Impact of igneous intrusion on the thermal evolution of source rocks with different maturities: a case study of Fangzheng Fault Depression and Suibin Sag in north-eastern China

Two intruded sections, respectively from the Fangzheng Fault Depression and the Suibin Sag in the north-eastern China, were continuously sampled and geochemically analyzed in order to decipher the thermal impact of igneous intrusion on source rocks with different thermal maturities. The changes of organic matter maturity and hydrocarbon generation characteristics of source rocks affected by igneous intrusion were studied according to the geochemical indicators of the rocks. The results showed that the thermal impact of igneous intrusion is distinguishable with sample organic maturity or diagenesis phase at the time of intrusion. When a source rock in its early diagenesis phase with a low organic maturity is intruded by magma, the thermal aureole is on average the thickness of the intrusion body and the maximum enhanced maturity can be up to 2.5% R_o. In contrast, when a source rock in its middle-late diagenesis phase with a high organic maturity is intruded, the thermal aureole is on average twice the thickness of intrusion body and the maximum enhanced organic maturity can be up to 4.0% R_o. It was suggested that in the early diagenesis phase, source rock bears a high specific heat capacity and a low thermal conductivity, and the thermal impact is less; whereas in the middle-late diagenesis phase, source rock bears a low specific heat capacity and a high thermal conductivity, and the thermal impact is greater.