation condition is well. And consequently, the favorable zones for deep gas exploration in Western Qaidam are Oil Spring- Katmerica in Mangya Depression and the surrounded Nanyishan.