Hydrocarbon expulsion potential of source rocks and its influence on the distribution of lacustrine tight oil reservoir, Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwest China

Abstract

The petroliferous Permian system of the Junggar Basin in northwest China is predominantly a conventional oil exploration region. To assist in the unconventional tight oil reservoir exploration, the hydrocarbon expulsion potential of source rocks and its influence on the tight oil distribution of the Middle Permian Lucaogou Formation (P2l) in the Jimsar Sag are discussed. The researches are based on geological and geochemical characteristics such as the distribution, abundance, type, thermal maturity of source rocks, and “sweet spots”. Meanwhile, hydrocarbon expulsion intensity and quantity were evaluated with hydrocarbon generation potential method. The Middle Permian Lucaogou source rocks have wide distribution and high thickness (mostly thicker than 100m). The source rocks exhibit low to high TOC (mainly ranges from 1.0wt% to 5.0wt%) and primarily contain type-II kerogen, resulting in significant hydrocarbon expulsion potential under moderate mature thermal evolution (Ro mainly ranges from 0.55% to 1.05%). According to the hydrocarbon expulsion modelling, the source rocks reached the hydrocarbon expulsion threshold at 0.73% Ro and the hydrocarbon expulsion rate became greatest at 0.9% Ro. The comprehensive hydrocarbon expulsion efficiency was approximately 48%. About 18.7×108t of hydrocarbons were expulsed from the Lucaogou source rocks in the Jimsar Sag. The hydrocarbon expulsion intensities are relatively large and rang from 150×104t/km2 to 400×104t/km2 in the center and west area of the Sag, where is the superimposed area of source rock thicker than 200m, TOC greater than 3.5% and Ro higher than 0.85% in the sag. There are 17 exploratory wells obtaining commercial oil flow in the Sag. 16 of the 17 exploratory wells are located in the area with hydrocarbon expulsion intensity greater than 50×104t/km2. By contrast, other exploratory wells outside the hydrocarbon expulsion coverage area have slight show of oil, even dry. In addition, there is positive correlation between the hydrocarbon expulsion intensity and the daily output of well testing. It is concluded that the tight oil exploration should focus on the area with hydrocarbon expulsion intensity greater than 100×104 t/km2, which is a favorable area for commercial hydrocarbon accumulation and exploration.