Mineral composition and organic geochemistry of the Lower Cretaceous Xiagou Formation source rock from the Qingxi Sag, Jiuquan Basin, Northwest China

Gang Gao,Guo Chen,Wei-Wei Zhang,Guo-Fu Ma,Tao Li,Ling-Zhi Hu,Zhi-Ming Yang,Jun Yang,Jian-Guo Wang

doi:10.1007/s12182-017-0213-y

Gang Gao, Guo Chen + Show 7 more

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https://doi.org/10.1007/s12182-017-0213-y

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Abstract

The Lower Cretaceous Xiagou Formation contains the major source rocks for the crude oils discovered in the Qingxi Sag and the South Uplift in the Jiuquan Basin, northwestern China. The Xiagou Formation source rock was formed in a closed, anoxic, reducing, alkaline lacustrine environment with a high salinity. Its high content of brittle minerals is favorable for the fracturing of reservoirs in source rock formations in the Qingxi Sag. The Xiagou Formation contains a great number of fair to excellent source rocks, and their organic matter (OM) came chiefly from plankton/algae and high plants as well as possibly bacterial organisms. The Xiagou Formation source rocks mainly contain Type II OM and some Type III and Type I OM, with good oil-generating potential. The source rock maturity is mainly in the early-mature and mature stages, and its Ro value corresponding to oil peak is about 0.8%, which is lower than classic oil peak Ro value of 1.0%; therefore, a great deal of hydrocarbon was generated before the classic oil peak Ro = 1.0%. Mature source rock in the Xiagou Formation tends to be distributed in the older members and at a greater depth. There is a better exploration potential of tight oil in the deep Qingxi Sag.

Highlights

Since the Laojunmiao oilfield was discovered in 1938, the Jiuquan Basin has had a history of oil exploration and development for nearly 80 years (Chen et al 2001)
The present study focuses on systematical research into mineral components, abundance, type, maturity of organic matter, and biomarker geochemistry of the source rocks, in order to improve the understanding of the oil exploration potential in the deep sag
The current study systematically analyzes the mineral composition and organic geochemistry of the Cretaceous Xiagou Formation source rocks, and it suggests the following major conclusions: Mineral composition and biological marker analyses show that the Xiagou Formation source rock was formed in a closed, reducing, anoxic, and alkaline lacustrine environment with higher salinity, with more brittle minerals which are favorable for fracture reservoirs within source rock formations in the Qingxi Sag

Summary

Introduction

Since the Laojunmiao oilfield was discovered in 1938, the Jiuquan Basin has had a history of oil exploration and development for nearly 80 years (Chen et al 2001). The Qingxi Sag, which covers an area of 230 km, is located in the southwest of the West Depression of the Jiuquan Basin (Chen et al 2001). It is one of the most important hydrocarbon-rich sags in the Jiuquan Basin, and it contains more than 0.16 billion tonnes of proved oil resources. It occupies 76% of the Western Depression of the Jiuquan Basin (You et al 2011) (Fig. 1a). In the South Uplift, there are two unconformities between basement and Oligocene, and between the Miocene and the Pliocene (Fig. 1b)

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