Geochemical characterization of black shales from the Tarfaya Basin

Organic geochemical and petrological investigations were carried out on Cenomanian/Turonian black shales from three sample sites in the Tarfaya Basin (SW Morocco) to characterize the sedimentary organic matter. These black shales have a variable bulk and molecular geochemical composition reflecting changes in the quantity and quality of the organic matter. High TOC contents (up to 18wt%) and hydrogen indices between 400 and 800 (mgHC/gTOC) indicate hydrogen-rich organic matter (Type I-II kerogen) which qualifies these laminated black shale sequences as excellent oil-prone source rocks. Low Tmax values obtained from Rock-Eval pyrolysis (404-425 MC) confirm an immature to early mature level of thermal maturation.
Organic petrological studies indicate that the kerogen is almost entirely composed of bituminite particles. These unstructured organic aggregates were most probably formed by intensive restructuring of labile biopolymers (lipids and/or carbohydrates), with the incorporation of sulphur into the kerogen during early diagenesis. Total lipid analyses performed after desulphurization of the total extract shows that the biomarkers mostly comprise short-chain n-alkanes (C16-C22) and long-chain (C25-C35) n-alkanes with no obvious odd-over-even predominance, together with steranes, hopanoids and acyclic isoprenoids. The presence of isorenieratane derivatives originating from green sulphur bacteria indicates that dissolved sulphide had reached the photic zone at shallow water depths (~100m) during times of deposition. These conditions probably favoured intensive sulphurization of the organic matter. Flash pyrolysis GC-MS analysis of the kerogen indicates the aliphatic nature of the bulk organic carbon. The vast majority of pyrolysis products are sulphur-containing components such as alkylthiophenes, alkenylthiophenes and alkybenzothiophenes. Abundant sulphurization of the Tarfaya Basin kerogen resulted from excess sulphide and metabolizable organic matter combined with a limited availability of iron during early diagenesis. The observed variability in the intensity of OM sulphurization may be attributed to sea level-driven fluctuations in the palaeoenvironment during sedimentation.

针对摩洛哥西南部塔法亚盆地（Tarfaya Basin）3个采样点的森诺曼阶/土仑阶（Cenomanian/Turonian）黑色页岩开展了有机地球化学与岩石学研究，以表征其沉积有机质特征。这套黑色页岩的整体与分子地球化学组成存在差异，反映了有机质数量与品质的变化。其总有机碳（Total Organic Carbon, TOC）含量较高（最高可达18wt%），氢指数介于400~800 mgHC/gTOC之间，指示富氢有机质（I-II型干酪根（kerogen）），表明这套纹层状黑色页岩序列属于优质的生油型烃源岩。通过Rock-Eval热解（Rock-Eval pyrolysis）得到的低Tmax值（404~425℃，原文"MC"应为笔误修正），证实其热成熟度处于未成熟至早成熟阶段。
有机岩石学研究表明，干酪根几乎完全由沥青质体（bituminite）颗粒组成。这些无结构有机聚集体极有可能是由不稳定生物聚合物（labile biopolymers，脂类（lipids）和/或碳水化合物（carbohydrates））经强烈改造形成，且在早成岩作用（early diagenesis）过程中硫元素被结合进入干酪根结构。对总抽提物进行脱硫处理（desulphurization）后开展的总脂类分析结果显示，生物标志物（biomarkers）主要包括短链正构烷烃（n-alkanes，C16~C22）与长链正构烷烃（C25~C35），二者均无明显的偶碳优势，同时还检出了甾烷（steranes）、藿烷（hopanoids）及无环类异戊二烯烃（acyclic isoprenoids）。源自绿色硫细菌（green sulphur bacteria）的异瑞尼烷（isorenieratane）衍生物的存在，表明沉积时期溶解硫化物曾在约100m的浅水环境中抵达透光带（photic zone）。此类环境可能促进了有机质的强烈硫化作用。对干酪根开展的闪热裂解气相色谱-质谱联用（Flash Pyrolysis GC-MS）分析结果表明，整体有机碳以脂肪族结构为主。绝大多数热解产物为含硫组分，如烷基噻吩（alkylthiophenes）、烯基噻吩（alkenylthiophenes）及烷基苯并噻吩（alkybenzothiophenes）。塔法亚盆地干酪根发生强烈硫化作用的原因是早成岩阶段存在过量硫化物与可代谢有机质，且铁元素供给有限。观测到的有机质硫化作用强度差异，可归因于沉积时期海平面变化驱动的古环境（palaeoenvironment）波动。