Formation mechanism and enrichment conditions of Lucaogou Formation shale oil from Malang sag,Santanghu Basin

Tight reservoirs were found in the Permian Lucaogou Formation of Malang sag in Santanghu Basin,which is a great challenge to the conventional oil-gas exploration theory.Based on the core observation and thin section identification,then combined with geological conditions,analytical data and testing for oil,we investigated in detail the formation mechanism and enrichment condition of Lucaogou Formation shale oil.The research result showed that Lucaogou Formation shale oil is a kind of low-maturity oil generated at the low-maturity to early-mature stage of source rocks and retained in-situ.The oil was characterized by high density,high viscosity,low saturated/aromatic hydrocarbon ratio and high non-hydrocarbon/asphalt ratio.There are two main forms for the occurrence of the shale oil in source rocks,one is an absorbed state on kerogen surface and the other is a free state in matrix pores,interlayers between laminae and flat seams,and microfissures.Besides,the shale oil is continuously distributed in a large scale in source rocks,with no obvious oil-water boundary and being poor in oil/water differentiation.The oil-water ratio in production depends on the thermal maturity of source rocks and wells cannot get the deliverability without artificial transformation.The yield of a single well is low but stable for a long time.There are two controlling factors for the shale oil enrichment,one is good conditions for generating and entrapping large amounts of oils,and the other is favorable preservation conditions of oils in source rocks due to undeveloped faults for oil expulsion.