Organic geochemical characteristics and thermal evolution maturity history modeling of source rocks in Eocene Hetaoyuan Formation of Biyang Sag, Nanxiang Basin

In the Biyang Sag of the Nanxiang Basin, effective hydrocarbon source rock system mainly develops in the lagoon or salt lake facies strata in the lower and upper parts of the 3rd member of the Hetaoyuan Formation. The source rocks include dark gray, brown gray, gray mudstones, calcareous mudstones and dolomitic mudstones. The average organic carbon contents of the lower and the upper parts of the 3rd member of the Hetaoyuan Formation are 1.60% and 2.23%, respectively. Kerogens of the lower part of the 3rd member of the Hetaoyuan Formation are mainly of type Ⅱ, and part of them are of type Ⅲ. In the upper part of the 3rd member of the Hetaoyuan Formation, kerogens are mainly of type Ⅱ₁, and part of them are of types Ⅰ and Ⅲ. Pr/Ph ratio which illustrates depositional environment is low and gammacerane content is high. Organic matter maturity values (R_o) range from 0.5% to 1.4%, indicating for oil generation stage with current threshold depth of approximately 1 820 m. In the southern steep slope and deep depression, source rocks at the bottom of the 3rd member of the Hetaoyuan Formation enter the threshold for hydrocarbon generation at 37 Ma, and the corresponding depth and temperature are 2 500 m and 100℃. At the top of the upper part, the threshold for hydrocarbon generation is at 23 Ma, and the corresponding depth and temperature are 1 950 m and 90℃. The average organic carbon content of the 2nd member of the Hetaoyuan Formation is 1.85%. Organic matter types are mainly Ⅰ and Ⅱ, having not entered the mature threshold.