Characterization of organic matter and depositional environment of Tertiary mudstones from the Sylhet Basin, Bangladesh
Characterization of organic matter and depositional environment of Tertiary mudstones from the Sylhet Basin, Bangladesh
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DOI：
10.1016/j.orggeochem.2009.04.009
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发表时间：
2009-07-01
影响因子：
3
通讯作者：
Roser, Barry P.
中科院分区：
文献类型：
--
作者：
Hossain, H. M. Zakir;Sampei, Yoshikazu;Roser, Barry P.
孟加拉国锡尔赫特盆地是孟加拉盆地的一个次盆地。它包含非常厚（长达22公里）的第三纪地层序列，主要由砂岩和泥岩组成。锡尔赫特序列按升序分为Jaintia群（古新世-晚始新世）、Barail群（晚始新世-早中新世）、Surma群（中晚中新世）、Tipam群（晚中新世-上新世）和Dupitila群（上新世-更新世）。通过对从钻芯和地表露头采集的 60 个泥岩样品进行 C、N、S 元素分析、Rock-Eval 热解和气相色谱-质谱 (GC-MS) 分析，评估了有机质 (OM) 的来源和沉积的古环境。总有机碳（TOC）含量范围为0.11%至1.56%。大多数样品中的硫含量较低。锡尔赫特序列 TOC 含量随沉积速率的变化而系统变化，TOC 较低是由于喜马拉雅山快速抬升和侵蚀造成的高沉积速率产生的碎屑稀释造成的。该序列中的 OM 的特征是降植烷/植烷（Pr/Ph）、齐墩果烷/C-30 藿烷、n-C-29/n-C-19 烷烃、Tm/Ts 的系统变化[17]始新世中期至更新世期间α(H)-22,29,30-三降藿烷/18 α(H)-22,29,30-三降藿烷]和甾烷C-29/(C-27 + C-28 + C-29)比率。根据生物标志物代理，锡尔赫特序列的沉积环境可分为三个阶段。第一个时期（始新世中期到中新世早期），沉积完全发生在以海水为主的含氧条件下，有大量的陆地高等植物（包括被子植物）输入。第二阶段（中新世中期至晚期）主要由淡水缺氧条件组成，根据海平面变化，海水影响较小，稀释的有机质来自浮游植物，陆地高等植物的影响较小。第三阶段（中新世晚期）普遍存在缺氧淡水条件。这一阶段浮游有机质相对丰富，同时被子植物大量涌入。约 T 最大值450°C，镜质体反射率（R-o）约为。 0.66% 和甲基菲指数 (MPI 3) 约。 1 表示 OM 已成熟。地层序的下部（始新世中期到中新世早期）具有中等 TOC 含量和主要是陆地 OM，可能在研究区域及其周围产生了一些凝析油和石油。 (C) 2009 Elsevier Ltd. 保留所有权利。
The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene-late Eocene), Barail (late Eocene-early Miocene), Surma (middle-late Miocene), Tipam (late Miocene-Pliocene) and Dupitila Groups (Pliocene-Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography-mass spectrometry (GC-MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya.The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C-30 hopane, n-C-29/n-C-19 alkane, Tm/Ts [17 alpha(H)-22,29,30-trisnorhopane/18 alpha(H)-22,29,30-trisnorhopane] and sterane C-29/(C-27 + C-28 + C-29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. T-max values of ca. 450 degrees C, vitrinite reflectance (R-o) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area. (C) 2009 Elsevier Ltd. All rights reserved.