近日，坦桑尼亚达累斯萨拉姆大学Gama, John William团队，报道了坦桑尼亚东南部Mandawa盆地Bajocian-Bathonian序列(Mtumbei组)的烃源岩潜力。2024年9月23日，国际知名学术期刊《地球化学学报》发表了这一成果。

据悉，在过去的几十年里，坦桑尼亚海岸盆地的中侏罗世沉积物在区域古地理尺度上引起了人们的关注。这些沉积物，包括厚的黑色页岩，是在冈瓦纳超大陆断裂引发的广泛海侵之后沉积的。先前的研究表明，这些页岩具有良好到极好的源潜力，是有潜力的区域性烃源岩。然而，没有详细的地球化学研究对它们在Mandawa盆地中进行评估。

研究人员对两口井的岩屑样品进行地球化学评价，(1)限制Mandawa盆地Bajocian-Bathonian阶黑色页岩的有机质丰度、类型和质量，(2)重建Bajocian-Bathonian阶黑色页岩的热演化和沉积条件。通过总有机碳(TOC)分析测定其有机质丰富度，通过程序热解评估有机质质量和热成熟度。古氧化还原条件由能谱伽马射线测井数据得出的Th/U比值确定。地球化学数据显示，该区有机质含量(TOC)普遍偏低，且存在空间差异。

研究所分析的TOC含量沿地层波动，取值范围为0.13-3.59wt%，平均值为0.92wt%。然而，干酪根产率(S2)和氢指数(HI)普遍较低；S2和HI范围分别为0-1.95(mg HC/g)，平均为0.29(mg HC/g)；和0-92(mg HC/g TOC)，平均为24(mg HC/g TOC)。有机质主要由III型(易生气)至IV型(惰性)组成，经历了从边缘成熟到过成熟的广泛热蚀变。通过对比岩屑样品的TOC、Th/U和沉积物组成数据(碎屑和碳酸盐)，研究发现，有机质在低氧条件下发生了幕式沉积。这主要受海平面波动、构造和碎屑稀释作用的控制。

附：英文原文

Title: Source rock potential of the Bajocian-Bathonian succession (Mtumbei Formation) in the Mandawa Basin, SE Tanzania

Author: Gama, John William, Kiswaka, Emily Barnabas

Issue&Volume: 2024-09-23

Abstract: Over the past few decades, Middle Jurassic sediments in the Tanzanian Coastal Basin have gained attention on a regional palaeogeographical scale. These sediments, including thick black shales, were deposited following a widespread marine transgression initiated by the breakup of Gondwana supercontinent. Previous studies indicate that these shales possess good to excellent source potential, making them promising regional source rocks. However, no detailed geochemical studies have assessed them in the Mandawa Basin. In this study, geochemical assessment was caried out on cutting samples from two wells to: (i) constrain organic matter richness, type, quality, and: (ii) reconstruct thermal evolution and depositional conditions of the Bajocian-Bathonian black shales in the Mandawa Basin. Organic matter richness was measured using Total Organic Carbon (TOC) analysis, while the quality and thermal maturity of the organic matter were assessed through programmed pyrolysis. Paleo-redox conditions were determined from Th/U ratios derived from Spectra Gamma Ray Logs data. Geochemical data reveal that organic matter content (TOC) is generally low and varies spatially. The analysed TOC content fluctuates along stratigraphy with values ranging from 0.13 to 3.59 wt% with an average of 0.92 wt%. Whereas, Kerogen yield (S2) and Hydrogen Index (HI) are generally low; S2 and HI ranges from 0 to 1.95 (mg HC/g) with an average of 0.29 (mg HC/g), and 092 (mg HC/g TOC) with an average of 24 (mg HC/g TOC), respectively. Organic matter is mainly composed of types III (gas prone) to IV (inert) that have been subjected to wide range of thermal alteration ranging from marginally mature to over mature. Comparison of TOC, Th/U and sediment composition data (clastics and carbonates) derived from cuttings samples suggests episodic deposition of organic matter under sub-oxic conditions. These were mainly controlled by fluctuation in sea level, tectonics and clastic dilution.

DOI: 10.1007/s11631-024-00738-7

Source: https://link.springer.com/article/10.1007/s11631-024-00738-7