Impact of hydrocarbon expulsion efficiency of continental shale upon shale oil accumulations in eastern China

Abstract

Core samples were taken from two formations (the Qingshankou Formation in northern Songliao Basin and the third member of the Shahejie Formation of Zhanhua Sag) to carry out a study of impact of hydrocarbon expulsion from continental shale upon shale oil accumulations in eastern China. Total Organic Carbon (TOC) and pyrolysis data of these samples were used to establish an evaluation criterion with absolute and relative oil content indexes for determining oil potential of shale. As a result, a correspondence relationship between shale oil content and organic types was established. Factors that may affect oil content of shale were discussed through study of fractures, pore development, minerals and lithological assemblage of the samples. The results show that oil content of mature petroleum source rocks is firstly controlled by fracture growth and secondly by pore development. They played both negative and positive roles in hydrocarbon flow and accumulation in shale. Overpressure caused by hydrocarbon generation and existence of organic acid corrosion are two important factors affecting the formation of fractures and pores in shale, respectively. Intervals with type I organic matter are not favorable shale oil exploration targets because they contain little oil. Shale with type I organic matter tends to generate large amount of hydrocarbon, but these hydrocarbon will escape with a high efficiency through over-pressurized fractures caused by hydrocarbon generation and lack of pore space, which in turn is the result of absence of oxygen that is essential for forming organic acid to create secondary pores in shale. On the contrary, intervals with type II organic matters are potential shale oil exploration targets for that they offered just right conditions for oil to stay inside: higher content of oxygen that is helpful in forming organic acid, moderate hydrocarbon generation capacity that will not cause over-pressurized fractures in shale with secondary pores. While intervals with type III organic matter bear no exploration significance due to the fact that this type of kerogen produces less oil because of low hydrocarbon generation capacity and unfavorable molecular configuration. Shale intervals with thin sandstone interbeds provide favorable passageways for dissolved carbonate by organic acid to flow out, avoiding secondary precipitation. The pores left behind can be excellent space for generated hydrocarbon to linger around. These interbeds are also favorable for later reservoir stimulation measures such as hydraulic fracturing. Shale oil exploration shall be focused on finding sweet spots with greater pores, which are the main reservoir space.