Formation conditions and distribution patterns of N1 tight oil in Zhahaquan Area, Qaidam Basin, China

Abstract

A systematic study on the formation conditions and distribution patterns of N1 tight oil in Zhahaquan Area of the Qaidam Basin was conducted by summarizing and analyzing documents related to geochemistry, sedimentary petrology, petroleum geology, lithofacies paleogeography, oil production test, well drilling and the well logging, etc. The four favorable conditions for the formation, migration, and accumulation of tight oil in the Zhahaquan Area are as follows: good source rocks, high quality reservoirs, display of the reservoirs, and abnormal high pressure of the stratum. Firstly, there are three sets of high quality source rocks that lay a good foundation for the formation of tight oil in this area. Secondly, high quality reservoirs consisting of multiple types of sand, such as beach sand, gravity current sand, and beach-dam sand are widely distributed, thickly accumulated and continuous, thus providing reserving space for the formation of tight oil in this area. Thirdly, the reservoirs of beach-bar sand links with the source rocks directly or links with the source rocks like fingers distributed in the deep or semi-deep lake or are interbedded with source rocks. Above source and reservoir dispositions are favorable for the formation of tight oil. Lastly, in the N1 downstream, tight oil reservoirs are at an abnormal high pressure of 1.3–1.5, which provides enough pressure for tight oil charging and reservoir forming. The study shows that tight oil is mainly distributed in the sag center and the slopes around the sag and has a huge resource potential.