Rock-Eval analysis of the Lower Cretaceous (Middle Aption-Early Albian) Shahezi Formation in the Xujiaweizi Rift Depression, Songliao Basin, NE Asia

Well SK-2, drilled as a part of the International Continental Scientific Drilling Program (ICDP), is located at the Xujiaweizi Rift Depression, Songliao Basin, northeastern Asia. The Lower Cretaceous (Middle Aptian-Early Albian) Shahezi Formation was entirely recovered by SK-2, spanning a depth range from 3337 m to 5960 m. Rock Eval pyrolysis was performed on 247 core pieces of whole rock shales and mudstones from the Shahezi Formation, following the Chinese National Standard (GB/T 18602 2012, Rock-Eval analysis). The Rock-EVAL 6 was employed to measure the pyrolysis parameters at the Experimental Research Center of Wuxi Research Institute of Petroleum Geology of SINOPEC. Samples were first heated at 300 ℃ to release volatile hydrocarbons (S1) and then temperature programmed to 600 ℃ at 25/min to release pyrolytic hydrocarbons (S2). The Tmax is the temperature corresponding to the maximum yield of S2 hydrocarbon, which can indicate the thermal maturity of kerogen. CO2 released from the thermal degradation of organic matter (S3) is trapped and measured. The hydrogen index (HI, mg HC/g TOC) is obtained by dividing S2 × 100 by TOC. Dividing S3 × 100 by TOC yields the oxygen index (OI, mg HC/g TOC).

SK-2井作为国际大陆科学钻探计划（International Continental Scientific Drilling Program, ICDP）的钻探项目之一，坐落于东北亚的松辽盆地徐家围子断陷。该井完整钻取了下白垩统（阿普第阶中期—阿尔布阶早期）沙河子组的岩心，取心深度范围为3337米至5960米。研究团队对采自沙河子组的247块全岩页岩及泥岩岩心样品开展了Rock Eval热解分析，测试严格遵循中国国家标准GB/T 18602-2012《Rock-Eval分析》的要求。实验在中国石化（SINOPEC）石油地质研究院无锡实验研究中心完成，采用Rock-EVAL 6型仪器测定热解参数：样品首先在300℃条件下加热以释放挥发性烃类（S1），随后以25℃/分钟的升温速率程序升温至600℃，释放热解烃类（S2）。Tmax指S2烃类产率达到峰值时对应的温度，可用于指示干酪根的热成熟度。由有机质热降解产生的二氧化碳（S3）会被收集并定量检测。氢指数（HI，单位为mg HC/g TOC）通过将S2乘以100后除以总有机碳（Total Organic Carbon, TOC）计算得到；氧指数（OI，单位为mg HC/g TOC）则通过将S3乘以100后除以总有机碳（TOC）计算得到。