TIGHT OIL FORMATION AND DISTRIBUTION OF MIDDLE PERMIAN LUCAOGOU FORMATION IN JIMSAR SAG, JUNGGAR BASIN, NORTHWESTERN CHINA

The tight oil in Middle Permian Lucaogou Formation,Jimsar sag in eastern Junggar Basin,is an important target of unconventional oil in Chinese terrestrial facies. The abundant tight oil resources in Lucaogou Formation is characterized by continuous hydrocarbon accumulation in thick self-sourced reservoirs composed of thin inter-layers.

The excellent source rocks, widely distributed reservoirs, optimal source-reservoir-caprock combination, and abnormal overpressure are key factors to tight oil formation．

Firstly, the source rocks are thick and widely distributed, with major lithologies of carbonate mudstone and siliceous mudstone,the average thickness is more than 150 meters. The TOC contents of the source rocks are mostly higher than 4% and the kerogen is mainly of Type II. Their Ro values range from 0.6% to 1.1% in oil generation window.

Secondly, the reservoirs are generally tight and comprised of carbonate rocks, carbonate sandstone and siliceous sandstone, with the porosity mostly of 6%–12% and the air permeability less than 0.1mD.The pore space is mainly composed of intra-granular pores and inter-granular dissolved pores which are connected by pore throats with the diameter from dozens of nano-meters to hundreds of nano-meters.

Thirdly,the source rocks and tight reservoirs are widely distributed and form an optimal combination through space and time，which are good for tight oil accumulation.

Finally, the formation pressure coefficient is 1.10-1.50, which provides a drive force for tight oil migration and charging.

Oil shows are common in the whole Lucaogou Formation and the tight oil is mainly accumulated in reservoirs related to excellent level source rocks in vertical. The tight oil is mainly distributed in the areas with thick source rocks(＞200 meters)，high TOC(＞3.5%)and thick reservoirs(＞15 meters), and controlled by source rocks and reservoirs in situ.