Source and reservoir characteristics of the Upper Triassic lacustrine Chang 6 tight oil play in the Zhangjiagou area, Ordos Basin, China

To explore the source and reservoir characteristics of Chang 6 tight oil in the Zhangjiagou area, we have extracted a suite of Chang 6 tight sandstones and the source rocks from the seventh to ninth members of the Upper Cretaceous Yanchang Formation in the Ordos Basin, China, respectively, using chloroform. We examined group components by fractionations of extracted organic matter. Using low-pressure gas adsorptions and gas chromatography-mass spectrometry, respectively, we analyzed the pore structure of the studied samples before and after extraction and the oil source of the separate saturated hydrocarbon components. The results indicate that the porosity of the Chang 6 tight sandstone is mainly distributed in the 8%-14% range, averaging 10.5%, the permeability of the studied reservoir is only approximately 0.16 x 10(-3) mu m(2), and the pore-throat radius is mainly less than 2 mu m. The major type of pores of the reservoir includes the residual intergranular pore, secondary intergranular dissolved pore, and intragranular dissolved pore. The micropore volume of the Chang 6 tight sandstone is in the range of 0.0071-0.0092 cm(3)/g, and the mesopore volume of the Chang 6 tight sandstone is in the range of 0.0237-0.0343 cm(3)/g. The micropore volume and micropore surface area significantly increased after chloroform extractions, and soluble hydrocarbons could be stored in micropores of the Chang 6 tight sandstone. The three sets of source rocks from the seventh to ninth members of the Upper Cretaceous Yanchang Formation are high quality by the evaluation of source rocks, and the Chang 7 has the highest value of source rocks, followed by Chang 9 and Chang 8. The pentacyclic triterpene characteristics (Ts-C30H-C-30*) of Chang 6 crude oil are similar to those of Chang 7 source rock, and the tight oil of the Chang 6 member in the Zhangjiagou area originated from Chang 7 source rocks.

Automated summary

The study on the source and reservoir characteristics of Chang 6 tight oil revealed that the porosity of the reservoir ranges from 8% to 14%, with major pore types including residual intergranular pore, secondary intergranular dissolved pore, and intragranular dissolved pore. The results indicate a good storage condition for the Chang 6 reservoir in the Sichuan Basin.