Characteristics and exploration prospects of low-maturity oil reserves in the Bohai Bay Basin: A case study on the southern sag of the Liaoxi Depression

Source rocks and geological conditions favorable for forming low-maturity oil and gas are noted in the Bohai Sea. However, systematic research has not been performed on these oil and gas formations and resource potentials. Thus, to understand the low-maturity oil and gas formation mechanisms in the Bohai Sea and to assess their resource potentials and exploration prospects, the characteristics and low-maturity hydrocarbon generation potential of source rocks in the southern sag of the Liaoxi Depression were examined via experimental analyses, such as chromatography-mass spectrometry of saturated and aromatic hydrocarbons. Meanwhile, typical reserves were systematically analyzed in vertical and horizontal directions and compared to reservoirs in adjacent sags on the same planes. The formation mechanisms of low-maturity oil and gas were investigated in-depth, and the contributions of these oil and gas in the southern sag of the Liaoxi Depression were determined. The following conclusions were drawn: (1) The Shahejie Formation in the southern sag of the Liaoxi Depression was formed in saline environments so that the resulting source rocks can generate large amounts of liquid hydrocarbons rich at low maturity. These source rocks are characterized by their potential to yield low-maturity oils at early stages. (2) The deep LD4 and LD5 oil reserves in the steep slopes of the southern sag of the Liaoxi Depression are of low maturity. The low-maturity oils were produced from the saline source rocks in the Shahejie Formation. With the help of faults connecting the source rocks and the traps, oil and gas generated at low-maturity stages migrated into the traps along the steep slopes to form low-maturity oil and gas reserves.