Research On Oil And Gas Accumulation Characteristics Of Yanchang Formation Of Jiyuan Area In Ordos Basin

Jiyuan Area at the junction of the Shanxi-Gansu-Ningxia is the key exploration area to Changqing Oilfield in recent years, the achievements of oil and gas exploration in Yanchang Formation are plentiful, oil and gas accumulation of low permeability attracts people's attention.on base of inclusions and diagenesis , the mechanism of Low permeability oil and gas accumulation was discussed. The study collected core samples, Grinded into inclusion slices, identified oil and gas inclusions in slices, proposed characteristics and time sequence of inclusions and diagenetic mineral generation, identified different minerals diagenesis Components by electronic probe and laser Raman , measured temperature and analysed laser Raman components of single inclusion. Based on geological analysis ,the relation between time sequence of diagenetic mineral generation and oil-gas migration to reservoir was researched, characteristics of oil and gas accumulation of low permeability were discussed.Through careful observation to the slices and identification of c minerals,it find that, diagenesis of sandstones in reservoirs of the study area Yanchang Formation are complex, diagenetic minerals are varied, and diagenesis have multi-phase and multi-type features. Of these, the most important and obvious features is that cementations were developed in the late diagenetic phase, which representation is that the albites, crystal calcites and muddy cements were formed and distributed. Crystal Calcites are generally distributed in double Double-crystal shape. Albites have two kinds,albite from plagioclases and self-albites,where large numbers of oil and gas inclusions are seen. It shows that the compaction has great influence to the porosity of low-permeability reservoirs, but does not affect the normal flow of oil and gas; early cementation has littte influence to the porosity, and dose not affect the oil and gas migration; a lot of albites, calciums And siliceous cements formed in Late cementation, reservoirs eventually became to dense layers, which affected the normal migration of oil and gas.There are three phases of fluid inclusions in No 4 +5 reservoirs in Yanchang Formation of JiYuan area. The first phase of fluid inclusions are Organic fluid inclusions with high homogenization temperature,related to Phase II fluid inclusions are non-fluorescent Organic fluid inclusions, which represent the early event of oil and gas accumulation and reconstruction; Phase III oil and gas inclusions have Fluorescence characteristics of hydrocarbons absorbed in pores, distributed in albites, calcareous and siliceous cements and other late cements, which has eventually led to become into dense reservoirs and low permeability reservoirs. This shows that dense reservoirs with low permeability formed in the same period of a massive injection of oil and gas.A large number of Albites are secondary minerals, which formed after environmental changes of fluids in pores caused by injection of oil and gas. Secondary porosities formed by dissolution of albites and calcites is the main space of migration and conservation of oil and gas Development of a lot of albites is a symbol of migration and accumulation of oil and gas ,is also a new symbol of the prediction to low permeability reservoirs in Yanchang Formation. Research show that oil and gas injected into reservoirs and changed the chemical properties of fluids in reservoirs, diagenic fluids in reservoirs precipited and formed lots of cements in the process of oil and gas injection, Reservoirs became to dense and low-permeability reservoirs, dense lithologic reservoirs also formed.