Geological characteristics and shale oil potential of alkaline lacustrine source rock in Fengcheng Formation of the Mahu Sag, Junggar Basin, Western China

Abstract

Shale oil is the main field of unconventional hydrocarbon exploration and exploitation. Additionally, deep (>4500 m) petroleum exploration in sedimentary basins is a worldwide frontier topic. The Fengcheng Formation in the Junggar Basin is deeply buried and formed in the special sedimentary environment of alkaline lakes. Studying this set of source rocks will provide insights into the deep unconventional hydrocarbon exploration in other basins with similar geologic settings worldwide. Therefore, this research takes Fengcheng shale as the object to systematically analyze the geochemical characteristics, reservoir quality, and shale oil resources’ potential based on total organic carbon (TOC), Rock-Eval pyrolysis, vitrinite reflectance (Ro), X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and other experimental methods.The Fengcheng shale has moderate organic matter abundance (0.5%–2.0%) and high hydrocarbon generation potential (averaging 3.95 mg/g). The shale is mainly comprised of type II kerogen, which can easily generate oil. Meanwhile, the source rock has reached the mature to high-mature thermal evolution stage. The mineral composition reveals the shale of Fengcheng Formation mainly contains higher values of carbonate minerals and lower quantities of clay minerals. Shale has poor physical properties, with porosity ranging from 1.0 to 12.0% (3.37% on average) and permeability generally lower than 0.1mD. However, the Fengcheng shale has a significantly higher brittleness than the North American marine shales and has good conditions for hydraulic fracturing. The pores developed in the shale are mainly intergranular, micro-fractures, and intragranular pores, while organic pores are rare. Generally, the hydrocarbon of the shale mainly exists in inorganic pores. Fractures are well developed in shale, and many micro-fractures and macro-fractures can be seen in the core. These fractures are important for hydrocarbons migration and accumulation. The shale oil saturation index OSI and S1 show that despite the migration of some hydrocarbons from the source rocks of the Fengcheng Formation, a large amount of crude oil is still retained, and a considerable part of the shale oil is movable oil. It indicates that Fengcheng shale is rich in shale oil resources and is a favorable target for shale oil exploration and exploitation.