Source rock characteristics of Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwest China, and its significance on tight oil source and occurrence

Abstract

Significant progress has recently been made in tight oil exploration within the Permian Lucaogou (P2l) Formation of the Jimusar Sag. However, current tight oil exploration deployment of the P2l Formation is mainly based on reservoir prediction, which is high risk for tight oil exploration. In this study, the geological and geochemical characteristics of the P2l Formation source rocks, including the distribution, sedimentary environment, organic matter abundance, kerogen types and thermal maturity were investigated. Hydrocarbon generation and expulsion intensity were evaluated through an improved hydrocarbon generation potential methodology, and the significance of source rocks in tight oil source and occurrence was systematically investigated. Results indicate that P2l Formation source rocks with total organic carbon >1.0 wt% occur widely (an area up to 1500 km2), are thick (up to 160 m), were deposited in a lacustrine weakly reducing sedimentary environment with relatively low salinity, have a high total organic content with a mean value of 3.12 wt%, are dominated by type II kerogen and have reached the early mature to mature stage. Modelling results indicate that the source rocks reached the hydrocarbon generation threshold and hydrocarbon expulsion threshold at 0.48% and 0.86% vitrinite reflectance, respectively. The comprehensive hydrocarbon expulsion efficiency was approximately 30%, and the maxima of hydrocarbon generation and expulsion intensities for P2l Formation source rocks are 1200 × 104 and 425 × 104 t/km2. The tight oil is sourced from adjacent source rocks that are interbedded with, or are close to, the reservoirs. The migration of oil generated from the source rocks occurs over very short distances. The oil filling degree index (oil bearing thickness/P2l Formation thickness) is higher at a closer proximity to the source rocks, and where it is higher the hydrocarbon generation intensity of the source rocks is also elevated. In addition, the greater the hydrocarbon expulsion intensity of the source rocks, the higher the daily oil production values (ton/day) from prospect wells. Copyright © 2016 John Wiley & Sons, Ltd.