Geochemistry and kinetic models od Cretaceous samples

The source rock potential of Cretaceous organic rich whole rock samples from deep sea drilling project (DSDP) wells offshore southwestern Africa was investigated using bulk and quantitative pyrolysis techniques. The sample material was taken from organic rich intervals of Aptian, Albian and Turonian aged core samples from DSDP site 364 offshore Angola, DSDP well 530A north of the Walvis Ridge offshore Namibia, and DSDP well 361 offshore South Africa. The analytical program included TOC, Rock-Eval, pyrolysis GC, bulk kinetics and micro-scale sealed vessel pyrolysis (MSSV) experiments. The results were used to determine differences in the source rock petroleum type organofacies, petroleum composition, gas/oil ratio (GOR) and pressure-volume-temperature (PVT) behavior of hydrocarbons generated from these black shales for petroleum system modeling purposes. The investigated Aptian and Albian organic rich shales proved to contain excellent quality marine kerogens. The highest source rock potential was identified in sapropelic shales in DSDP well 364, containing very homogeneous Type II and organic sulfur rich Type IIS kerogen. They generate P-N-A low wax oils and low GOR sulfur rich oils, whereas Type III kerogen rich silty sandstones of DSDP well 361 show a potential for gas/condensate generation. Bulk kinetic experiments on these samples indicate that the organic sulfur contents influence kerogen transformation rates, Type IIS kerogen being the least stable. South of the Walvis Ridge, the Turonian contains predominantly a Type III kerogen. North of the Walvis Ridge, the Turonian black shales contain Type II kerogen and have the potential to generate P-N-A low and high wax oils, the latter with a high GOR at high maturity. Our results provide the first compositional kinetic description of Cretaceous organic rich black shales, and demonstrate the excellent source rock potential, especially of the Aptian-aged source rock, that has been recognized in a number of the South Atlantic offshore basins.本研究针对西南非洲近海深海钻探计划（Deep Sea Drilling Project, DSDP）井中采集的白垩系富有机质全岩样品的烃源岩潜力，采用整体热解与定量热解技术开展了系统分析。样品取自安哥拉近海DSDP 364站位、纳米比亚近海瓦尔维斯脊以北DSDP 530A井以及南非近海DSDP 361井中阿普第期、阿尔布期和土仑期的富有机质岩芯段。本次分析测试项目涵盖总有机碳（Total Organic Carbon, TOC）、Rock-Eval热解分析、热解气相色谱（pyrolysis GC）、整体动力学实验以及微型密封容器热解（Micro-scale Sealed Vessel Pyrolysis, MSSV）实验。所得结果用于明确上述黑色页岩所生成烃类的烃源岩石油类型有机相、石油组分、油气比（Gas/Oil Ratio, GOR）以及压力-体积-温度（Pressure-Volume-Temperature, PVT）行为差异，以服务于油气系统模拟工作。 研究证实，阿普第期与阿尔布期富有机质页岩发育优质海相干酪根。其中DSDP 364井中的腐泥页岩展现出最高的烃源岩潜力，其有机质以均一的Ⅱ型干酪根与富有机硫ⅡS型干酪根为主，可生成烷烃-环烷烃-芳香烃（P-N-A）低蜡原油以及高油气比的富硫原油；而DSDP 361井中富含Ⅲ型干酪根的粉砂质砂岩则具备生成气/凝析油的潜力。对上述样品开展的整体动力学实验结果显示，有机硫含量会影响干酪根的转化速率，其中ⅡS型干酪根的稳定性最差。瓦尔维斯脊以南的土仑阶地层以Ⅲ型干酪根为主；而瓦尔维斯脊以北的土仑阶黑色页岩则发育Ⅱ型干酪根，具备生成低蜡与高蜡P-N-A型原油的潜力，后者在成熟度较高时可产出高油气比的烃类。 本研究成果首次实现了白垩系富有机质黑色页岩的组分动力学表征，同时证实了南大西洋近海诸多盆地中已被识别的优质烃源岩潜力，其中尤以阿普第期烃源岩的潜力最为突出。